Significant ideas:

• Climate determines the type of biome in a given area, although individual ecosystems may vary due to many local abiotic and biotic factors.

• Succession leads to climax communities that may vary due to random events and interactions over time. This leads to a pattern of alternative stable states for a given ecosystem.

• Ecosystem stability, succession and biodiversity are intrinsically linked.

The big picture

Each one of us lives in a biome, you may not notice it because you probably live in a city or town, separated from the natural vegetation and environment. If you are not sure of what biome you are living in – look it up. Biomes cover large areas of the planet and you will recognise some of the characteristics in your own biome. However, biomes are not uniform and local abiotic and biotic factors will cause variations within the biome.

If you didn't notice the biome you will have noticed the climate around you and that will be the most important influence in the biome. Climate is the average atmospheric conditions that are experienced in a particular place on earth; weather on the other hand is what you get on a day-to-day basis. Climate is what you expect to get (based on the mean of past condition), weather is what you do get. In order to live in a particular biome, organisms will have certain characteristics suited to the climate and conditions of a that biome. For example desert plants are xerophytes – drought resistant, whilst tropical rainforest plants must shed water quickly as too much may damage their leaves.

Our climate is determined by many factors e.g. latitude, altitude and ocean currents; and all of them function within the atmosphere. All planets have an atmosphere and earth is no exception. Our atmosphere moderates the impact of solar radiation, makes life on earth possible and affects the climate.

Succession is the process in which an area changes through time. An area that is devoid of any living organisms will not stay that way for very long. Consider a volcanic eruption that covers an area of land with lava. Initially there will be nothing in the area – no plant and no animals. If you could take a time-lapse photograph every month for the next 100-150 years you would see a marked change. The bare land will be colonised by plants of increasing complexity. Eventually a climax community of plants and animals that are in balance with the climate will have developed.

As succession proceeds the abiotic factors stabilise the environment and the vegetation community matures and becomes more complex. More vegetation adds complexity to the ecosystem and provides greater habitat diversity which encourages species and genetic diversity. Succession leads to higher biodiversity and greater stability through the addition of nutrients and energy pathways.

Biome location I

Biomes are a major association of vegetation that share similar climate characteristics and so biome distribution is largely determined by the climate. To understand and explain the distribution of biomes we need to understand what determines climatic variation. Climatic variation is influenced by many factors including atmospheric circulation and latitude, the tilt of the earth’s axis in its orbit, ocean currents and topography. Some of these operate globally, others locally and as with everything else in ESS these are interlinked.

Atmospheric circulation and latitude

Heating of the earth’s surface is not the same spatially (latitude) or temporally (through the year). The sun is overhead between 23°N and 23°S of the equator therefore that is where heating is most intense. As you move away from the equator into higher latitudes the angle at which the suns rays arrive (the angle of incidence) is more oblique (slanted) and the heating less intense. This idea is shown in Figure 2. The lower the angle of incidence the greater the area over which the heating is spread = less warmth for the earth’s surface.

Methodology:

1. Stand In darkened room
2. Shine torch with narrow focused beam on a piece of paper at your feet.
3. Draw round the light beam as accurately as possible.
4. Put same piece of paper two meters away from you.
5. Shine torch on it.
6. Draw round the light beam again.

What do you observe?

You should see a difference in the area the beam of light covers, the more oblique the beam of light the larger the area it hits. In planetary terms that means the sunbeam is spread over a bigger area of land but still has the same amount of heating power. Therefore the heat is more dispersed and can not cause as much warming.

Theory of Knowledge

In this case inductive reasoning (specific - general) is probably pretty accurate. Consider the roles of inductive reasoning when applied to explaining why the biomes are where they are.

The impact of this on climate is seen in the amount of solar radiation received at the earth’s surface. The graph below shows that the greatest mean solar radiation is at the equator and the least is at the poles. But what does this mean for the climate?

The tri-cellular model

Fast facts

You do not need to remember these facts but they will help with your understanding of the tri-cellular model.

• Rising air creates low pressure at the surface of the earth and high pressure in the troposphere.
• Sinking air creates high pressure at the surface of the earth (low pressure in the troposphere).
• Air moves from high pressure to low pressure.
• The spin of the earth deflects air movement to the right of its path of motion in the northern hemisphere and to the left in the southern hemisphere (coriolis force). This is shown in the diagram below where none of the winds blow straight.

The tri-cellular model of atmospheric circulation explains how thermal energy is distributed around the planet and why the major biomes are where they are. It is composed of three large-scale cells that have remained fairly constant over millions of years. Weather patterns associated with the mid-latitude depression vary in the short term but their general location is controlled by the tri-cellular model and so, is predictable.

Theory of Knowledge

According to chaos theory a butterfly flapping its wings in New Mexico can cause a hurricane in China. How then can we predict the behaviour of the atmosphere?

The three cells are the Hadley cell, the Ferrel cell and the Polar cell (diagram below). Their size and characteristics are determined by the size of the earth, the depth of the atmosphere, the speed of earth’s rotation and heating patterns. Thermal differences drive the Hadley and Polar cells. The cells create the planetary wind belts which comprise of Trade winds, Westerlies and Polar Easterlies.

The Hadley cell

The Hadley cell is centred on the thermal equator, that is the point of greatest heating. This may or may not be the equator we know and recognise. The seasons are determined by the tilt of the earth. Hence the thermal equator shifts with the seasons.

SUMMARISE THE FOLLOWING INTO A CARTOON OR SONG LYRICS FOLLOWING THE JOURNEY OF A PARCEL OF AIR.

The equator receives the highest mean solar radiation that causes intense heating at the surface. This creates rising air and an equatorial low-pressure zone called the ITCZ (Inter Tropical Convergence Zone). This warm moist air rises to the troposphere where it then travels poleward as the jet stream. The poleward journey causes it to cool and at latitude 30°N or S it descends back to the surface creating the high-pressure zones characteristic of the subtropics. Once at the surface the air moves towards the equator as the Trade winds.

Impact on climate

The Hadley cell has a number of impacts on the climate and therefore the distribution of biomes.

