An integrative approach to understanding host–parasitoid population dynamics in real landscapes

Host-plant availability drives the spatiotemporal dynamics of interacting metapopulations across a fragmented landscape

The dynamics of ecological communities depend partly on species interactions within and among trophic levels. Experimental work has demonstrated the impact of species interactions on the species involved, but it remains unclear whether these effects can also be detected in long‐term time series across heterogeneous landscapes. We analyzed a 19‐yr time series of patch occupancy by the Glanville fritillary butterfly Melitaea cinxia, its specialist parasitoid wasp Cotesia melitaearum, and the specialist fungal pathogen Podosphaera plantaginis infecting Plantago lanceolata, a host plant of the Glanville fritillary. These species share a network of more than 4,000 habitat patches in the Åland islands, providing a metacommunity data set of unique spatial and temporal resolution. To assess the influence of interactions among the butterfly, parasitoid, and mildew on metacommunity dynamics, we modeled local colonization and extinction rates of each species while including or excluding the presence of potentially interacting species in the previous year as predictors. The metapopulation dynamics of all focal species varied both along a gradient in host plant abundance, and spatially as indicated by strong effects of local connectivity. Colonization and to a lesser extent extinction rates depended also on the presence of interacting species within patches. However, the directions of most effects differed from expectations based on previous experimental and modeling work, and the inferred influence of species interactions on observed metacommunity dynamics was limited. These results suggest that although local interactions among the butterfly, parasitoid, and mildew occur, their roles in metacommunity spatiotemporal dynamics are relatively weak. Instead, all species respond to variation in plant abundance, which may in turn fluctuate in response to variation in climate, land use, or other environmental factors.

Effects of Patch-Matrix Composition and Individual Movement Response on Population Persistence at the Patch Level

Fragmentation creates landscape-level spatial heterogeneity which in turn influences population dynamics of the resident species. This often leads to declines in abundance of the species due to increased susceptibility to edge effects between the remnant habitat patches and the lower quality "matrix" surrounding these focal patches. In this paper, we formalize a framework to facilitate the connection between small-scale movement and patch-level predictions of persistence through a mechanistic model based on reaction-diffusion equations. The model is capable of incorporating essential information about edge-mediated effects such as patch preference, movement behavior, and matrix-induced mortality. We mathematically analyze the model's predictions of persistence with a general logistic-type growth term and explore their sensitivity to demographic attributes in both the patch and matrix, as well as patch size and geometry. Also, we provide bounds on demographic attributes and patch size in order for the model to predict persistence of a species in a given patch based on assumptions on the patch/matrix interface. Finally, we illustrate the utility of this framework with a well-studied planthopper species (Prokelisia crocea) living in a highly fragmented landscape. Using experimentally derived data from various sources to parameterize the model, we show that, qualitatively, the model results are in accord with experimental predictions regarding minimum patch size of P. crocea. Through application of a sensitivity analysis to the model, we also suggest a ranking of the most important model parameters based on which parameter will cause the largest output variance.

Dispersal in Host⁻Parasitoid Interactions: Crop Colonization by Pests and Specialist Enemies

Interactions of insect pests and their natural enemies increasingly are being considered from a metapopulation perspective, with focus on movements of individuals among habitat patches (e.g., individual crop fields). Biological control may be undercut in short-lived crops as natural enemies lag behind the pests in colonizing newly created habitat. This hypothesis was tested by assessing parasitism of cereal leaf beetle (Oulema melanopus) and alfalfa weevil (Hypera postica) larvae at varying distances along transects into newly planted fields of small grains and alfalfa in northern Utah. The rate of parasitism of cereal leaf beetles and alfalfa weevils by their host-specific parasitoids (Tetrastichus julis (Eulophidae) and Bathyplectes curculionis (Ichneumonidae), respectively) was determined for earliest maturing first generation host larvae. Rates of parasitism did not vary significantly with increasing distance into a newly planted field (up to 250–700 m in individual experiments) from the nearest source field from which pest and parasitoid adults may have immigrated. These results indicate strong, rapid dispersal of the parasitoids in pursuing their prey into new habitat. Thus, across the fragmented agricultural landscape of northern Utah, neither the cereal leaf beetle nor the alfalfa weevil initially gained substantial spatial refuge from parasitism by more strongly dispersing than their natural enemies into newly created habitat. Additional studies, including those of colonization of newly planted crops by generalist pests and natural enemies, are called for in assessing these results with a broader perspective.

Invasion in patchy landscapes is affected by dispersal mortality and mate-finding failure

Range expansions are a function of population growth and dispersal, and nascent populations often must overcome demographic Allee effects (positive density dependence at low population densities) driven by factors such as mate-finding failure. Given the importance of individual movement to mate finding, links between landscape structure and movement may be critical to range expansion; however, landscape effects on other factors including mortality may be equally or more important. In one of the most comprehensive investigations of the interactions of these processes to date, we combined field experiments, simulation modeling, and analysis of empirical spread patterns to investigate how landscape structure affected the spread of the gypsy moth in Virginia and West Virginia. In experiments designed to assess how landscape attributes affect mate finding, we found adult males resisted leaving forest patches and the probability of locating a pheromone source declined more rapidly over distance in non-forest matrix than in forest. We used these findings to develop individual-based simulation models of gypsy moth population dynamics and spread in complex patch-matrix landscapes. The models produced an Allee effect that strengthened with reductions in forested area, but owing more so to dispersal mortality than to effects on mate location. Predicted maximum rates of population spread grew with increases in forest area due to increasing success of long-distance transport events. Evaluations of empirical data showed relationships between spread rates and landscape structure largely consistent with model predictions. We conclude rates of spread were largely driven by long-distance dispersal events, the success of which was influenced primarily by dispersal mortality of larvae in unsuitable matrix, and that landscape effects on mate location played a secondary role. Though influences of landscape structure on mate location appear to be unimportant to the spread of the gypsy moth, we predict they would have stronger effects on more dispersive species.