Understanding the processes that limit species' ranges is a leading challenge in ecology and a necessary step towards predicting how ranges will shift under global climate change. If the environmental conditions that support population viability can be defined, then these conditions can be mapped onto physical space to generate expectations for how and where range limits should arise. This process is complicated by the fact that many species exhibit intra-specific niche differentiation, especially with respect to sex. Females and males of the same species often respond differently to the same environmental drivers, which may cause skew in the sex ratio, especially in extreme environments. Traditional theory focuses exclusively on females, and therefore suggests that female responses to environmental variation, alone, should determine range limits. We asked: does male mate limitation at range edges, arising from sexual niche differentiation, contribute to range boundary formation? Or, does the female environmental niche alone determine geographic distribution? We addressed these questions using a dioecious grass (Poa arachnifera) that was experimentally planted into common garden environments throughout its range in the southern Great Plains, combined with demographic modeling to infer the environmental limits of population viability.
Results/Conclusions
We found that demographic performance was greatest in the arid environments of the western Great Plains and declined in the more mesic environments of the east. This qualitative pattern was consistent across all vital rates and between both sexes, leading to an expected eastern range limit (population growth < 1) that matched the observed longitudinal boundary. However, males suffered a greater reduction in reproductive performance from west to east than females did, causing a geographic cline in operational sex ratio (OSR) of the common gardens that matched observed OSR variation in natural populations. The decline in male reproductive performance caused mate limitation in female seed production. We used decomposes the contributions of females and males to range limitation and found that, while mate scarcity can strongly limit female fertility near the range boundary, it was the decline in female survival and growth that most strongly contributed to the loss of population viability beyond the range limit, with very little contribution from males. While mate availability can strongly limit female fertility, female fertility only weakly limits geographic distribution, with more important contributions from non-reproductive vital rates. Thus, range boundaries were determined solely by the environmental niche of females.