Ecosystems and communities are part of a bigger ecological organisational structure (see below). Species are the basic unit of this system and they group together to form populations, which in turn interact to form communities. Communities come together and interact with the physical environment and so you have ecosystems. Biomes are the next stage up and they are a collection of ecosystems. The largest unit in the organisational structure is the biosphere, or planet earth.

Within this ecological hierarchy species interact and as a result energy and nutrients flow through the systems. As solar energy enters the system the plants use the light energy in the process of photosynthesis, which combines inorganic substances to transform them into organic matter. Once there is organic matter the chemical energy can flow through the systems and the nutrients cycle around it. Remember - the biosphere is a closed system so whilst we have a constant stream of new energy coming into the biosphere (sunlight) there is no additional matter coming in so everything must recycle.

Photosynthesis uses light energy to synthesise inorganic substances and chemical energy whilst respiration reverses the process. Animals take in the chemical energy made by the plants to grow, that is then converted into muscle to form chemical energy. Animals move around and perform live processes that need energy. Respiration releases energy for activity and heat that is lost into the environment.

This process is the start of the feeding relationships between species, which can be shown as simple food chains or more complex food webs (below). These models show the links between organisms that feed on each other. Food chains and food webs are one way to show feeding relationships - who is doing the eating and what is being eaten. However this is not the whole story and there is more we need to know. Like how many organisms there are of each type, how much energy or matter there is in the system.

Ecosystem basics - key players

The living aspects of the ecosystem are the species and populations and they come together to form communities.

Definition

Ecosystem is a community of interdependent organisms and the physical environment they interact with. A community is a group of populations living and interacting with each other in a common habitat.

Producers

All ecosystems on earth, and in fact all life on earth depend on this group of organisms. Producers convert inorganic compounds into food. Hence they are known as autotrophs or self-feeders, they obtain their food by making it for themselves. Producers are at the base of the food chain and so they are referred to as primary producers.

The majority of the primary producers are green plants that manufacture their own food through photosynthesis. They take nutrients from the soil and use solar energy to change light energy into chemical energy. Plants do not only grow on soil, they may grow on:

• Other plants, epiphytes e.g. orchids grow on trees.
• Rock surfaces, succession starts with plants that grow on rocks e.g. lichens.
• Dead organic matter, fungi grow on dead trees and start the decomposition cycle.

A small percentage of primary production is provided by chemosynthesis. Certain bacteria and a group of organisms called archaea take the energy released by inorganic chemical reactions to make sugar (similar to the way green plants take sunlight to make sugar). This reaction takes place in total darkness mainly in hot springs and the deep ocean hydrothermal vents.

Plants have a number of important roles in the ecosystem:

• They provide food for all other life on earth, herbivores eat plants and then carnivores and omnivores eat the herbivores.
• Plants regulate the hydrological cycle by taking water in and releasing it in to the atmosphere through transpiration.
• They maintain the balance of gases in the atmosphere by absorbing CO2 and releasing oxygen. This may also help redress global warming.
• Plants provide habitats for many animals.
• The roots of plants help bind the soil and reduce erosion.

Consumers

Consumers are referred to as heterotrophs – meaning other-feeders, they cannot manufacture their own food so they must obtain it by consuming other organisms. There is a variety of ways in which consumers derive their energy and nutrients.

• Herbivores eat just plants – leaves, flowers, fruits and nuts or stems and wood. E.g. Capybara, a large South American rodent eats grasses, aquatic plants, fruit and tree bark.
• Carnivores eat only meat/other animals and these are the predators. E.g. African lions eat wildebeest, zebras, buffalo and warthogs.
• Omnivores eat both plants and other animals e.g. Red panda eats bamboo, small mammals, birds, eggs, flowers and berries.

Decomposers and detritivores

The terms detritivores and decomposers are often used interchangeably. However these terms are used they are the clean up crew for ecosystems as they obtain their energy and nutrients from dead plant and animal material and waste.

Technically detritivores are the first stage of the decomposition cycle. They gain their nutrients by consuming detritus – plant and animal parts and feces. Detritivores such as millipedes, woodlice, worms and maggots ingest lumps of matter and pass them through their bodies to be further dealt with by decomposers.

Decomposers such as bacteria and fungi absorb and metabolise waste and dead matter on a molecular level then release it as inorganic chemicals that can be recycled through the ecosystem via plants.

