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1 Disturbance may cause community composition across sites to become more or less homogenous, depending on the importance of different processes involved in community assembly. In north-eastern North America and Europe local (alpha) diversity of forest plants is lower in forests growing on former agricultural fields (recent forests) than in older (ancient) forests, but little is known about the influence of land-use history on the degree of compositional differentiation among sites (beta diversity). 2 Here we analyse data from 1446 sites in ancient and recent forests across 11 different landscapes in north-eastern North America and Europe to demonstrate decreases in beta diversity and in the strength of species-environment relationships in recent vs. ancient forests. 3 The magnitude of environmental variability among sites did not differ between the two forest types. This suggests the difference in beta diversity between ancient and recent forests was not due to different degrees of environmental heterogeneity, but rather to dispersal filters that constrain the pool of species initially colonizing recent forests. 4 The observed effects of community homogenization and weakened relationships between species distributions and environmental gradients appear to persist for decades or longer. The legacy of human land-use history in spatial patterns of biodiversity may endure, both within individual sites and across sites, for decades if not centuries.

Effective management of invasive species requires that we understand the mechanisms determining community invasibility. Successful invaders must tolerate abiotic conditions and overcome resistance from native species in invaded habitats. Biotic resistance to invasions may reflect the diversity, abundance, or identity of species in a community. Few studies, however, have examined the relative importance of abiotic and biotic factors determining community invasibility. In a greenhouse experiment, we simulated the abiotic and biotic gradients typically found in vernal pools to better understand their impacts on invasibility. Specifically, we invaded plant communities differing in richness, identity, and abundance of native plants (the \"plant neighborhood\") and depth of inundation to measure their effects on growth, reproduction, and survival of five exotic plant species. Inundation reduced growth, reproduction, and survival of the five exotic species more than did plant neighborhood. Inundation reduced survival of three species and growth and reproduction of all five species. Neighboring plants reduced growth and reproduction of three species but generally did not affect survival. Brassica rapa, Centaurea solstitialis, and Vicia villosa all suffered high mortality due to inundation but were generally unaffected by neighboring plants. In contrast, Hordeum marinum and Lolium multiflorum, whose survival was unaffected by inundation, were more impacted by neighboring plants. However, the four measures describing plant neighborhood differed in their effects. Neighbor abundance impacted growth and reproduction more than did neighbor richness or identity, with growth and reproduction generally decreasing with increasing density and mass of neighbors. Collectively, these results suggest that abiotic constraints play the dominant role in determining invasibility along vernal pool and similar gradients. By reducing survival, abiotic constraints allow only species with the appropriate morphological and physiological traits to invade. In contrast, biotic resistance reduces invasibility only in more benign environments and is best predicted by the abundance, rather than diversity, of neighbors. These results suggest that stressful environments are not likely to be invaded by most exotic species. However, species, such as H. marinum, that are able to invade these habitats require careful management, especially since these environments often harbor rare species and communities.

Invasion of native ecosystems by exotic species can seriously threaten native biodiversity, alter ecosystem function, and inhibit conservation. Moreover, restoration of native plant communities is often impeded by competition from exotic species. Exotic species invasion may be limited by unfavorable abiotic conditions and by competition with native species, but the relative importance of biotic and abiotic factors remains controversial and may vary during the invasion process. We used a long-term experiment involving restored vernal pool plant communities to characterize the temporal dynamics of exotic species invasion, and to evaluate the relative support for biotic and abiotic factors affecting invasion resistance. Experimental pools ( n = 256) were divided among controls and several seeding treatments. In most treatments, native vernal pool species were initially more abundant than exotic species, and pools that initially received more native seeds exhibited lower frequencies of exotic species over time. However, even densely seeded pools were eventually dominated by exotic species, following extreme climatic events that reduced both native and exotic plant densities across the study site. By the sixth year of the experiment, most pools supported more exotics than native vernal pool species, regardless of seeding treatment or pool depth. Although deeper pools were less invaded by exotic species, two exotics ( Hordeum marinum and Lolium multiflorum ) were able to colonize deeper pools as soon as the cover of native species was reduced by climatic extremes. Based on an information-theoretic analysis, the best model of invasion resistance included a nonlinear effect of seeding treatment and both linear and nonlinear effects of pool depth. Pool depth received more support as a predictor of invasion resistance, but seeding intensity was also strongly supported in multivariate models of invasion, and was the best predictor of resistance to invasion by H. marinum and L. multiflorum . We conclude that extreme climatic events can facilitate exotic species invasions by both reducing abiotic constraints and weakening biotic resistance to invasion.

