From the universe as a whole down to the atoms: distinctions between community and ecosystem ecology

Tools of the trade: ecosystem ecology

In one of the more nerve-wracking moments of my early career I was informed that the ecosystem concept was “putting food on the family table.”  I was to stick to the methods of ecosystem ecology and not to waste time with community level questions.  By this I understood that I should not indulge an interest in individual organisms nor attempt to identify these to the species level – certainly this was consuming a lot of my time. I should not, I was told, become overly interested in species diversity, another community-level phenomenon.  Though ecology is rapidly changing, nevertheless as recently as a decade and a half ago you picked your favourite flavour early – if you chose chocolate it is frowned on if you started adding dollops of vanilla.

The ecosystem, as we have discussed, is a compound constituted by all the organisms of a region interacting with the non-living environment – soil, water, and atmosphere.  In modern ecology the ecosystem concept has become the foundation for functional studies of the environment, often at the landscape scale, where ecologists study the processes contributing to the flow of energy (primarily starting with the photosynthetic “fixing” of the sun’s energy by plants) and the cycling of nutrients.  For instance, a present-day ecosystem ecologist might be interested in how the rates of leaf litter decomposition in a woodland affects the availability of nutrients in the soil, thus influencing subsequent plant growth.  This functional interpretation of the ecosystem is contrasted with the approach taken by population and community ecologists who study the interaction of individual organisms, the growth of populations and the interactions that occur between individual of multiple species coexisting in the same place at the same time. 

The differences between these sub-disciplinary specialties is reflected quite strongly in the way these ecologists go about their business – for instance, the functional ecologist often disregards the identity of the species contributing to the processes being examined (grouping these species into categories like decomposers, producers etc.), and she may spend her day measuring the concentration of nitrogen and phosphorus in a prairie soil, whereas her colleague studying community level questions may spend that same day on her hands and knees identifying plant species and estimating their abundance in a one-meter squared sampling unit in that prairie, or tallying the abundance of individual insects pollinating flowers.  

From an historical perspective a demand, such as the one I got early in my career, to avoid the methods of community ecology in perusing ecosystem questions was a peculiar one.  The ecosystem, proposed by Arthur Tansley in a paper entitled “The use and abuse of vegetation concepts and terms” (1935) was understood by him to be a relatively stable entity of a series of entities ranging from the universe to the atom.  The ecosystem concept was proposed in a discussion concerned largely with early concepts of the ecological community.  One might therefore wonder why in the ecology of the 1990s ecosystem ecology was considered so emphatically distinct from community ecology that a junior scientist crossed boundaries at his peril.  The answer in part is that the functional orientation of ecosystem studies was a later addition to the ecosystem studies and was not the perspective developed in Tansley’s theoretical paper.  Most of Tansley’s field work, assessing the vegetation patterns of the British Isles, would have received a disapproving wag of the finger from today’s ecosystem ecologists.  The functional orientation of ecosystem studies was a later addition by researchers like Raymond Lindeman in the 1940s and Eugene Odum in the 1950s and 60s who sought to implement the concept in concrete empirical studies. 

In addressing the distinctions and commonalities between ecosystem and community ecology it is especially important to ask what exactly the units of community and ecosystem studies are – and then to ask why that entity might need distinct tools for examining it.  Do these entities have properties in common with individual organisms of which they are partially comprised?  We have already seen that friction about the properties of communities was what, in part, motivated Tansley’s 1935 paper.  Indeed, one of the abuses that Tansley identifies in his paper was the suggestion that vegetation on large spatial scales had properties in common with individual organisms making them complex organisms (or “superorganisms”).  In dismissing the complex organism, Tansley did not deny that an analogy between individuals (aggregates of cells, tissue, organs etc) and ecosystems (aggregates of plant, animals, soil etc.).  Instead he called the community a quasi-organism.  Vegetation considered on a regional scale could be well-integrated and has many of the characters of organisms, in the way that human societies, Tansley remarked, have such properties.  However, the dissimilarities between the mature plant community and the organism are so compelling that, according to Tansley, “the ridicule poured on the proposition that vegetation is an organism are easily understood.” 

In discussing the properties of the ecosystem as just one category of entity among other physical systems that range in size from the universe on down, Tansley was drawing upon bodies of theory in science and philosophy that were themselves in a primordial form.  These included discussions of “holism” and the philosophy of Jan Christiaan Smuts, and perhaps more influentially the work of Alfred North Whitehead, the British mathematician whose “process philosophy”, especially as presented in Science and the Modern World (1925) has had some influence on ecologists and is mentioned in passing by Tansley. Another and possibly more influential body of thinking, systems theory, was taken up in earnest by ecologists in the 1980s and remains influential.  Hierarchy theory, a type systems thinking, is crucial for an understanding of how we can study entities such as ecological systems, comprised of parts within part.  Hierarchy theory justifies, or at least make obvious, why a given level of organization requires the development of an appropriate suite of techniques, that is, why the ecosystem ecologist studies nitrogen and the community ecologist studies pollination.  Just as powerfully, it suggests ways in which we can link disciplines, asking questions about how the structure and dynamics of the vegetation can affect ecosystem properties like decomposition and nutrient cycling.  Examining relationships between biological diversity (a community concept) and ecosystem function (like productivity) are hip newer themes for ecology, but the questions might have seemed obvious to Arthur Tansley.  These days contemplating such questions are putting food on my family’s table.