Document: LIN-3-63-22

A conceptual clarification of complexity and its application in small aquatic systems.

PUTH, L.M.* and T.F.H.ALLEN

University of Wisconsin, Madison, WI 53706, USA. ¹

Abstract:
Although there is a great deal of literature exploring the effects of complexity on community and ecosystem stability, there is little consensus on the magnitude or direction of these impacts. Much of this disparity stems from confusion in the meaning of the terms complexity and stability. While cleaving the general term stability into resistance, resilience, and other measures has rendered it more precise, an analogous division of complexity has yet to occur. Here, we parse complexity into two fundamentally distinct components: complicatedness, elaboration at a single level of a hierarchy, and hierarchical organization, elaboration of levels within a hierarchy. Theory predicts that changes in these two aspects of complexity should cause different behaviors in ecosystem function across an ecologically relevant gradient. For example, the addition of a new species with functional equivalents, a change in complicatedness, should produce little change in ecosystem function, while the addition of a new functional group, a change in hierarchical organization, is likely to show major effects. We propose a method to determine whether ecological variables represent complicatedness or hierarchical organization. Measures that represent complicatedness should change continuously, while variables representing hierarchical organization should change discontinuously (e.g., step functions) across any salient gradient. These differences can be distinguished using linear regression techniques. We compared model predictions to trends for a suite of limnological variables in a set of naturally occurring rock pools in northern Wisconsin and Minnesota across a gradient of rock pool volume. This method yielded reliable distinction between complicated and hierarchically organized variables. Variables such as phytoplankton and zooplankton species richness showed positive and continuous relationships with rock pool volume, while variables such as the presence or absence of permanent sediment show positive but discontinuous relationships with system size. This research should help clarify the complexity-stability debate by identifying the different components of complexity and allowing new evaluation of the contradictory results of previous studies.

Keywords: complexity, complicatedness, hierarchical organization

This abstract is being presented at: 1:00 PM in session:
Oral Session #11: Trophic Cascades.