Loss of cached foods is a constant threat to animals that hoard food, and pilfering plays an important role in the evolution of hoarding strategies. Although pilfering is known to affect the behavior of scatter hoarders, pilfering has been assumed to be less important for larder-hoarding animals. In this study, I used a mark-recapture study of cached Norway spruce (Picea abies) cones to quantify pilfering rates in larder-hoarding red squirrels (Tamiasciurus hudsonicus). Red squirrels stole 26% of the cones that they ate and lost 25% of the cones cached in their middens. Most squirrels stole cones (97%) and lost cones to stealing (92%). However, individual squirrels stole 1-100% of the cones that they ate and lost 1-84% of the cones cached in their middens. Numbers of cones gained by and lost to stealing were not related to the age or sex of individual squirrels or the distances between or numbers of cones cached in individual territories. Squirrels with small middens, however, ultimately gained cones, and those with large middens lost cones. Because food is frequently a limiting resource for red squirrels, these changes in food abundance may directly affect the fitness of individual squirrels. Thus, pilfering likely plays an important role in shaping the hoarding and defensive behaviors of larder-hoarding animals.

Loss of cached foods is a constant threat to animals that hoard food, and pilfering plays an important role in the evolution of hoarding strategies. Although pilfering is known to affect the behavior of scatter hoarders, pilfering has been assumed to be less important for larder-hoarding animals. In this study, I used a mark-recapture study of cached Norway spruce (Picea abies) cones to quantify pilfering rates in larder-hoarding red squirrels (Tamiasciurus hudsonicus). Red squirrels stole 26% of the cones that they ate and lost 25% of the cones cached in their middens. Most squirrels stole cones (97%) and lost cones to stealing (92%). However, individual squirrels stole 1–100% of the cones that they ate and lost 1–84% of the cones cached in their middens. Numbers of cones gained by and lost to stealing were not related to the age or sex of individual squirrels or the distances between or numbers of cones cached in individual territories. Squirrels with small middens, however, ultimately gained cones, and those with large middens lost cones. Because food is frequently a limiting resource for red squirrels, these changes in food abundance may directly affect the fitness of individual squirrels. Thus, pilfering likely plays an important role in shaping the hoarding and defensive behaviors of larder-hoarding animals.

Aim Land-use history can be an important determinant of ecosystem characteristics, even in landscapes that outwardly appear 'natural'. In New England, like much of the eastern United States, the natural reforestation of agricultural lands over the past 150 years has created a predominantly forested landscape. Understanding the physiographical and historical factors controlling forest structure and composition is a major challenge to ecologists, conservationists and land managers. Location We studied the forest structure and composition of Petersham, Massachusetts, which is located in the Central Upland Physiographical province of New England. Like much of New England, Petersham was largely cleared for agriculture by the mid-1800s, but most of the agricultural fields were abandoned and naturally reforested in the late-1800s and early 1900s. The modern landscape is > 90% forested by a mosaic of primary and secondary woodlands. Methods At seventy-four randomly selected 0.04-ha plots, we measured the abundances of all vascular plants in the overstory and understory and nine physiographical and historical variables. Results Species richness was primarily related to landform: species-rich communities occurred in poorly drained basins and species-poor communities occurred on well-drained glacial outwash. Distributions of the sixty-nine most common species were associated with (in order of importance) landform, past land use and elevation. Many species were restricted to specific physiographical conditions, but no species were restricted to specific past land uses. Nine plant associations identified by two-way indicator species analysis were associated with elevation, landform, soil texture and past land use. Ordination by canonical correspondence analysis indicated that the vegetation reflected two principal gradients: a physiographical gradient, defined by landform and soil texture, and a land-use gradient. Main conclusions These analyses suggest the following ranking of factors controlling forest structure and composition in this landscape at this time: landform > agricultural history > elevation > hurricane = fire = logging. Even in this physically heterogenous landscape, land-use history continues to play an important role in shaping forest vegetation 100-150 years after agricultural abandonment and reforestation.

