Organisation of a North American ecosystem

Organisation within ecosystems

Food webs demonstrate predatory relationships

Relationships within ecosystems

Communities are made up of different organisms grouped together at a given time and place. On a larger scale, these communities interact with other communities and their physical surroundings and are classified as ecosystems.

For example, Wollemi National Park in the Greater Blue Mountains World Heritage area, NSW is a dynamic ecosystem with communities of flora and fauna such as grey box eucalypts and wombats interacting with their physical environment of sandstone gorges, canyons and river valleys.

Ecosystems are biological communities, composed of abiotic and biotic factors which interact to sustain life.

Abiotic factors that affect plant growth

Impact of abiotic

Common elements that act as limiting factors for

A limiting factors is an environment condition that will reduce the size of a population, by limiting its growth. For example, the availability of sunlight in a forest would be limited for plants low to the ground, so their growth is reduced. In this way, abiotic factors can slow the rate of growth of certain populations within an ecosystem.

Different abiotic factors will have varying impacts depending on the type of ecosystem. Terrestrial ecosystems primarily need the right balance of temperature and rainfall, whereas aquatic ecosystems are highly effected by salt concentrations, sunlight, and availability of oxygen and nutrients.

Whether or not an organism can tolerate changes to their ecosystem will also influence their success. For example, ectotherms are animals which rely on external influences to maintain a constant internal environment so they will be greatly affected by abiotic changes in temperature. Contrastingly, endotherms are able to regulate their bodies to maintain an internal bodily temperature, and so can survive a wider range of temperature changes.

The secondary consumer relies on dissolved chemicals

Organisms will fit into one of these three categories

Biotic factors

Examples of biotic factors:

• Competition for resources

• Predator/prey relationships

• Disease

• Parasitism

• Symbiosis

Components of the biosphere:

Producers:

Are organisms which form the bottom of the food chain, autotrophs. This includes plants, which use photosynthesis to 'fix carbon' and turn light energy into chemical energy for other organisms' consumption. They produce food not only for themselves, but other organisms in the ecosystems.

Consumers:

Are organisms which eat the producer organisms as a food source, to gain energy and carbon. These organisms form the higher tiers on the food chain, and may also eat other consumers (carnivores). Organisms which only eat producers are primary consumers (herbivores).

Decomposers:

Are organisms which break down dead materials, returning the nutrients back to the soil to be taken up again by producers. They link the food chain together, ensuring that nutrients are recycled through ecosystems. Decomposers include microorganisms and some fungi.

Any disruption to the biotic factors of the ecosystem will have impacts for the entire food chain.

The Australian Dingo is at the top of the food chain

Ring-tailed possums and Brush-tail possums compete for territory

Symbiotic relationships

Clown fish live in symbiosis Sea Anemones

Trophic levels

Consequences of predation, competition, symbiosis, and disease:

Predation:

When one organism kills and then consumes another within an ecosystem. This relationship is beneficial to the predator, as they obtain nutrients to survive and reproduce, but detrimental to the prey. There are many examples of predation, and they exist on many levels. Carnivores such as lions hunt zebras, plants like Venus fly traps consume insects and bacteria like Vampiro vibrio feed on eukaryotic cells such as Chlorella.

The process of predation can influence an ecosystem by forcing adaptation of organisms. Predators adapt to become better at catching their prey, through natural selection. The organism naturally best at obtaining a food source will reproduce and so the population changes. Predators therefore exhibit enhanced traits such as claws, or speed, which help in obtaining food. In response, prey adapt to become better at avoiding predators. Their traits have evolved to help them survive whilst avoiding detection, traits such as camouflage or minimal movement. In this way, the continual predator/prey cycle changes an ecosystem, as organisms adapt to survive.

Competition:

When organisms interact to the detriment of both species, due to a limiting factor (either abiotic or biotic). Competition can affect the structure of an ecosystem community, and influence what organisms are able to survive within a set ecological niche. The competitive exclusion principle means that even if competition exists, it will eventually lead to either the extinction of a competition species, or adaptation to new sources. This however may not be as drastic if the ecosystem is quite large. Competition therefore is necessary to the process of evolution, as it drives radiation and selects for new mutations and the environment changes.

Competition is when two organisms fight for the access to the same resource, meaning one may miss out on access. Competition may be between members of their own species (intraspecific competition) or between members of different species (interspecific competition).

Symbiosis:

When organisms interact closely or in a long-term relationship. Symbiosis can be beneficial or detrimental, depending whether it is a mutualistic, communalistic, or parasitic relationship.

