Abstract:

There are at least four mathematically developed models that predict unimodal diversity–
productivity relations in local communities. I attempt to distinguish among these theories
using data from surveys of planktonic organisms in 31 fishless ponds in southern Michigan
by relating plant and herbivore diversity and composition to pond nutrient levels. The density
of plants (phytoplankton) was positively correlated with nutrient levels and the density of
herbivores (zooplankton) was positively correlated with the density of plants. Species richness
of plants and of herbivores were declining or unimodal functions of nutrient levels. The
composition of each trophic level changed significantly with eutrophication as indicated by
significant correlations between nutrient levels and site scores (obtained by reciprocal averaging
ordination on occurrences). A concomitant lack of correlation between site scores and
species richness for both trophic levels further shows that the patterns of species distributions
form gradients of species replacements rather than nested subsets. Taxonomic ordination scores
also showed that the algae found at low nutrient levels consisted disproportionately of small,
unprotected forms (thought to be fast-growing but grazer-susceptible), whereas the algae found
at high nutrient levels were larger and often sheathed or gelatinous (thought to be slow-growing
but more resistant to grazers). The results of this analysis of changes in the patterns of distribution of planktonic organisms are consistent with a hypothesis of productivity-dependent
‘keystone-predation’ causing the unimodal relation between diversity and productivity