Kyle J. Haynes

THE MATRIX ENHANCES THE EFFECTIVENESS OF CORRIDORS AND STEPPING STONES

Conservation strategies often call for the utilization of corridors and/or stepping stones to promote dispersal among fragmented populations. However, the extent to which these strategies increase connectivity for an organism may depend not only on the corridors and stepping stones themselves, but also on the composition of the surrounding matrix. Using an herbivore–host-plant system consisting of the planthopper Prokelisia crocea and its sole host plant, prairie cordgrass (Spartina pectinata), we show that the effectiveness of corridors and stepping stones for promoting planthopper dispersal among patches depended strongly on the intervening matrix habitat. In a low-resistance matrix (one that facilitates high rates of interpatch dispersal), both stepping stones and corridors promoted high connectivity, increasing the number of colonists by threefold relative to patches separated by matrix habitat only. The effectiveness of stepping stones and corridors was significantly lower in a high-resistance matrix...

MATRIX COMPOSITION AFFECTS THE SPATIAL ECOLOGY OF A PRAIRIE PLANTHOPPER

To date, there is a lack of well-controlled field experiments that disentangle the effects of the intervening matrix from other landscape variables (e.g., patch geography or quality) that might influence animal dispersal among patches. We performed a field experiment to investigate how the movement of a delphacid planthopper (Prokelisia crocea) among discrete patches of prairie cordgrass (Spartina pectinata) is affected by the composition of the matrix (mudflat, native nonhost grasses, and the introduced grass smooth brome [Bromus inermis]). Within each matrix type, marked planthoppers were released onto experimental cordgrass patches that were made identical in size, isolation, and host plant quality. We found that the emigration rate (planthoppers lost per patch per day) was 1.3 times higher for patches embedded in the two nonhost grass matrix types than for patches in mudflat. The rate of immigration (immigrants per patch per day) into patches isolated by 3 m was 5.4 times higher in the brome than in the mudflat matrix. Patches in the native grass matrix had intermediate immigration rates. In addition, a survey of planthopper distributions in nature revealed that both the within- and among-patch distributions of the planthopper were related to the composition of the matrix. Within patches, individuals accumulated against mudflat edges (relative to patch interiors) but not against nonhost grass edges. Among patches, incidence and density increased with the proportion of the matrix composed of open mud. The matrix was equal to that of patch geography (size and isolation) in its ability to explain the distribution of the planthopper. We suggest that the low permeability of the mudflat relative to a nonhost grass edge may explain these planthopper distributional patterns. Also, because natural cordgrass patches in mudflat were richer in nutrients than those in nonhost grasses, planthoppers may have been more likely to remain and build up densities on the former patches. We predict that the displacement of native matrix types by invasive brome will result in increased connectivity and greater spatial synchrony in densities of planthoppers among cordgrass patches.

Climatic warming disrupts recurrent Alpine insect outbreaks.

Climate change has been identified as a causal factor for diverse ecological changes worldwide. Warming trends over the last couple of decades have coincided with the collapse of long-term population cycles in a broad range of taxa, although causal mechanisms are not well-understood. Larch budmoth (LBM) population dynamics across the European Alps, a classic example of regular outbreaks, inexplicably changed sometime during the 1980s after 1,200 y of nearly uninterrupted periodic outbreak cycles. Herein, analysis of perhaps the most extensive spatiotemporal dataset of population dynamics and reconstructed Alpine-wide LBM defoliation records reveals elevational shifts in LBM outbreak epicenters that coincide with temperature fluctuations over two centuries. A population model supports the hypothesis that temperature-mediated shifting of the optimal elevation for LBM population growth is the mechanism for elevational epicenter changes. Increases in the optimal elevation for population growth over the warming period of the last century to near the distributional limit of host larch likely dampened population cycles, thereby causing the collapse of a millennium-long outbreak cycle. The threshold-like change in LBM outbreak pattern highlights how interacting species with differential response rates to climate change can result in dramatic ecological changes.

SPIDER EFFECTS ON PLANTHOPPER MORTALITY, DISPERSAL, AND SPATIAL POPULATION DYNAMICS

Nonlethal (trait-mediated) effects of predators on prey populations, particularly with regard to prey dispersal, scarcely have been considered in spatial ecological studies. In this study, we report on the effects of spider predators on the mortality, dispersal, and spatial population dynamics of Prokelisia crocea planthoppers (Hemiptera: Delphacidae) in a prairie landscape. Based on a three-generation survey of host-plant patches (Spartina pectinata; Poaceae), the density of cursorial and web-building spiders declined significantly with increasing patch size (a pattern the opposite of that for the planthopper). Independent of patch size effects, an increase in the density of web-building and cursorial spiders had a negative effect on planthopper density in one of three generations each. Finally, the likelihood of extinction of local (patch) populations of planthoppers increased significantly with an increase in the density of web-building spiders. Planthoppers in small host-plant patches with high densit...

