Quantifying temporal variation in dietary niche to reveal drivers of past population declines; stable isotope, harvest trends, and land use

1. Given the long and dynamic history of anthropogenic disturbances to
ecosystems, it is difficult to determine the drivers of past population
declines. These uncertainties dilute the efficacy of conservation efforts
and might hinder species and ecosystem recovery. 2. Niche quantification
can be a useful tool for understanding drivers of past population
declines. Niche parameters reflect key resources used, providing insight
into the conditions needed to achieve population stability. By
reconstructing a population’s niche position and space over a period of
decline and comparing to historic baselines, shifts in the realized niche
of a species can be assessed. Comparing shifts to historic information on
resource availability and timing of declines can allow practitioners to
identify probable drivers of species decline. 3. We demonstrated the
utility of this technique by reconstructing parameters of isotopic dietary
niche over a 130 year period and comparing isotopic niche reconstructions
to land use and crop harvests during this same period
via regression and Bayesian standard ellipsoid (SIBER) analyses.
We use a formerly widespread but now endangered species, the eastern
spotted skunk (Spilogale putorius), addressing the hypothesis that land
use change and agricultural intensification led to a historical collapse
of key dietary resources which correlates with population declines in this
species. To help control for isotopic variability unrelated to population
decline, we compare trends to those of a secure, but ecologically similar
generalist mesocarnivore, the striped skunk (Mephitis mephitis), across
the same spatiotemporal scale. 4. We present evidence that historic
dietary changes occurred in spotted skunks in the early 1900s but not to
the same degree in striped skunks. Changes in isotopic composition
correspond with the temporal period of decline and are explained by
concurrent changes in land use. These results support the hypothesis that
loss of key dietary resources as a result of land use change and
agricultural intensification played a significant role in population
declines of spotted skunks in this region.