Birds are more Vulnerable to Climate Change Impacts than Small Mammals in the Mojave Desert

Exposure to climate change drives stability or collapse of desert mammal and bird populations

E.A. Riddell, K.J. Iknayan, L. Hargrove, S. Tremor, J.L. Patton, R. Ramirez, B.O. Wolf, S.R. Beissinger

Summarized by Anna Geldert

What data were used? Researchers compared climate change responses in desert species, including 34 small mammal species and 135 bird species. Surveys were conducted at 151 sites throughout the Mojave Desert, concentrated mostly in Death Valley National Park, Mojave National Preserve, and Joshua Tree National Park (California, USA). Modern observations were compared to historical observations by Joseph Grinnell and colleagues in the early 20th century to assess change over time.

Methods: The authors used a dynamic multi-species occupancy model to determine how the proportion of sites that a species occupied changed over time. In summary, this approach assessed the probability of detecting a species  at different time periods, and used this data to determine the change in occupancy (likelihood of a species occupying a site), change in species richness (number of species at a site), colonization probability (likelihood of expanding to new sites), and persistence (long-term survival of a species at a site) probability. This model also factored in the impacts of climate change and habitat loss. The authors also estimated the degree of exposure (or how greatly an organism is affected by climatic changes) in small mammals and birds by simulating the “cooling costs” of each species. Cooling costs represent the water required for evaporative cooling to maintain a stable body temperature and were based on the species’ behavior, morphology, and microhabitat conditions.

Results: Overall, modern bird species declined in occupancy when compared to historical records, while small mammal occupancy remained relatively consistent. The model estimated that the occupancy of 29% of bird species decreased, 70% were unchanged, and only 1% increased. Meanwhile, only 9% of small mammals saw an occupancy decrease, while 79% stayed constant and 12% increased. Similarly, bird species richness decreased at 90.1% of sites and only 3.3% of sites for small mammals. The authors also found that bird populations experienced higher exposure to climate change than small mammals. The exposure model estimated that cooling costs were approximately 3.3 times higher in birds than they were in mammals, with this number projected to increase to 3.8 times by 2080. Finally, the level of adaptation and specialization among species of both groups had little influence on changes in cooling costs, suggesting that microhabitat conditions and their behavioral ability to “buffer” against climatic changes had a much greater impact.

The figure shows a histogram graph, labeled ‘B’, which represents the change in species occupancy over time for both birds and small mammals. The x-axis is labeled “change in occupancy,” and ranges from -0.6 to 0.4, increasing by a factor of 0.5. Two y-axises appear stacked vertically on top of one another so that data on birds and small mammals can be graphed separately; both are labeled “number of species.” On the top right corner of each graph is the black silhouette of a bird on the top graph, and a small rodent on the bottom graph. The top axis, which shows data for birds, ranges from 0 to 30. Gray bars (roughly 70% of total) represent no significant change in occupancy compared to historical records, while red bars (roughly 30%) represent significant increases and decreases. Occupancy bars for birds are concentrated left of zero, indicating an overall decrease in species occupancy. The number of species is highest for changes in occupancy of -0.1 and -0.05, which each have about 25 species. As change in occupancy continues to decrease, the number of species slopes off rapidly, with only 5 species or less for occupancies lower than -0.35. Only 3 bird species have a positive change in occupancy, with probability values at 0.1, 0.15, and 0.4. The bottom y-axis ranges from 0 to 15, and represents data for small mammals. Gray bars (roughly 80% of total) again represent no significant change, while blue bars represent significant increases or decreases. Change in occupancy for small mammals is much less skewed than occupancy for birds. The change in occupancy of 0 has the highest number of species, at roughly 15. All other occupancies have 7 species or less, and quickly decrease to zero on either side by -0.2 and 0.3 change in occupancy. Small mammals, therefore, have a much lower range in change of occupancy probability than birds. Occupancy probabilities are also much more similar to historical records for small mammals than for birds.
Fig. 1 Change in occupancy (modern – historical) of bird and small mammal species in the Mojave desert. Changes in occupancy were estimated using a dynamic multi-species occupancy model based on survey data collected during two different time periods: first, by Joseph Grinnell and colleagues in the early 20th century (historical), and second, by the authors of this paper in 2007-2018 (modern). The gray bars represent the number of species with no significant change in occupancy between modern and historical records, while colored bars (red for birds; blue for small mammals) indicate significant increases or decreases over time.

Why is this study important? This study counters the traditional approach of assessing impacts from climate change, which often assumes that exposure within an ecosystem is uniform across all species. This study revealed that in the same locations birds were more severely impacted by climate change than small mammals, as shown by the lower occupancy probability, lower species richness, and higher cooling costs in birds. Additionally, this study highlighted the importance of microhabitat buffering potential, which may be a driving factor as to why small mammals were sheltered in their burrows during the day  from the worst of the impacts of heat, while birds were not.

The big picture: As the impacts of climate change on animal populations progress, desert communities remain especially vulnerable. In order to minimize these impacts, it is important to understand how ecosystems respond to climate changes. This study suggests that impacts should be considered at the population level, rather than the community level, as species responses varied greatly even within the same ecosystem. Furthermore, the results suggest that microhabitat buffering is especially important in reducing impacts from climate change, and should be given greater attention in conservation efforts and future studies.

Citation: Riddell, E. A., Iknayan, K. J., Hargrove, L., Tremor, S., Patton, J. L., Ramirez, R., … Beissinger, S. R. (2021). Exposure to climate change drives stability or collapse of desert mammal and bird communities. Science, 371(6529), 633–636. https://doi.org/10.1126/science.abd4605

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