Effects of Predator Diversity on the Strength of Trophic Cascades in an Oceanic Benthic Ecosystem

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Intellectual Merit. Predator diversity has decreased dramatically in the world's oceans due to overfishing, anthropogenic habitat destruction and possibly climate change. Yet, still unknown for most ecosystems is the importance of predator diversity and abundance for ecosystem functioning. One of the most pervasive community-wide consequences of top predators is the Trophic Cascade (TC), where herbivores are suppressed, which releases plants from consumption, thus increasing plant productivity. Recent studies have shown that the diversity of predators may reduce, increase, or have no effect on the strength of trophic cascades. The small number, to date, of experimental tests of predator diversity effects on cascade strength precludes broad generalizations vital to the development of predictive theory. Such research is limited by the lack of experimental realism due to the small number of predator species that can be manipulated in simplified mesocosms. Without more realistic species numbers, it is impossible to extrapolate results to natural ecosystems that experience losses of predator diversity. To meet these challenges, and to better understand the consequences of present and changing levels of predator diversity in marine ecosystems, a series of experimental manipulations will be conducted on natural levels of predator diversity and their herbivorous sea urchin prey. The hypotheses test the ultimate effects on benthic algae, as a measure of cascade strength in oceanic benthic ecosystems of the Galapagos Marine Reserve (GMR). Because of years of protection from industrial fishing as a UNESCO World Heritage Site, and of local conservation protection as the GMR, there are diverse guilds of higher trophic level predators, such as large fish and sharks. Likewise, there is high diversity of intermediate-level fish and invertebrates that prey on sea urchins, creating an unusual opportunity for testing and developing predator diversity and Biodiversity Ecosystem Functioning theory. The overarching questions addressed in this project are: How do naturally occurring large ranges of oceanic predator diversity influence the strength of trophic cascades? and How does environmental variation and conservation protection influence these processes? The first question will be addressed in experiments manipulating both horizontal (within trophic level; urchin herbivores) and vertical (across trophic level; predators) consumer diversity and in another experiment manipulating the diversity of predatory fish and invertebrates guilds. The experiments employ open fenced treatments containing urchins but allowing access by fish and invertebrate predators of the urchins. To record natural levels of fish and invertebrate predator richness encountering the treatments, consuming the urchins and interacting with each other, the entire experimental layout will be video-recorded for up to several weeks at a time. The time-lapse cameras/lighting system is capable of day and night imaging without affecting predator behavior. A simplified manipulation to measure the influence of predator diversity on cascade strength will be replicated and video-recorded at 16 sites -- representing different levels of upwelling and conservation protection -- to place the mechanistic understanding gleaned from detailed experiments at local sites into a broader (mesoscale) context. Broader Impacts. Several major impacts of the proposed research on education are expected beyond the implications for conservation and ecosystem-based management. For instance, the program will provide hands-on training for one graduate student and two undergraduates in experimental ecology, sub-tidal community ecology, statistics, marine reserve science, scientific writing and oral presentation skills. The PI is committed to educational and public outreach activities beyond the typical use of the information gained on predators and food webs by using it to enhance lectures in his large undergraduate Ecology class at Brown University. One of these outreach activities is to help train the next generation of teacher leaders by collaborating with the Director of the Brown University's Masters in Teaching (MAT) program to offer a science internship for one MAT student per year. The general information on food webs, ecology of charismatic Galapagos megafuana and images of marine predators in action obtained from the time-lapse video would be shared with the MAT interns to inspire their lesson plans for elementary and high school students. In summary, the broader educational impacts of the proposed research will be to enhance the workforce of globally trained doctoral students in the life sciences, help prepare undergraduate students for graduate school in marine ecology, augment the training of the next generation of K-12 teachers and enhance the understanding of one of the most pressing environmental issues -- the ecosystem consequences of over-harvesting top predators and concomitant loss of predator diversity in the sea.