Erik S. Jules

Are wolves saving Yellowstone's aspen? A landscape-level test of a behaviorally mediated trophic cascade

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.

Habitat fragmentation and demographic change for a common plant : Trillium in old-growth forest

I studied the influence of forest fragmentation on an understory herb, Trillium ovatum, in the Siskiyou Mountains of Oregon, where logging practices over the past 35 yr have created a mosaic of fragments surrounded by clearcuts and tree plantations. The age of trillium plants can be estimated by counting the annual constrictions on their rhizomes. Based on data collected by Whittaker in 1949 (i.e., pre-fragmentation) and a survey I conducted in 1995, I estimated that the process of clearcutting and subsequent conifer planting results in the mortality of almost all trillium (∼97.6%). In general, the remaining plants are not recruiting new individuals, even in sites clearcut 30 yr ago. Thus, trillium is restricted to smaller amounts of remnant, uncut forest. My study also demonstrated that populations in forest remnants that were within ∼65 m of forest-clearcut edges have had almost no recruitment of young plants since the time of the adjacent clearcutting, while forest interior populations contained higher recruitment levels. Projections based on these recruitment estimates indicated that edge populations will decline in size and interior populations will not decline. This study provides the first evidence of demographic changes in plant populations resulting from habitat fragmentation, and it offers evidence for the mechanisms responsible for such demographic changes.

A broader ecological context to habitat fragmentation: Why matrix habitat is more important than we thought

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.

SPREAD OF AN INVASIVE PATHOGEN OVER A VARIABLE LANDSCAPE: A NONNATIVE ROOT ROT ON PORT ORFORD CEDAR

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-km2 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 t...

OF MICE AND MEN AND TRILLIUM: CASCADING EFFECTS OF FOREST FRAGMENTATION

Cascading ecological effects of anthropogenic habitat fragmentation have been studied primarily in extreme cases (e.g., the isolation of habitat fragments in a novel habitat matrix such as suburban developments, reservoirs, or agricultural fields), with less attention to more subtle and widespread cases, such as habitat fragmentation due to timber harvest. Few studies have used rigorous demographic data to demonstrate the direct and indirect effects of habitat fragmentation. We trapped deer mice (Peromyscus maniculatus) at five sites over two years in southwest Oregon, USA, and used multi-state capture- recapture models to estimate deer mouse survival and movement in clearcuts, forest-frag- ment edges, forest-fragment interiors, and contiguous forests. We also estimated deer mouse densities in fragmented and unfragmented forests and combined deer mouse demographic studies with trillium (Trillium ovatum) seed predation trials to link deer mouse changes to reduced trillium recruitment previously observed at the same study sites. Mouse survival was highest in clearcuts, intermediate in forest fragments, and lowest in unfragmented (control) forests. Mouse movement among clearcuts, forest edges, and forest interiors was common over short time intervals. Collectively, demographic rates led to mouse densities that were 3-4 times higher at forest-fragment sites than at unfragmented sites. Trillium seeds were ;3 times more likely to be depredated in areas of elevated relative mouse abundance than in areas of lower relative abundance. Forest fragmentation has favored mouse populations, resulting in increased seed predation that may decrease recruitment rates and increase local extinction risks for trillium.

Multiple plant traits shape the genetic basis of herbivore community assembly

Summary 1. Community genetics research has posited a genetic basis to the assembly of ecological communities. For arthropod herbivores in particular, there is strong support that genetic variation in host plants is a key factor shaping their diversity and composition. However, the specific plant phenotypes underlying herbivore responses remain poorly explored for most systems. 2. We address this knowledge gap by examining the influence of both genetic and phenotypic variation in a dominant host-plant species, Salix hookeriana, on its associated arthropod herbivore community in a common garden experiment. Specifically, we surveyed herbivore responses among five different arthropod feeding guilds to 26 distinct S. hookeriana genotypes. Moreover, we quantified the heritability of a suite of plant traits that determine leaf quality (e.g. phenolic compounds, trichomes, specific leaf area, C : N) and whole-plant architecture, to identify which traits best accounted for herbivore community responses to S. hookeriana genotype. 3. We found that total herbivore abundance and community composition differed considerably among S. hookeriana genotypes, with strong and independent responses of several species and feeding guilds driving these patterns. We also found that leaf phenolic chemistry displayed extensive heritable variation, whereas leaf physiology and plant architecture tended to be less heritable. Of these traits, herbivore responses were primarily associated with leaf phenolics and plant architecture; however, different herbivore species and feeding guilds were associated with different sets of traits. Despite our thorough trait survey, plant genotype remained a significant predictor of herbivore responses in most trait association analyses, suggesting that unmeasured host-plant characteristics and/or interspecific interactions were also contributing factors. 4. Taken together, our results support that the genetic basis of herbivore community assembly occurs through a suite of plant traits for different herbivore species and feeding guilds. Still, identifying these phenotypic mechanisms requires measuring a broad range of plant traits and likely further consideration of how these traits affect interspecific interactions.

