Collaborative Research: Linking Climate, Disease, and Demography To Understand Extinction Risks in Ectotherms

Ectotherms are organisms like amphibians, insects, and plants, whose metabolic processes, including immune system functioning, are largely controlled by environmental temperature. Climate can therefore strongly influence the interactions between ectotherm hosts and their pathogens. In recent decades, amphibian populations have been strongly impacted by disease and many species are now extinct.

A primary goal of this project is to build mathematical models that help us understand and predict how a changing climate will influence the severity of disease outbreaks, and whether we expect disease to lead to long-term population declines in ectotherms. To do this, we must build models that incorporate the effect of climate on host traits that influence pathogen transmission, as well as the effect of climate on the timing and success of host reproduction.

In this project we build such models, and we test the models using experiments and field observations of a salamander host and the prevalent Ambystoma tigrinum virus. Testing our models against these data will help us narrow down the ways in which climate most strongly impacts disease and long-term patterns of population abundance. Then, we can use our well-tested models to make forecasts of how a changing climate might affect disease and host population stability in the future.

We will also work with collaborators in state and federal wildlife agencies to better understand the threat of Ambystoma tigrinum virus to natural amphibian populations. Moreover, we will work to increase the participation of historically underserved groups in science by creating and refining a program that recruits undergraduates from underserved populations, trains these students in grant writing and scientific hypothesis testing, and incorporates them as members of our research team, where they will seek to answer complementary questions.

We will also build web-based modeling resources in which the general public can manipulate versions of our disease dynamics models while learning about the effects of climate and disease on ectotherm populations across the world.

Lethal pathogens often regulate how host population abundances fluctuate over time, which can cause host population declines, local extirpation, and even species extinctions. For ectothermic host species, climate regulates traits that affect host demography and pathogen transmission, such that climate can mediate how disease affects host population dynamics.

Yet key knowledge gaps impede reliable predictions of how a changing climate will interact with disease to drive long-term fluctuations in host abundance. We ultimately require more robust host-pathogen models that forecast long-term host population dynamics in response to disease and climate, combined with robust tests of this theory. Here we build novel host-pathogen models that more realistically describe how climate simultaneously affects transmission, host demography, and host phenology.

Further, our models allow host individuals to respond idiosyncratically to temperature fluctuations in the environment. This is important because we hypothesize that the size of disease outbreaks and the risk of population declines are ultimately determined by whether temperature controls the degree to which traits vary among host individuals. To test our theoretical developments, we conduct laboratory experiments and observe natural disease outbreaks in a salamander-virus system.

We also follow an overarching, Bayesian framework to estimate model parameters and compare competing models, which allows us to identify the specific climate-dependent mechanisms that matter most for explaining large-scale patterns in the field. We then use our well-tested model to project how a changing climate and viral disease will alter the future population dynamics of salamanders in the US Southwest.

We will also develop an undergraduate outreach program to enhance engagement in STEM fields. Specifically, with the Equal Partners in Inclusive Community (EPIC) program at Northern Arizona University, we will create a grant-writing workshop for students who are under-represented in STEM. Finally, we will develop a web portal to broadly disseminate the results of our work, which will allow the public to interact with graphical model output and explore the effects of climate change and disease on amphibian populations in different ecoregions.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.