A key challenge for ecology is the development of predictive models to guide our responses to ongoing environmental change. To more effectively inform future conservation and resource-use policy we need to understand the mechanisms by which species – and the interactions among them – affect the dynamics of their communities and the functioning of their ecosystems.  While we have made tremendous advances in our understanding, there still exist large gaps between theory and our empirical knowledge of nature’s complex ecological systems; our forecasting powers are still in their infancy.

How and when do species and their interactions affect the structure, dynamics, and functioning of their communities? How can we best predict the response of ecological systems to acute and chronic perturbations, including not only extinctions and invasions, but also more subtle changes in ecological processes?

Primary topics of research include (i) the development of methods for quantifying the strength and functional form of species interactions, (ii) understanding the influence of indirect effects in simple and complex interaction networks, and (iii) characterizing patterns of intraspecific variation (esp. diet specialization) to understand its consequences at the population and community level.

Ongoing and past projects include…

Estimating species interaction strengths
Efforts to estimate the strength of species interactions in species-rich, reticulate food webs are hampered by the multitude of direct and indirect interactions such systems exhibit. They have also been limited by an assumption that pairwise interactions display linear functional forms.  Empirical estimates of interaction strengths are nevertheless important for characterizing keystone species and parameterizing mathematical food web models.

I am interested in developing methods that avoid these problems to further our understanding of the processes regulating communities.  For example, Tim Wootton and I developed a logistically feasible observational method that avoids the indeterminacy of indirect effects and estimates the nonlinear strength of trophic interactions in species-rich food webs (i.e. the approach assumes a type II functional response rather than a linear type I functional response).  I subsequently confirmed the empirical accuracy of this method by comparing its species-specific attack rate estimates with those obtained by independent experimental manipulations (i.e. caging experiments) of a New Zealand intertidal whelk, Haustrum scobina.
Current work is extending these approaches to (i) additional functional response formulations (including consumer-dependent and ratio-dependent functional responses), (ii) characterizing the deterministic and stochastic contributions to interaction strength estimation uncertainty, and (iii) describing the tempo and form of adaptive foraging behaviour in local intertidal whelk and freshwater sculpin populations.

Prediction limits in complex food webs
Effective ecosystem-based management necessitates knowing the limitations of our predictive accuracy.  To understand these limits, a NCEAS working group and I have considered which approaches to describing species interactions may be most successful at making predictions regarding how species will respond to disturbances elsewhere in their community.  Our contributions to this effort have focused in particular on the effects of network complexity and interaction strength uncertainty on the utility of qualitative and quantitative matrix methods (i.e. Loop Analysis and the so-called Community matrix).  Current work is extending this line of research to understand the nuances of when, where, and how network topology, species-, and interaction-attributes influence predictive success.

Patterns and consequences of intraspecific diet specialization
Animals are choosy in what they eat.  I am a selective omnivore and don’t eat seafood or beef.  My brother is was a vegan, my father a “carnivore”, and my mother avoids wheat. For Homo sapiens, it’s clear that what we eat and how we specialize affects not only our own bodies, but collectively has important ramifications for the ways we affect the world around us.
Other animals exhibit diet specialization as well, but the scope, patterns, and community-level consequences of this intraspecific variation are much less understood.  In collaboration with Tim Tinker and others, I am studying how individuals go about choosing their prey. My ultimate goal is to use this knowledge to understand how diet specialization within a population may affect food web dynamics and the structure of ecological communities.  I have also been involved in a NIMBioS working group to synthesize more general theory regarding the consequences that ignoring such variation may have for population and community ecology.  We organized a symposium on the topic at the 2011 ESA meeting.

Omnivorous food webs across a productivity gradient
Trophic omnivores – species that feed at multiple trophic levels – are central to our understanding of food webs.  Many analyses have now shown that omnivores are ubiquitous and often over-represented in ecological communities.  Their presence in food webs complicates the predictive power of trophic cascades and undermines the utility of the trophic level concept itself.  This is particularly true when omnivores engage in intraguild predation (IGP) by feeding on a second consumer species with whom they also share a prey.  A now well-developed theory of IGP systems offers interesting predictions regarding the mechanisms governing species coexistence in omnivorous food webs and how species abundance patterns should change across gradients of system productivity.  However, although the IGP module is perhaps the best theoretically studied of all food web modules, its applicability to real, species-rich food webs remains largely unknown.