• The rising air at the ITCZ creates a band of high rainfall and regular thunderstorms characteristic of the tropical rainforests e.g. Amazonia. Clouds block some of the sunlight causing a slight drop in temperatures.
• As the air descends over the subtropics it warms. Descending air is very stable and dry, hence the large deserts areas e.g. Sahara. Clear skies mean very high temperatures (higher than tropical rainforests).
• The shifting of the thermal equator moves the ITCZ and the low and high-pressure zones slightly creating the tropical rainforest, the savanna and the desert climates.
  • The tropical rainforest is always under the influence of the ITCZ and thus always experiences high rainfall.
  • The desert always sits under the falling limb of the Hadley cell creating hot dry conditions.
  • The savanna experiences the influence of both. In June when the Hadley cells moves into the northern hemisphere the ITCZ delivers the savannas rains. Then in December the shift of the ITCZ takes the rains to the southern savannas.

The Polar cell

This is another simple, thermally driven cell. Mid latitude regions (around 60°N and S) are the starting point for this cell. Warm air in the region rises to the troposphere where it tracks poleward and cools. Cooling in this cell is extreme (some say it is the cold that drives this cell not the heat) and at the poles the air descends and dries creating high-pressure zones. Air then moves out from the poles as the Polar Easterlies. The high pressure creates stable conditions and clear skies. The polar cell acts as a very effective heat sink, the coldest temperatures on earth have been recorded in Antarctica (-89°C). This balances out the incoming solar radiation at the equator.

The Ferrel cell

This is a slightly odd cell in that it appears to defy the laws of physics - it has rising air in cooler regions and sinking air in the warmer latitudes. To understand this you have to know about eddies and how they work, it is not necessary to do so for this course but if you are interested then carry out some independent research and come and talk to me about it. All you really need to know is that the Ferrel cell is the average motion of air in the mid-latitudes and that it creates the mid latitude westerlies.

Examiner Tip

You will not have to know the names of the cells but you may be asked to explain why the biomes are where they are and why they have the climate they do.

Biome location II

Earths tilt and seasons

The earth rotates on its own axis and it revolves around the sun, these two facts have a number of impacts:

• The rotation creates day and night.
• The revolution on the tilted axis create the seasons.
• The combination of rotation and revolution gives varying day length.

All of which impact the climate on a global and local scale.

​Rotation

The earth rotates on its axis once every 24 hours creating day and night. Due to the tilt of the axis day length varies, at the equator days and night are about 12 hours all year. At the poles day light can be 24 hours in summer and 0 in winter. This impacts the climate and the adaptations the organisms in the biomes have to make. Longer days mean more solar heating and higher temperatures, shorter days or no daylight at all means no heating for months at a time. In winter there is no solar radiation and no heating - this is one of the factors that contributes to the excessive cold of polar regions. In summer although the sun shines 24/7 the angle of incidence is very low so the heating power of the sun is very low too.

Revolution

As the earth revolves around the sun on an elliptical orbit it results in the seasons. The tilt of the earth’s axis results in the sunbeams hitting the surface at different angels. The diagram below shows four key positions of the earth relative to the sun. Lets take the southern hemisphere. In December it is tilted towards the sun so the angle of incidence of the solar radiation is higher and it will experience summer. In June it is tilted away from the sun, the solar energy is spread over a wider area and temperatures are lower.

This impact is less extreme the closer you are to the equator. At the equator the sun is close to overhead all the time so there is very little seasonal variation – tropical rainforests do not experience summer and winter. As you move towards the poles the length of the seasons change. Summer gets progressively shorter and winter progressively longer.

Biomes are affected by the length of the seasons. Mid latitude biomes such as temperate forests have a significant dormant season (winter) when low temperatures and limited availability of water puts the biome on hold as many of the life processes slow down.

Ocean currents and the maritime effect

The great ocean conveyor belt moves heat around the planet, moderates global climates and is largely responsible for supplying heat to the polar regions, thus controlling sea ice formation. Ocean currents are warm or cold and they flow at the surface or in the deep ocean. Surface currents have very clear impacts on climate. Some currents are well known for their impact on climate.

The Gulf Stream is a warm current that travels from the Gulf of Mexico up the Northwestern coast of Europe. It makes the climate of the area up to 4°C warmer than similar places on the same latitude.

The cold Humboldt Current that flows up the coast of South American past the Galapagos Islands, Ecuador and Peru has a cooling effect on the climate of the area. Due to the fact that prevailing winds blowing across the Humboldt Current cannot pick up any moisture (cold water does not evaporate) the current is also responsible for the dry coastal climates of the area and Atacama Desert.

International-mindedness

Ocean currents move water around the globe, that means they move pollutants around the globe too.

Proximity to the ocean in general has an impact on climate. Water has a specific heat capacity approximately double that of land. That means it takes twice as long to heat 1 kg of water as it takes to heat 1 kg of land. It takes a long time for the oceans to heat up in summer, keeping summer time temperatures lower. The reverse is true in winter when it takes a long time for the ocean to cool down, keeping coastal areas warmer. Maritime locations tend to have warm summers (19°C) and mild winters (1°C) whereas continental locations have cold winters (-20°C) and hotter summers (25°C).

Continental locations do not just have different temperatures to coastal location on the same latitude, they also have different rainfall patterns. Coastal locations will be influenced by the rain bearing winds that come off the ocean. As the winds first hit land they will have a heavy moisture load which they will drop as they pass over land. Continental locations are a long way from the ocean so by the time the air masses reach the continental interior they have lost most of their moisture. The Turkestan and Gobi deserts are both in continental interiors where the prevailing winds bring no moisture.

Topography

This is to do with the shape of the land, mountains, altitude and aspect. The impacts these features have on climate are at the local scale.

Mountains and altitude

In general the higher you ascend the lower the temperature. For every 100 m rise the temperature falls by 1°C. The reason is you are moving further away from the atmosphere’s source of heating. This is somewhat counterintuitive but the source of the atmospheres heat is the earth. Incoming shortwave light radiation is re-radiated as outgoing long-wave heat.

It is like moving up through the latitudes but on a smaller scale. For instance as you travel up Mt Kilimanjaro (Tanzania and Kenya) you will go through rainforest at the base, into heathland, grassland, moorland, alpine desert and finally glacier and snow.

Mountains and precipitation

This video gives a simple explanation of the relief rainfall and the rain shadow effect.

If a mountain range is long enough, wide enough and high enough it will form a barrier to the passage of air masses.

