The word 'ecology' was first used by Ernest Haeckel in 1869. Paraphrasing Haeckel we can describe ecology as the scientific study of the interactions between organisms and their environment. The word is derived from the Greek oikos, meaning 'home'. Ecology might therefore be thought of as the study of the 'home life' of living organisms. A less vague definition was suggested by Krebs (1972): 'Ecology is the scientific study of the interactions that determine the distribution and abundance of organisms'. Notice that Krebs' definition does not use the word 'environment'; to see why, it is necessary to define the word. The environment of an organism consists of all those factors and phenomena outside the organism that influence it, whether these are physical and chemical (abiotic) or other organisms (biotic). The 'interactions' in Krebs' definition are, of course, interactions with these very factors. The environment therefore retains the central position that Haeckel gave it. Krebs' definition has the merit of pinpointing the ultimate subject matter of ecology: the distribution and abundance of organisms - where organisms occur, how many occur there, and why. This being so, it might be better still to define ecology as:
the scientific study of the distribution and abundance of organisms and the interactions that determine distribution and abundance.
As far as the subject matter of ecology is concerned, 'the distribution and abundance of organisms' is pleasantly succinct. But we need to expand it. The living world can be viewed as a biological hierarchy that starts with subcellular particles, and continues up through cells, tissues and organs. Ecology deals with the next three levels: the individual organism, the population (consisting of individuals of the same species) and the community
(consisting of a greater or lesser number of species populations). At the level of the organism, ecology deals with how individuals are affected by (and how they affect) their environment. At the level of the population, ecology is concerned with the presence or absence of particular species, their abundance or rarity, and with the trends and fluctuations in their numbers. Community ecology then deals with the composition and organization of ecological communities. Ecologists also focus on the pathways followed by energy and matter as these move among living and nonliving elements of a further category of organization: the ecosystem, comprising the community together with its physical environment. With this in mind, Likens (1992) would extend our preferred definition of ecology to include 'the interactions between organisms and the transformation and flux of energy and matter'. However, we take energy/matter transformations as being subsumed in the 'interactions' of our definition.
There are two broad approaches that ecologists can take at each level of ecological organization. First, much can be gained by building from properties at the level below: physiology when studying organismal ecology; individual clutch size and survival probabilities when investigating the dynamics of individual species populations; food consumption rates when dealing with interactions between predator and prey populations; limits to the similarity of coexisting species when researching communities, and so on. An alternative approach deals directly with properties of the level of interest - for example, niche breadth at the organismal level; relative importance of density-dependent processes at the population level; species diversity at the level of community; rate of biomass production at the ecosystem level - and tries to relate these to abiotic or biotic aspects of the environment. Both approaches have their uses, and both will be used in each of the three parts of this book: Organisms; Species Interactions; and Communities and Ecosystems.
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