• Mutualism: a positive interspecies relationship where both benefit. Mutual relationship may be obligatory (meaning organisms are interdependent, and require the other for survival) or facultative (they do not require the other organisms for survival, but it's still nice anyway). Examples of mutualism include gut bacteria (bacteria within the gut have a constant food source, and aid in breaking down nutrient for the host), coral reefs (many coral species have a plethora of microbial species living in and on them to provide nutrients in return for shelter, and clown fish (obtain shelter from anemones, and in return protect the anemone from fish that may eat it).

• Commensalism: a relationship between two species where one benefits and the other is unaffected. Examples of these relationships include transportation around an ecosystem, shelter, or use of an organism after death (e.g. hermit crabs using shells after the death of the previous inhabitants).

• Parasitism: a relationship between species in which one organisms obtains a benefit, and one organism obtains a detriment. This could be a result of endoparasites (inside of the body) or ectoparasites (outside of the body). If the relationship causes death of an organism, it is called necrotrophic. If the relationship requires the continued survival of the affected organism, it is called biotrophic. Biotrophic relationships are extremely successful for parasites, as they gain continual nutrients throughout their lifetime. Microscopic parasites cannot be seen without a microscope, whilst macroscopic parasites can be.

These relationships are found throughout ecosystems, and after by the types of organisms present in a population (biotic factors), as well as availability of nutrients (abiotic factors).

Disease:

When an organism is subject to a disorder in their structure or function, not simply a result of physical injury. Diseases can greatly effect ecosystems, particularly when they are fatal to an organism. Biotic factors are connected by food webs, and so if one link in this web is killed, it can affect the entire ecosystem. Diseases can be parasitic (caused by a pathogen), a result of nutrient deficiency, due to chemicals and pollutants, or a result of climate.

Populations will not often have uniform distributions

Measurement of populations

Organisms are not always distributed evenly over an area. They can be randomly assorted, or found in particular patterns, like clumped in colonies. Demography is an important scientific study, as it allows us to track how populations, and therefore ecosystems, change over time. This can help us to understand whether ecosystems are being particularly effected by human activity, and allows u to undertake conservation efforts.

Studying the size and distribution of populations can help us understand how they function. For example we know that a big population will probably be more stable, as there are many individuals with lots of genetic variability which will allow the organisms to adapt. Another example is low-density populations, which might have distinct mating patterns, because they are spread out and have difficulty finding a mate.

We measure the size of populations using the quadrat method for slow moving and small organisms. Scientists mark a small plot, and then count the number of organisms of a certain type within this area. This is repeated to account for random variations, and then from this information they extrapolate the population over the whole habitat.

For larger organisms which are more mobile, the mark-recapture method is used. This is particularly effective for populations of birds, fish and mammals. A small sample of a population is taken. The individuals are given a visible marking (usually a tag), and then releasing them into the wild. When a new sample is collected, the ratio of marked to unmarked individuals can give scientists a rough estimation of how many organisms exist in the wild.

The follow calculation is used:

From this we obtain a general idea of population numbers, although the method is not always perfect. Sometimes populations can rapidly expand, or the same organisms are preferentially trapped. This is why newer technologies, such as electronic tracking, are being developed, to help us better understand how ecosystems function.

Capture-mark-recapture method

Quadrat determining weed density

Different methods of adding tracking devices

Predator and prey populations will be influence by each other

Consequences in populations

Predation:

Predators affect the distribution and abundance of their prey. This is one of nature's means of population control. However, if the prey species can reproduce as fast as it is predated, its population will remain stable. In natural communities, the abundance of a predator and its prey can fluctuate through time, with the predator numbers copying those of the prey. When there are large numbers decline, leading to a shortage of food for the predators, whose numbers then decline.

Factors that affect predator/prey populations:

• Number of predators competing for the same prey

• Availability of the prey's food

• reproduction rate, depending on:

  • The age of reproductive maturity

  • number of reproductive episodes per lifetime

  • Fertility (likelihood of fertilization as a mating)

  • Fecundity (number of offspring per mating)

• Death rate

• Ratio of males to females

• Size of the ecosystem for supporting numbers

• Movement between ecosystems

• Number of shelter sites available.

Competition:

Competition between species for resources affects reproduction and survival rates. Population fluctuations can be directly linked to the competing species and their resource. If the resource is a common food source, such as food sources become more readily available the abundance of both species increases. As food sources decrease, so may the abundance of both competing species.

When two species compete for a resource, the short-term effect is a decrease in population numbers of one or both species. Some species may be more successful competitors than others. Depending on the continued success of one species over the other, this trend may continue. However, depending on the supply of the resource, the ability of the 'losing' species to adapt to occupy a different niche, this trend may change or even reverse.