• The mountain range will force air to rise over it and as air rises it expands and cools.
• Cool air holds less moisture so it condenses to form clouds and it will rain or snow (depending on the height and latitude of the mountain range).
• Once over the mountain range the air flows down the other side and as it does it compresses and warms.
• The air mass has already lost moisture on the windward side* of the mountain and warm air holds more moisture so it can not form clouds or rain on the leeward side** .
• A rain shadow develops on the leeward side of the mountain range (diagram below) and deserts common in these areas.
• The effect is more marked in coastal mountain ranges e.g Mojave Desert, California.

* Windward side is the side of the mountain range that faces the direction the wind is coming from.

** Leeward side is the other side of the mountain, away from the wind.

Aspect

Aspect is the direction in which the slope face so it has a very localised impact on climate. In the northern hemisphere the north side of a slope is often more shaded and shaded for longer. The south-facing slope receives more solar radiation because the slope is tilted towards the sun and is not shaded. The further you get form the equator the more pronounced the impact is. This can affect the microclimate. The south facing slopes will be warmer and drier with higher levels of evapotranspiration, this will naturally have an impact on what vegetation can grow and cause variations within biomes.

Biomes

The video below gives a brief overview of the biomes topic:

Definition

A biome is a collection of ecosystems that are classified according to their predominant vegetation; they share similar climatic conditions and organisms that have adaptations to the environment.

Biomes are one level down from biosphere and they cover very large areas of the earth’s surface. Each biome will have a particular set of abiotic factors and characteristic limiting factors, productivity and biodiversity. Within the biome there will be numerous ecosystems.

There is little agreement about the classification of biomes but for the purposes of this course there are five categories of biome that you need to know about:

• Aquatic - which are further subdivided into:
  • Freshwater: ponds and lakes, streams and rivers and wetlands such as bogs and swamps.
  • Marine: deep ocean, coral reefs, estuaries and mangrove swamps.
• Forest – tropical rainforest, temperate forests and boreal or taiga.
• Grassland – savanna and temperate.
• Desert – hot, coastal and cold.
• Tundra – arctic and alpine.

The map above is an approximate representation of where most of the major biomes are located. As you can see, there are more than five categories, some have been combined, others not and the aquatic biomes are missing totally. The Internet has a few maps showing aquatic biomes – try this one.

Biome distribution

The importance of abiotic factors such as temperature, precipitation (water) and insolation (sunlight). So lets see how they come together to influence biome distribution.

Theory of Knowledge

The diagram below shows the distribution biomes according to rainfall and temperature. How do we know these are the two most important things in determining the location of biomes?

The diagram above shows how mean annual temperature and precipitation combine to effect biomes distribution. Temperatures above 20°C can have a wide range of biomes from hot desert through to tropical rainforest and precipitation levels between 150 and 200 cm can support various types of forest. The graph shows that in general forests need higher levels of precipitation but can survive at pretty low temperatures. Deserts on the other hand are found in areas ranging from 30 to -7°C. The reasons for the high temperatures are discussed in Case study: Desert biome. If you want to learn more about cold deserts you could go to University of California Museum of Paleontology.

Examiner Tip

You could be given a graph like the one below and asked to describe what it shows. Pick out trends as the previous paragraph has.

Qu.) Figure 1 shows the temperatures in four different biomes. What are the trends? What are the similarities and the differences?

Answer: The graph shows the mean annual temperatures in four very different biomes. The two drier climates (desert and tundra) show more extreme temperature ranges and they both show maximum temperatures around June to September. The tropical rainforest shows no seasonal variations in temperature and the savanna shows lower temperatures when deserts and tundra show their highest.

Not all precipitation that falls will be available to the vegetation. In the tundra the precipitation is snow, the snow falls to the ground where it remains frozen until the summer thaw. In hot deserts precipitation may not even make it to the ground as the high temperatures cause it to evaporate before it reaches the surface. Even if the precipitation makes it to the ground temperatures are high enough to cause evaporation of soil and surface water.

Precipitation/evaporation (P/E) is a way of assessing how much water stress there is in a biome:

• P/E below 1 is indicative of water shortages (deserts). Rainfall may soak into the soil but high evaporation rates will draw it back to the surface and possibly cause salinization.
• P/E ratio of 1 will have good soil moisture conditions leading to fertile soil and good water available to the plants.
• P/E ratio above 1 suggests waterlogged or heavily leached soils.

Productivity

Productivity varies considerably between biomes. Net primary productivity (NPP) of biomes is high at the equator due to ideal growing conditions of high temperatures with plentiful sunlight and water supply. NPP drops towards the poles as the growing conditions become less favourable. Deserts are close to the equator with favourable temperature and sunlight levels but they have low NPP due to water scarcity. Grasslands have lower productivity due to seasonal rainfall patterns and a dry season. At the poles NPP is low due to low levels of sunlight, low temperatures and lack of water – it is frozen most of the year.

International-mindedness

The diagram above shows the open oceans have the highest NPP - most of the ocean is not "owned" or controlled by any particular country. Does that mean we can exploit it at will?

Climate change and biomes

Climate change is impacting the distribution of biomes and it is estimated that between one tenth and a half of the earth will be effected by biome shifts.

Changes in precipitation, temperature and humidity are making some areas less favourable for the vegetation of some biomes. The result is a move to higher latitudes (poleward) and higher altitudes (up mountains). E.g. in the African Sahel, woodlands are being replaced by grasslands and in the Arctic, scrublands are moving in to tundra areas.

As ice and permafrost melts in high altitudes and latitudes new areas of land are exposed. The new areas may support new tundra and alpine biomes or accommodate currents ones as they shift out of their existing locations.

Melting ice caps causes rising sea levels. This will impact both aquatic and terrestrial biomes. As shallow coastal environments get deeper the shallow water coastal biomes such as coral reefs will be destroyed or shift. Mangrove swamps will be flooded but can move in to the new shallow water (that used to be land).

Increase in water temperatures may destroy corals reefs by bleaching. Increased acidification of the oceans will damage plankton and have major impacts on all aquatic biomes. Changes in the great ocean conveyor belt and the associated ocean currents are likely to bring benefit to some biomes but destroy others.

Examiner Tip

To score well in exam questions about biomes you need to be able to give examples in as much detail as possible. Case studies help this but a good study aid is fact sheets. Produce a half to one page fact sheet for the five major classes of biomes. On it include statistics and brief notes on:

• Location
• Climate
• Limiting factors
• Productivity
• Biodiversity
• Vegetation structure
• Named plants and animals

Pick examples from biome areas that are close to you or that you find interesting - it helps you remember the facts!