AN INVASIVE PLANT PROMOTES UNSTABLE HOST–PARASITOID PATCH DYNAMICS

In theory, the rate of interpatch dispersal significantly influences the population dynamics of predators and their prey, yet there are relatively few field experiments that provide a strong link between these two processes. In tallgrass prairies of North America, the planthopper, Prokelisia crocea, and its specialist parasitoid, Anagrus columbi, exist among discrete host-plant patches (prairie cordgrass, Spartina pectinata). In many areas, the matrix, or habitat between patches, has become dominated by the invasive exotic grass, smooth brome (Bromus inermis). We performed a landscape-level field study in which replicate cordgrass networks (identical in number, size, quality, and distribution of cordgrass patches) were embedded in a matrix composed of either mudflat (a native matrix habitat) or smooth brome. Mark–recapture experiments with the planthopper and parasitoid revealed that the rate of movement among cordgrass patches for both species was 3–11 times higher in smooth brome than in mudflat. Within three generations, planthopper and parasitoid densities per patch were on average ∼50% lower and spatially 50–87% more variable for patches embedded in a brome as compared to a mudflat matrix. A brome-dominated landscape also promoted extinction rates per patch that were 4–5 times higher than the rates per patch in native mudflat habitat. The effect was more acute for the parasitoid. We suggest that the differences in population dynamics between networks of patches in brome and those in mudflat were driven by underlying differences in interpatch dispersal (i.e., patch connectivity). To our knowledge, this is the first experimental study to reveal that matrix composition, in particular, the presence of an invasive plant species, affects the spatial and temporal dynamics of an herbivore and its natural enemy.

Landscape context outweighs local habitat quality in its effects on herbivore dispersal and distribution

Past studies with spatially structured herbivore populations have emphasized the primacy of intrinsic factors (e.g., patch quality), patch geometry (e.g., patch size and isolation), and more recently landscape context (e.g., matrix composition) in affecting local population abundance and dispersal rate. However, few studies have examined the relative importance of each factor, or how they might interact to affect herbivore abundance or dispersal. Here, we performed a factorial field experiment to examine the independent and interactive effects of patch quality (plant biomass, leaf protein, leaf phenolics) and matrix composition [mudflat or non-host grass (Bromus inermis)] on planthopper (Prokelisia crocea) emigration from host-plant patches (prairie cordgrass, Spartina pectinata). In addition, a field survey was conducted to examine the relative importance of patch quality, geography, and matrix composition on planthopper occupancy and density. In the experiment, we found that rates of emigration from low and intermediate quality patches were, on average, 21% percent higher for patches embedded in brome than mudflat. In contrast, the emigration rate was unaffected by matrix composition in nutrient-rich patches. Within matrix types, plant quality had little effect on emigration. In the survey, planthopper density and the patch occupancy rate of planthoppers increased nonadditively with increasing patch size and the percentage of the surrounding matrix composed of mudflat. This study suggests that landscape-level factors, such as the matrix, may be more important than factors intrinsic to the patches.

Confounding of patch quality and matrix effects in herbivore movement studies

Although the landscape matrix is increasingly incorporated into spatial-ecological population studies, little consideration has been given to the likely possibility that patch quality is confounded with the composition of the matrix surrounding each patch. For example, the nutritional quality of host-plant patches to an herbivore may be highly correlated with matrix composition, consequently obfuscating the importance of the matrix itself to interpatch dispersal. From a literature survey of the effects of the matrix on herbivore movement among host-plant patches, we found that 55% of the studies (6/11) failed to experimentally or statistically isolate the effects of the matrix from potential patch-quality effects on dispersal. Most studies consisted of mark-recapture experiments in natural landscapes where patch equality was not controlled or manipulated. Of the few studies that evaluated the relationship between matrix composition and patch quality, all of them (3/3) found that these two landscape factors covaried. These data suggest that in most matrix studies, effects of the matrix on dispersal may be wholly, or in part, due to underlying differences in patch quality.

