Seminars

November 8 2017

12:00 LSB 2320

Matthew Bracken
Department of Ecology and Evolutionary Biology, UC Irvine

Top-down modification of bottom-up processes: nutrient recycling by consumers enhances algal growth in marine ecosystems Top-down modification of bottom-up processes: nutrient recycling by consumers enhances algal growth in marine ecosystems

Summary

Understanding the roles and relative importance of top-down and bottom-up factors on primary producers is a central goal of ecology. At first glance, the effects of nutrients and herbivores on primary producers seem straightforward: nutrients enhance producer abundance, whereas herbivores consume producers, reducing abundance. However, herbivores’ effects on primary producers are not limited to consumption. For example, molluscan grazers on rocky shores enhance nutrient availability and algal recruitment, both of which have the potential to increase resource availability. Here, I describe experiments in which I partitioned consumers’ top-down and bottom-up effects on marine algae. First, I show how community context mediates the top-down versus bottom-up effects of snails on rocky intertidal seaweeds. In particular, I highlight the necessity of considering simultaneous effects of grazers on multiple co-occurring seaweed species. Second, I show that positive effects of grazers on algae are not only associated with local-scale nutrient recycling. I manipulated grazer abundances at the level of whole tide pools but controlled access of those grazers to experimental plots within each pool. I found that grazers’ consumptive effects were paradoxically greatest in pools where grazer densities were reduced and that positive effects scaled with grazer abundance. Grazers benefitted algae not only by recycling nutrients but also by facilitating recruitment of algae into the system. Slime trails left by molluscs acted as adhesive traps for algae, enhancing settlement. Our understanding of the effects of consumers and nutrients on primary producers is incomplete without accounting for the top-down modification of bottom-up processes

 

 

 

 

 



 

 

 

 

 

 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



 



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