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Christine Sample, Joanna A. Bieri, Benjamin Allen, Yulia Dementieva, Alyssa Carson, Connor Higgins, Sadie Piatt, Shirley Qiu, Summer Stafford, Brady J. Mattsson, Darius J. Semmens, Jay E. Diffendorfer, and Wayne E. Thogmartin (August 2020)
A new metric quantifies a habitat’s contribution to population growth in the face of environmental change
There are few places in the world unaffected by humans. Conservation of species in this human-altered environment, which continues to change at a rapid pace, is difficult. Migratory species offer particular challenges because they use multiple habitats over the course of their migration, and these habitats can span several geopolitical boundaries. Consequently, the observed population level in any given location may depend on environmental changes and management actions in far-away areas. A key question is this: Which habitats are most important to the survival and growth of a population facing environmental change?
Using a network model, a team of mathematicians and ecologists from the United States and Austria have developed a metric to assess the importance of habitats and movement pathways used by a population. Their metric predicts which habitats and pathways are most critical to short-term population growth. As a case study, the team has investigated the migratory population of monarch butterflies in eastern North America. They find that the central breeding region is the most critical node of the migratory network under small to moderate disturbances. Conditions in Mexico, the sole wintering habitat, become increasingly important when the habitat undergoes very large harmful changes. The metric can be applied to a wide variety of animal populations, and can help wildlife managers proactively target the most critical areas for conservation.
The consequences of environmental disturbance and management are difficult to quantify for spatially structured populations, because changes in one location carry through to other areas due to species movement. We develop a metric, G, for measuring the contribution of a habitat or pathway to network-wide population growth rate in the face of environmental change. This metric is different than other contribution metrics as it quantifies effects of modifying vital rates for habitats and pathways in perturbation experiments. Perturbation treatments may range from small degradation or enhancement to complete habitat or pathway removal. We demonstrate the metric using a simple metapopulation example and a case study of eastern monarch butterflies. For the monarch case study, the magnitude of environmental change influences ordering of node contribution. We find that habitats within which all individuals reside during one season are the most important to short-term network growth under complete-removal scenarios. Whereas the central breeding region contributes most to population growth over all but the strongest disturbances. The metric G provides for more efficient management interventions that proactively mitigate impacts of expected disturbances to spatially structured populations.