Geographical variation in the spatial synchrony of a forest-defoliating insect: isolation of environmental and spatial drivers

Despite the pervasiveness of spatial synchrony of population fluctuations in virtually every taxon, it remains difficult to disentangle its underlying mechanisms, such as environmental perturbations and dispersal. We used multiple regression of distance matrices (MRMs) to statistically partition the importance of several factors potentially synchronizing the dynamics of the gypsy moth, an invasive species in North America, exhibiting outbreaks that are partially synchronized over long distances (approx. 900 km). The factors considered in the MRM were synchrony in weather conditions, spatial proximity and forest-type similarity. We found that the most likely driver of outbreak synchrony is synchronous precipitation. Proximity played no apparent role in influencing outbreak synchrony after accounting for precipitation, suggesting dispersal does not drive outbreak synchrony. Because a previous modelling study indicated weather might indirectly synchronize outbreaks through synchronization of oak masting and generalist predators that feed upon acorns, we also examined the influence of weather and proximity on synchrony of acorn production. As we found for outbreak synchrony, synchrony in oak masting increased with synchrony in precipitation, though it also increased with proximity. We conclude that precipitation could synchronize gypsy moth populations directly, as in a Moran effect, or indirectly, through effects on oak masting, generalist predators or diseases.

Spatial synchrony propagates through a forest food web via consumer-resource interactions

In many study systems, populations fluctuate synchronously across large regions. Several mechanisms have been advanced to explain this, but their importance in nature is often uncertain. Theoretical studies suggest that spatial synchrony initiated in one species through Moran effects may propagate among trophically linked species, but evidence for this in nature is lacking. By applying the nonparametric spatial correlation function to time series data, we discover that densities of the gypsy moth, the moths chief predator (the white-footed mouse), and the mouses winter food source (red oak acorns) fluctuate synchronously over similar distances (approximately1000 km) and with similar levels of synchrony. In addition, we investigate the importance of consumer-resource interactions in propagating synchrony among species using an empirically informed simulation model of interactions between acorns, the white-footed mouse, the gypsy moth, and a viral pathogen of the gypsy moth. Our results reveal that regional stochasticity acting directly on populations of the mouse, moth, or pathogen likely has little effect on levels of the synchrony displayed by these species. In contrast, synchrony in mast seeding can propagate across trophic levels, thus explaining observed levels of synchrony in both white-footed mouse and gypsy moth populations. This work suggests that the transfer of synchrony among trophically linked species may be a major factor causing interspecific synchrony.

Induced plant defenses, host–pathogen interactions, and forest insect outbreaks

Significance Many forest insects undergo outbreaks, in which their densities rise from undetectable to extremely high. Outbreaks are widely assumed to be driven by specialist natural enemies such as infectious pathogens, but gypsy moth outbreaks show alternating severe and mild outbreaks in forests with a high percentage of oaks, a pattern that cannot be explained by host-pathogen models. We used an experiment to show that induced defenses in red oak reduce heterogeneity among gypsy moth larvae in the risk of virus infection, and extending standard models to allow for this effect produces alternating outbreaks, matching the data. The ability of our model to reproduce this complex pattern suggests that the role of induced defenses in insect outbreaks has been underestimated. Cyclic outbreaks of defoliating insects devastate forests, but their causes are poorly understood. Outbreak cycles are often assumed to be driven by density-dependent mortality due to natural enemies, because pathogens and predators cause high mortality and because natural-enemy models reproduce fluctuations in defoliation data. The role of induced defenses is in contrast often dismissed, because toxic effects of defenses are often weak and because induced-defense models explain defoliation data no better than natural-enemy models. Natural-enemy models, however, fail to explain gypsy moth outbreaks in North America, in which outbreaks in forests with a higher percentage of oaks have alternated between severe and mild, whereas outbreaks in forests with a lower percentage of oaks have been uniformly moderate. Here we show that this pattern can be explained by an interaction between induced defenses and a natural enemy. We experimentally induced hydrolyzable-tannin defenses in red oak, to show that induction reduces variability in a gypsy moth’s risk of baculovirus infection. Because this effect can modulate outbreak severity and because oaks are the only genus of gypsy moth host tree that can be induced, we extended a natural-enemy model to allow for spatial variability in inducibility. Our model shows alternating outbreaks in forests with a high frequency of oaks, and uniform outbreaks in forests with a low frequency of oaks, matching the data. The complexity of this effect suggests that detecting effects of induced defenses on defoliator cycles requires a combination of experiments and models.