Assessing the effects of elephant foraging on the structure and diversity of an Afrotropical forest

Abstract

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 with control trees, the area around elephant trees had the following: (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.Abstract in French is available with online material. K E Y W O R D S diversity, Elephant, Gabon, herbivory, Loxodonta cyclotis, physical damage, structure, tropical forest | 503 ROSIN et al.

The challenging coexistence of forest elephants Loxodonta cyclotis and timber concessions in central Africa

“…Although forest elephants feed on hundreds of tree and liana species, a relatively low number of them constitute the majority of their diet (Merz 1981, Blake 2002). By browsing on small trees and debarking larger ones, forest elephants shape landscapes (Rosin et al 2020), both in the interior and at the edge of forests (Cardoso et al 2019). For instance, a study conducted in Mount Cameroon showed reduced tree diversity and shorter trees, but with larger diameter at breast height, in areas disturbed by forest elephants (Maicher et al 2020).…”

“…Although forest elephants help the regeneration of many tree species, their feeding behaviour and large size also cause serious damage to the vegetation. They trample seedlings (Short 1981, Piiroinen et al 2017, Rosin et al 2020), snap saplings (Omeja et al 2014, Terborgh et al 2015), break branches (Wing & Buss 1970), uproot trees, and strip their bark (Wing & Buss 1970, Blake 2002). Besides fruits and grass, terminal twigs, leaves, and bark are dominant in the forest elephant's diet.…”

Vertical stratification and seasonal changes of the avian community in Mount Cameroon lowland rainforest

“…A relatively abundant population of forest elephants thrive in Mount Cameroon forest and their impact on vegetation structure is clear. Forest sites regularly visited by elephants are subjected to disturbance (Amahowé et al, 2012; Rosin et al, 2020) and are characterised by a dense herbaceous understorey (Carroll, 1988). This also explains the character of the vertical profile, with high vegetation cover in the understorey, gradually decreasing towards the canopy.…”

Fruit trees drive small‐scale movement of elephants in Kibale National Park, Uganda

“…These well‐used trails likely incur heavier and more consistent damage to vegetation, which may offset elephant seed dispersal. While studies have attempted to determine the effects of elephants on forest structure and composition (i.e., Rosin et al, 2020), it is difficult to separate elephants’ impacts from one another and from other confounding factors.…”

“…Yet, elephants are prolific seed dispersers, with areas of high elephant activity usually showing greater seed deposition (Campos‐Arceiz & Blake, 2011; Powell, 1997). Dispersal by elephants is also known to improve the germination and recruitment of seeds (Chapman et al, 1992), though seedlings near elephant fruit trees are more likely to be damaged by elephants which may impact their survival (Rosin et al, 2020). Given that trails can be several decades old (Agnew, 1966), seed dispersal by elephants and other species likely leads to some of the aggregation of trees around trails.…”