Doug Jackson

Spatial and Temporal Dynamics of a Fungal Pathogen Promote Pattern Formation in a Tropical Agroecosystem

Recent studies have shown that the spatial pattern of nests of an arboreal ant, Azteca instabilis (Hymenoptera: Formicidae), in a tropical coffee agroecosystem may emerge through self-organization. The proposed self-organization process involves both local expansion and density-dependent mortality of the ant colonies. We explored a possible mechanism for the density-dependent mortality involving the entomopathogenic fungus Lecanicillium lecanii. L. lecanii attacks a scale insect, Coccus viridis (Coccidae, Hemiptera), which is tended by A. instabilis in a mutualistic association. By attacking C. viridis, L. lecanii may have an indirect, negative effect on ant colony survival. To explore this hypothesis, we conducted investigations into the spatial and temporal distributions of L. lecanii. We measured incidence and severity at 4 spatial scales: (1) throughout a 45 hectare study plot; (2) in two 40 X 50 meter plots; (3) on coffee bushes within 4 m of two ant nests; and (3) on individual branches in a single coffee bush. The plot-level censuses did not reveal a clear spatial pattern, but the finer scale surveys show distinct patterns in the spread of infection over time. We also developed a simple cellular automata model of the coupled ant nest-L. lecanii system which is able to produce spatial patterns qualitatively and quantitatively similar to that found in the field. The accumulated evidence suggests that L. lecanii may very well be responsible for the density-dependent control thought necessary for spatial pattern formation of ant nests in this system.

Ecological complexity in a coffee agroecosystem: spatial heterogeneity, population persistence and biological control.

Background Spatial heterogeneity is essential for the persistence of many inherently unstable systems such as predator-prey and parasitoid-host interactions. Since biological interactions themselves can create heterogeneity in space, the heterogeneity necessary for the persistence of an unstable system could be the result of local interactions involving elements of the unstable system itself. Methodology/Principal Findings Here we report on a predatory ladybird beetle whose natural history suggests that the beetle requires the patchy distribution of the mutualism between its prey, the green coffee scale, and the arboreal ant, Azteca instabilis. Based on known ecological interactions and the natural history of the system, we constructed a spatially-explicit model and showed that the clustered spatial pattern of ant nests facilitates the persistence of the beetle populations. Furthermore, we show that the dynamics of the beetle consuming the scale insects can cause the clustered distribution of the mutualistic ants in the first place. Conclusions/Significance From a theoretical point of view, our model represents a novel situation in which a predator indirectly causes a spatial pattern of an organism other than its prey, and in doing so facilitates its own persistence. From a practical point of view, it is noteworthy that one of the elements in the system is a persistent pest of coffee, an important world commodity. This pest, we argue, is kept within limits of control through a complex web of ecological interactions that involves the emergent spatial pattern.

Population Responses to Environmental Change in a Tropical Ant: The Interaction of Spatial and Temporal Dynamics

Spatial structure can have a profound, but often underappreciated, effect on the temporal dynamics of ecosystems. Here we report on a counterintuitive increase in the population of a tree-nesting ant, Azteca sericeasur, in response to a drastic reduction in the number of potential nesting sites. This surprising result is comprehensible when viewed in the context of the self-organized spatial dynamics of the ants and their effect on the ants’ dispersal-limited natural enemies. Approximately 30% of the trees in the study site, a coffee agroecosystem in southern Mexico, were pruned or felled over a two-year period, and yet the abundance of the ant nests more than doubled over the seven-year study. Throughout the transition, the spatial distribution of the ants maintained a power-law distribution – a signal of spatial self organization – but the local clustering of the nests was reduced post-pruning. A cellular automata model incorporating the changed spatial structure of the ants and the resulting partial escape from antagonists reproduced the observed increase in abundance, highlighting how self-organized spatial dynamics can profoundly influence the responses of ecosystems to perturbations.

Fine-scale spatial genetic structure of a fungal parasite of coffee scale insects

The entomopathogenic fungus Lecanicillium lecanii persists in a highly dynamic network of habitat patches (i.e., a metapopulation) formed by its primary host, the green coffee scale Coccus viridis. Lecanicillium lecanii is an important biological control of both C. viridis and the coffee rust, Hemileia vastatrix. Successfully managing this biocontrol agent will depend on an increased understanding of the characteristics of its dispersal, as migration between occupied and unoccupied patches is essential for the persistence of this metapopulation. In the present study, we employ a population genetics approach, and show that in our study system, a coffee farm in the Soconusco region of southern Mexico, L. lecanii is characterized by clear spatial genetic structure among plots within the farm but a lack of apparent structure at smaller scales. This is consistent with dispersal dominated by highly localized transport, such as by insects or rain splash, and less dependence on longer distance dispersal such as wind transport. The study site was dominated by a few multi-locus microsatellite genotypes, and their identities and large-scale locations persist across both study years, suggesting that local epizootics (outbreaks) are initiated each wet season by residual propagules from the previous wet season, and not by long-distance transport of propagules from other sites. The index of association, a measure of linkage disequilibrium, indicates that epizootics are primarily driven by asexual, clonal reproduction, which is consistent with the apparent lack of a teleomorph in the study site and the presence of only a single mating type across the site (MAT-1-2-1). Although the same predominant clonal genotypes were found across years, a drastic difference in genotypic diversity was witnessed across two sites between the two years, suggesting that interclonal selection was occurring. In light of the dispersal limitation of L. lecanii, spatial structure may be an essential axis of management to ensure the persistence of L. lecanii and preserve the ecosystem services provided by this versatile biocontrol agent in this and similar coffee farms.

Species complementarity in two myrmecophilous lady beetle species in a coffee agroecosystem: implications for biological control

Natural enemy diversity may be beneficial, through species complementarity, or detrimental, through antagonistic interactions, such as competition or intraguild predation, for the biological control of agricultural pests. We studied two coexisting myrmecophilous coccinellid beetles, Azya orbigera (Mulsant) (Coleoptera: Coccinellidae) and an undescribed species in the genus Diomus (Coleoptera: Coccinellidae), in a coffee agroecosystem in Chiapas, Mexico. As both beetles specialize on the same prey, the green coffee scale pest, Coccus viridis (Green) (Hemiptera: Coccidae), we studied the beetles’ behavior and distribution to determine if they niche partition in order to avoid extreme competition. Through field surveys and lab experiments we detected spatial segregation but not resource partitioning among A. orbigera and Diomus sp. We posit that the presence of both species can lead to improved biocontrol of C. viridis populations through species complementarity. Our work supports the growing evidence that natural enemy diversity can provide enhanced conservation biological control.