DOI: 10.5061/dryad.x95x69pdr
title: Assessing the effects of elephant foraging on the structure and diversity of an Afrotropical forest

description: African forest elephants (Loxodonta cyclotis) are ecosystem engineers that
 browse and damage large quantities of vegetation during their foraging and
 movement. Though elephant trail networks and clearings are conspicuous
 features of many African forests, the consequences of elephant foraging
 for forest structure and diversity are poorly documented. In this study in
 northeastern Gabon, we compare stem size, stem density, proportional
 damage, species diversity, and species relative abundance of seedlings and
 saplings in the vicinity of seven tree species that produce
 elephant-preferred fruits (“elephant trees”) relative to control trees
 that do not. Across 34 survey trees, with a combined census area of 2.04
 ha, we recorded data on 26,128 woody stems in three sizes classes.
 Compared to control trees, the area around elephant trees had: a) a
 significantly greater proportion of damaged seedlings and a marginally
 greater proportion of damaged saplings (with 82% and 24% greater odds of
 damage, respectively); b) no significant difference in stem density or
 species diversity; and c) a significantly greater relative abundance of
 seedlings of elephant tree species. Increasing distance away from focal
 elephant trees was associated with significantly reduced sapling stem
 damage, significantly increased sapling stem density, and significantly
 increased sapling species diversity. Considered in sum, our results
 suggest that elephants can affect the structure and diversity of
 Afrotropical forests through their foraging activities, with some
 variation based on location and plant size class. Developing a more
 complete understanding of elephants’ ecological effects will require
 continued research, ideally with manipulative experiments.
description: <i>Study Area and Species Selection</i>
             We conducted this study in the Ogooué-Ivindo province
 of northeastern Gabon.  The region is dominated by lowland forest, and
 receives approximately 1700 mm of rain annually, with two rainy seasons
 (September-December and March-June).  The study area includes the northern
 section of Ivindo National Park and the Ipassa Field Station, located
 within the park’s buffer zone.             We selected
 seven tree species that produce fruits that have been observed with high
 frequency in elephant dung to serve as our focal elephant trees:
 <i>Annonidium mannii</i>, <i>Baillonella
 toxisperma</i>, <i>Dacryodes buettneri</i>,
 <i>Gambeya lacourtiana</i>, <i>Klainedoxa
 gabonensis</i>, <i>Mammea africana</i>, and
 <i>Panda oleosa</i> (White <i>et al.</i> 1993,
 Poulsen unpublished data).  As many of these fruits are also consumed by
 apes and other mammals (White <i>et al.</i> 1993), we selected
 individual adult trees of each species that had visible impacts of
 elephant browsing around their trunks, in order to maximize the likelihood
 that the observed effects were caused by elephants rather than by other
 species.  In total, we conducted plant surveys around three individuals of
 each elephant tree species (21 total trees), and 13 control trees with the
 following criteria: a) located within 200 m of one of the elephant trees,
 b) had similar canopy-level height as the elephant trees, and c) did not
 produce fruits commonly consumed by elephants.  The control trees included
 two <i>Alstonia boonei</i>, two <i>Celtis
 tessmannii</i>, three <i>Dialium pachyphyllum</i>, one
 <i>Lophira alata</i>, two <i>Newtonia sp</i>., and
 three <i>Pterocarpus soyauxii</i>.
 <i>Survey Methods</i>            
 For each of the 34 survey trees, we recorded diameter at breast height
 (DBH, cm) and measured height (m) using a hypsometer. We also estimated
 canopy size by measuring the distance from the trunk to the end of the
 canopy at eight locations, and then calculating the corresponding area. At
 the base of each tree, we established three 5 x 40 m plots radiating out
 along the 0°, 120°, and 240° axes.  We marked each plot with ribbon at the
 corners, and delimitated them with Topofil thread so that the sides of the
 plot were clearly distinguishable.  We then subdivided each plot into
 eight 5 x 5 m quadrats.             For each woody stem
 within the plots, we identified the plant to genus or species (see below)
 and recorded it as one of three size class categories for further
 measurement: seedling (0.5 m – 2 m in height), sapling (&gt;2 m in
 height but &lt; 6 cm DBH), or adult (≥6 cm DBH). We excluded lianas
 from our study with the exception of those within the seedling size class,
 which had not yet exhibited the typical climbing liana growth form and
 thus were as susceptible to elephant trampling damage as were tree
 seedlings. For seedlings, we measured the diameter of the plant at 5 cm
 and measured height from the ground to the terminal bud. For saplings and
 adults, we recorded only DBH. In the 0° plot, we measured all stems of all
 three size classes.  In the 120° and 240° plots, we measured all saplings
 and adult trees, but only counted (and identified) the seedlings. In
 addition to species identification and stem measurement, we assessed all
 stems for the presence of damage, including breakages and irregular
 regrowth that may have occurred after being bent or snapped.  All
 taxonomic identifications were conducted in the field by the same local
 botanist, in order to maintain consistency across survey trees. Plants
 that could not be identified to genus or species were assigned a unique
 code; all individuals that apparently belonged to the same unknown
 taxonomic group were given the same code, so that classification could be
 as specific as possible even when specific taxonomic identification was
 not possible. <i>Statistical
 Analysis</i> We used linear mixed (LMM) or
 generalized linear mixed (GLMM) models to test our hypotheses, including
 the focal tree species as a random effect in all models. For LMMs, we
 evaluated model fit by examining residuals and selected the best model
 based on the coefficient of determination, R<sup>2</sup>.  For
 GLMMs, we employed the negative binomial distribution when the data were
 strongly overdispersed and selected among models using the Akaike
 Information Criterion (AIC).  We performed statistical analyses in R 3.5
 (R Development Core Team 2018) using the lme4 package (Bates <i>et
 al.</i> 2015), and followed the general recommendations for GLMMs
 outlined by Bolker et al. (2009). To test whether elephant activity
 affects stem damage (H1) and density (H2), we modeled the proportion of
 elephant-damaged seedlings and saplings (binomial distribution) and the
 number of seedlings and saplings (negative binomial distribution) as
 functions of tree type (elephant tree vs. control tree) and distance from
 the tree (by quadrat), with species of the focal tree as a random
 effect. To test
 whether elephant activity affects species diversity (H3), we computed
 Shannon-Wiener diversity indices (Magurran 2004) for seedlings and
 saplings around elephant trees and control trees.  We then assessed the
 effect of tree type (elephant tree vs. control tree) on diversity for both
 size classes, with species of the focal tree as a random effect. We did
 not account for differing levels of taxonomic identification in this
 diversity analysis, as even unidentified stems were given a unique
 identifying code in the field, and were considered in the analysis as
 unique “species.” To test whether elephant activity
 increases the proportion of elephant-dispersed species (H4), we modeled
 the proportion of seedlings and saplings of elephant tree species
 (binomial distribution) as a function of tree type (elephant tree vs.
 control tree) and distance from the tree (by quadrat), with species of the
 focal tree as a random effect.  Although there are many other species that
 can be prominent in elephant diets, we considered only the relative
 abundance of the seven focal elephant tree species in this
 analysis.