Case study: Forest

A forest is any large area covered by woody vegetation. They are the dominant biome making up 80% of earth’s biomass and producing 75% of its gross primary productivity. There are three major forest biomes (Table 1).

Revision: can you sketch a world map and indicate on teh location of these biomes from memory? How accurate were you?

Temperate deciduous forest

Deciduous trees are broad leaf trees which lose their leafs in winter e.g. oak, ash, beech and elm.

Location

• Mid latitudes between 40 and 60° north and south of the equator.
• Europe, Asia, Southern slope of the Himalayas and North and South America.
• E.g. Sherwood Forest, UK.

• Climate
• Rainfall:
  • High at 500 – 1,500 mm yr-1.
  • Falls throughout the year with a summer maximum.
• Temperatures:
  • Winter temperatures are cool but usually stay above 0°C.
  • Summer temperatures are between 20 and 25°C.
• There are four seasons, winter, spring, summer and autumn.
• Growing season is between 140 – 275 days.

Productivity

Primary productivity is relatively high at just over 100 billion Kcal yr-1. Most forests have high productivity due to the large, dense, layered vegetation. The deciduous forest canopy is more open the tropical rainforests and so there is a rich understory layer.

Limiting factors

As with all forests sunlight is limited to the lower layers. The canopy is not continuous in temperate deciduous forests but it does partially shade out the understory and so productivity is reduced below the canopy.

Winter temperatures drop to around zero. That is not conducive to photosynthesis so the vegetation goes into a dormant phase where primary productivity is very low.

Nutrient cycle

In temperate deciduous forests Gersmehl nutrient cycles is relatively well balanced with no one store holding the majority of the nutrients. The diagram below explains the balance of flows and stores.

Plants and animals

The dominant vegetation type will vary depending on the continent but they typically have three or four layers, authorities do not agree on how many or what they are called.

• Canopy layer: A moderately dense layer made up the taller deciduous - maple oak, birch, or gums depending on the location. Not many animals live in the canopy because conditions are harsh. The layer is exposed to winds and airborne predators.
• Shrub layer: The canopy allows sunlight to penetrate so smaller trees, saplings and low growing woody plants such as rhododendrons, blackberries and azaleas are found here. Animals are frequent in this layer; they are protected from airborne attack by the canopy and high enough above ground to avoid forest floor predators. The animals found here are dependent on the location but include numerous birds and insects, squirrels, porcupines and other small mammals.
• Herb layer: Here you will find grasses, ferns, lichens and wildflowers. Along with snakes, mice, amphibians and insects.
• Ground layer: Mosses, lichens and liverworts.

Theory of Knowledge

To what extent does the lack of agreement of naming the forest layers hinder our knowledge and understanding?

Plant adaptations

• During times of low temperatures deciduous trees shed their leaves to avoid moisture loss and tissue damage. The nutrient and water supply to the leaf is cut off, it dies, changes colour and falls to the ground.
• In spring the first leaves are adapted to catch light and photosynthesize and grow quickly. The leaves are thin and broad to capture lots of light and grow quickly.

Animal adaptations

The winter season present problems to animals as well as plants so they have a few options:

• Many of the birds migrate to warmer climes. For instance the song thrush migrates from Britain to the Iberian Peninsula.
• Many of the mammals do not have the option to migrate so they hibernate e.g. deer mice and hedgehogs hibernate.
• Some mammals and birds store food in the months of plenty and use the stores to feed during winter. Squirrels are probably the most famous for losing their stash of nuts.

Biodiversity

Biodiversity will vary between deciduous forests but in general diversity is high for plants, invertebrates and small mammals. There is also a wide variety birds, reptiles and amphibians. Variations in soil can lead to specilised plants in the herbaceous layer as well as additional invertebrates.

Problems

The temperate mid-latitudes have seen human occupation for thousands of years resulting in forest destruction. Initially land was cleared for agriculture, then expanding towns and industry. This type of clearance continues today and we are left with severely fragmented woodlands. In addition to deliberate clearance, large tracts of forest are being lost to acid deposition, which destroys forests and poisons the soil.

Non-native plant and animal species can destroy the balance of the forests. Humans introduce plants and animals to solve an existing problem only to create another one as the introduced species out-competes native species. Black Locusts were introduced to Wisconsin in the early 1900s, it grew rapidly and with no natural controls took over areas where deciduous forests should have developed. Similar problems are seen with introduced insects.

Global warming, disease and hunting are also causing problems in temperate deciduous forests. Each factor on its own is a problem but when they act together in concert the effects can be devastating.

Important

This is one case study for the forest biome. Now it is your turn. You need a case study of a contrasting biome, that could be the boreal forest or the tropical rainforest. There is plenty of information about the tropical rainforest in subtopic 1.3 and 3.3. You could use this format with the side headings as a base for a second forest case study.

nternational-mindedness

People in different parts of the world will have different EVSs to forests, how do this impact how they are protected?

Case study: Grassland

Grassland biomes are large rolling areas of grasses, sedges, rushes, flowers and herbs. They are found on every continent except Antarctica. Latitude, soil and local climatic variation will determine the characteristics of a particular area within the biomes. Rainfall is enough to support grasses but not trees and drought and fire ensure very few trees grow. Many grasses need fire as a natural part of their life cycle in order to burn off the dead above ground biomass and leave the roots ready for fresh growth. There are two types of grassland:

• Tropical grassland or savanna located between the tropical rainforest and the deserts. The major savanna areas are in African, India and Australia.
• Temperate grassland located between the tropical deserts and the temperate forests. They spread throughout central Asia and Europe (Steppes) South America (Pampas) and North America (Prairies).

Tropical grasslands: Australian savanna

Location

The Australian savanna is found between 10° and 20° south of the equator and makes up 23% of Australia’s land mass. It is located in the northern sections of Western Australia, Northern Territory and Queensland (Figure 2). Although the areas are referred to as tropical grasslands, grass is not the only vegetation, it is just the dominant one.

Theory of Knowledge

Australian Aborigines are famous for "Dreamtime" the start of creation. Consider the role that these grasslands may play in aboriginal knowledge. It will be very different to the role grasslands play for most of us.

Figure 2. Location of Australian savanna.

Climate

Figure 3. Climate graph for Australian Savanna.

The climate has two seasons:

Hot and wet season

• November – April: approximately 800 mm rain.
• Rain falls in heavy bursts from thunderstorms and monsoonal depressions associated with the tropical cyclones of the areas. This can cause problems with flooding.
• Temperatures are general in the high 30s (°C) but can get up in to the high 40s; so conditions are hot and humid.
• This is the growing season for plants.

