DOI: 10.7291/d1mm59
title: Herbivory mediates direct and indirect interactions in long-unburned chaparral

description: 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 &lt;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.
description: We worked in a 242 ha reserve on the former U.S. Fort Ord Army
 base, now part of the University of California Natural Reserve System. The
 on-going herbivore exclusion experiment regulated deer and rabbit access
 to chaparral plants. The experiment had open plots as controls and two
 types of caged plots. Control plots were uncaged, with natural levels of
 browsing by deer and rabbits (+D+R). One cage type prevented deer from
 browsing in a plot but allowed access to rabbits (-D+R), and the other
 cage type excluded both deer and rabbits (-D-R). Deer were excluded by 2 m
 high fences, with 20 cm x 10 cm mesh. Rabbits were excluded by 1.5 cm mesh
 (aviary wire) on the lower 1 m of half of the exclosures. Each plot was 9
 m2. Four experimental blocks, each with a control and one of each cage
 type, were randomly located at each of two randomly selected sites ~1 km
 apart (sites called F and N in this paper are designated F4, N1 and N2 in
 the FONR mapping system). In total, there were 24 plots among eight blocks
 over two sites. Baseline data collection began immediately after the cages
 were completed in August 1996. We censused and measured all shrubs in late
 summer or early autumn, annually from 1996-2000 and 2002-2007, and in
 2011. We also recorded browsing intensity and shrub mortality during
 censuses as well as each spring and sporadically at other times of the
 year. Observations variously related to individual-,
 species-, or plot-level response metrics. Univariate and multivariate
 response variables were modeled separately as they generally corresponded
 to different time periods. Unless otherwise noted, we assessed trends only
 over years 1997-2007, since we did not measure plants between 2007 and
 2011. We modeled individual-, species-, and plot-level
 response variables with generalized linear mixed models (GLMMs) and
 explored directional community composition changes among the three
 dominant shrub species with non-metric multidimensional scaling (NMDS)
 ordination. The initial experimental design established four blocks, each
 nested in two spatially distinct random sites; however, sites explained
 zero variation so were removed from model random structures. We modeled
 GLMMs in PROC GLIMMIX in SAS® 9.4 (SAS Institute Inc. 2013) and NMDS with
 package vegan 2.4-4 in R 3.4 (Moen et al. 1993, Oksanen et al. 2017, R
 Core Team 2017).