Chapter 26: Interactions Involving Conditions
A particularly significant and interesting aspect of organismal life is its interaction with its surrounding environment. As indicated by the term, interaction implies that both components affect each other: the environment affects organisms and organisms affect the environment. The ‘environment’ includes components that are living, i.e., other organisms (the biotic environment), and components that are non-living such as rocks, clouds, water, dead organisms or parts of dead organisms. The environment has properties, or conditions, such as temperature and oxygen concentration that are commonly considered ‘the abiotic environment’ or the ‘physical environment’. Conditions are the consequences of physical processes such as radiation, diffusion, convection; and these processes are sometimes strongly influenced by biotic processes (Fig. 1), such as respiration (generates heat and carbon dioxide, removes oxygen) and photosynthesis (absorbs light, adds oxygen and water, removes carbon dioxide). The conditions present on a site can dictate whether or not an organism can exist and, if it exists, how it behaves. This is sometimes described as an interaction between organisms and the physical, or abiotic, environment.
Because organisms can affect conditions, they can interact with other organisms as a result. In addition to the interaction between organisms mediated through conditions, there are more direct interactions between organisms, such as one organism eating another or an insect transporting pollen from one plant to another. Interactions are fundamental to the discipline of ecology. Indeed one definition of ecology is ‘a study of the interactions between living things and their environment’.
Another common definition of ecology is ‘a study of the distribution and abundance of organisms’ (Fig. 2). The distribution and abundance of organisms are controlled by interactions between organisms and their biotic and abiotic environment. For example, the distribution of a plant might be limited by the intensity of light. The amount of light available is a consequence of both the physical environment (latitude, degree of cloudiness, whether the site is facing south or north, etc.) and the biotic environment (presence or absence of tree species to intercept the light). The distribution and abundance of a particular plant (species ‘A’) might also be limited to the presence or absence of an organism that eats the plant or perhaps by an organism that pollinates the plant. In either of the latter cases, one might consider the factors that determine the distribution and abundance of the second species to be controlling the distribution and abundance of species A.
Another definition of ecology is “a study of biological organization above the level of individual.” As we have seen, organisms have a structure (form, size, organization of component parts), and also organisms have functions, they do things such as develop, r