Warm and dry season

• May – October: less than 50 mm rain throughout the dry season.
• This is cooler (though not cool) with temperature in the low 30s (°C) but can dip in to the 20s.

The seasons are reversed in the northern hemisphere savanna.

• May to November is the wet season.
• April to September is the dry season.

Productivity

Productivity is considerably lower than that of the forest biomes at only 40 billion Kcal yr-1. Grasslands lower productivity can be attributed to the fact that the dominant vegetation is grass, which has a much lower biomass than forests. The Savanna also has a long dry season when the vegetation dies back completely and there is no productivity.

Figure 4. Australian savanna.

Limiting factors

Savanna soils are poor and lacking in nutrients and organic matter because the high rainfall in the wet season washes out the nutrients. This limits the variety of plant species that can survive in the savanna.

Rainfall only occurs for half the year and in the dry season there is almost no rain and very limited water availability. Rivers and streams may dry up forcing migration to other areas. The only plant species that can survive are those with deep root systems that can access ground water.

During the dry season wildfires are common and it is this that limits tree growth. Human induced or natural wildfires sweep through savannas very quickly. Fire is beneficial to grasses as it burns off the dead organic matter that accumulated in the wet season but died back in the dry season. Many scientists believe that it is the fires that maintain the open grasslands by limiting tree growth.

Nutrient cycle

Figure 5. Nutrient cycle for savanna.

The nutrient cycle is a little more complicated in the savanna as there is a great deal of seasonal variation. Check out the section on plants adaptation, as there is more in there to explain the nutrient cycle.

Plants and animals

Savannas are very complex with a wide range of ecosystems that are dependent on local factors. Different soil and rainfall patterns will produce different plants. There are many grasses, shrubs, sedges and vines. Examples include Pink Pimelea – a small spreading plants that forms clumps, Tunbridge Buttercup, Rose Mallee – a low tree that reaches about four meters in height.

Trees in the savanna are small and widely spaced due to inadequate rainfall and frequent fires. Watercourses support larger trees. The majority of the trees are eucalyptus that remain green throughout the year.

Marsupials are the dominant group of animals in this area. Marsupial young are born underdeveloped then move in to the mother’s pouch where they attach to a nipple and continue to grow. Examples include Gray kangaroo, koala, wallaby, possums, sugar gliders and qoull. Most of the animals live in or near trees that provide them with water, food and shade. Mammals in this biome are predominantly marsupials but there are also bats such as the flying fox.

Figure 6. Northern quoll.

The Australian savanna supports numerous birds, many of them endemic to the area. Seed-eating birds include parrots, pigeons, quails and finches. Predatory birds such as the brown falcon make the most of the regular fires to pick off insects and snakes as they escape the flames. The kookaburra and the emu are also found in the savanna.

Reptiles, for which Australia is famous, are common in the savanna. Crocodiles, lizards and venomous and non-venomous snakes are all present. The eastern brown snake is the second most venomous terrestrial snake in the world and it is found in the Australian savanna.

In addition to these larger obvious animals there are also over 100 species of frogs found in the savanna region. Insects are the clean up crew of the savannas and the huge termite fields of Australia are well known to many people. Termites that build these mound clear up much of the dead organic matter in the savanna.

Figure 7. Termite mounds.

Plant adaptations

All organisms in the savanna must adapt to the strong seasonality of rainfall:

• Grasses have deep roots to access deep soil moisture and avoid fire damage. This puts a large proportion of the nutrient below ground.
• Many shrubs and trees need the heat of the fire to burst the seedpods, seeds can then make the most of the nutrients liberated by the fire.
• Grasses are able to grow new green shoots from their roots after fire has passed by.
• Flowers are brightly coloured to attract insects for pollination.
• Plant leaves are narrow to minimise moisture loss.

Animal adaptations

• Many animals are well camouflaged in the grasses to avoid predation. Kangaroos use speed to escape predators.
• Some animals are nocturnal to avoid predators and the heat of the day.
• Animals can survive cool fast moving fires by hiding in their burrows; slower hotter fires will kill animals unless the animals can escape.
• Burrows also protect them from the heat of the day.
• Goannas (type of monitor lizard) need very little water and get most of their moisture needs from the animals they eat.

Food web

Figure 8. Australian savanna food web.

Problems

• Non-native pests and weeds are causing problems to the native species of plants and animals. Weeds such as mimosa are out-competing native grasses and shrubs. Feral animals such as buffalo, pigs and cane toads are out competing the native wildlife.
• Human induced fires are often set at the wrong time but the fires burn too hot and are set too frequently so cause damage greater than the native plants can cope with.
• Large areas of the savanna are being cleared for cattle grazing.
• Overgrazing by cattle is damaging the environment and making it impossible for plants to seed and mature.

International-mindedness

The grassland biome stretches across continents. Consider the different approaches to grassland management in different areas of the world.

Figure 9. Feral pigs in Northern Australia.

Important

This is one case study for the grassland biome. Now it is your turn. You need a case study of a contrasting biome, that would be the temperate grasslands. You could use this format with the side headings as a base for a second grassland case study.

Case study: Deserts

In simple terms deserts are areas of the world where there is less than 250 mm of precipitation in a year. The aridity (dryness) is caused by:

• Descending air – the descending limb of the Hadley and Polar cells generate some of the biggest deserts in the world. E.g. Sahara (Africa), Great Sandy Desert (Australia), Antarctic Desert.
• Coastal rain shadows – areas of the world that are on the leeward side of mountains receive little or no rain. E.g. Atacama and Patagonian (South America), Mojave (North America).
• Continentally – some deserts are in the continental interior where the prevailing winds bear no moisture. E.g. Turkestan and Gobi Deserts.
• Cold ocean currents - air masses that pass over cold ocean currents are unable to pick up any moisture so they have no chance of bringing any rainfall to the land they pass over. E.g. Namib Desert (Africa) and Atacama Desert (South America).

International-mindedness

Deserts are found on every continent. How do different people view deserts? What EVS's apply to deserts?

Desert classification is very complicated so to keep it simple; here we are just going to look at hot and cold deserts. Some deserts are both hot and cold depending on the season. The worlds deserts are located on the map in Biomes: A general introduction.

