Current Research


Evolution and climate change
Pika metapopulations
Cocos Island sharks
Bahamas shark modeling

Evolution and plasticity in a changing world

For many species their seasonal timing of life history events, or phenology, is important for both ecological and evolutionary dynamics (Forrest and Miller-Rushing 2010). While it is well established that the phenological timing of many species has advanced in response to climate change, it remains unclear if these responses suffice for population persistence (Merilä and Hendry 2014). For example, the reproductive timing is often restricted to certain parts of the years. This timing depends on factors like snowfall, food availability, and predation pressures, which not only change from year to year, but may also follow trends caused by climate change (Reed et al. 2011).


Pairwise invasibility plot – shows regions where a mutant with a particular trait value (e.g. body size) is able to invade a resident population with it’s own particular trait value

Two processes may help species in response to climate change. First, phenotypic plasticity allows an organism to modify its trait depending on the environment (Merilä and Hendry 2014). In the context of reproductive timing, an organism could adjust its timing depending on the environmental conditions of that particular year. The ability of plasticity to help organisms cope with environmental change depends on costs of plasticity, reliability of cues, and the fitness effects gained by being plastic (Reed et al. 2011, Chevin et al. 2010). Besides plasticity, a population may also genetically adapt to changing environmental conditions (Merilä and Hendry 2014). Here the speed of genetic change (as determined by mutation rate, population size, generation time) compared to the speed of environmental change will determine if a population will persist.

I will study simple mathematical models of reproductive timing. I will apply this model to two well-studied populations of small mammals: the Yukon red squirrel and the Collared pika (Williams et al. 2014, Franken and Hik 2004). Both of these populations have seen recent declines with climate change as a potential driver.

Pika metapopulations

We study the ecology and evolution of the American pika (Ochotona princeps) living on a American-pika-collecting-vegetationmetapopulation at Bodie State Historic Park. The metapopulation is comprised of patches made up of abandoned ore dumps. Near the end of the 19th century, pikas colonized these ore dumps creating a metapopulation of about 80 patches. Around 1990, the southern half of the study site had collapsed and few pikas have been censused there since. We developed a simulation model to better understand the dynamics of the Bodie population. We have found that spatial structure (the arrangement of the patches in the landscape) and demographic stochasticity both important aspects of the Bodie site. We are currently writing up these results for publication.

Here is my 2015 ESA talk on pika metapopulation dynamics. We are also interested in questions related to the evolution of dispersal in metapopulations.

Here is a simulation of metapopulation dynamics at Bodie over the last 40 years. The size of the circles represents how many pikas are on a given patch in each year.

simulation of pika metapopulation at Bodie

Population trends of sharks and rays in the tropical Eastern Pacific


Hammerhead sharks at Cocos Island

Worldwide, many species of sharks and rays have seen large declines in population size. Approximently, 25% of all chondrichthyan (shark, ray, chimaeras) species around the world are threatened according to IUCN criteria (Dulvy 2014). These species are threatened from overfishing, habitat degradation and pollution. Sharks and rays are often important in marine ecosystems. Specifically, these species act as predators and drive population dynamics of species lower in the food chain. (Heithaus et al. 2010). In response to declining populations of sharks and rays, many countries have instituted marine protected areas (MPA). An MPA is a conservation and management tool that is designed to let species recover by providing them with an area of refuge, free from fishing. However, it is not clear if MPAs are effective for large mobile predators, like sharks and rays.

We are studying sharks and rays at Cocos Island. The island is located 550km off of the mainland of Costa Rica. The island has been a marine protected area for the past two decades. We use data collected by the dive company Undersea Hunter, to get a sense of the population status of 12 shark and ray species that are commonly seen at Cocos Island. Here is the paper Shifting elasmobranch community assemblage at Cocos Island – an isolated marine protected area and here is a corresponding press release.


Field estimates and model predictions of yearly abundance, or probability of occurrence, for several shark and rays species at Cocos Island, 1993-2013.


Shark Population Modeling

Several shark species use nursery areas to give birth to offspring. These nursersharkpeny areas are able to provide protection to juvenile sharks (Heupel et al 2007). In order to understand the role of nursery areas, it is important to understand the basic population dynamics of juvenile sharks in these areas. The Bimini Biological Field Station has conducted research on the lemon shark population in Bimini, Bahamas for over 20 years (Jennings et al 2012). We developed a series of mathematical models to better understand the population of juvenile lemon sharks found in the lagoons of Bimini. We found that the variance in population size of juvenile sharks, is mostly driven by the environment. Therefore, predators and food resources may not be as important factors in driving population dynamics as once thought. The paper can be found here: Modeling the population dynamics of lemon sharks.