If the trend successfully out-competes a species, the long periods of decreased reproduction rates and increased deaths will eventually lead to the elimination of the 'losing' species in that area, and on the larger scale lead to possible extinction.

Symbiosis:

The process of symbiosis has profound consequences for all life on Earth. Scientists recognize the potential of symbiosis to:

• Increased evolutionary diversification - biodiversity - which allows for more resilient ecosystem

• The development of new species from the integration of their genetic material with each other - symbiogenesis - it is thought that early eukaryotic cells absorbed primitive mitochondria-like organisms and lived in symbiosis, allowing for aerobic respiration, thus forming a new species.

• Sources of new capabilities for organisms, which enhance evolutionary 'fitness' - many legume plants live with nitrogen-fixing bacteria. These bacteria live in the root nodules of plants. The plant supplies the bacteria with their nutritional requirements (sugar) and the bacteria supplies the plant with nitrogen in the form of ammonia (converted from nitrogen gas in the atmosphere).

Disease:

Disease can be defined as any process that adversely affects the normal functioning of tissues in a living organism. This includes infectious and non-infectious causes. In wild ecosystems, the greatest threats are generally infectious diseases. For a disease outbreak to occur, the pathogen must be introduced into a new host population from where the disease spreads through direct or indirect means, or it must be given a selective advantage by a change in the abiotic and biotic factors. The effect of an emerging disease on an ecosystem is to alter the balance of food webs, sometimes dramatically. Affected species will suffer a decline in numbers, and this has consequences for both their prey as well as their predators.

The most well known of the extinctions as it wiped out the dinosaurs

Cretaceous-Paleogene extinction

The Cretaceous-Paleogene extinction event was a global extinction event which eliminated about 76% of all species on Earth during that period. It occurred about 66 million years ago, extinguishing a number of organisms prevalent during the Mesozoic Era (250-66 million years ago). It is the third most severe extinction event in Earth's history. Sedimentation evidence suggests that the extinction was relatively short, only about 1-10 thousand years in length.

Loss of biodiversity:

• Photosynthetic organism populations significantly decreased due to a reduction in sunlight. This consequentially impacted herbivorous organisms severely. However, insectivores and omnivores were better suited, and therefore survived the extinction.

• Approximately 57% of plants in North America were eliminated

• Non-avian dinosaurs were completely wiped out

• A number of mammals, birds, lizards, insects, and plants were eradicated, although they fared altogether better than dinosaurs, and later these species grew to populate the Earth

• Almost all tetrapods (four legged vertebrates) over 25kg were killed (except some ectotherms like turtles and crocodiles)

• Oceanic populations including fish, sharks, mollusks and plankton were severely impacted

• Approximately 98% of corals living in tropical waters became extinct, due to reliance on photosynthetic symbionts

• Significant reduction in marine invertebrates.

Cause of the extinction:

The now generally accepted cause of this mass extinction is the impact of a comet or asteroid with Earth. This theory was proposed by Luis and Walter Alverez in 1980. It is thought that the impact was around 10km wide, and that it had widespread impacts on the globe. It is theorized that the impact caused atmospheric particles to enter the Earth's atmosphere and block sunlight, possible for months. Additionally, this will have caused reduced solar energy reaching the Earth, cooling the climate significantly.

This theory is supported by the high levels of iridium found in geological deposits from this period. Iridium is very rare to find on Earth, but is found abundantly in asteroids. Additionally, in the early 1990s, the Chicxulub crater, a 180km wide crater, was discovered in the Gulf of Mexico. This is thought to be the site of impact, and provided evidence that the clay deposits from this period contained debris from an asteroid impact. The mass extinction of species coincides with the estimated time of impact, suggesting it as the cause of the loss of biodiversity. Despite the evidence, some scientists still believe that other factors contributed to the widespread extinction, including volcanic eruptions, change in the climate, and change to the sea level.

The Chicxulub crater

Effects of the extinction:

The Cretaceous-Paleogene extinction event had widespread impacts upon the Earth's evolutionary course. The extinction of many species allowed for new evolutionary opportunities. Adaptive radiation (rapid diversification of species) occurred on a large scale, as organ isms were suddenly able to move into new ecological niches. In particular, mammals became very diversified, filling the space that dinosaurs had left within the Earth's ecosystem. New species such as horses, bats, whales and primates began to evolve. In addition, the widespread extinction of terrestrial plants allowed for the proliferation of ferns, leading to a fern spike in the fossil records of plant life.

