Home field advantage: Do species' vital rates decline towards range limits and does competition play a role?

Background/Question/Methods

Common sense would suggest that a species’ survival, growth and reproduction rates must be lower as you approach that species' range limits.  However, we have little quantitative information about how these vital rates change at range limits or whether biotic interactions, such as competition, play a role.  We studied the abrupt upper elevation range limit of a low elevation sagebrush species, Artemisia tripartita, and the abrupt lower range limit of a higher elevation species, A. tridentata ssp. vaseyana, across a gradual 200 m elevation gradient with no apparent abiotic discontinuity in eastern Idaho, USA.  We tested the hypothesis that each species’ growth, survival and reproductive output would respond in opposite directions to the elevation gradient.  We also tested whether the effect of competition was greater towards each species’ range limit.  To test these hypotheses we manipulated competition around individual plants across the gradient and measured survival, growth and flowering over two years.  

Results/Conclusions

We did not find support for our main hypothesis that the vital rates of these two species change in different ways along the elevation gradient.  Survival of the lower elevation species decreased with elevation, while flower production of both species was greater at higher elevations.  Growth rates did not change across the gradient.  Support for our second hypothesis, that competition would be stronger towards each species’ range limit, was weak.  For the survival of the lower elevation species, we found that neighbors had a facilitative effect at lower elevations but a competitive effect towards the upper elevation range limit.  For the upper elevation species, we found that competition reduced flower production more towards the lower elevation range limit—although only in the first year.  Rates of small mammal herbivory were high at upper elevations and appeared to contribute to mortality of both species.  The increase in rates of herbivory and decrease in survival with elevation may help explain the upper range limit of the lower elevation species.  In general the vital rates of these two species either responded weakly to the elevation gradient or in similar ways, leaving the explanation for their contrasting distributions unresolved.