On the existence of ecological communities

Abstract

uncontroversial assumption that living organisms arepresent – because organisms live, consume, reproduce,and die; nutrients arrive by various means, and nutrientsleave by various means. Lindeman therefore defined theecosystem into existence. Although he certainly be-lieved that some processes regulated ecosystem struc-ture, his ideas of regulation did not impinge upon thedefinition of ecosystem. Ecosystems are not defined asintegrated, discrete entities.We suggest that community should be defined in aparallel manner as “the living organisms present withina space-time unit of any magnitude”. The statement“communities exist” is no longer a fruitless ontologicalproblem (as Keddy 1993 suggested) but a tautology. Byusing an operational definition, we can now ask ques-tions concerning the properties of communities, and theforces underlying variation in communities.Wilson (1991) asked whether “plant communitiesexist in a more meaningful sense, as integrated, discreteentities”. Keddy (1993) argued that this question isunscientific. Our operational definition transformsWilson’s question to “Are there any communities whichare integrated and discrete?” This reformulation shouldbe scientifically valid by Keddy’s standards, as long asprocedures for defining and detecting integration anddiscreteness can be identified.Wilson implies that the general usage of the termcommunity connotes a strong degree of integration.Keddy believes that it is possible (but scientificallyvacuous) to list criteria which distinguish whether asystem exists as a community; these criteria are notlisted, but presumably involve some sort of integrationbetween species/and or environment. Some ecologistsdo indeed believe that integration is the key to defini-tions of community. In attempting to establish an episte-mology of ecology, Scheiner (1993) considered com-munities as natural entities which are defined by ‘link-ing processes such as interactions among populations.Allen & Hoekstra (1990) recognized that there is muchKeywords: Biome; Community concept; Communityecology; Heterogeneity; Homogeneity.IntroductionWilson (1991) raised the question of whether plantcommunities exist. He included the existence of assem-bly rules, niche limitation, discreteness, discontinuity,and integratedness as potential criteria for the existenceof communities. In response, Keddy (1993) argued thatthough it may be possible to draft a list of criteria for theexistence of communities, the debate over existence ofcommunities is an ontological and epistemological gamewhich is peripheral (and, Keddy argued, even harmful)to scientific progress. We suggest that community ecolo-gists define community operationally, with as little con-ceptual baggage as possible, so that we can put thedebate about their existence behind us.An operational view of the communityDefinitionsLindeman (1942) defined the ecosystem as “thesystem composed of physical-chemical-biological proc-esses active within a space-time unit of any magnitude”.As such, it is a perfect operational definition. We beginby considering the ‘space-time unit’. The space aspectof this space-time unit could be completely arbitrary,such as a 0.1 hectare quadrat, a cubic meter of lakewater, or a political district such as a province or nation,or it could be somewhat less arbitrary, such as a lake, awatershed, or an island. The time aspect of the unitcould be measured arbitrarily as seconds or decades, orsomewhat less arbitrarily as days or years.Whatever space and time units are chosen, physical-chemical-biological processes will be present – with the