76% of life was extinguished in the Cretaceous-Paleogene extinction

The estimated size of the asteroid impact

Comparison of the wombat and its megafauna counterpart the Palorchestes

Australian megafauna

Australia's extinct megafauna

Australian megafauna

During the Pleistocene epoch, Australia was home to a group of giant animals known as Australian Megafauna. Examples included Diprotodon (like a large wombat), Megalania(like a large goanna) and Procoptodon (like a giant, flat-faced kangaroo). Megafauna existed in Europe and North America (woolly mammoths, saber-toothed cats) as well as Africa (giraffes, hippopotamus, elephants). In fact, Africa is the only continent to retain most of its megafauna.

There have been many 'ice ages' throughout the 4.5 billion year history of Earth. The last ice age is only one in which humans were present. The continents were in their current positions, and the climate was very cold and dry. Humans and many other mammals survived this epoch, but many species went extinct. The epoch we are in now is referred to as the Holocene epoch, which started 10 000 years ago. It is a period of warming called an inter-glacial period.

North America's extinct megafauna include the largest terrestrial animals to ever live

There is an ongoing debate as to what led to the changes in the Australian flora and fauna, particularly the extinction of the Australian megafauna. Until recently, many supported the theory that it was the result of climate change associated with the last ice age. Opponents of this theory proposed that it was humans alone who have caused almost all extinctions of animals throughout the world. Current researchers tend to think it may be a combination of the two - initiated by the change in climate, with human impact delivering the final blow.

Theory 1: Climate change

• • The continent dried out due to the ice age.

  • Rainforests were contracting due to a drying climate. Because rainforests had stored moisture and returned an enormous amount to the atmosphere, monsoon rains once penetrated south and kept the rivers and lakes full. As the rainforests diminished they were eventually replaced by eucalypt forests, but these were less efficient at retaining water. As a result, less water was returned to the atmosphere.

  • As the climate became hotter and drier, fires broke out and the drier vegetation caught fire easily. Those plants and animals that could survive drought and fire produced and flourished, bringing about a change to the flora and fauna.

Supporting evidence:

• • Large animals such as megafauna, which were dependent on large supplies of food and water, would have died out when both became scarce.

  • They may also have died out due to temperature change as this may have affected their breeding seasons.

Contradicting evidence:

• • The last ice age was probably like previous ice ages. If so, why would the last one have had such an immense effect, when there is no evidence that the previous ice ages had a similar result?

  • The earlier extinctions seem to have occurred before the peak of the last ice age.

  • Climate change today does not seem to select against large, slow-moving species.

Theory 2: Human intervention

• • Aboriginal people arrived in Australia 65 000 years ago. They probably 'island-hopped' from the north. They were extremely successful predators.

  • They used fire to burn back the bush (used to help regenerate grasses and control bush fires). Increasing population numbers of animals (as a result of the regenerated bush) meant that there would be more available for hunting.

  • Evidence from the Madjedbebe site (300km east of Darwin) suggests that humans hunted the megafauna and, because the larger animals were slower, they were the ones that were killed. The smaller, faster animals that escaped survived to pass on their genes.

  • It appears that the original Indigenous people hunted Australian terrestrial animals that were larger than they were. The introduction of the dingo from Asia about 4000 years ago may have also led to a decrease in the diversity of carnivore predators. It has been suggested that the dingos drove the Thylacine and Tasmanian devil to extinction on the mainland.

Supporting evidence:

• • The main evidence for the theory that humans were involved in the increase in fires is that the increased carbon deposits in fossils are about the same age as the oldest archaeological sites beyond northern Australia. The smaller species of megafauna that become extinct had short limbs, which would have made them slow; the largest surviving of our present-day species are also among the fastest (e.g. kangaroos).

Contradicting evidence:

• • There is no fossil evidence of kill sites and very little evidence of humans and megafauna coexisting. p

  • If you consider the size of the animals, there is an overlap in the size of the smallest extinct species and that of the largest present-day species.

Thylacine (Tasmanian Tiger)

Theory 3: Nutrient access

• • A third theory accounting for the survival of smaller animals is that there were such low levels of nutrients in the soils in Australia that this may have caused a nutrient depletion throughout the food web, resulting in smaller animals. The smaller size of mammals in Australia compared with their counterparts on other continents today could provide evidence.

Evidence of coexistence:

• • When two species are found at a fossil site near each other, it can be inferred that they coexisted and possibly interacted with each other. Cuddie Springs is a fossil site in central NSW where the bones of megafauna and stone tools made by humans have been found in close proximity. There are in fact several different layers in the Cuddie Springs deposit, each with its own unique insights into different times.

  • Some of the specific finds include:

    • A kangaroo leg bone with signs of butchering by tools (30 000 years old)

    • Stone tools (around 18 000 years old)

    • Mixtures of megafauna bones

    • Charcoal from camp fires (around 28 000 years old)

    • A sandstone grinding stone (around 30 000 years old).