Ecosystems
Dr. Kelli Anderson
As you might already know, healthy ecosystems form the foundation of all life on Earth and are essential for our survival. But what is an ecosystem exactly? How are they structured? And how do they function?
At a fundamental level, ecosystems are made up of biotic (living) and abiotic (non-living) components. In terms of biota, the number and diversity of species (species richness) within an ecosystem can be dependent on many factors including climate (both stability and severity), resource availability, and species-to-species interactions. For example, a relatively shallow coral reef that is rich in nutrients and has a variety of habitat types will typically have much higher species richness than a pelagic (open-water) environment where shelter may not be available, and the temperature/pressure experienced will be more extreme as depth increases.
Groups of individuals within a species make up a population, and populations that interact with each other form a community at a given location. The way that different populations interact determines the structure of the community. The diagram below (left) depicts a simple food chain where only a few species are present. In this kind of community, it is easy to see that if species number 2 is taken out of the equation then the abundance of species 1 and 3 will be dramatically affected. The bottom right of the diagram shows a comparatively species-rich community. In contrast to the community on the left, a more complex community may be able to compensate for the disturbance/exploitation of a single species due to the large number of population interactions. Having said that, the ability of a community to function and survive after a disturbance depends on the magnitude and duration of the disturbance, and which specie(s) is affected. For example, imagine a simplified community where dugongs eat seagrass, and tiger sharks eat dugongs. In such a community, the extinction of the tiger shark may cause over-grazing of seagrass by dugongs, which in turn would reduce the amount of nursery habitat for juvenile fish and therefore cause a decline in their numbers. In this scenario, a trophic cascade occurred after the removal of a top predator and the abundance of seagrass, dugongs and fish would have been affected. Thus the removal or decline of one important (‘keystone’) species can have far reaching effects, even in complex food webs. When an ecosystem is affected by interference with a top predator, it is said that there was a ‘top-down’ effect on trophic levels (see below) as opposed to a ‘bottom-up’ effect when factors such as nutrient input are altered.
Simple food chain vs comparatively complex food web
Energy flow
All species belong to a ‘trophic level’ within a food web, and the trophic level they are grouped into depends on the method they use to obtain energy. For example, the primary trophic level contains species that are able to produce their own food (autotrophic) from sunlight (in a process called photosynthesis) or other compounds. In the left panel of the above diagram, species number 3 would be a primary producer such as seagrass or phytoplankton. The next tropic level includes those organisms that consume other species to obtain energy (heterotrophic). They may feed on primary producers such as plants if they are herbivores or omnivores, or other animals if they are carnivores or omnivores. In the simple food chain (above), species 2 and 3 are consumers; thus, the arrows on the diagram indicate which way the energy flows as it is transferred from one species to another. The third trophic level belongs to the decomposers who obtain their energy from dead plants and animals. Decomposers such as bacteria or fungi break down dead tissue and are essential for nutrient cycling (covered below).
As the arrows on the above food chain/web diagram imply, energy moves through an ecosystem in a unidirectional fashion. As such,
ecosystems require a continuous input of energy which comes from sunlight in most ecosystems. In a process called photosynthesis, primary producers ‘catch’ sunlight and use it (in combination with atmospheric carbon dioxide- CO2) to create carbohydrates which are a form of chemical energy that is then available to other consumer species. After one animal consumes the tissue of another, approximately 90% of the energy is lost as heat and other waste products (faeces/urine). As such, energy transfer through multiple trophic levels is a rather inefficient process and only a small percentage of the original energy is left for decomposers. The rate at which primary producers convert atmospheric CO2 and sunlight to biomass (living tissue) determines the ecosystem’s ‘primary productivity’. It is typically measured as the amount of carbon (in grams) assimilated per meter squared in a year in a given ecosystem. As you might expect, rainforest and coral reef ecosystems are the top performers in terms of primary productivity, while desert, open oceans, streams and grasslands assimilate comparatively less carbon.
As the arrows on the above food chain/web diagram imply, energy moves through an ecosystem in a unidirectional fashion. As such,
ecosystems require a continuous input of energy which comes from sunlight in most ecosystems. In a process called photosynthesis, primary producers ‘catch’ sunlight and use it (in combination with atmospheric carbon dioxide- CO2) to create carbohydrates which are a form of chemical energy that is then available to other consumer species. After one animal consumes the tissue of another, approximately 90% of the energy is lost as heat and other waste products (faeces/urine). As such, energy transfer through multiple trophic levels is a rather inefficient process and only a small percentage of the original energy is left for decomposers. The rate at which primary producers convert atmospheric CO2 and sunlight to biomass (living tissue) determines the ecosystem’s ‘primary productivity’. It is typically measured as the amount of carbon (in grams) assimilated per meter squared in a year in a given ecosystem. As you might expect, rainforest and coral reef ecosystems are the top performers in terms of primary productivity, while desert, open oceans, streams and grasslands assimilate comparatively less carbon.
Nutrient cycling
Various nutrients, such as nitrogen, carbon and phosphorus are essential for the survival of living organisms. In the previous section we saw how energy is continually lost from an ecosystem and for this reason a consistent input of energy from the sun is required. In contrast, nutrients can be used over and over again in range of biogeochemical (biological, geological and chemical) processes. Nutrients may exist within an exchange pool or reservoir within an ecosystem. Nutrients within the exchange pool can be taken up by primary producers, and can then be transferred through the various trophic levels of a food web as part of the biomass. During this process, some carbon will be released back into the atmosphere in form of carbon dioxide (through the gills for fish or via the lungs for other animals when they breathe) as the result of respiration- respiration is the process by which organisms break down molecules such as carbohydrates to produce energy. To complete the nutrient cycle, decomposers break down dead tissue and release the nutrients back into the exchange pool. Alternatively, nutrients may become part of a reservoir where they are ‘locked up’ for a period of time. Common examples of reservoirs include coral reefs, coal and limestone.