Evolution of tolerance to deer browsing in Impatiens capensis
Browsing by overabundant white-tailed deer (Odocoileus virginianus) has altered ecological relationships in forest communities across eastern North America. Recent but limited work suggests that deer browsing also selects for particular plant defensive traits. We hypothesized that browsing by deer has imposed selection on defensive traits in an annual native wildflower, orange jewelweed (Impatiens capensis). To test this hypothesis, we collected individuals from 26 natural populations across a 5000 km2 area in New York State, USA. Half of these populations were historically protected from deer and half were exposed to heavy browsing. We planted individuals in common gardens subjected to natural deer browsing or no browsing.
Individuals from historically browsed populations exhibited significantly higher tolerance than those from historically protected populations. Herbivory by deer reduced lifetime fruit production by only 20% in historically browsed populations, as opposed to 57% in historically protected populations. Two mechanisms were correlated with this increased tolerance: increased number of flowering days and increased fruits per flowering node. The increased tolerance of historically browsed populations suggests that these populations evolved increased tolerance or that historically protected populations lost tolerance over time. Variation in tolerance traits in native plant species may allow them to persist in the face of rapid ecological change.