Ecology refers to the study, or the nature, of the relationships organisms have with each other and their environment, and an ecosystem is a community of interacting organisms and the environment in which they live. Here we give a brief overview of some key concepts in ecology which underpin what we are exploring through drawing in the Grant Museum.
Trophic Chains and Cycles
Trophic refers to feeding and trophic chains show how food, in the form of material and energy, moves through ecosystems, defining the relationships between things and what they eat. All of life on earth is built on the foundation of primary producers or autotrophs, which are organisms that extract energy and matter from the abiotic, non-living or geological, environment. On land plants are the most apparent members of this fundamental trophic level. They produce their own food using sunlight to power photosynthesis. In the sea this base level of food production is mostly carried out by algae, which are often single celled but can also be multicellular, like the more familiar seaweed. There are also other, less abundant, primary producers called chemoautotrophs which derive energy from chemicals and often live in otherwise inhospitable environments, such as highly acid or salty lakes.
The next level of the trophic chain is made up predators, animals that eat other living things. Herbivores eat plants, which are in turn eaten by carnivores, which themselves may be eaten by other predators. Finally, there are the detritovores, which eat non-living organic matter (detritus). This cascade of energy and matter from the level of abiotic (non-living) environment, through subsequent types of organisms, can be seen as a cycle when we consider that the detritovores break everything down into its constituent parts which are then reincorporated into living matter at the level of primary production. In fact there are many cycles, not every piece of organic matter moves from one end of a chain to the other in a linear fashion – detritovores eat primary producers too, and matter which is not incorporated into living bodies, but excreted, is also recycled.
One organism feeding on another is clearly harmful to one party, as is competition for limited resources. However, nature also throws up many instances of cooperation. Mutualism or symbiosis are instances when both interacting species benefit – for example: many plants rely on fungi to provide access to elements which they are unable to extract from the environment themselves; herbivorous animals require microorganisms within their guts to break down plant material so that they can digest it; flowering plants provide food to insect pollinators, which facilitate fertilisation; Remora fish feed on parasites that infest sharks thereby increasing their fitness.
There are also many indirect interactions whereby the actions of one species affect another via its effects on another. For example: predators reduce the population size of the herbivores and can therefore benefit plants; if one plant which is more successful at attracting a pollinator than another it indirectly harms other plant by reducing its reproductive success. These indirect effects are often not apparent and food webs of interacting species can be very complex.
Sometimes predator and prey populations are coupled, and can oscillate together. When there are more predators the prey population is driven down, this then reduces the predator population because it has less to food available. Decreasing predator numbers mean that the prey species population can subsequently recover, leading to greater numbers of prey and therefore more food for predators, driving up predator numbers. These cycles may be due to a direct predator and prey relationship, however, there are many cautionary tales which remind us that correlation does not show cause. If there is true coupling, then when one population stops cycling so does the other, if there is no real link then one may continue to cycle despite the other not doing so.
Keystones are particularly important species in communities, when their population changes everything changes. For example, when top predators are removed their prey can become so abundant that the resultant overgrazing leads to food shortages and a subsequent collapse of herbivore numbers due to lack of food. Other organisms, such as beavers or corals, significantly alter their habitat, leading to increased biodiversity.
Another important concept is succession, which underscores the dynamic and unbounded nature of ecosystems. When environments change catastrophically, like after a volcanic eruption, some aspects of recolonisation of an area are predictable. Early succession is marked by low biodiversity, low stability, and smaller plants with higher growth rates, whereas later succession environments are highly biodiverse and stable, with large slow growing plants. This is partially due to facilitation, the process whereby the action or presence of early species changes the environment, thereby facilitating colonisation by later species which are better adapted to the enriched environment and can therefore out compete the early species. These gradients of species types that make up a community can be seen through time, as in recolonisation after catastrophe, or over space, for example on a sea shore.
There are many other factors which affect how ecosystems develop and evolve, and ecology is becoming increasingly complicated. Things such as the random dispersal of species, interactions with the abiotic cycles, and the spread of disease all serve to produce unexpected synergies and knock-on effects. These complex systems tend to develop emergent properties, those which cannot be predicted from considering the system’s component parts. The lessons of ecology are endlessly surprising and nature’s richness reminds us not to be complacent in how we interpret our own actions within complex systems. Human intervention often leads to a disruption of natural services, such as water resources or food stocks, and one of the biggest challenges for the future is how to maintain a healthy environment in the face of increasing human populations, pollution, and climate effects.
See the links below for some freely available web resources.