Testing assumptions of the climatic envelope for narrow and broadly distributed species using empirical methods

Background/Question/Methods

Although narrowly distributed species occupy smaller geographic ranges than their broadly distributed congeners, narrowly distributed species do not necessarily have narrower climatic tolerances.  This distinction is important because of the increasing use of species distribution models to predict how climate change may influence a species distribution. Species distribution models frequently use species occurrences to derive climatic associations and thus climatic tolerances for species of interest. If a species’ perceived climatic tolerance is narrower than its actual tolerance, predictions from species distribution models will underestimate potential suitable habitat and overestimate extinction risk. I tested the assertion that species climatic tolerances, and thus climatic sensitivity, cannot be postulated based on a species’ distribution size. I compared fitness responses to abiotic climate variables for congeneric pairs of narrowly distributed and broadly distributed species. Water and temperature treatments were derived from a combination of meteorological stations, global climate models, and IPCC scenarios for the year 2080. I applied three temperature treatments and seven water treatments to broadly distributed Mimulus guttatus and Clarkia purpurea, and narrowly distributed Mimulus nudatus and Clarkia gracilis, ssp. Tracyi. I measured fitness as total seed head count at senescence for all species.

Results/Conclusions

All species showed a positive response to water treatments and varied responses to temperature treatments. However, narrowly distributed M. nudatus showed less sensitivity to water and more sensitivity to temperature than broadly distributed M. guttatus, while both Clarkia species responded similarly to both water and temperature. Both Mimulus species responded positively to increases in water and negatively to increases in temperature while both Clarkia species positive responses to increases in water and temperature. Results suggest climatic responses to temperature and water are dependent on species and not distribution size. Furthermore, this research is not consistent with commonly accepted expectations that species occurrences accurately represent climatic tolerances of species. Caution is recommended in using occurrence data in predicting responses to climate change, especially for the management and triage of rare species.