Inferences from spatial and temporal variability in ecosystems: long-term zooplankton data from lakes
Ecosystem patterns and processes exhibit spatial and temporal variability. Analysis of this variability can reveal which aspects of a system are specific to sites or to years. Patterns in this variability yield insights into the forces structuring ecosystems. As an example of this approach, we examined an unpublished zooplankton data set collected by Birge, Juday, and coworkers in the summers of 1929 through 1941 from five neighboring northern Wisconsin lakes. Eight taxa were common to the five lakes: two rotifers (Keratella cochlearis and Kellicottia), four cladocerans (Daphnia pulex, Daphnia galeata mendotae, Diaphanosoma, and Bosmina), and two copepods (cyclopoid and calanoid). Taxa differed in their consistency in depth distribution and seasonal timing. For example, Diaphanosoma peaked at the same relative depth and time in each of our five study lakes. In contrast, Bosmina varied among lakes in the timing of its maximum abundance, and calanoid copepods varied in the depth of their maxima. In general, among-year variability was greater in zooplankton abundance than in seasonal timing, which, in turn, was more variable than depth distribution. Among-year variability in maximum abundance was related to a taxon’s potential for rapid population increase, r\$\_\max\$. Variability among years was greater in rotifer taxa than in cladoceran taxa, which were more variable than copepod taxa. The abundance and depth distribution of copepods and cladocerans exhibited relatively greater variability among lakes than among years, suggesting that conditions specific to lakes are important in controlling these parameters. This conclusion was supported by data from Lake Washington; except in one case, the same taxa that showed significant site specificity in Wisconsin changed significantly in abundance in Lake Washington after sewage diversion. In contrast, we inferred that variation in weather was an important determinant of seasonal timing for all eight taxa and of the maximum abundance and depth of maximum abundance of the two rotifers. An ability to predict general site-specific and year-specific ecosystem parameters has significant practical and theoretical importance. For example, taxa or parameters exhibiting large variability among sites are sensitive indicators of change in an ecosystem, whereas those parameters or taxa exhibiting large temporal variation should be studied to understand factors influencing seasonal or yearly dynamics. We suggest that similar analyses of spatial and temporal variability could provide useful comparisons of ecosystems ranging from aquatic to terrestrial habitats and comprising markedly different taxa.