Behaviorally mediated trophic cascades (BMTCs) occur when the fear of predation among herbivores enhances plant productivity. Based primarily on systems involving small-bodied predators, BMTCs have been proposed as both strong and ubiquitous in natural ecosystems. Recently, however, synthetic work has suggested that the existence of BMTCs may be mediated by predator hunting mode, whereby passive (sit-and-wait) predators have much stronger effects than active (coursing) predators. One BMTC that has been proposed for a wide-ranging active predator system involves the reintroduction of wolves (Canis lupus) to Yellowstone National Park, USA, which is thought to be leading to a recovery of trembling aspen (Populus tremuloides) by causing elk (Cervus elaphus) to avoid foraging in risky areas. Although this BMTC has been generally accepted and highly popularized, it has never been adequately tested. We assessed whether wolves influence aspen by obtaining detailed demographic data on aspen Stands using tree rings and by monitoring browsing levels in experimental elk exclosures arrayed across a gradient of predation risk for three years. Our study demonstrates that the historical failure of aspen to regenerate varied widely among stands (last recruitment year ranged from 1892 to 1956), and our data do not indicate an abrupt cessation of recruitment. This pattern of recruitment failure appears more consistent with a gradual increase in elk numbers rather than a rapid behavioral shift in elk foraging following wolf extirpation. In addition, our estimates of relative survivorship of young browsable aspen indicate that aspen are not currently recovering in Yellowstone, even in the presence of a large wolf population. Finally, in an experimental test of the BMTC hypothesis we found that the impacts of elk browsing on aspen demography are not diminished in sites where elk are at higher risk of predation by wolves. These findings suggest the need to further evaluate how trophic cascades are mediated by predator-prey life history and ecological context.

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Abstract. Consequences of habitat fragmentation have garnered much attention over the past few decades. The resulting literature has been useful for understanding how land‐use changes influence population viability and community structure, but we are still hampered by a major aspect of the conceptual framework within which most fragmentation work arises. Specifically, habitat between fragments (‘matrix’) is usually treated as uniform and ecologically irrelevant. However, recent work on animals shows that matrix habitat can profoundly influence within‐fragment dynamics. We review related evidence for plants. Various matrix types (e.g. clear‐cutting, agriculture, or urbanization) can act in different ways to alter resource availability and movement of pollinators, seed dispersers, and herbivores. Inclusion of matrix qualities in fragmentation studies is further complicated since most matrices are not static; sites in which timber harvesting or agriculture occur develop through succession or change as crops are rotated, respectively, such that their influence on within‐fragment processes vary temporally. Also, many plants are not restricted to remnants of original habitat. Using studies of forest understory plants, we summarize work showing how diversity can change significantly through time in matrix. Understanding the persistence of a species across fragmented landscapes will require more attention to matrix habitat, and to the species utilizing the matrix.

Understanding biological invasions requires information on the history of spatial spread, as well as measures of landscape and biotic features that control habitat invasibility. Because invasive species often spread quickly over large areas, attaining these two sets of information simultaneously is uncommon. We studied the spread of a fatal nonnative root pathogen, Phytophthora lateralis, across a heterogeneous landscape of its host, Port Orford cedar (Chamaecyparis lawsoniana). Within our 37-km 2 study area in southwestern Oregon and northwest California, Port Orford cedar populations are generally restricted to riparian zones along creeks. The pathogen is spread between watersheds in two ways: (1) by spore-infested material being dislodged from vehicles, and (2) by animals or people moving infested mud (i.e., via foot traffic). Using dendrochronological techniques, we determined the date of infection for dead cedars and reconstructed spread history across our study area from 1977 to 1999. Twenty-six of the 36 (72%) separate infection events we identified were caused by dispersal via vehicles along roads, and the remainder by foot traffic. Survival analysis demonstrated that cedar populations in creeks crossed by roads were more likely to be infected than those creeks that were not crossed by roads. Also, a comparison of minimum dispersal distances showed infections that moved via road moved significantly farther than those vectored by foot traffic, and the distance infection traveled declined significantly through time. We also coupled our spread history with measures of landscape and host features, including abundance of potential host trees, the distance from the road surface to the nearest potential host, length of road in immediate contact with the riparian zone, catchment area (a measure of stream flow), elevation, slope, and solar radiation. Our results show that catchment area, host abundance, and proximity to the nearest tree are significantly and positively associated with infection risk. Our study demonstrates that increased connectivity between invasible sites created by the presence of roads can increase invasion success of a plant pathogen. We also document that successful pathogen invasion can be governed by both physical landscape features and attributes of host plant populations.

Studies of plants in fragmented habitats have focused on single ecological processes, such as seed production or seed dispersal, that may be altered by increased fragmentation and that operate as the mechanism(s) that increase extinction probability. We examined a suite of potential mechanisms to explain demographic shifts toward extinction in populations of Trillium ovatum, a long‐lived herbaceous perennial found in the understory of western North American conifer forests. Past work has shown that populations of T. ovatum found within ∼65 m of forest‐clearcut edges in southwestern Oregon have had almost no new recruitment since the edges were formed. We hypothesized that changes in abiotic conditions and biotic interactions present along edges are responsible for reduced recruitment. In eight populations in eight separate fragments, we evaluated the relationship of distance of the populations to the forest edge with respect to six processes: flowering phenology (timing), seed production, pollination‐ and resource‐limitation of seed set, seed dispersal, seed predation, and germination. Those factors that showed a significant relationship with edge distance were then compared with recruitment of younger age classes. Two processes were significantly different near edges and were highly correlated with decreased recruitment: decreased seed production due to changes in pollination and increased seed predation by rodents. Our study (in conjunction with previous studies) suggests that several ecological processes show no significant relationship with edge distance and can be eliminated as possible mechanisms of reduced recruitment: flowering phenology, resource‐limitation of seed set, seed dispersal, germination, herbivory, and survivorship of established plants. Edges influence some but not all components of a plant's life history. Thus, determining shifts in only one part of a life history will be inadequate for testing the prediction of increasing extinction probabilities in fragmented landscapes. Future studies should include enough information to conduct comprehensive analyses, such as matrix projections and sensitivity analyses.