While most studies of the relationship between biodiversity and ecosystem function have examined randomized diversity losses, real biodiversity loss generally occurs progressively rather than randomly. Progressive, realistic biodiversity losses through time are necessarily strongly nested – that is, each decreasing level of biodiversity represents a subset of the previous higher level of diversity. Studies that have examined realistic biodiversity loss have generally found higher effect sizes on ecosystem functioning relative to randomized diversity loss. However, nestedness within realistic loss orders is a potentially confounding effect because the randomized diversity losses to which they are being compared are usually much less nested. In this study we addressed the question of whether the difference between randomized and realistic biodiversity loss is generated by the degree of realism or the degree of nestedness in the loss order. We isolated the effect of nestedness by performing post-hoc analyses on data from an experimental, field-based biodiversity manipulation in a serpentine grassland at Coyote Ridge, California. Using plots with randomized assemblages of 2, 5, 8, 12, and 16 species, we assigned plots to create 32 highly nested (though non-realistic) loss orders and also generated 32 non-nested orders by random draw from the same set of plots.
Results/Conclusions
Using two measures of nestedness (temperature and NODF), we compared the effect that the degree of nestedness within a loss order has on the relationship between diversity and aboveground productivity. We found that the degree of nestedness does not influence the relationship between diversity and productivity (r2 = 0.00 and P>0.05 for both nestedness temperature and NODF). Additionally, nested loss orders demonstrated strong species- and functional trait-specific influences on the productivity-diversity relationship that were not evident from random loss orders. For instance, high persistence of perennials in a nested loss order leads to a positive productivity-diversity relationship while low persistence of perennials does not. Somewhat surprisingly, high persistence of a nitrogen-fixing forb did not lead to higher productivity at lower levels of diversity as has been noted in other experiments. Understanding the relationship between the expected order of species loss and functional traits is becoming increasingly important in the face of ongoing biodiversity loss worldwide. These results suggest that species composition and the order of biodiversity loss rather than nestedness per se drive the productivity-diversity relationship in these experiments.