Aim To better understand patterns of community invasibility, we examined exotic plant invasions along an environmental gradient found in vernal pools in the Central Valley of California. Specifically, we analysed the distribution and abundance of exotic plants in relation to pool size, seasonal inundation, soil and nutrient properties, the native plant community, and disturbance history. Location We studied patterns of exotic plant invasions in a complex of natural and restored vernal pools at Travis Air Force Base near Fairfield, California. Methods In thirty natural and thirty restored pools, we conducted field surveys to measure the distribution and abundance of exotic and native plants and several descriptors of the abiotic environment. We used multiple linear regressions to analyse exotic species richness and cover and the abundances of individual exotic species in relation to pool type, fire history, pool size, water depth, native plant cover, and six soil and nutrient properties. Results Exotic species richness was greater in burned than unburned pools, and exotic plant cover was greater in natural than restored pools. Both exotic species richness and cover decreased with increasing water depth. Exotic species richness was not related to native plant cover, but exotic plant cover decreased with increasing native plant cover. Eighteen of the nineteen most common exotic species showed significant relationships with pool type (twelve species), water depth (eleven species), fire history (nine species), native plant cover (six species), and pool size (five species). Most species increased in abundance with decreasing water depth (nine species) or were more abundant in natural (eight species) or burned pools (six species). Main conclusions These results suggest that both abiotic factors, especially those associated with seasonal inundation, and interactions with the native plant community play important roles in determining the invasibility of vernal pool plant communities. In particular, abiotic constraints may limit exotic plant invasions in more stressful environments, and interactions with the native plant community may limit invasions in more benign environments.

Wilderness areas are protected and valued in part for recreation; recreational use, however, can negatively impact these areas. In particular, recreational use can facilitate transport of non-native propagules and create open sites for establishment of non-native species. We examined the role of recreational portage trails in the introduction and establishment of non-native plants into the Boundary Waters Canoe Area Wilderness of northern Minnesota (U.S.A.). On 20 portages, we sampled non-native plant richness and cover at four distances (0, 10, 25, and 50 m) from trails. Non-native richness and cover were not related to distance from wilderness entry point. Non-native richness and cover were, however, negatively related to distance from trails. All six non-native species we observed were either directly on or within 1 m of trails. These results suggest that recreational trails act as corridors facilitating invasions of non-native plants into wilderness areas. It remains unclear, however, whether these effects are caused by dispersal of propagules, creation of bare ground, or changes in the native plant community.

We developed a description of a central New England deciduous hardwood forest based on the distribution within the community of morphological and life history traits (N= 34) and environmental factors. Classification by TWINSPAN of 186 species based on morphological and life history traits identified six major functional groups of species largely corresponding to growth form. A data matrix of plots x traits was ordinated using PCA. Each of the resulting four PCA axes was associated with a major environmental gradient: drainage, site exposure, disturbance due to past land use and degree of disturbance in the 1938 New England Hurricane (24.9, 19.8, 11.7 and 8.4% of the variation respectively). Two patterns suggested that a suite of potentially functional traits, rather than a few key characters (e.g. vital attributes), govern the distribution of species in this community: (1) each of the four axes was largely associated with a different group of traits and (2) each axis was associated with several traits that appeared to sort independently (i.e. not to co‐occur within species). Evaluating one often‐examined trait, there was no evidence that dispersal ability limited the colonization of species into secondary woodlands. We also found that landscape‐scale abundance was associated with a small number of traits. Production of fleshy fruits and few diaspores per plant were positively associated with landscape‐level abundance. Our results suggest that attempts to understand the overall structure and function of this plant community based on a few key characters, such as dispersal ability, will meet with limited success. However, when focusing on one aspect of the community, such as frequency across the landscape, relatively few characters may be important.