Energetic compensation breaks down over time in a desert rodent community

Renata M. Diaz, School of Natural Resources and Environment, University of Florida, Gainesville, FL and S.K. Morgan Ernest, Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL

Background/Question/Methods

Energetic compensation occurs when declines in energy use from some species in an assemblage are offset by gains from others. When observed, energetic compensation is consistent with zero-sum dynamics, and renders assemblage-level function resilient to species’ fluctuations. In strongly niche structured communities, compensation is possible when species overlap in both their resource use and their match to current biotic and abiotic conditions – such that, if one species declines, another can access those resources without being limited by another niche axis. Shifting environmental conditions may modulate the degree of overlap between species, causing compensation to vary over time – even within a single assemblage.

Some of the strongest evidence of energetic compensation comes from long-term experiments on the rodent community near Portal, AZ. Since 1977, kangaroo rats have been excluded from experimental plots. Partial energetic compensation from smaller granivores was observed immediately, and near-complete compensation occurred after 1996, following the establishment of the complementary Chaetodipus baileyi. We explore whether the historically documented energetic compensation has persisted despite major transitions in the habitat and rodent community structure over time, including a community reorganization event that occurred following a drought and community-wide population crash in 2010.

Results/Conclusions

Since 2010, C. baileyi has gone from being the dominant species on exclosure plots (accounting for ~65% of energy use on exclosures, and ~10% on controls) to being scarce (~25% of energy use on exclosures, and absent on controls). This coincided with a decline in energetic compensation from small granivores for kangaroo rat removal, from an average of ~52% between 1996-2010 to an average of ~20% after 2010 – comparable to the level observed prior to C. baileyi’s establishment at the site (~16%). Other species of small granivore did not increase in abundance to absorb the available resources and offset the loss in assemblage-level function following the decline in C. baileyi.

In this community, energetic compensation appears to have been driven by C. baileyi’s fortunes at the site. We suggest that C. baileyi’s ability to exploit resources made available by kangaroo rat removal depended on favorable conditions at the site that have since shifted. Our results highlight that energetic compensation is a dynamic, contingent phenomenon even within a consistent set of species. Assemblage-level resilience to species loss, and zero-sum competitive dynamics, may therefore be temporary and manifest most clearly over long temporal scales – in this case, spanning decades.