Important

It is important to note that whilst energy flows through the ecosystem (trophic levels) nutrients cycle around it. This is why decomposers and detritivores are so important.

Decomposers and detritivores are vital because they:

• Clear ecosystems of dead bodies.
• Prevent the spread of disease by disposing of dead bodies.
• Facilitate the continued functioning of ecosystems by releasing the nutrients that were locked up in the organic matter and making them available again (see Figure 3).

The relationship between these groups of animals can be illustrated through trophic levels, food chains, food webs and ecological pyramids. ​

Photosynthesis

Green plants are able to take light energy from the sun and use it to make chemical energy – which is just as well for us as we cannot make our own energy we have to take it in when we eat. In fact all life on earth (including plants) need to take the chemical energy and use it for life processes.

Be Aware

Photosynthesis is a very complex set of chemical reactions and if you do biology you will need to know these processes in much greater detail. For ESS you just need to know the basics.

Plant leaves have chloroplast and within these chloroplasts there is an important green pigment called chlorophyll. Within the chlorophyll there is a special protein that is able to absorb the light energy from the sun. The light energy is used to combine water (H2O) and carbon dioxide (CO2) to produce oxygen (O2) and glucose (C6H12O6). The glucose is stored as plant tissue and thus forms the basis of plant biomass.

During respiration energy is transformed from chemical energy to kinetic energy and eventually dissipated into the environment as heat energy. This demonstrates the second law of thermodynamics – the entropy (disorder) of a system increases in over time. The transformation of energy is never 100% efficient so as light energy is converted to chemical energy entropy increases, as chemical energy turns into heat energy entropy increases again. The consumption of food and the burning of energy in a living body keeps organisms alive and maintains low entropy in their bodies but increases entropy in the overall system.

Important

You do not have to know the chemical formulae for these equations, you can use the word equations.

Trophic levels

This section investigates the relationships between the "key players" discussed in ecosystem basics. Each organism is classified according to how it catches its food and that places it at a particular trophic level. Trophic is derived from the Greek word for food or feeding - so trophic level is the feeding level of an organism.

Definition

Trophic level is the position an organism (or group of organisms in a community) occupies in the food chain.

Figure 1 shows a simplified grazing food chain with the trophic levels. The primary producers (PP) are at trophic level 1 (T1), primary consumers (PC) are T2, secondary consumers (SC) T3, tertiary consumers (TC) T4 and quaternary consumers (QC), the top predators are T5.

Food chains

Food chains model trophic levels (see below), they start at T1 with the primary consumer and finish at T4 or T5. In terrestrial ecosystems T4 is usually the highest level due to significant losses of energy between trophic levels. The number of steps an organism is from the start of the food chain indicates its trophic level. The flow of energy along the food chain is in the direction of the arrows and points towards where the energy is going not where it has come from. In the diagram the energy moves from the primary producer to the consumer to the secondary consumer and so on.

Food chains are a very simplistic representation of the relationship between the trophic levels. It is rare that a herbivore eats only one type of plant or that a carnivore only eats one type of herbivore. In reality it is far more likely that a species will have more than one food option.

The diagram below shows a food web for the British countryside. You can see that with a food web, organisms can occupy more than one trophic level. The dog for instance is at T3 if you follow the food chain that goes from grass - rabbit - dog. However if you follow the food chain that goes from grass - grasshopper - lizard - dog, then the dog is T4. As you can see, you can extract food chains from food webs.

Try for yourself!

In the diagram below there are two food chains of four trophic levels. Find one of them and draw it. Use this format:

organism 1 - organism 2 - organism 3 - organism 4

Energy in the food chain

Food chains demonstrate the first and second laws of thermodynamics. Energy is neither created nor destroyed (1st law) in the food chain and as energy passes along the food chain entropy increased (2nd law).

Energy enters the food chain as light energy form the sun. It is transformed into chemical energy in the form of chemical bonds in organic matter. It then passes along or transfers along the food chain as chemical energy. Most people think that if a gazelle goes into a lion as gazelle and come out as feces it is a transformation but believe it or not it is a transfer. That is because when you look at it in terms of energy it goes in as organic matter (muscle etc.) and comes out as organic matter (feces). There is one more transformation and that is when the chemical bonds in the food are broken and used for movement some of the energy is transformed into heat – the least useful energy there is for the ecosystem. Heat dissipates into the surrounding environment.

