Assemblages

The concept of communities as superorganisms received considerable criticism in the US and Russia. Much of this criticism was apparently ignored or otherwise had little impact. There were compelling reasons to maintain a body of theory about integrated communities. For example, without a widespread use of communities as objects to study, there was no compelling need for theories about the ecology of communities (such as some of the theories about deterministic processes and patterns in natural succession). Ecologists might apparently have had less work to do.

There were, nevertheless, important features of developing ecology that made it inevitable that concepts about communities would be questioned. One of these was undoubtedly that animal ecologists in the early decades of the 1900s were finding some difficulties not shared by plant ecologists. Animals, at least many of them, were actually more difficult to observe (they ran or flew away, they actively hid when disturbed, etc.). As a result, the passive, motionless structures of sets of plants (i.e., communities) were not so evident for sets of animals. Seeing apparent patterns was not such a preoccupation for ecologists studying animals.

A second area of developing ecology had this problem in a more severe form. Early marine ecologists could not actually see the animals under the water and were, willy-nilly, forced to examine them in dredges and grabs that would bring pieces of substratum, with attached or embedded animals to the surface. Any structure or relationship of species into groupings had to be inferred from these samples. Furthermore, early marine biologists were compelled to recognize that their units of study were not natural (nor even contrived descriptions of) communities. Instead, they were studying species in artificial and arbitrarily designed sampling units - dredges and grabs. This did not stop them describing an equilibrial type of community - a biocenosis. This term was used to describe the collection of organisms found together in an oyster bed. It was, however, important that the unit of study was clearly a sample or representation of reality and not a naturally defined collection of species. It took some years before the sample unit became the standard unit of terrestrial ecological study.

Inevitably, quantitative sampling pervaded more and more of terrestrial (and plant) ecology. Objectively sampled data along environmental gradients provided data that did not fit easily into a structure of defined plant communities. This was often resolved by designating the anomalous samples as transitional or atypical or as identifying 'mixed stands' of plants. The latter is a somewhat obscure way of dealing with anomalous results, given that the tightly integrated community cannot realistically be considered to be mixed with another community.

Eventually, sufficient evidence accumulated to indicate that evidence for communities was outweighed by contrary evidence. Whittaker's sampling of vegetation up the environmental gradient of height in the Great Smoky Mountains is a deserved classical study to demonstrate this. Instead of accepting the evidence of communities of species of plants, as was the widely held view, he sampled at intervals up the mountains, to determine which species were present and their vertical distribution. As a result, he found that most, if not all, species were distributed independently up the gradient. There were not tightly knit, coherent communities.

The result was definitely not the anticipated outcome. Communities up a mountainside should have consisted of a set of species which have (or, of which, most have) upper limits to distribution at about the same height. They should have reasonably coincident lower limits to distribution. Otherwise, there can be no organized community of co-occurring species. Instead, Whittaker in 1956 found that upper boundaries and lower boundaries were scattered, without any obvious order, up the mountain (see later, Figure 1).

The interpretation of this result was difficult because the concept of communities as integrated units was so

Position along environmental gradient

Position along environmental gradient

Position along environmental gradient

Lower

123 1 2111 1 122

Upper

Lower

212 12111 111111 1 1 112 1 111 12 111111

Upper

Figure 1 Sampling along an environmental gradient (e.g., distance from a river, height up a mountain) to detect communities of species. (a) Most species are arranged in discrete communities, with very few occurring across boundaries of communities. Edges of distribution of the species in one community are coincident and there is little overlap with an adjacent community. (b) Species are arranged independent of each other and there are no communities. Sampling at intervals along the gradient (at the positions shown by arrows) would reveal different assemblages of species whether or not they were actually structured into communities. (c) and (d) The numbers of lower and upper boundaries per quadrat of the 18 species are shown for (a) and (b), respectively. The black bars represent the quadrats used along the gradient. Boundaries appear more clumped in (c) than in (d) (there are more boundaries in some quadrats in (c) than in (d); see text for details), identifying the difference between the two sets of distributions of species.

widespread that Whittaker had to resurrect the term 'individualistic concept' to describe the independent patterns of distribution of species.

Many subsequent analyses of assemblages of species, particularly across gradients, have described species being distributed according to their physiological tolerances. The distributions are, however, modified (usually, their extent or range is reduced) by interactions with other species (such as pathogens, competitors, and predators). Assemblages of such species intergrade along a gradient and do not display the abrupt, ecotonal changes supposedly identifying communities. Such analyses do not require ad hoc argumentation about the intergrading or missing or partial nature of communities.

So during the 1960s, work casting doubt on the super-organismic communities that had been popular with plant ecologists was being published. It is ironic that, at the same time, animal ecologists were developing new rationales for integrating the ecologies of species into integrated communities. A very sophisticated theory was beginning to be developed about coevolution and diversity of species in communities. 'Assembly rules' that were supposed to maintain the structure of communities began to appear.

A new unit of study - the ecosystem - was popularized. This was based not just on a set of integrated species being a community, but on a more holistic ecosystem, which included the biota and all the physical and chemical components of habitat within which they interact.

The definition of an ecosystem can be difficult if not impossible. It is supposedly a community, but not just consisting of an interactive, integrated set of species. Instead, an ecosystem is a set of species interacting intimately with all the inputs and outputs of resources and energy. Given that, for most assemblages, vital resources of food are autotrophs which require sunlight, the connection of use of solar energy must extend to all organisms on the planet that are dependent on solar energy. Thus, in an attempt to use a rational definition, all of life on Earth, except for chemotrophs (e.g., organisms around deep-sea vents of hot water), should be considered to be in a single ecosystem. In its most extreme form, the world as a single ecosystem has achieved considerable impetus from the well-publicized concept of Gaia. This is the Earth conceived to be responding to change as though it were a single entity. Despite this concept, it is, however, not clear that the whole planet has yet been demonstrated to be a necessary, nor useful unit of study for ecologists.

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