Rahn, O. J.; Hargreaves, A. L.

Species' range edges provide excellent arenas for testing which ecological constraints prevent expansion into new environments. Theory predicts that ranges can be constrained by declines in the quality or amount of habitat, but their relative impact is unknown because empirical studies are seldom designed to quantify habitat amount. Here, we propose a simple modification of across-the-range-edge transplant experiments that enables tests for declines in both habitat quality and amount. Using this design, we show that quality and amount of suitable microhabitat both decline across the high-elevation range edge of the herb Rhinanthus minor. Using simulation models parameterized with field data, we show that either decline is sufficient to impose range limits, and both declines contribute to limiting R. minor's high-elevation range. We end with three simple suggestions for the design and presentation of across-the-range-edge transplant experiments that would clarify how often and severely declines in habitat amount limit species' ranges.