Understanding why plant diversity declines with increasing soil resource availability: Tests of the important hypotheses.

摘要

One of the most striking patterns within ecological assemblages is the decline in species richness with increasing resource availability, often referred to as the paradox of enrichment. Currently, we do not understand the mechanisms underlying this ubiquitous pattern. Further, this paradoxical decline in richness may be related to a more general unimodal pattern of species richness and productivity. Specifically, worldwide, across habitats from a few meters to thousands of square kilometers, species richness of a wide variety of plant and animal taxa often peaks at moderate levels of productivity and is lower when productivity is either very low or very high. While there is substantial debate regarding the ubiquity of the unimodal richness productivity curve, however, there is virtually no debate on the general effect of enrichment in plant assemblages: as resource availability rises, species richness declines, and we do not know why. This project seeks to test two of the most prominent mechanisms that have been proposed to explain this widely cited, paradoxical decline in species richness.; Understanding the mechanisms for the decline in richness with increasing fertility remains a challenge to ecologists partly because of the paucity of experimental empirical tests. I have tested two important and general hypotheses that have been proposed to explain this pattern: spatial resource heterogeneity , and assemblage level thinning. Although many other hypotheses exist, these two hypotheses are well grounded in recent theoretical and empirical evidence and each tests an important and basic simplifying assumption of resource competition theory. The experiments and analyses presented herein constitute the first rigorous tests of much long standing dogma in plant ecology. In contrast to long standing, untested dogma, I found that spatial resource heterogeneity played no role in maintaining coexisting among species. Rather, species richness was simply positively related to the average quantity of the limiting resource. Further, I found little evidence to support resource niche differentiation, and strong evidence suggesting that species richness is first and foremost merely a byproduct of the number of individuals in a sample.