Research Interests

RESEARCH INTERESTS

My research focus within ecology and conservation biology is an examination of what happens to a community’s productivity, structure, trophic interactions, and native species composition when ecosystems are modified via human perturbations that result in biodiversity loss, species invasions, habitat alteration, and changes in food availability. As outlined below, my research combines field, laboratory, and modeling approaches to address a variety of related topics.

CURRENT RESEARCH

Determining ecological factors driving trophic cascades

In my post-doctoral position, I am using field studies in streams coupled with laboratory experiments to determine correlates between ecological factors and the strength of trophic cascades. We are finding that algal diversity is negatively correlated with trophic cascade strength. This allows us to move closer to understanding the ecological consequences of ecosystem alterations, especially diversitly loss, as they relate to the structure and function of food webs.

RECENT RESEARCH

Determining how invasive animals structure ecosystems and alter trophic connections

The natural experiment opportunities presented by the presence of invaded and non-invaded replicates within a single system such as islands within an archipelago can be a powerful tool with which to measure the role of trophic interactions in structuring communities. These natural experiments can also determine the trophic function of introduced species which can lead to direct conservation action. Vertebrate predators introduced to oceanic islands frequently shape communities directly by significantly reducing native species abundance. It has proven more difficult to demonstrate the indirect trophic effects mediated by invaders and the extent to which these trophic interactions alter native community composition, especially at landscape levels.

I examined the effects of invasive rats on marine bird and intertidal communities in the Aleutian Islands, Alaska and found evidence of a landscape-level trophic cascade driven by rats that altered the intertidal communities. Islands with rats had an order of magnitude fewer marine birds, half as much algal cover, and significantly higher densities of invertebrates than islands without rats. Utilizing stable isotope modeling, I found that rats eat birds that forage in the intertidal which releases marine invertebrates and increases grazing pressure on algae. Rats significantly altered the intertidal community structure in the islands via their trophic interactions with native birds. My work contributed to the recent project conducted by several agencies that successfully removed rats from Rat Island in the Aleutian Islands. This is the sort of concrete conservation action that I wish to continue in my future research.

Modeling temporal and spatial variation in trophic interactions

There has been an explosion in the use and development of primarily stable carbon and nitrogen isotope ratios to study aspects of animal ecology in the past 30 years. The technique has grown from a method to estimate diet to an increasingly nuanced and widely used tool to: track animal movements, measure trophic level and degree of omnivory, trace temporal changes in foraging ecology over life stages and historic and geologic time, indicate periods of drought and nutritional stress, detect marine and terrestrial components in diets, measure degree of marine or terrestrial input into coastal ecological systems, and estimate levels of toxic exposure.

I have worked with stable isotope technology for 12 years beginning with my master’s thesis, and continuing through my position as a Research Wildlife Biologist for the National Marine Mammal Laboratory (NMML) and in my collaborations and doctoral research. I utilize δ¹⁵N and δ¹³C signatures to: track spatial and temporal variations in trophic function of marine vertebrates, estimate degree of omnivory and forage type in diets of invasive mammalian species, and measure isotope gradients in ecosystems. Each of my projects has had strong conservation applications that relate to assessing the effects of human alterations of ecosystems. For example, my work at the NMML on endangered and threatened marine mammals helped determine the extent to which northern fur seals and Steller sea lions were interacting with commercially important fish species that are being removed from pinniped feeding grounds in significant quantities.

Investigating animal habitat use and movement patterns

My research on the foraging ecology of marine mammals conducted as a Research Wildlife Biologist for the NMML provided considerable insight into the trophic niches and migratory patterns occupied by these top predators. For my dissertation research, I built upon my expertise to assess foraging and marine versus terrestrial habitat use of invasive rats on the Aleutian Islands, and I did so over considerable spatial and temporal scales using four-source concentration dependent stable isotope mixing models with isotope values from several rat tissues and their prey.

I also conducted a study published in Marine Eoclogy Progress Series (MEPS) demonstrating that dietary classifications of endangered Steller sea lions are driven by regional differences as illustrated by sea lion δ¹⁵N signatures and by known geographic differences in δ¹³C signatures throughout Alaskan waters. I have also examined significant temporal and geographic variations in stable isotope ratios of marine prey from Alaskan waters. The differences we found in isotope values from fish and squid caught in 1997, 2000, and 2005 demonstrate how best to use these data to categorize and measure habitat use and migratory patterns in apex predators. This work is currently in review at MEPS.

Acquiring stable isotope modeling parameters via laboratory experiments

Studies conducted on captive animals held on experimentally controlled diets are crucial for the most accurate modeling of wild animal trophic ecology using stable isotope techniques. I conducted such experiments on captive seals and laboratory rats. My research helped determine critical parameters allowing ecologists to best interpret the trophic function of animals in the wild.

Determining causality of species change through examination of historic trends in foraging ecology

Historic trophic shifts can be measured using stable isotope analyses of museum tissue samples. In a collaborative project to determine why threatened marine mammal populations have declined significantly over the last 40 years in northern oceans, we analyzed stable isotope signatures of historic collections of fur seal teeth to determine if species changes could be due to declines in North Pacific Ocean and Bering Sea primary productivity. We found a historic decrease in δ13C values which was not attributable to declines in primary productivity, but to human induced alterations of carbon input in the form of increased fossil fuel burning. We clarified the use of isotopes to examine other questions of historic species changes in marine communities and our findings were published in 2007 in MEPS.

FUTURE RESEARCH

I will continue to persue research projects that fit within and logically expand upon the studies outlined above.