Dr Daniel Okamoto1, Dr Lynn Lee2, Nathan Spindel1, Jaasaljuus Yakgujanaas4, Niisii Guujaaw2, Dr Aaron Galloway3
1Department of Biological Science, Florida State University, Tallahasse, USA, 2Parks Canada Gwaii Haanas, Skidegate, Canada, 3Oregon Institute of Marine Biology, University of Oregon, Charleston, USA, 4Council of the Haida Nation, Skidegate, Canada
Patterns of growth, reproduction, maintenance, and activity in animals are central components in both life history theory and resource management. Yet for most species, models used to characterize these properties are either phenomenological and/or mechanistic models not capable of reproducing empirical dynamics (e.g., von Bertalanffy or classic energy budget models). Moreover, they generally have little capacity to project how dynamics may shift with substantial changes in food availability (such as in urchin barrens) or with changes in climate. To address this gap we constructed a mechanistic, size-specific energy budget model of growth, maintenance, and reproduction that allows for shifts in energetic allocation with ontogeny, temperature, and food availability. We then parameterize the model using sea urchins in field and laboratory experiments. We conducted field mark and recapture experiments in urchin barrens and kelp forests in Haida Gwaii, Canada in tandem with controlled laboratory feeding experiments. We scaled energetic inputs and expenditures by measuring metabolism, energetic density, and grazing rates. The resulting model both explains empirical patterns and yields key insights into life-history trade-offs and sensitivities to environmental variation. First, the model predicts a non-asymptotic (i.e., indeterminate) growth form that matches empirical dynamics of sea urchins (as well as many marine invertebrates and fish, as well as trees). Second, our model correctly predicts that reductions in food availability produce greater impacts on larger individuals in terms of growth and reproduction with little impact on smaller individuals. As a result, the models and experiments combine to show how smaller urchins can persist and thrive in barrens, while larger individuals either perish or shift all energy to maintenance. Importantly, we show how a mechanistic model can be both practical and flexible enough to predict empirical physiological dynamics in response to changes in ecosystem productivity or climate change.
Presentation Slides – Daniel Okamoto