Heterogeneity Shapes Invasion: Host Size And Environment Influence Susceptibility To A Nonnative Pathogen

Theoretical study of invasion dynamics has suggested that spatial heterogeneity should strongly influence the rate and extent of spreading organisms. However, empirical support for this prediction is scant, and the importance of understanding heterogeneity for real-world systems has remained ambiguous. This study quantified the influence of host and environmental heterogeneity on the dynamics of a 19-year disease invasion by the exotic and fatal pathogen, Phytophthora lateralis, within a stream population of its host tree, Port Orford cedar (Chamaecyparis lawsoniana). Using dendrochronology, we reconstructed the invasion history along a 1350-m length of infected stream, which serves as the only route of pathogen dispersal. Contrary to theoretical predictions, the temporal progression of the disease invasion was not related to a hosts downstream spatial position, but instead was determined by two sources of heterogeneity: host size and proximity to the stream channel. These sources of heterogeneity influenced both the epidemic and endemic dynamics of this pathogen invasion. This analysis provides empirical support for the influence of heterogeneity on the invasion dynamics of a commercially important forest pathogen and highlights the need to incorporate such natural variability into both invasion theory and methods aimed at controlling future spread.

Genetic specificity of a plant–insect food web: Implications for linking genetic variation to network complexity

Significance We know that the gain or loss of species can have cascading effects on food-web complexity; however, it is less clear whether the gain or loss of genetic variation within species, an often overlooked component of biodiversity, will similarly affect food-web structure. Here, we empirically identify how genetic variation within a host plant directly and indirectly affects its associated insect food web, resulting in distinct trophic interactions occurring on each host-plant genotype. Moreover, simulations of our empirical data suggest that higher levels of host-plant genetic variation lead to a more complex plant–insect food web. Our results suggest that preserving genetic variation within key species may be critical for maintaining complex and robust food webs under future environmental change. Theory predicts that intraspecific genetic variation can increase the complexity of an ecological network. To date, however, we are lacking empirical knowledge of the extent to which genetic variation determines the assembly of ecological networks, as well as how the gain or loss of genetic variation will affect network structure. To address this knowledge gap, we used a common garden experiment to quantify the extent to which heritable trait variation in a host plant determines the assembly of its associated insect food web (network of trophic interactions). We then used a resampling procedure to simulate the additive effects of genetic variation on overall food-web complexity. We found that trait variation among host-plant genotypes was associated with resistance to insect herbivores, which indirectly affected interactions between herbivores and their insect parasitoids. Direct and indirect genetic effects resulted in distinct compositions of trophic interactions associated with each host-plant genotype. Moreover, our simulations suggest that food-web complexity would increase by 20% over the range of genetic variation in the experimental population of host plants. Taken together, our results indicate that intraspecific genetic variation can play a key role in structuring ecological networks, which may in turn affect network persistence.

Host heterogeneity influences the impact of a non-native disease invasion on populations of a foundation tree species

Invasive pathogens are becoming increasingly important in forested ecosystems, yet they are often difficult to study because of their rapid transmission. The rate and extent of pathogen spread are thought to be partially controlled by variation in host characteristics, such as when host size and location influence susceptibility. Few host-pathogen systems, however, have been used to test this prediction. We used Port Orford cedar (Chamaecyparis lawsoniana), a foundation tree species in riparian areas of California and Oregon (USA), and the invasive oomycete Phytophthora lateralis to assess pathogen impacts and the role of host characteristics on invasion. Across three streams that had been infected for 13–18 years by P. lateralis, we mapped 2241 trees and determined whether they had been infected using dendrochronology. The infection probability of trees was governed by host size (diameter at breast height [DBH]) and geomorphic position (e.g., active channel, stream bank, floodplain, etc.) similarly across streams. For instance, only 23% of trees <20 cm DBH were infected, while 69% of trees ≥20 cm DBH were infected. Presumably, because spores of P. lateralis are transported downstream in water, they are more likely to encounter well-developed root systems of larger trees. Also because of this water-transport of spores, differences in infection probability were found across the geomorphic positions: 59% of cedar in the active channel and the stream bank (combined) were infected, while 23% of trees found on higher geomorphic types were infected. Overall, 32% of cedar had been infected across the three streams. However, 63% of the total cedar basal area had been killed, because the greatest number of trees, and the largest trees, were found in the most susceptible positions. In the active channel and stream bank, 91% of the basal area was infected, while 46% was infected across higher geomorphic positions. The invasion of Port Orford cedar populations by P. lateralis causes profound impacts to population structure and the invasion outcome will be governed by the heterogeneity found in host size and location. Models of disease invasion will require an understanding of how heterogeneity influences spread dynamics to adequately predict the outcome for host populations.

Disturbance response across a productivity gradient: postfire vegetation in serpentine and nonserpentine forests

Disturbances such as wildfire play a major role in the diversity, structure and composition of plant communities, however, little is known about the differential impacts of fire across landscapes that vary in characteristics such as soil nutrients and site productivity. Theory predicts that productivity can mediate the impacts of fire for reasons related to broad ecological processes and differential selective forces. For instance, ecosystems with lower site productivity are less limited by space and light and consequently experience less pronounced changes in these resources following a disturbance. Moreover, resource availability related to disturbance and productivity can affect the proportion of plants with competitive versus stress-tolerant life history strategies. In this study, we utilized a model system for testing predictions about productivity and disturbance that included a mixed conifer forest across a gradient of edaphically harsh, ultramafic “serpentine” soils and “nonserpentine” soils in th...