title: 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.
authors.affiliation: Department of Ecology & Evolutionary Biology, Princeton University, Princeton, NJ, 08544, USA.|Mpala Research Centre, PO Box 555, Nanyuki, Kenya.
authors.forename: Robert M
authors.initials: RM
authors.lastname: Pringle
authors.affiliation: Mpala Research Centre, PO Box 555, Nanyuki, Kenya.|Department of Natural Resources, Karatina University, Karatina, Kenya.
authors.forename: Duncan M
authors.initials: DM
authors.lastname: Kimuyu
authors.affiliation: Mpala Research Centre, PO Box 555, Nanyuki, Kenya.|Department of Biological Sciences, Goshen College, Goshen, IN, 46526, USA.
authors.forename: Ryan L
authors.initials: RL
authors.lastname: Sensenig
authors.affiliation: Mpala Research Centre, PO Box 555, Nanyuki, Kenya.|Department of Biology, University of Florida, Gainesville, FL, 32611, USA.
authors.forename: Todd M
authors.initials: TM
authors.lastname: Palmer
authors.affiliation: Mpala Research Centre, PO Box 555, Nanyuki, Kenya.|Department of Zoology and Physiology, University of Wyoming, Laramie, WY, 82071, USA.
authors.forename: Corinna
authors.initials: C
authors.lastname: Riginos
authors.affiliation: Mpala Research Centre, PO Box 555, Nanyuki, Kenya.|Department of Wildland Resources and Ecology Center, Utah State University, Logan, UT, 84322, USA.
authors.forename: Kari E
authors.initials: KE
authors.lastname: Veblen
authors.affiliation: Mpala Research Centre, PO Box 555, Nanyuki, Kenya.|Department of Plant Sciences, University of California, Davis, CA, 95616, USA.
authors.forename: Truman P
authors.initials: TP
authors.lastname: Young
mesh_terms: D000818:Animals; D001000:Ants / Q000502:physiology*; D004606:Elephants / Q000502:physiology*; D005390:Fires; D020387:Food Chain; D065948:Grassland; D007630:Kenya; D008116:Lizards / Q000502:physiology*; D014197:Trees / Q000254:growth & development
keywords: browsing megaherbivores; compounded disturbances; context-dependent responses; contingency; controlled burning; ecosystem engineers; habitat facilitation; keystone species; large mammalian herbivores; transiently synergistic indirect effects