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Dryad

Herbivory mediates direct and indirect interactions in long-unburned chaparral

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Jun 07, 2022 version files 203.83 KB

Abstract

Community interaction webs describe both direct and indirect interactions among species. Changes in direct interactions often become noticeable soon after a perturbation, but time lags in responses of many species may delay the appearance of indirect effects and lead to temporal and/or spatial variation in interaction webs. Accurately identifying these shifts in the field requires time-specific, spatially-differentiated interaction webs. We explore how variation in browsing affects interaction webs in a long-unburned chaparral shrubland near the central California coast. Most prior work in chaparral focused on rapid changes for <5 yrs after a wildfire that have been assumed to determine community patterns until the next fire. Here, we report results of the first 15 years of an on-going experiment monitoring how interaction webs in long-unburned chaparral (at least 100 yrs postfire) respond to experimental variation in browsing by deer and rabbits on dominant shrubs (Arctostaphylos pumila, Ceanothus cuneatus var. rigidus and Ericameria ericoides). We hypothesized that variation in browsing would directly affect foodplants, indirectly modify growth and survival of other shrubs and impact habitat needed by herbaceous plants. We found a dynamic web of plant-herbivore and plant-plant interactions that responded rapidly to changes in deer browsing on Ceanothus followed by indirect interactions that continued developing over several years, affecting shrubs, open space, herbaceous plants, and small mammals. Experimental variation in the intensity of deer browsing led to temporal and spatial changes in interactions that produced three different community interaction webs. With deer, community webs were complex, having numerous direct and indirect interactions. Removing deer simplified the community web, changed outcomes of interactions, and reduced open space and herbaceous plants densities. Finally, changes in Ceanothus morphology without deer allowed woodrats to browse these shrubs, with negative impacts on Ceanothus growth and survival. General field observations also showed that all three alternative interaction webs occur naturally at our fieldsite, varying across space and over time. Long-unburned chaparral communities browsed by deer maintain high biological diversity, but maintenance of this diversity involves many key direct and indirect biotic interactions.