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

Abstract

Large mammalian herbivores (LMH) strongly shape the composition and architecture of plant communities. A growing literature shows that negative direct effects of LMH on vegetation frequently propagate to suppress the abundance of smaller consumers. Indirect effects of LMH on the behaviour of these consumers, however, have received comparatively little attention despite their potential ecological significance. We sought to understand (i) how LMH indirectly shape small-mammal habitat use by altering the density and distribution of understorey plants; (ii) how these effects vary with climatic context (here, seasonality in rainfall); and (iii) the extent to which behavioural responses of small mammals are contingent upon small-mammal density. We tested the effects of a diverse LMH community on small-mammal habitat use using 4 years of spatially explicit small-mammal trapping and vegetation data from the UHURU Experiment, a replicated set of LMH exclosures in semi-arid Kenyan savanna. Small-mammal habitat use was positively associated with tree density and negatively associated with bare (unvegetated) patches in all plots and seasons. In the presence of LMH, and especially during the dry season, small mammals consistently selected tree cover and avoided bare patches. In contrast, when LMH were excluded, small mammals were weakly associated with tree cover and did not avoid bare patches as strongly. These behavioural responses of small mammals were largely unaffected by changes in small-mammal density associated with LMH exclusion. Our results show that LMH indirectly affect small-mammal behaviour, and that these effects are influenced by climate and can arise via density-independent mechanisms. This raises the possibility that anthropogenic LMH declines might interact with changing patterns of rainfall to alter small-mammal distribution and behaviour, independent of numerical responses by small mammals to these perturbations. For example, increased rainfall in East Africa (as predicted in many recent climate-model simulations) may relax constraints on small-mammal distribution where LMH are rare or absent, whereas increased aridity and/or drought frequency may tighten them.

Conservation lessons from large‐mammal manipulations in East African savannas: the KLEE, UHURU, and GLADE experiments

“…Keesing showed that the exclusion of large herbivores in KLEE led to a rapid and sustained doubling of small‐mammal abundance. Subsequent research in UHURU has revealed a pattern that is congruent with, and of similar magnitude to, Keesing's results . These effects appear to stem chiefly from exploitation competition: the release of herbivorous and omnivorous rodents from competition for food 59 —especially seeds, which are the primary food for the dominant small‐mammal species, and the production of which is suppressed by ungulate herbivory .…”

Searching for cover: soil enrichment and herbivore exclusion, not fire, enhance African savanna small‐mammal abundance

“…It is also possible that S. mearnsi could be drawn to the greater availability of seeds of forbs and particularly Acacia spp., living trees of which were denser on the unburned plots a few years after the fire (Long et al. ), although body mass was the same for this species on burned as unburned patches.…”

“…, Long et al. ). Similarly, enhanced insect availability and enhanced visual cover may explain why 14 of 15 captures of the diurnal insectivore E. rufescens were inside or adjacent to exclosures in red sands.…”

Indirect effects of a large mammalian herbivore on small mammal populations: Context‐dependent variation across habitat types, mammal species, and seasons

“…The authors also found a significant rainfall × land‐use interaction, with rainfall affecting the responses of small mammal in agricultural and pastoral areas but not in areas without agriculture or livestock. Long et al () detected an interaction between large herbivores and rainfall on habitat selection by small mammals in Kenya. That study pooled captures of three species of small mammals, from both bare ground and tree‐covered patches, in plots that were either accessible to large herbivores or from which large herbivores had been excluded.…”

“…Biotic factors, such as the introduction or range expansion of a non-native plant species, can also alter the indirect interactions between consumers in the invaded areas (Pearson, 2009). The consequences of indirect interactions can also depend on climatic conditions that shift over time (Brown et al, 2001;Heske, Brown, & Mistry, 1994;Long, Wambua, Goheen, Palmer, & Pringle, 2017). This variability in outcomes, combined with the rapidly changing global climate (Rosenzweig et al, 2008) and the increasing prevalence of exotic species in ecosystems (Hellmann, Byers, Bierwagen, & Dukes, 2008;Rosenstock, 1996), make long-term experiments of critical importance for examining context-dependent interactions among consumers.…”