Project: Long-term and interactive effects of seaweed diversity and herbivory on intertidal community structure and functioning

Description from NSF award abstract:
Stimulated in part by rapid changes in the biological composition and richness of most of Earth's ecosystems, considerable theoretical and empirical research has focused on understanding the effects of species richness on communities and ecosystems. This research has resulted in major advances and has shown that diversity can affect ecosystem processes. However, we are currently limited in our ability to draw generalities regarding these effects that transcend ecosystems and to apply this knowledge to conservation and management efforts because: (1) most experiments are of short duration and relatively few are conducted under field conditions on natural substratum; and (2) most experiments manipulate only diversity and so cannot assess the strength of diversity relative to other known drivers of ecosystem functioning. Previous NSF-funded work found that species diversity of seaweeds does increase the accumulation of producer biomass and the diversity and richness of mobile invertebrates associated with those seaweeds. In this project, the investigator will examine the interactive effects of plant and grazer richness on seaweed productivity and assemblage recovery from disturbance by (a) continuing our ongoing diversity manipulations (b) nesting additional experiments manipulating grazers within these plots, and (c) performing new experiments to examine effects of grazer richness on algal assemblages. The main experiment consists of 72 1.5 m diameter plots in the rocky intertidal at Bodega Bay, California, split into 6 different treatments: a monoculture of each of the 4 algal species that comprise >85% of the algal cover in our system; a 4-species polyculture, and an unmanipulated control plot. The PI created these "diversity" plots by weeding under the auspices of a previous NSF grant.

Objectives of this project are:
(1) To nest within the diversity plots smaller cleared areas that either allow or prohibit access by grazers in order to test the interactive effects of algal diversity and herbivory on recovery of the algal assemblage from disturbance (or resilience).
(2) To conduct a separate combined additive and replacement series experiment to examine the effects of herbivore identity and richness on algal recruitment and succession.
(3) While the nested grazer-exclusion experiment is in progress, to continue weeding and collecting data in the uncleared portion of these plots as a continued test of the univariate effect of algal richness on the cover of algae and sessile invertebrates, and on mobile invertebrate abundance, richness, and diversity. Together with previous results, this will produce a 7 year record of the effects of seaweed richness on rocky intertidal communities, which will then be correlated with archived environmental data.
(4) After the nested grazer exclusion experiment is complete, to cease weeding plots and track their reinvasion by weeded species in order to evaluate the mechanisms maintaining diversity of perennial algae.

Understanding the consequences of simultaneous changes in grazer abundance or diversity and algal diversity as well as the long-term effects of both on the resilience of algal communities in the face of disturbance will provide a better understanding of both the consequences of human encroachment into these habitats and the appropriate management efforts needed to minimize impacts and maintain ecosystem functioning. This is especially critical given that harvesting and trampling by humans are altering the diversity and composition of herbivores and algae in the California rocky intertidal zone.