Poa arachnifera demography and sex ratio surveys

Understanding the mechanisms that generate biogeographic range limits is a
long-standing goal of ecology. It is widely hypothesized that
distributional limits reflect the environmental niche, but this hypothesis
is complicated by widespread potential for intra-specific niche
heterogeneity. In dioecious species, sexual niche differentiation
may cause divergence between the sexes in their limits of environmental
suitability. We studied range boundary formation in Texas
bluegrass ( Poa arachnifera ), a perennial dioecious
plant, testing the alternative hypotheses that range limits reflect the
niche limits of females only versus the combined contributions of females
and males, including their inter-dependence via mating. Common
garden experiments across a longitudinal aridity gradient revealed
female-biased flowering approaching eastern range limits, suggesting that
mate limitation may constrain the species'
distribution. However, a demographic model showed that declines
in λ approaching range limits were driven almost entirely by female vital
rates. The dominant role of females was attributable to seed
viability being robust to sex ratio variation and to low sensitivity of λ
to reproductive transitions. We suggest that female-dominant range limits
may be common to long-lived species with polygamous mating systems, and
that female responses to environmental drivers may often be sufficient for
predicting range shifts in response to environmental change.