PARENT SESSION
Poster Session # 21: Biogeography, Biodiversity, Populations, and Genetics.

Thursday, August 7 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Can species pool size and composition explain different productivity-diversity relationships?

Houseman, Gregory^{*,1, 2}, ¹ Michigan State University, East Lansing, MI² Kellogg Biological Station, Hickory Corners, MI

ABSTRACT- Although the influence of plant diversity on ecosystem function (i.e. productivity) has garnered much attention, understanding how diversity varies across productivity gradients remains an important way to examine how underlying differences in environmental conditions influence community dynamics. Recent reviews have found that, while unimodal patterns are common, positive, negative or no relationship between diversity and productivity also occur with surprising frequency. If species pools strongly affect local diversity, then different species pools found among sites may explain the disparate productivity-diversity results. I tested this idea by adding 0, 5, 15, 30, or 45 new species to low, medium and high productivity (146, 338, 556 g live biomass/m² respectively) portions of a natural productivity gradient in a southwest-Michigan oldfield. I assumed that the naturally occurring species pool was relatively homogeneous because the gradient occurred over a relatively small spatial scale (200m). Patterns of colonization and persistence through one growing season demonstrated that local diversity increased with increasing species pool size. Consistent with predictions from Huston (1999), the diversity of colonists was unimodally related to productivity for larger species pools. Simulations based on the experimental results demonstrated that randomly selected species pools of equal size from each portion of the gradient could result in negative, unimodal or no relationship between productivity and diversity. I also determined whether each species could germinate and persist in the absence of competitors by creating monoculture plots along the gradient. Environmental conditions strongly reduced (filtered) species pools at low productivity and suggested that even when the pool sizes are identical unimodal productivity-diversity patterns will not always occur because the pattern also depends upon the particular composition of the pool. Thus, historical and landscape level factors that influence the size and composition of species pools can explain different productivity-diversity relationships.

Key words: Productivity, Grasslands, Diversity, Species Pools