Energy efficiency in the food chain

The diagram below shows how the laws of thermodynamics act on the food chain and we know that energy transformations are not 100% efficient. But just how inefficient are they? Well as a general rule they are approximately 10% efficient. That means that only 10% of the energy moves from one trophic level to the next, not a lot. This has significant implications for the number of organisms and the amount of biomass found at each trophic level (revise ecological pyramids).

You may have to calculate the efficiency of a food chain so lets do the maths.

Solar input to primary producers = 21,000

Primary producer output to primary consumers = 3,330

What percentage of the suns energy is actually passed to the primary consumer?

One way to do the calculation is as follows:

3,330 / 21,000 × 100 = 15.9%

So the energy efficiency is about 16%.

Watch this video to learn about ecological pyramids and how they show feeding relationships quantitatively.

Pyramid of numbers

As the name suggests this ecological pyramid shows the number of organisms at each trophic level in the food chain and the unit is whole numbers. This sounds straightforward – what could be simpler than counting the number of organisms at each trophic level? After all, we can all count. There are some challenges though, how do you count the blades of grass in a field? In fact how do you count the blades of grass in a small square? Is each blade of grass separate or are they part of the same plant? How do you count the number of insects or small mammals if they are the herbivores? These questions will be answered in "Measuring biomass and energy".

Figure 2a. Pyramid of numbers for a temperate grassland.

Figure 2b. Inverted pyramid of numbers for a temperate woodland.

Figure 2c. Inverted pyramid of numbers for an aquatic ecosystem.

Figure 2c is not that common but it is possible. There are a few reasons that a pyramid of numbers may show this pattern. If the organisms lower on the food chain are microscopic they will have very different life cycles to those higher up the food chain. In Figure 2c the phytoplankton may have just reproduced (giving larger number) whilst the zooplankton are at the other end of their life cycle (giving low numbers). Some of the organisms may be migratory and not present in large numbers when the count was taken.

Advantages

• Non-destructive method of data collection.
• Good for comparing changes in an ecosystem over time.

Disadvantages

• All organisms are included regardless of their size.
• Numbers can be so big that it is hard to represent them accurately.
• Does not allow for juveniles or immature forms of the species (they may look very different).

Biomass is measured as dry weight; this eliminates the variation in water content between organisms. To find out what the biomass of a particular trophic level is a sample is collected, dried and then weighed – this is obviously very destructive, not to mention unacceptable if you are dealing with animals.

Pyramid of productivity (energy)

The pyramid of productivity solves some of the problems of the other two types of pyramid as it shows the turnover of biomass at each trophic level. It is not a snapshot in time as it shows the flow of energy over a period of time. Each bar in this pyramid represents the amount of energy that is generated and available as food for the next trophic level. The units are given as energy or mass per unit area per unit of time – joules per meter squared per year or J m-2 yr-1. How the data is collected is covered in "Measuring biomass and energy".

Unlike the other ecological pyramids the pyramid of productivity in a healthy ecosystem is always pyramid shaped. This is due to the 10% rule and the energy efficiency discussed in "Trophic levels, food chains and food webs".

Advantages

• These are the most accurate pyramids as they show actual energy available and the rate of production over a period of time.
• Ecosystems can be compared.
• Solar input can be added to the model.

Disadvantages

• Data collection is not easy as you need to know the rate of biomass production over time.
• Species can be difficult to assign to a particular trophic level (this is a problem in all the pyramids). As we saw in food webs organisms can be in more than one trophic level.

Apex predators in trouble

The apex predator or top carnivore is most affected by the poor efficiency in the food chain due to the two previous points. The amount of biomass and energy decreases up the food chain therefore the top predators have far less food available to them under normal circumstances. If there is a problem lower down in the food chain, say an animal is hunted by humans and lost from the food chain; this will have a knock on effect on the predators. The loss of one species form the food chain means less food for all and with smaller populations the top predators will suffer a greater impact. Also the effects of biomagnification grow exponentially worse as you go up the food chain. By the time the toxins reach the top predators they are at dangerous levels. At high concentrations the toxins cause a variety of problems for the top predators that were not in evidence at the lower concentrations of the lower trophic levels.