Research

My research program broadly addresses ecology, conservation and restoration of marine and estuarine systems. My work addresses questions at the intersection of community and ecosystem ecology that specifically include the impacts of human activities. The approaches I take reflect my training as a marine community ecologist, and ultimately I am interested in what determines the abundance and diversity of species in coastal systems. I am particularly interested in how human impacts have shaped community and ecosystem processes and how we can ensure the future health of coastal ecosystems through enlightened management based on good science.

Current work includes the following areas:

(1) Ecology and Evolution of Introduced Species
(2) Population Dynamics and Restoration of Native Olympia Oysters
(3) Green Crab Eradication


Ecology and Evolution of Introduced Species

One of the most important human impacts in coastal marine systems is the introduction of non-native or introduced species. These species can have a wide range of effects on native species from little or none to quite profound changes. Not only do the invading species influence native species, but they can also undergo changes themselves. A large part of my interest in introduced species involves understanding how they affect native species by altering fundamental community and ecosystem processes. In addition, my work has addressed how the invasion process has influenced the population genetic structure of introduced species (collaborations) and the influence on characteristics of invading populations such as body size and behavior.

My earlier work has addressed the impacts of the European green crab Carcinus maenas in food webs in central California estuaries. This work has documented dramatic changes in native clams and crabs, but these effects did not (so far) cascade upwards to influence populations of native shorebirds that use these species as prey. My work has also shown that in the process of drastically reducing some very abundant native clams, green crabs have accelerated the invasion of a non-native that had been kept in check for decades by the native clams. This indirect effect of predation by the green crab is an example of a positive feedback mechanism by which one introduced species may facilitate the invasion of other invasive species. It also suggests the possibility that some of the many hundreds of benign introductions that have accumulated in coastal waters might become management problems in response to changing conditions brought about by new introductions.

Recent publications:
Grosholz, E. D. 2005. Recent biological invasion may hasten invasional meltdown by accelerating historical introductions. Proceedings of the National Academy of Sciences U.S.A. 102: 1088-1091.

Grosholz, E. D., G. M. Ruiz, C. A. Dean, K. A. Shirley, J. L. Maron, and P. G. Connors. 2000. The impacts of a nonindigenous marine predator in a California bay. Ecology 81: 1206-1224.

Much of my recent work over the last several years has involved measuring the community and ecosystem impacts of the invasive salt marsh cordgrass Spartina on a broad range of organisms from primary producers to shorebirds. This has been a collaborative project funded by the Biocomplexity Program of the National Science Foundation involving researchers from UC Davis, Scripps, and SFEI. Our work has addressed the impacts of Spartina invasion on a range of processes, particularly those involving benthic invertebrate communities and the sediment environment. Our results have shown that this species is truly an ecosystem engineer and that the structure of the plant itself changes the physical and chemical environment in ways that strongly influence benthic communities. Spartina attenuates light transmission to the substrate, reduces photosynthesis of benthic microalgae, and reduces water flow which results in decreased recruitment and growth of benthic suspension feeders. The plant structure also increases sedimentation rates and accumulation of detritus, which together with increased below ground plant biomass influence sediment chemistry and metabolism. The increased below ground biomass of Spartina also directly reduces benthic abundance and diversity. These physical and chemical changes result in dramatic shifts in benthic communities from larger surface-feeding taxa to subsurface detritivores, which are smaller and less available to higher trophic level consumers such as crabs, fish and birds. Understanding this balance of these effects will us develop a more general understanding of the impacts of introduced plants in coastal systems and will also help guide ongoing and future programs aimed at their eradication. For more information about invasive Spartina, see the following websites:

NSF Biocomplexity Spartina project

Invasive Spartina in Willapa Bay, Washington

Spartina Project

Recent publications:
Neira, C., E. D. Grosholz, L. A. Levin, and R. Blake. Mechanisms generating modification of benthos following tidal flat invasion by Spartina (alterniflora X foliosa). Ecological Applications (in press).

Brusati, E. D. and E. D. Grosholz. Native and introduced ecosystem engineers produce contrasting effects on estuarine infaunal communities. Biological Invasions (in press).

Levin, L. A., C. Neira, and E. D. Grosholz. 2006. Invasive cordgrass modifies wetland trophic function. Ecology 87: 419-432

Neira, C., L. Levin and E. D. Grosholz. 2005. Benthic macrofaunal communities of three Spartina-hybrid invaded sites in San Francisco Bay, with comparison to uninvaded habitats. Marine Ecology Progress Series 292: 111-126.



Population Dynamics and Restoration of Native Olympia Oysters

An important focus of my work involves the ecology and potential for restoration of native Olympia oysters Ostreola conchaphila (previously Ostrea lurida) in western estuaries. Estuaries in California have been heavily impacted by human activities which have resulted in substantial loss of habitat, invasion by non-native species, inputs of sediments and contaminants and other stressors that have resulted in substantial declines in ecosystem function. One important approach to restoring these functions is the restoration of native species. Historically, native oysters likely provided the same ecosystem services that have been documented for native oysters in the eastern U.S. including the provision of habitat for native invertebrates and fishes and improving local water quality. We have only indirect measures of the historic abundance of native Olympia oysters as indicated by their dominance of bivalve fossil assemblages in West Coast estuaries. Also, the depth of native oyster shell in cores from San Francisco Bay and their extensive presence in Native Americans middens also suggest that the native oysters were once plentiful in central California. Unfortunately, uncontrolled harvest, declining habitat quality and non-native species introductions likely led to the decline of native oysters by the early 1900s. These populations have still not recovered despite a lack of harvesting pressure for nearly a century. Luckily, there is now substantial interest on the part of many local, state, and federal agencies and natural resource managers in improving the health of California estuaries by restoring native oyster populations.

Our work examines some of the ecological factors that may be limiting the recovery of native oyster populations in Tomales Bay, CA and that may also limit the success of restoration programs. The work in my lab, particularly by graduate student David Kimbro, has focused on variation aspects of reproduction and recruitment, food availability and growth, and predator abundances and impacts of survival. We have found that non-native predators such as the European green crab and the Atlantic oyster drill are important predators of native oysters. There are sharp gradients of temperature and salinity in Tomales Bay and consequently the distributions of these predators, as well as native predators are sharply bounded. Much of the variation in the demography of native oysters parallels the distribution of these predators. These differences are also magnified by gradients in food availability and recruitment, which are predictably features of the well studied hydrodynamics of Tomales Bay.

We are also involved in developing a pilot scale restoration project to restore native oyster populations in Tomales Bay. The introduced predators such as oyster drills (including the Japanese oyster drill Ceratostoma inornatum) that have been in the system for many years may be partly responsible for the lack of recovery of native oysters. We also suspect that they may provide an obstacle to successful restoration of native oysters in bays where these species have been introduced. Our work here is to develop a system that will encourage high levels of recruitment and survival of native oysters in the hopes of expanding these methods on larger scale to restore a significant fraction of the original native oyster population.

Recent publications:
Kimbro, D. L. and E. D. Grosholz. Disturbance influences richness, evenness, but not diversity in a native California oyster community. Ecology (in press).