Causes and consequences of biodiversity change

We seek to understand the factors that determine how many species live in a particular location and how that diversity, in turn, influences key biogeochemical processes. Research in the Bracken Lab has examined how nutrients, consumers, and physical stress interact to determine the richness, evenness, biomass, and community composition of seaweeds on rocky shores (Bracken & Nielsen 2004Altieri et al. 2009Bracken et al. 2011, Williams et al. 2013). These changes in richness, biomass, and evenness can have major effects on local-scale nutrient uptake and photosynthesis in seaweed assemblages (Bracken & Stachowicz 2006Bracken et al. 2008Altieri et al. 2009Bracken et al. 2011Bracken & Williams in press). We have found that realistic changes in diversity, such as those associated with certain species’ susceptibility to disturbance, can have much different effects on nutrient uptake than random changes (Bracken et al. 2008) and that for realistic (as opposed to random) assemblages of species, a gradient in species richness is required to maximize multiple functions (e.g., nutrient uptake and photosynthesis; Bracken & Williams in press). Collectively, these results highlight the need to appreciate the causes of diversity change in natural ecosystems. Furthermore, realistic losses of rare sessile species – particularly seaweeds – from the base of an intertidal food web disproportionately impact the diversity and abundance of consumers at higher trophic levels (Bracken & Low 2012).

Top-down modification of bottom-up processes

Herbivores do not just eat primary producers, they can also dramatically affect those producers’ access to critical limiting nutrients. This can occur in two basic ways: (1) via nutrient inhibition, where consumers affect the abilities of producers to access nutrients and (2) via nutrient facilitation, where consumers add nutrients to an ecosystem, facilitating producers at local scales. Our research examines both aspects, including projects that have examined how herbivores selectively graze on seaweeds, reducing the abilities of both individuals and entire algal assemblages to acquire nitrate (Bracken & Stachowicz 2007Bracken et al. 2011). We have also shown that consumers excrete ammonium, enhancing seaweed diversity and growth on even the most wave-swept rocky shores (Bracken 2004Bracken & Nielsen 2004Bracken et al. 2007Aquilino et al. 2009).

Recent grants supporting our work

Mechanisms for success and potential impacts of an invasive seaweed

Matt Bracken and Carol Thornber

Woods Hole Sea Grant, February 2012-January 2014

Heterosiphonia japonica is a rapidly spreading marine invasive macroalgal species in New England coastal waters, where it achieves abundances much higher than those in its native range. However, we currently lack detailed information on the in-situ distribution, abundance, and impacts of this invader. Little is known about the ecological interactions of Heterosiphonia anywhere, likely because it became noticeably abundant in Europe relatively recently and is inconspicuous in its native range in eastern Asia. We are (1) determining the presence and abundance of Heterosiphonia at sites spanning its entire invaded range on the New England coastline (Newton et al. 2013); (2) quantifying habitat characteristics across the invaded range to assess physical and biological attributes correlated with the success of Heterosiphonia; (3) quantifying the impacts of Heterosiphonia on recipient communities; (4) determining growth rates of Heterosiphonia relative to native taxa; and (5) characterizing rates of herbivory on Heterosiphonia and native taxa.

Context-dependency of marine biodiversity-ecosystem function relationships

Matt Bracken and Geoff Trussell

National Science Foundation, March 2010-February 2014

Because species mediate key biogeochemical processes and play unique ecological roles, changes in the diversity and composition of species in an ecosystem can alter how that system functions. A growing body of work has demonstrated that changes in biodiversity can have profound effects on the functioning of marine ecosystems. However, key unresolved issues remain with respect to relationships between marine biodiversity and ecosystem functioning. In particular, few studies have evaluated interactions between producer diversity and herbivore diversity, especially in the field. Furthermore, the vast majority of experiments have evaluated the consequences of biodiversity changes at only one location, so we currently have very little knowledge of how diversity-function relationships are modified by environmental context.

We are examining, in the field, how environmental context shapes marine diversity-function relationships, and particularly the interaction between producer diversity and herbivory, at 9 rocky intertidal sites, 3 sites in each of 3 regions spanning 500 km of the New England coastline. These factorial experiments involve manipulations of seaweed diversity and grazer abundance at sites in the vicinity of East Point, Massachusetts; the Boothbay region of Maine; and Quoddy Head, Maine.

Influences of nonrandom biodiversity change on marine ecosystem functioning

Susan Williams and Matt Bracken

National Science Foundation, March 2006-February 2010

Because the diversity of species in an ecosystem is influenced by a variety of factors, it is important to understand both how diversity is altered by these factors and how the resulting changes in diversity influence the transformation and flux of energy and matter in that system. Historically, most research linking diversity and ecosystem-level processes was done in terrestrial ecosystems and used random assemblages of species to experimentally assess diversity’s influences on ecosystem functioning. While these previous studies suggested that diversity influences processes such as productivity, nutrient cycling, and energy flow, they did not evaluate the consequences of nonrandom changes in biodiversity associated with factors such as nutrient availability, consumers, biological invasions, abiotic stressors, local extinction, and habitat loss.

We studied the consequences of nonrandom biodiversity changes by conducting observations and experimental manipulations of intertidal seaweed assemblages to determine the influences of abiotic stress, consumers, and nutrients on diversity (Williams et al. 2013). Rocky intertidal seaweeds are an optimal study system for evaluating these effectors of biodiversity change, as previous research by ourselves and others has demonstrated dramatic impacts of disturbance, herbivory, and nutrient availability on seaweed diversity. We performed complementary field and laboratory experiments in the Bodega Marine Reserve and at the Bodega Marine Laboratory on the northern California coast to assess the influences of these nonrandom biodiversity changes on ecosystem-level processes, including seaweeds’ growth, photosynthesis, and nutrient uptake rates (Bracken et al. 2008, Bracken et al. 2011Bracken & Williams in press).

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