The Local Scale                                       The Regional Scale
 

Local-regional patterns-    The relationship between local and regional species richness may hold clues to the mechanisms that maintain diversity in communities.  If local diversity increases with the richness of the regional pool, then the supply of colonists may limit local diversity.  Alternatively, if local and regional richness are independent over some of their range, then local biotic and abiotic processes may be most important.  One problem is that the appropriate definition of the regional scale is ambiguous.  My collaborators and I developed a method for examining the shape of local and regional patterns that is independent of regional extent.  Our method found that local and regional zooplankton richness are positively related whereas the standard “scale dependent” method found saturated patterns.

J.B. Shurin, J.E. Havel, M.A. Leibold, and B. Pinel-Alloul.  Local and regional zooplankton species richness: a scale-independent test for saturation. Ecology 81: 3062-3073.   Abstract  PDF
 
 

J.B. Shurin.  Dispersal limitation, invasion resistance, and the structure of pond zooplankton communities. Ecology 81: 3074-3086.  Abstract  PDF

Community invasibility- Positive relationships between local and regional diversity might mean that local assemblages are open to invasion and that local interactions exert weak influence over species distributions.  I tested these conclusions in Michigan ponds by introducing the regional species pool in large enclosures.  The communities resisted invasion by >90% of the species in the region.  Invasibility declined with native species diversity, and the invaders were far more successful when the density of native zooplankton was reduced, showing that species interactions excluded many potential invaders.  These results contrast with the patterns, suggesting that dispersal does not strongly limit zooplankton diversity and that biotic interactions play a major role.

Theory of local and regional diversity-    To resolve the contrast between the patterns and experimentsl, Emily Allen and I modeled the effects of indirect facilitation via shared predators on the relationship between local and regional diversity.  The model predicts that predators expand the conditions for linear, positive patterns of local and regional richness among competitors.  It also predicts that predators can both promote regional diversity and depress local diversity.  These predictions were supported by an experiment that looked at the effects of insect and fish predators on plankton communities that were either connected to or isolated from the regional zooplankton species pool.

J.B. Shurin.  Interactive effects of predation and dispersal on zooplankton communities. Ecology 82:3404-3416.   Abstract   PDF
J.B. Shurin and E.G. Allen.  Effects of competition, predation and dispersal on local and regional species richness.
    American Naturalist 158:624-637.   Abstract   PDF

Variable trophic cascades among ecosystems-  Trophic cascades (indirect cntrol of plant biomass by predators via herbivory) are often thought to vary among ecosystems.  Much heated debate has concerned the question of whether top-down control is more important in aquatic than terrestrial ecosystems.  Along with Eric Seabloom, I have explored theory that looks at the roles of body sizes of plants and consumers and productivity of plants in the strength of trophic cascades.  Trophic casacades are strongest when herbivores are larger than plants, when plants have high growth rates.  Since these conditions are more characteristic of aquatic plants than terrestrial, we have good reasons to expect trophic cascades to be more important in water than on land.

To test these predictions, we performed a meta-analysis of 99 trophic cascade experiments in six different ecosystems.  The strongest cascades were in marine and freshwater benthos, and the weakest in terrestrial ecosystems and marine plankton.  The results agree well with many of the predictions from the theory.  J.B. Shurin, E.T. Borer, E.W. Seabloom, K. Anderson, C.A. Blanchette, B. Broitman, S.D. Cooper, and B. Halpern.  A cross-ecosystem comparison of the strength of trophic cascades.  Ecology Letters 5: 785-791. PDF

The spread of exotic species-    Exotic species represent a major threat to ecosystem integrity.  The effectiveness of methods for controlling invasive species depends on whether their ranges are limited primarily by dispersal or local factors.  John Havel and I are studying the expansion of Daphnia lumholtzi, an invasive zooplankter, throughout the United States.  We are using data on physical factors, the biotic community and spatial position to test spatial vs. local models of spread. J.E. Havel, J.B. Shurin and J. Jones.  Estimating dispersal from patterns of spread: spatial and local control of invasion by Daphnia lumholtzi in Missouri lakes. Ecology 83: 3306-3318.    PDF

I was also recently involved in a debate in The Scientist on the subject of ecology angst, along with other NCEAS post docs.  Here is s letter we wrote in response to an earlier letter.  Enjoy.