Is there reduction in disease and predispersal seed predation at the border of a host plant’s range? - field and herbarium studies of Carex blanda

Abstract

1. Small, isolated populations at species’ borders have been postulated to be less likely to have specialist pathogens and predators. Field and herbarium surveys were thus used to determine if two pathogens (a smut and a rust) and a predispersal seed predator were less common at the western range limit of the forest sedge Carex blanda in Kansas, USA. 2. Host plant size, reproduction, and density did not decline at the western border of the range. In fact, plants at two western sites had unusually large size and seed production. 3. Host populations at the edge of the range were more likely to be disease-free or lack the predispersal seed predator. Where the smut, seed predator, and rust were found, the proportion of infected or infested plants was not related to longitude, latitude, or percent forest cover. 4. More of the peripheral populations lacked the smut than the rust, as expected given the more localised nature of smut spore dispersal and the limited period when smut infection can occur. 5. In the adjacent, more highly forested state of Missouri, there were no geographic patterns in the incidence of the smut or seed predator in herbarium data. 6. The smut and rust increased in frequency over the 129 year span of herbarium collections. 7. Although field and herbarium distributional data were not identical (for example, smut infection was found much farther west in the field than in the herbarium data), the qualitative agreement between the two datasets suggests herbarium data can be used more broadly for studies of natural enemy distributions. 8. Limited dispersal by pathogens and seed predators is probably the reason why small, isolated western populations were less likely to have natural enemies. Peripheral host populations may thus have different ecological and evolutionary trajectories compared to more central populations. This conclusion, as well as the considerable variation among peripheral populations, is relevant to geographical studies of coevolution and to research on climatic effects on plants inhabiting ecotonal regions.