Hot desert: Sahara

A short video introduction to the Sahara Desert with some interesting facts and figures.

Location

The Sahara Desert occupies the northern third of Africa and covers the majority of Algeria, Egypt, Libya, Mauritania and Niger along with parts of Chad, Mali, Sudan and Tunisia. Running from the Atlantic Ocean (West) to the Red Sea (East) up to the Mediterranean in the North and the Sahel in the South it cover over 9 million square km.

Figure 1. Location of the Sahara and other African deserts.

Climate

Rainfall

• Minimal, most of the Sahara receives less than 100 mm yr-1 but over half of it gets less than 25 mm annually for many years in a row.
• Northern and southern areas along with the mountains do get more rainfall.
• Rain usually falls in short torrential downpours.
• Rain that does fall is usually in the summer months as the ITCZ swings into the northern hemisphere.

Temperature

• Mean daily maximum temperatures are around 38°C between May and July.
• The highest recorded temperature in the Sahara Desert was 58°C in 2012.
• Some areas experience temperatures in excess of 40°C for 3 – 5 months of the year.
• Diurnal temperature ranges can be up to 20°C, much greater than the mean annual temperatures. This is due to the lack of cloud cover and low humidity. There is nothing to block incoming solar radiation all day and nothing to keep in the out going heat.

Figure 2. Climate graph for Timbuktu (southern edge of the Sahara).

Productivity

As you would expect the productivity levels in deserts is very low – less than 200 Kcal m-2 yr-1. Lack of water restricts plant growth and when it does rain it is in the hottest part of the year meaning that the P/E ratio is unfavourable. Much of the rainfall evaporates before the plants can use it.

Limiting factors

Nearly all resources are limited in the desert, water, soil nutrients, food etc.

• Due to the low water availability the primary producers are very limited and only a few plants (and animals) are adapted to survive in such conditions. This means that the base of the food chain is highly restricted and productivity (NPP and NSP) is very low.
• The Sahara has very high temperatures which few species can tolerate.
• High temperature mainly due to the impact temperatures have on moisture loss.
• Soil is poor quality due to high levels of salinity. P/E ratio is below 1 and that means the dominant water movement is upwards and nutrients are drawn to the surface where the water evaporates leaving behind mineral salts.

Nutrient cycle

Some authorities contend that the nutrient stores and flows in the desert are so small that the nutrient cycle is not applicable.

Figure 3. Desert biome nutrient cycle.

Plants and animals

Although productivity in the Sahara is low the diversity is reasonably high. There are 2,800 vascular plants of which about 25% are endemic. Acacia trees and palms are the dominant trees but only where water is sufficient (oases), there are also succulents (cacti), spiny shrubs and grasses.

Surprisingly, animals are very varied in the Sahara.

• Numerous species of mammals:
  • Foxes - Fennec fox, pale fox, sand fox.
  • Small mammals – gerbil, desert hedgehog, shrew, slender mongoose, jerboa mouse, hyrax.
• Antelopes – addax, dorcas gazelle, dama deer.
• Predators - Saharan cheetah (endangered), North eastern African cheetah, spotted hyena, jackal.
• Reptiles – monitor lizards, spiny- tailed lizard, sand viper, horned viper, sidewinder and desert crocodiles.
• Birds – ostrich, Nubian bustard, fan-tailed raven, finches.
• Insects – death stalker scorpion, harvester ants, grasshoppers, beetles and a variety of spiders.

Figure 4. Fennec foxes close to burrow.

Plant adaptations

• Low profiles – growing close to the ground avoids the strong desert winds that increase evapotranspiration rates and desiccate plants very quickly.
• Store water in thick stems - cacti and other succulents.
• Long roots to find deep ground water.
• Lateral roots to catch as much water as possible when it rains.
• Thick leaves or needles to deter herbivores from grazing and minimise water loss.
• “Fur” on cacti traps morning dew.

Figure 5. Cacti "fur" can catch the morning dew.

Animal adaptations

• Nocturnal habits avoid predators and the heat of the day e.g. kangaroo rats.
• Many animals do not have to drink as they get all the water they need from plants e.g. Jerboa.
• Large ears allow for heat exchange and acts as a cooling mechanism e.g. Fennec fox (Figure 4).
• Ostriches have long legs, which keep them above the extreme heat of the sand and allow them to spot predators.
• Sidewinders move in a sideways motion and minimise the amount of body in touch with the hot sand.
• Flat feet of numerous herbivores prevents them from sinking in the soft sand e.g. camels and addax antelope.
• Thick hairs in nose and ears along wth thick eyelashes keeps out sand e.g. camels.
• Many animals have fat stores in their tails (lizards) or humps (camels).

Food web

Figure 6. Sahara Desert food web.

Problems

The desert is an inhospitable biome so threats are somewhat limited, especially in the truly hostile interiors. However along the margins and around oases the pressure for resources is significant. Here humans and wildlife are in conflict. People want the water for their own use but the natural inhabitants also need water. Projects to pump water for irrigation from underground aquifers is causing salinization and soil degradation.

Many desert species populations have been severely reduced by hunting. Antelopes and gazelles such as the addax are hunted. The addax is critically endangered due to its magnificent horns. The Egyptian tortoise is hunted for the pet trade. The Saharan cheetah is now critically endangered due to habitat loss and hunting.

Figure 7. The addax antelope.

Theory of Knowledge

To many people the hot deserts are useless - a barren waste land of no value. Consider how the indigenous people view the deserts.

Survival strategies

Below is an excellent video explaining survival strategies and the survivorship curve.

Before we move on to succession we need to understand survival strategies. There are different approaches to life’s question - “what is the best way to get your genes into the next generation?” The answer comes down to a trade off between quantity and quality.

There are K-strategists (K-selected species) which produce very few offspring but they increase the quality of them by investing in a lot of parental care. In this case quality means fit for purpose – survive long enough to reproduce themselves. On the other hand r-strategists (r-selected species) focus on increased quantity of offspring at the expense of quality. With little or no parental care survival chances are low but high numbers of off spring ensures at least some survive.

Figure 3. Sunflowers; r-selected species.

Both strategies are successful but in different types of environment. The ability to reproduce large numbers of offspring quickly (r-strategist) is beneficial in unstable, unpredictable environments. The strategy is – flood the habitat with as many offspring as you can so that some are bound to survive. The early stages of succession are unstable, harsh environments thus r-selected species are common in the pioneer stages.

