How selective are elephants as agents of forest tree damage in Bwindi Impenetrable National Park, Uganda?

Abstract

Elephants are locally concentrated in Bwindi Impenetrable National Park. Vegetation damage attributable to elephants appears to be increasing and may result in the modification of the forest. We examined the implied selectivity of stem damage due to elephants. We followed 26.84 km of recent elephant trails and used 122 plots to document tree damage in relation to species, stem sizes and locations. Of 897 trees (DBH ≥2 cm), 542 (60.4%) were intact, 22 (2.5%) debarked, 274 (30.5%) toppled and 172 (19.2%) had broken branches. Small trees were more likely to be pushed over or have their branches broken, whereas large trees were more commonly debarked. The species most frequently selected for damage included mid‐successional species such as Newtonia buchananii, Myrianthus holstii and Chrysophyllum albidum. These species may be vulnerable to increasing elephant numbers. Our analyses using general linear models indicate that elephants are selective concerning where, how and what tree stems they damage. We found a higher incidence of elephant damage per‐tree stem in open areas than in more closed areas, suggesting feedback in which elephants maintain open habitats that may be conducive for other species such as mountain gorillas. More work is needed to better determine how changing elephant numbers may influence Bwindi's conservation values.

Damage to artificial seedlings across a disturbed Afromontane forest landscape

“…Damage intensity also decreased with distance from water, possibly due to higher animal activity near water sources. For example, such effects are seen with elephants, which are known to stay close to the Mubwindi Swamp during droughts (Babaasa 2000, Ssali et al., ). Seedling damage also increased with progression from hilltop sites down to lower topographic settings, again reflecting higher animal abundance in these lower‐slope habitats.…”

“…Bwindi's larger herbivores include elephant Loxodonta africana Blumenbach, mountain gorilla Gorilla beringei beringei Matschie, olive baboon Papio anubis Lesson, L'hoest's monkey Cercopithecus l'hoesti P. Sclater, bush pig Potamochoerus larvatus F. Cuvier, black‐fronted duiker Cephalophus nigrifrons Thomas, and yellow‐backed duiker Cephalophus silvicultor Afzelius (Mugerwa et al., ; Olupot & Sheil, ; Ssali et al., ).…”

“…(b) Human activities: We expected more frequent damage farther from roads and from the park edge since larger animals often tend to avoid these features (Mugerwa, Sheil, Ssekiranda, Van Heist & Ezuma, ; Olupot & Sheil, ). (c) Slope position: We expected less damage where slopes impede ready access to larger mammals (Ssali et al., ).…”

“…The intensity of damage likely varies across the landscape, with more animal‐related damage where there are more animals (e.g., near water and in valley bottoms) and more debris‐related damage in locations with more plant debris (in regrowth vegetation). We hypothesized that the frequency of damage is influenced by multiple factors, including the following: (a) Canopy cover: We expected that damage to artificial seedlings would be more frequent in open areas where ground‐feeding animals tend to be more abundant than in the more closed forest (Laws, Parker & JohnsTONE, ; Ssali, Sheil & Nkurunungi, ). (b) Human activities: We expected more frequent damage farther from roads and from the park edge since larger animals often tend to avoid these features (Mugerwa, Sheil, Ssekiranda, Van Heist & Ezuma, ; Olupot & Sheil, ).…”

Assessing the role of megafauna in tropical forest ecosystems and biogeochemical cycles – the potential of vegetation models

“…In particular, there is a research gap concerning the physical impacts (ecosystem engineering) of megafauna in tropical forests. Studies of both African forest and Asian elephants Elephas maximus, and of Asian water buffalo Bubalus bubalis have shown that megafauna can influence plant composition and diversity, and forest structure (Braithwaite et al 1984, Pradhan et al 2007, Ssali et al 2013, Terborgh et al 2016, although, no effects on species composition have been reported in some cases (Omeja et al 2014). Forest elephants can also inhibit forest regeneration and increase understory openness.…”