I have been testing two key predictions of IGP theory by investigating species abundance patterns and the structure and interaction strengths of a series of species-rich omnivorous whelk food webs situated along a gradient of productivity present around New Zealand’s coastline.  I am finding that the intermediate predator (Haustrum scobina) is the superior competitor for shared prey species, as predicted by IGP theory.  Counter to theory, however, results suggest that it is the omnivore (Haustrum haustorium) that is the superior competitor when both shared and unshared prey are considered.  In further contrast to theory, I am documenting an increase in the abundance of the intermediate predator with increasing productivity.  My data nevertheless reveal clear and remarkably regular cross-gradient shifts in the food web structure and strengths of species interactions, and suggest that adaptive and optimal foraging behaviour, and interactions among basal prey species, play an important role in structuring communities.  These empirical insights offer hope that future modeling efforts which incorporate such processes will lead to a theory that can predict the emergent properties of natural food webs.

Forecasting the effects of an impending re-colonization
California’s kelp forests rank among the most productive ecosystems in the world. Their algae, invertebrates and fishes sustain numerous commercial and recreational fisheries of both economic and social importance. With funding from the NSF/NOAA CAMEO program, my collaborators at the USGS, PISCO, and NOAA and I are using spatial and temporal investigations of the empirical structure and dynamics of central and southern Californian nearshore kelp forest communities to inform and compare the performance of multi-species approaches to modeling the complex dynamics of these systems.  We have also initiated a large-scale compilation of the existing literature on kelp forest species and their interactions in the form of the online Kelp Forest Database.  Our goals are to develop tools to facilitate ecosystem-based decision making, and to forecast how marine reserves and the impending re-colonization of sea otters to southern Californian waters will affect the region’s valued fisheries.

Stability of nonlinear species interactions
The study of predator foraging behaviours such as prey choice, relative prey preferences, and the manner in which predator feeding rates respond to changes in prey abundance (i.e. functional responses) has long been a mainstay of modern ecology.  The nonlinear nature of trophic interactions that such behaviours introduce has important implications for the dynamics of populations and the structure and stability of food webs.  Populations of specialist predators, for example, fluctuate more than do those of generalist predators, but the mechanisms promoting the stability of generalist predator-prey dynamics are difficult to investigate.  The saturating functional responses which most predators exhibit, for example, destabilize predator-prey dynamics in theory, begging the question of how whole food webs persist.

Using data on New Zealand’s whelk food webs, I am addressing this question by determining (i) to what degree the feeding rates of whelks are saturated with respect to the density of their prey, (ii) the extent to which prey-attributes can be used to predict prey-specific contributions to the nonlinearity of a predator’s functional response, and (iii) how a predator’s diet richness affects the degree to which its overall feeding rate is saturated.  By using empirical data to parameterize an extension of the classic Rosenzweig-MacArthur model of predator-prey interactions, I am asking whether the degree of saturation observed within New Zealand’s whelk populations is nonlinear enough to affect the stability of their predator-prey interactions.  Results indicate that (i) whelk feeding rates are not strongly saturated, (ii) that most prey species contribute little to their predator’s saturation, and (iii) that increasing diet richness has a non-additive effect on a predator’s saturation such that the addition of alternative prey has a stabilizing effect on predator-prey dynamics.  This work is thereby offering a new mechanism by which generalist predators may stabilize the dynamics of their species-rich food webs, and an explanation for why predator-removal experiments typically result in linear responses in prey populations despite the inherent nonlinearity of trophic interactions.

Behavioural consequences of intraguild interactions
On the rocky shores of the Gulf of Maine, the American lobster, Homarus americanus, the Jonah crab, Cancer borealis, the Rock crab, Cancer irroratus, and the Green crab, Carcinus maenas, compose a guild of highly mobile predators.  Although these decapods are potential competitors that consume the same prey and utilize the same shelters, lobsters also prey on crabs (i.e. lobsters are intraguild predators of crabs).  During daytime low tides, crabs are also preyed upon by Larus spp. gulls.  I have investigated the importance of avian and intraguild predation in influencing the diel (day/night) and spatial (depth) patterns of decapod activity in the low intertidal and subtidal zones of the Isles of Shoals archipelago. This work has suggested that the modern overfishing of coastal fishes has increased the importance of intraguild interactions between lobsters and crabs, which in turn has caused crabs to become active during the day (a novel behaviour to the Cancer genus).  Having thereby become more susceptible to predation by gulls, crabs now contribute a stronger link between marine and terrestrial ecosystems.