K-selected species tend to produce few offspring at a time but they invest a lot of time and energy in looking after them to ensure they survive. To be able to use this strategy the environment needs to be stable. In succession stability increases with time so K-strategists are more common in the climax community.

Figure 4. Elephants; K-selected species.

This shows the two extremes of the survival strategy, in actual fact it life is a continuum and organisms usually display a mixture of strategies.

The different survival strategies can be shown graphically in a survivorship curve (Figure 5). The graph shows the number of individuals in a population of 1,000, that are expected to survive to a certain age. The y-axis is on a logarithmic scale and shows 1,000 individuals. The x-axis is the organisms relative age as a percentage of their maximum lifespan.

• Type I survivorship curve: this is indicative of the K-selected species. The curve starts out very flat showing a high survival rate in early life. This long life expectance causes the line to have a sharp drop at the end as the mortality increases dramatically.
• Type II survivorship curve: this shows the middle ground with a more or less constant mortality rate throughout the organisms life. That is they are as likely to die at birth as they are at old age.
• Type III survivorship curve: this is typical of the r-selected species. The curve drops sharply immediately showing very low survival rates after birth. Very few individuals make it into later life.

Figure 5. Survivorship curve.

Theory of Knowledge

In the model of r-strategist and K-strategists there is no clear categories, just one long continuum. This type of model does not advance out knowledge and understanding. Discuss.

Succession and zonation

Succession and zonation are often confused. They are similar concepts but they are not the same thing.

• Succession is a change over time; zonation is a spatial change in response to changing conditions.
• In succession the changes in the plant community cause changes in the physical environment but in zonation the plant communities adapt to the different environmental conditions.
• Succession may occur alone – on fresh lava the whole area will go through succession through time.
• Zonation occurs without succession, the rocky shore demonstrates zonation due to tidal changes not succession.
• In a sand dune succession and zonation are seen together. The area closest to the sea is at the beginning of succession and is largely sand. As you move away from the sea the sand dunes succession has progressed further and you move through areas of grass and in to woodland (the oldest dunes). The physical conditions change as you move away from the sea so zonation also appears.

Figure 1. Sand dunes show succession and zonation.

Theory of Knowledge

Chicken and the egg - does succession cause changes in climate and soil or do changes in climate and soil cause succession?

​Zonation

Zonation is the change in a vegetation community along an environmental gradient. The change may be caused by changes in altitude, depth of water, tidal level, distance from the shore etc. Zonation changes are spatial and are determined by changes in the abiotic factors.

Altitudinal zonation is triggered by changes in climatic conditions with increased altitude. In tropical regions the top of a mountain may have the same vegetation communities as you would find in the tundra, e.g. Kilimanjaro. Changes up the mountain may include:

• Shortening of the growing season caused by lower temperatures and longer periods of freezing.
• More precipitation and a change from rain to snow this may result in longer periods of snow coverage.
• Higher rates of insolation.
• Strong winds for longer periods of time.

Figure 2. Altitudinal zonation.

As distance from the shore increase the vegetation communities change with changing conditions. Sand dunes show both zonation and succession (Figure 1) and as you move away from the sea the following changes occur:

• Soil changes:
  • Depth and humus content increase.
  • pH decreases moving from alkaline to neutral.
  • Moisture holding capacity improves.
• Sea spray and wind speeds decrease.

Succession

Succession is the predictable change in a vegetation community over time. It starts with a pioneer community then the vegetation transitions through various intermediate communities to the final climax community. During succession the ecological community will change in composition. Some species will start the process as the dominant species but then die out to be replaced by an alternative dominant species. Through time the vegetation becomes taller and the ecosystem more complex.

During succession a variety of changes take place and Table 1 provides a summary of a few of these changes. You need to be able to explain how and why these changes take place so watch out for the details in the following sections.

Types of succession

Ecological successions are made up of several communities – several intermediate plant communities, from pioneer to climax. There are a number of different examples of succession:

• Hydrosere: Succession in a body of freshwater. In this process small lakes may disappear and be replaced by the plant communities.
• Halosere: Succession in salt water marshes.
• Psammosere: Succession along sand dunes. This stabilises the dunes and stops them shifting.
• Lithosere: Succession starting from bare rock. This is seen most often on lava flows.
• Xerosere: Succession in dry areas.

Each these may be a primary or a secondary succession. Primary succession occurs in areas that have never been occupied by an ecological community, e.g. bare rock or sand dunes. Secondary succession is where there has been a natural or anthropogenic (human-made) disturbance and the soil is still in place, e.g. abandoned fields, deforested areas, storm damage, flooding.

What happens during succession?

Many ecologists debate the predictability of succession because a variety of environmental factors may affect the timing and the stages that develop. It could take anything from a hundred years to thousands of years for the process to be completed. There are certain identifiable characteristics of succession. It is a directional change as the plant community moves through a series of stages from colonization by pioneer species to the final climatic climax. There is no clear delineation between the stages and at times the vegetation community will possess characteristics of more than one stage.

Terrestrial succession starts with a bare rock surface. The environment is harsh and variable. Temperatures vary due to exposure and water supply is unreliable. There is no soil, but there is the raw material for it to develop.

Colonization is initiated by pioneer species that are adapted to the extreme conditions. These species are the r-strategists that are suited to the unstable environment of early succession. They are small organisms with short life cycles and they produce many offspring. These hardy species find enough nutrients with in the environment to kick-start an ecosystem. Soil starts to form as weathering begins to break down the rocks and plants contribute organic matter to the debris.

Figure 3. Lichen colonizes bare rock.

Establishment follows colonization. In this stage the ecosystem is just getting going as an ecosystem as opposed to a collection of constituent parts. The soil becomes deep enough to provide niches for invertebrates, they breakdown the dead organic matter to form humus which improves water holding capacity. The quantity of resources increases and food, shelter, water and habitats develop.

Competition is the next stage in succession. Throughout succession each species is adapted to a very specific set of abiotic conditions - the pioneer species thrive in the harsh conditions of the early stages of succession. However, the pioneer plants have now done their job. They have taken a harsh, barren area and made it suitable to sustain a wider variety of plant life. Abiotic conditions such as temperature, sun and wind are less extreme. This improvement in conditions means that more complex larger plants invade the area and outcompete the original species for space, light and nutrients. They provide shelter which changes conditions and the pioneers disappear. This environment is more stable and the K-strategy is more successful.

