Print PageAccelerating human-caused declines in biodiversity have spurred a large body of research investigating linkages between species richness and ecosystem function. With few exceptions, these experiments have randomly manipulated species richness, while observed patterns of species losses in real ecosystems are non-random with respect to traits. Recently, the importance of conducting more realistic biodiversity-ecosystem functioning experiments has been recognized and the few studies published have demonstrated that realistic species losses may have strikingly distinct functional consequences from randomized losses. However, most of these studies have relied on either modeling approaches or isolated experimental microcosms. We conducted a field-based study directly comparing the functional consequences of realistic and randomized species losses in a serpentine grassland at Kirby Canyon Reserve near San Jose, California. We used a pool of sixteen native serpentine grassland plant species planted in 90 plots (29 cm radius) in nine blocks varying in soil depth. Each plot contains a mixture of 2, 5, 8, 12, or 16 species either drawn at random or adhering to a realistic loss order based on nested subset analysis of 19 years of plant community composition data from our experiment site. We measured aboveground productivity and tissue nitrogen (N) concentration of dried biomass harvested from each plot, and used these data to calculate community-level N uptake and N-use efficiency.
Results/Conclusions
Randomized species losses had no effect on either N uptake (F = 1.94, p = 0.07) or N-use efficiency (F = 0.34, p = 0.85). In contrast, N uptake declined strongly with realistic species losses (F = 3.99, p = 0.009), by an average of ~ 36% from the 16-species treatment to the 2-species richness level. Realistic species losses also significantly affected N-use efficiency (F = 3.67, p = 0.015); the 8- and 12-species richness levels had the highest N-use efficiency while the 2-species richness level had the lowest N-use efficiency. Our findings illustrate that the order of species losses from a community can strongly influence the effects of declining species richness on ecosystem processes. In this ecosystem, a biodiversity-ecosystem functioning study based solely on randomized assemblages would have predicted a very different response of N uptake and N-use efficiency than what occurred under more likely, realistic biodiversity losses. We argue that the contrast between the effects of randomized and realistic species losses results because the latter involves concentrated loss of particular traits rather than a random loss of traits from across a spectrum.
