Chapter 3: Population and Demographic Considerations


Chapter 3 assesses feasibility from the perspective of population biology, examining some of the demographic variables that can determine the success of a reintroduction effort. To aid in this assessment, researchers have built upon previously published population models for sea otters to develop a quantitative modeling framework for evaluating probable outcomes (and associated uncertainty) of various reintroduction scenarios. An Oregon sea otter population model (ORSO) uses a stage-structured matrix model of sea otter demography to project future trends in abundance and changes in the spatial distribution of reintroduced sea otter populations. The model is structured and parameterized based on extensive data from other sea otter populations and reintroduction outcomes, and it incorporates density dependence, Allee effects, environmental and demographic stochasticity, and realistic dispersal and range expansion behavior. While any one simulation trajectory is unlikely to reliably predict future outcomes for a proposed reintroduction scenario, projections over many model iterations are likely to capture the most likely future outcomes.

This study used quantitative approaches to assess the population-level impacts of removing animals from putative source populations, and to assess the likely viability of an established Oregon population under various reintroduction scenarios. The model framework developed for this assessment (the ORSO web app) can be used by managers and a wide range of stakeholders to explore potential outcomes of alternative reintroduction scenarios, assess the relative risks and implications for coastal ecosystems and socioeconomic activities, and evaluate the factors likely to determine success or failure of a reintroduction. The study emphasizes that biological considerations (e.g., population viability and ecological impacts) represent just one category of variables to be factored into decisions about reintroduction. Biological factors must be placed in a broader context that includes social and economic considerations, legal and administrative considerations, and logistical considerations. The OSRO model can help in evaluating all these subject areas, as it provides a spatially and temporally explicit tool for visualizing the outcomes of sea otter reintroduction under different scenarios and assumptions. Finally, in thinking about the species-level implications of reintroduction, the study encourages the use of a broad perspective that considers the history and biogeography of past and current sea otter populations.