Succession is starting to stabilise and there are fewer new species entering the ecosystem. The rate of new species entering the system falls as trees now shade out the lower levels. K-strategists continue to dominate the ecosystem.

The final stage of succession is the climax community. The climax community is in steady state equilibrium with the climate and/or the soil. The succession has come as far as it can and will be self-perpetuating as long as prevailing climatic conditions remain.

Examiner Tip

You must know an example of succession with specific species names. You can pick any type of succession - hydrosere, halosere, psammosere, lithosere or xerosere. Pick one with names that you find easy to remember.

Theory of Knowledge

The only thing all lithoseres have in common is the fact that they start on rock - they vary depending on the climatic zone the are in. Consider whether or not this invalidates the model.

Succession: What changes?

In the previous section we learnt that succession causes changes over time. Pioneer communities are simple ecosystems with high productivity but limited biodiversity; whereas climax communities are large, complex ecosystems where productivity is less but biodiversity is high and the complexity results in stability of the system.

Overall the productivity in the low in the early stages due to the limited amount of vegetation present in the pioneer communities. Productivity rises quickly due to the fact that gross primary productivity is low as there few are producers present. However, the low density of producers means that losses through respiration are also low. Pioneer plants grow fast and biomass accumulates quickly giving high net primary productivity. By the climax community the production/respiration ratio (P/R) = 1, meaning that the gross productivity and respiration are balanced. Thus there is no longer a net accumulation of organic matter and energy production and use is balanced.

Figure 1. Changes through succession.

The early stages of succession are characterised by low levels of diversity, as there are few species that can tolerate the harsh conditions. This means that energy and nutrient cycling is limited, food webs and the system in general is simple. By the time the climax community is established there is no longer the pattern of one species being replaced by a species that is better adapted to the environment giving a stable species composition. The vegetation has developed to be in balance with the climate and in many cases that means a forest ecosystem. The multiple layers of a forest ecosystem vastly increases the number of available habitats and that means more animals, complex food webs and higher species and genetic diversity.

The soil will be mature with plenty of organic matter and good structure. This tends to make it well drained but with good moisture holding capacity. The soil is able to hold enough water for plant growth but excess water will drain away. The organic matter and structure also improves the soils nutrient holding capacity so the nutrient cycle is stable and is in balance.

Figure 2. Well developed soil.

Succession is controlled by the climate but it also has an impact on the climate. As the vegetation matures it alters the light conditions in the area. The low profile plants of the pioneer community have almost no impact on the light availability but as shrubs and trees invade the area, light conditions change. Temperatures become more even as the vegetation shades out sunlight and reduces wind speeds. Relative humidity will increase as there are more transpiration surfaces and lower wind speeds.

There is no single climax community in an area because local conditions will vary considerably. Most climax communities are in balance with the climate of the area and are often referred to as the climatic climax. However, within any climatic region there will be variations in bedrock and soil type that can have a significant impact on vegetation. If the bedrock is permeable (allows water to drain through) the soil will be well drained but if the bedrock is impermeable the soil can be waterlogged. These differences will give very different climax communities.

Theory of Knowledge

Succession is a model. Compare the value of three ways of knowing in compiling the succession model.

Why are climax communities stable?

Climax communities are stable and that stability is related the complexity of the system. In 1955 American ecologists Robert MacArthur noted that as the number of interactions in the ecosystem increases so did the stability. More complex = more stable, which is logical. In a complex food web where there are numerous organisms at each trophic level the disappearance of one organism will not cause any great impact. In Figure 3 (A) if any of the organisms disappeared every other organism would survive because they are all have alternative food sources and more than one predator. But in a simple food chain the removal of one species could be disastrous. In Figure 3 (B) the loss of any of the organisms would be a problem because they all have one food source.

Figure 3. Comparison of complex food webs and simple food chains.

Stability tends to give an ecosystem a higher level of resilience. Resilience is how well an ecosystem resists damage caused by a disturbance then how well it recovers. Disturbances may be natural such as floods, insect explosions etc. or anthropogenic (human induced) events like deforestation or fracking. Resilience does not mean that the ecosystem will return to exactly the same state as it was before but that it has the same function, structures and feedbacks. Therefore there can be more than one stable state.

Figure 4. Fracking Kern County, California.

The critical aspects of resilience include latitude, resistance and precariousness.

• Latitude – links to tipping points. This is maximum amount of disturbance that a system can tolerate before it losses its ability to recover. Once past that tipping point the system will not recover easily, if at all.
• Resistance – how easily the ecosystem resists the change.
• Precariousness – how close the ecosystem is to its limit/tipping point.

Thus an ecosystems ability to return to its previous state is dependent on its resilience. A system is more resilient if it has high biodiversity with complex food webs and nutrient and energy pathways.

Secondary succession

Human activities or natural disasters may arrest succession part way through or they can destroy the climatic climax and send the process of succession back to the beginning. This is often referred to as a secondary succession.

Figure 5. Disturbances to succession.

The amount of impact the disturbance has depends on the stage succession has reached and thus how stable the ecosystem is.

• A disturbance at point 1 is impacting a simple, unstable ecosystem. Such simplicity makes it easy to destroy the pioneer community causing a secondary succession to start from the ready-formed soil. If the disturbance is severe enough then the soil may be removed and succession starts over; that would be considered a primary succession.
• By point 2 the succession has progressed further and the ecosystem is becoming more complex and thus more stable. A disturbance at this point may just cause the system to revert back to the pioneer community and so secondary succession continues from that point.
• The climax community is a complex, well-balanced ecosystem so any disturbance would have to be of a large magnitude OR of long duration to upset the balance of the ecosystem. However, disturbances of large magnitude or long duration can push the ecosystem past the tipping point and the community reverts back to an intermediate stage or the pioneer community.

Stable secondary successions

The climatic climax is the ultimate climax community but it is not the only stable one. There are a number circumstances in which an intermediate stage is maintained either naturally or by man.

TYPICAL EXAM QUESTIONS:

Define succession and zonation.

Distinguish the difference between succession and zonation.

Describe the difference between primary and secondary succession.

Describe the process of succession with reference to a names habitat and specific species.

Outline what a plagioclimax is.

why might farmers want to maintain their crops at a plagioclimax?

Evaluate the costs and benefits of replacing natural ecosystems with crops or livestock grazing.

Why does a complex ecosystem provide stability? (hint include reference to nutrient and energy pathways, complex food webs, etc)