Synergistic effects of fire and elephants on arboreal animals in an African savanna

Abstract

Disturbance is a crucial determinant of animal abundance, distribution and community structure in many ecosystems, but the ways in which multiple disturbance types interact remain poorly understood. The effects of multiple-disturbance interactions can be additive, subadditive or super-additive (synergistic). Synergistic effects in particular can accelerate ecological change; thus, characterizing such synergies, the conditions under which they arise, and how long they persist has been identified as a major goal of ecology. We factorially manipulated two principal sources of disturbance in African savannas, fire and elephants, and measured their independent and interactive effects on the numerically dominant vertebrate (the arboreal gekkonid lizard Lygodactylus keniensis) and invertebrate (a guild of symbiotic Acacia ants) animal species in a semi-arid Kenyan savanna. Elephant exclusion alone (minus fire) had negligible effects on gecko density. Fire alone (minus elephants) had negligible effects on gecko density after 4 months, but increased gecko density twofold after 16 months, likely because the decay of fire-damaged woody biomass created refuges and nest sites for geckos. In the presence of elephants, fire increased gecko density nearly threefold within 4 months of the experimental burn; this occurred because fire increased the incidence of elephant damage to trees, which in turn improved microhabitat quality for geckos. However, this synergistic positive effect of fire and elephants attenuated over the ensuing year, such that only the main effect of fire was evident after 16 months. Fire also altered the structure of symbiotic plant-ant assemblages occupying the dominant tree species (Acacia drepanolobium); this influenced gecko habitat selection but did not explain the synergistic effect of fire and elephants. However, fire-driven shifts in plant-ant occupancy may have indirectly mediated this effect by increasing trees' susceptibility to elephant damage. Our findings confirm the importance of fire × elephant interactions in structuring arboreal wildlife populations. Where habitat modification by megaherbivores facilitates co-occurring species, fire may amplify these effects in the short term by increasing the frequency or intensity of herbivory, leading to synergy. In the longer term, tree mortality due to both top kill by fire and toppling by large herbivores may reduce overall microhabitat availability, eliminating the synergy.

Climatic variation modulates the indirect effects of large herbivores on small‐mammal habitat use

“…), and that also provide refuge for small animals (Pringle ; Pringle et al . ). This protected and comparatively low‐risk forage base is likely attractive to small mammals, which might strengthen the positive association between small‐mammal activity and tree canopies in the presence of elephants and other LMH; when high‐quality forage is abundant and more evenly distributed (i.e., during the wet season and in the absence of LMH; Fig.…”

“…The magnitude of LMH impacts on ecological properties and processes, however, is variable in space and time, and the factors contributing to this contingency remain incompletely understood (Pringle et al . , ; Goheen et al . ; Louthan et al .…”

Does primary productivity modulate the indirect effects of large herbivores? A global meta‐analysis

“…First, LMH may physically engineer habitat for specialist consumers; for example, frogs live in Amazonian peccary wallows (Beck, Thebpanya & Filiaggi ) and Sri Lankan elephant dung (Campos‐Arceiz ), while geckos and other reptiles find shelter among elephant‐damaged trees in African savannas (Pringle ; Nasseri, McBrayer & Schulte ; Pringle et al . ). Secondly, fertilization by LMH excreta can yield bottom‐up effects on consumer assemblages (e.g.…”

“…creation of micro‐habitat via ecosystem engineering; Pringle ; Pringle et al . ) effects on co‐occurring taxa. Several reviews of HIII have also been published (Suominen & Danell ; Pringle et al .…”

Woody plant biomass and carbon exchange depend on elephant‐fire interactions across a productivity gradient in African savanna

“…First, experimental studies have not simultaneously manipulated elephants and fire (but see Pringle et al . ()), while observational studies are potentially confounded by covariation and feedbacks among plants, herbivores and fire (Dublin, Sinclair & McGlade ; Barnes ; Holdo ; Staver et al . ; Shannon et al .…”