Stachowicz Lab University of California, Davis

Effects of Species Diversity on Communities and Ecosystems

While the relationship between species diversity and community stability has long been a topic of theoretical interest to ecologists, recent concern about the loss of biodiversity has prompted renewed interested in what the consequences of diversity loss might really be. We have worked on three fronts in this research area: documenting patterns of change in biodiversity in real systems, performing experiments to demonstrate the mechanisms by which diversity does and does not affect communities and ecosystems, and collaborating with other ecologists, conservation biologists and economists to test links between the loss of biodiversity and the provision of goods and services to humans by natural ecosystems.

A typical high diversity algal plot

Weeding the intertidal to create plots that differ in algal diversity
Predictions of the consequences of diversity loss are complicated by the fact that we often do not know exactly how diversity is changing, and that it may be changing in different ways at different scales. At the global scale, diversity is undoubtedly declining due to extinctions, many caused directly or indirectly by the activities of humans. At the regional scale diversity often increases due to introduction of new species, although work in our lab comparing invasions and extinctions in coastal ecosystems has shown that despite little change or even increases in total diversity, the composition of the biota has changed in important qualitative ways. Specifically, we find that extinctions occur mainly at the level of high-order predators and other carnivores, whereas invasions are dominated by species at lower trophic levels like suspension feeders and detritivores. A major focus of our lab's work is examining the consequences of these changes in diversity for the structure and function of local communities and the services they provide to humans.
One ongoing research project in the lab is a long-term experiment (4 years as of July 2008) examining the ecological consequences of altering seaweed diversity in the rocky intertidal zone at the Bodega Marine Lab. We find that diversity has strong effects on plant biomass and production, as well as on invertebrate diversity, but these effects emerge only in experiments that are long term and conducted in the field on natural substrate. Short-term field, and mesocosm experiments find little if any effect of diversity. This line of work has caused us to suspect that many current experiments have underestimated the effects of diversity, as they are often short term. This conclusion is supported by other researchers' re-analysis of experiments on land, which finds an increasing effect of diversity with increasing duration of an experiment. Still, we find that comparing the outcome of experiments of different duration, or those conducted in the field vs. in the lab provides a clearer picture of how diversity affects ecosystems because each type of experiment differs in the types of mechanisms they can detect.

A predator polyculture from our kelp mesocosm experiment. Kelp in these treatments loss less mass than those with lower predator diversity

The distribution of local and global predator extinctions in the oceans broken apart by trophic level. Most species going extinct are either top or secondary predators.
Another effort focuses on the consequences of "realistic" diversity loss. Our review of the literature on invasions and extinctions shows that in coastal ecosystems, species loss is highly skewed toward top trophic levels, whereas invasion are skewed toward suspension feeders and detritivores. We have followed this observation up with two separate experiments: one examines the effect of changing predator diversity on the control of kelp forest herbivores (a trophic cascade). While we generally found that increasing predator diversity increased herbivore suppression, our study also illustrated that the effects of predator diversity on trophic cascades depends on the diversity of herbivores in the system and that predator diversity can have effects on herbivores by altering their behavior as well as reducing density. Additional studies in the "fouling" community on docks and pilings are investigating the effects of increasing the diversity of suspension feeders and altering predator diversity within the same ecosystem.

The growing number of experimental studies in this area published in the past few years has led to a need for a review of this literature and a synthesis focused on answering the more applied question of whether diversity really affects the ability of ecosystems to provide goods and services such as food, tourism, etc.
Relevant Papers

Stachowicz, J. J., M. H. Graham, M. E. S. Bracken, and A. I. Szoboszlai. In Press. Diversity enhances cover and stability of seaweed assemblages: the importance of environmental heterogeneity and experimental duration. Ecology.

Worm, B., E. B. Barbier, N. Beaumont, J. E. Duffy, C. Folke, B. S. Halpern, J. B. C. Jackson, H. K. Lotze, F. Micheli, S. R. Palumbi, E. Sala, K. A. Selkoe, J. J. Stachowicz, and R. Watson. 2006. Impacts of Biodiversity Loss on Ocean Ecosystem Services. Science 314:787-790. [-pdf-]

Worm, B., E. B. Barbier, N. Beaumont, J. E. Duffy, C. Folke, B. S. Halpern, J. B. C. Jackson, H. K. Lotze, F. Micheli, S. R. Palumbi, E. Sala, K. A. Selkoe, J. J. Stachowicz, and R. Watson. 2007. Response to Comments on "Impacts of Biodiversity Loss on Ocean Ecosystem Services". Science 316:1285d-. [-zip-]

Stachowicz, J. J., J. F. Bruno, and J. E. Duffy. 2007. Understanding the effects of marine biodiversity on communities and ecosystems. Annual Review Of Ecology Evolution And Systematics 38:739-766. [-pdf-]

Byrnes, J. E. 2007, P. L. Reynolds, and J. J. Stachowicz. Invasions and Extinctions Reshape Coastal Marine Food Webs. PLoS One 2:e295. [-pdf-]

Byrnes, J. E., J. J. Stachowicz, K. M. Hultgren, A. R. Hughes, S. V. Olyarnik, and C. S. Thornber. 2006. Predator diversity strengthens trophic cascades in kelp forests by modifying herbivore behavior. Ecology Letters 9:61-71. [-pdf-]

Hughes, A. R., J. E. Byrnes, D. L. Kimbro, and J. J. Stachowicz. 2007. Reciprocal relationships and potential feedbacks between biodiversity and disturbance. Ecology Letters 10:849-864. [-pdf-]