Brown University Dept. of Ecology & Evolutionary Biology

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Robert P. Brown Professor of Biology
(401) 863-2280
Mark_Bertness@brown.edu

Wave I am interested in the biological and physical processes that generate pattern in natural communities and the role that experimental community ecology can play in improving the conservation and management of natural and human impacted ecosystems. I usually work in shoreline communities, since they are particularly good model systems to elucidate how biotic and physical factors interact to organize and structure natural communities. Much of my work is focused on salt marshes not just because they are good model systems, but because they are ecologically and economically important and in many parts of the world are seriously threatened by human population pressure.

SALT MARSH PLANT COMMUNITIES

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For over two decades my students and I have explored processes that generate spatial patterns in New England salt marsh plant communities. Our work on these ecologically critical habitats has examined the relative importance of plant competition, plant-animal interactions, physical disturbance and positive feedbacks in generating and maintaining the often-striking patterns in these systems. The relative simplicity of these systems makes them ideal for experimental manipulation and addressing questions about the general organization of communities. We have found that in New England while plant competition is largely responsible for the distribution patterns of plants across marsh landscapes, positive interactions among marsh plants are often responsible for making the physically harsh marsh habitat inhabitable. For example, in low marsh habitats, oxygen depleted, anoxic substrate can limit the success of plants. In these habitats, however, groups of plants can colonize and communally oxygenate anoxic substrate by supplying their roots with oxygen leading to high plant production. Analogously, at higher marsh elevations plant success can be limited by high soil salinities that are the result of evaporation of pore water in marsh sediments. In these habitats colonization by salt tolerant plants shade the soil, lower soil salinities and facilitate the invasion and success of plants that are not as salt tolerant. Much of my ongoing work with salt marshes is exploring the how common these types of positive feedbacks are in natural assemblages and examining the implications of these findings on our current understanding and conservation of shoreline communities.

HUMAN IMPACTS ON SALT MARSH PLANT COMMUNITIES

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Southern New England marshes

Our work with nutrient limitation in Southern New England salt marshes has revealed that the traditional plant zonation of New England salt marshes described early in the 20th century was largely driven by competition for nitrogen. Contemporary New England landscapes are being taken over by cordgrass (Spartina alterniflora) and invasive Phragmites australis due to shoreline development and eutrophication. Shoreline development removes vegetative buffers from marshes, triggering increased nitrogen input. This relaxes competition for nitrogen and consequently, cordgrass and Phragmites, dominant light competitors, competitively exclude all other plants reducing plant diversity dramatically and leading to a simplified landscape dominated by stands of these two plants. Thus our work in New England has lead to a simple mechanistic understanding of how humans are triggering extensive species composition shifts in these communities and suggests how these communities can be managed to minimize human impacts. We are currently exploring the generality of these results in other systems and exploring the links between eutrophication and consumer control in New England and South American marshes.

CLIMATE-DRIVEN PATTERNS IN THE ORGANIZATION OF SALT MARSH PLANT COMMUNITIES

Greenhouses

The role of climate in influencing the structure and organization of marsh plant communities is not well understood, but important if we are going to be able to predict how global warming over the next century will impact salt marshes and the ecological and societal services that they provide. We are currently exploring the potential role of climate in influencing marsh plant zonation patterns by affecting physical gradients across salt marsh landscapes. In particular, we are examining the hypothesis that by controlling evaporative processes and the potential accumulation of salt in marsh soils, climate may determine the importance of soil salinity in influencing the distribution and abundance of plants across marsh habitats and overall marsh primary production. A powerful approach to exploring the linkage between climate and the organization of communities is to examine natural latitudinal variation in community structure. Along the east coast of North America salt marsh plant communities are a common shoreline habitat from the Canadian Maritime provinces to central Florida, where marshes give way to mangrove forests, the tropical analog to salt marshes. Whereas marshes North of central Maine differ from more Southerly marshes due to the heavy, chronic impact of winter ice, marshes from Southern Maine to Florida are composed of a similar suite of plants. We are using these marshes as a model system to examine the effects of climate on plant community organization. We are using experimental greenhouses to test the hypothesis that climatic warming will increase the productivity of Northern marshes, but decrease the productivity of Southern marshes by increasing the size and geographic extent of low productivity salt pans. The role of climate in influencing the structure and organization of marsh plant communities is largely unexplored, but an important topic if we are going to be able to predict how global warming over the next century will impact salt marshes and the ecological and societal services that they provide. We are currently exploring the potential role of climate in influencing marsh plant zonation patterns by affecting physical gradients across salt marsh landscapes. In particular, we are examining the hypothesis that by controlling evaporative processes and the potential accumulation of salt in marsh soils, climate may determine the importance of soil salinity in influencing the distribution and abundance of plants across marsh habitats and overall marsh primary production.

