Is there reduction in disease and pre‐dispersal seed predation at the border of a host plant's range? Field and herbarium studies ofCarex blanda

Abstract

Summary1 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 pre-dispersal 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 pre-dispersal 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 percentage forest cover. 4 More of the peripheral populations lacked the smut than the rust, as expected given the more localized 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 geographical 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 data sets 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 with more central populations. This conclusion, as well as the considerable variation among peripheral populations, is relevant to geographical studies of co-evolution and to research on climatic effects on plants inhabiting ecotonal regions.

The effect of sterilizing diseases on host abundance and distribution along environmental gradients

“…However, the few precise distributional studies that have been done have shown contrasting patterns. Both decreased disease prevalence at margins (Brewer 1995;Alexander et al 2007) as well increased prevalence have been observed (Antonovics et al 2003;Briers 2003), but there is not enough information to match these contrasting patterns to the models presented here. Other than evidence of decreased species richness of parasites on islands (Gouy de Bellocq et al 2002) or in invasive species (Wolfe 2002) there seems to be little direct empirical evidence for disease-free zones around the edges of a species range.…”

Trophic interactions and range limits: the diverse roles of predation

“…There are certainly examples of nested distributions for specialist natural enemy-victim distributions (Shenbrot et al 2007). For instance, Alexander et al (2007) found that peripheral populations of the forest sedge Carex blanda in Kansas were more likely to lack two pathogens (a smut and a rust) and a seed predator (a chalcid), compared with central populations further east.…”

The niche, limits to species' distributions, and spatiotemporal variation in demography across the elevation ranges of two monkeyflowers

“…24-26), many others have not found expected differences (e.g., refs. [27][28][29]. In these latter cases, present-day range boundaries may not be at niche limits; instead, the boundaries may represent nonequilibrial edges of ranges, set by limits to dispersal over time or space.…”