A powerful approach to exploring the linkage between climate and the organization of communities is to examine natural latitudinal variation in community structure. Along the east coast of North America salt marsh plant communities are a common shoreline habitat from the Canadian Maritime provinces to central Florida, where marshes give way to mangrove forests, the tropical analog to salt marshes. Whereas marshes North of central Maine differ from more Southerly marshes due to the heavy, chronic impact of winter ice, marshes from Southern Maine to Florida are composed of a similar suite of plants. We are using these marshes as a model system to examine the effects of climate on plant community organization. We are doing this by using greenhouses at salt marsh that increase air temperature by ~3oC at study sites in Maine, Rhode Island, Georgia and Florida. We are testing the explicit hypothesis that climatic warming will increase the productivity of Northern marshes, but decrease the productivity of Southern marshes by increasing the size and geographic extent of low productivity salt pans.

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CONSUMER CONTROL OF MARSH PLANT PRODUCTION

Salt marsh ecosystems are widely considered to be controlled exclusively by bottom up forces, but there is mounting evidence that human disturbances are triggering consumer control in western Atlantic salt marshes, often with catastrophic consequences. The entrenched view that salt marshes are controlled largely by bottom-up forces was seriously challenged when, my former graduate student and current collaborator, Brian Silliman found that the snail Littoraria irrorata could potentially exert strong top-down, consumer control over marsh production. Since that time we have examined and found strong evidence for consumer control triggered by human disturbance on the Southeastern and Gulf coasts of the US by snails, on the Atlantic and Pacific coasts of South America by crabs and cattle gazing, respectively, and in New England by insects and a nocturnal, native crab. In many of these systems, once consumer control is triggered it is leading to run away consumption, salt marsh die offs and the loss of salt marshes and the services they provide. Together, these examples warn that while historically salt marshes may have been under bottom up control, human disturbances ranging from the use of nitrogen fertilizers, over-harvesting top predators, climate-change induced drought and exotic consumer invasions are stimulating consumer control in salt marsh ecosystems, sometimes with catastrophic results. Understanding the emerging role of consumers in human disturbed marshes, particularly as they relate to salt marsh die off phenomena, is a major current focus of my lab.

ROCKY INTERTIDAL COMMUNITIES

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Maine Rocky Shores

I am also interested in rocky intertidal communities and have a research program at the Darling Center of the University of Maine using Gulf of Maine rocky shores to explore community organization questions. In the past few years we have examined the biogeography of thermal stress ameliorating group benefits in barnacles and seaweeds, the role of flow in influencing marine benthic communities and the importance of predator (crab) cues in influencing consumer (snail) behavior and indirectly benthic community structure. We have also recently tested the idea that intertidal communities can represent alternate stable states on rocky shores in the Gulf of Maine. It has been suggested that intertidal mussel beds and seaweed canopies represent alternate community stable states on rocky shores in the Gulf of Maine. This theoretically appealing idea hypothesizes that these habitats are disturbance patch mosaics that can be dominated by either seaweed canopies or mussel beds, and that which community occurs in a given habitat is stochastic and dependent on the size of the original disturbance and recruit availability. Large disturbances are postulated to be dominated by mussel beds and barnacles that have widely dispersed larvae, while smaller disturbances are dominated by seaweeds, which have limited dispersal. Positive feedbacks are proposed to maintain the stability of these two distinct communities. We have experimentally exploring this idea and to date have found that in general mussel beds and seaweed canopies are highly deterministic states dictated by flow patterns and consumer pressure. We are currently trying to rectify these results with the alternate stable state hypothesis.

SOUTH AMERICAN SHORELINE ECOLOGY

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I am generally interested in the comparative ecology of South and North American shoreline communities. My students and I are working in Argentina with rocky intertidal shores in central Patagonia and with salt marshes in northern Argentina, southern Brazil central Chile. The goal of this work is to test the robustness of our North American-biased understanding of these systems on their much less studied South American counterparts, develop collaborations with South American faculty and graduate students and build scientific capacity in marine ecology and conservation biology in Latin America.

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Patagonian Rocky Shores

On rocky shores in Patagonia we are experimentally examining the roles of consumers, desiccation stress and wave exposure in generating the structure of South American intertidal assemblages. We have done similar studies in Maine intertidal systems for comparison. On Patagonian rocky shores we are also examining ecosystem engineering and the community structuring role of mussel beds on wave exposed headlands on Patagonian rocky shores which appear to facilitate the success of virtually all the other intertidal benthic organisms that live in this particularly physically stressful habitat.

South American Salt Marshes

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We are exploring the role of consumers in South American salt marsh systems to compliment our work in North American salt marshes that has demonstrated strong consumer control of marsh production. In Argentinean marshes we are examining the hypothesis that the exceedingly abundant crab, Chasmagnathus granulata, exerts strong top-down control on plant production in these poorly studied systems. On the Chilean coast we are examining the interactive roles of nutrient limitation and cattle grazing in controlling salt marsh plant production and distribution and abundance patterns.

To facilitate our work in South America we have collaborative relationships with the laboratories of Lobo Orensanz at the CONICET: Argentine Council for Science, Puerto Madryn, Oscar Irbarne at the University of Mar del Plata in Argentina, Cesar Costa at the National University of Rio Grande in Brazil and Kongo Farina at the Catholic University of Santiago, Chile. We are co-advising and collaborating with South American graduate students in each of these labs.

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