Savanna tree cover is dynamic due to disturbances such as fire and herbivory. Frequent fires can limit a key demographic transition from sapling to adult height classes in savanna trees. Saplings may be caught in a ‘fire trap’, wherein individuals repeatedly resprout following fire top‐kill events. Saplings only rarely escape the cycle by attaining a fire‐resistant height (e.g. taller than the minimum scorch height) during fire‐free intervals. Large mammalian herbivores also may trap trees in shorter size classes. Browsing herbivores directly limit sapling height, while grazing herbivores such as cattle facilitate sapling growth indirectly via grass removal. Experimental studies investigating how meso‐wildlife, megaherbivores and domestic livestock affect height of resprouts following fire are rare, but necessary for fully understanding how herbivory may reinforce (or counteract) the fire trap. In our study system, interactive fire–herbivore effects on transitions from sapling (<1 m) to adult tree (>1 m) height classes may be further influenced by plant defences, such as symbiotic ants. We used the Kenya Long‐term Exclosure Experiment ( Resprout height of saplings that were short prior to fire (<1 m) was reduced primarily by meso‐wildlife. Negative effects of elephants on post‐fire resprout height increased with pre‐fire tree size, suggesting that resprouts of the tallest trees (with the greatest potential to escape the fire trap cycle) were preferentially browsed and reduced in height by elephants. There were no significant cattle effects.
Whether wild herbivores confer biotic resistance to invasion by exotic plants remains a key question in ecology. There is evidence that wild herbivores can impede invasion by exotic plants, but it is unclear whether and how this generalises across ecosystems with varying wild herbivore diversity and functional groups of plants, particularly over long‐term (decadal) time frames. Using data from three long‐term (13‐ to 26‐year) exclosure experiments in central Kenya, we tested the effects of wild herbivores on the density of exotic invasive cacti, At the landscape level, modelled
- NSF-PAR ID:
- 10389956
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 111
- Issue:
- 1
- ISSN:
- 0022-0477
- Page Range / eLocation ID:
- p. 33-44
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract KLEE ) to investigate how post‐fire resprout size of a widespread monodominant East African tree,Acacia drepanolobium was influenced by (a) herbivory by different combinations of cattle, meso‐wildlife (15–1,000 kg) and megaherbivores (>1,000 kg) and (b) the presence of acacia–ant mutualists that confer tree defences. We sampled height, stem length and ant occupancy of resprouts exposed to different herbivore combinations before and after controlled burns.Synthesis . We provide experimental evidence for two potential pathways through which large herbivores exert control over sapling escape from the fire trap: (a) post‐fire meso‐wildlife browsing of short (<1 m) resprouts and (b) elephant browsing of the largest size class of resprouts, which would otherwise be most likely to escape the fire trap. -
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Abstract Cattle and other livestock graze more than a quarter of the world's terrestrial area and are widely regarded to be drivers of global biodiversity declines. Studies often compare the effects of livestock presence/absence but, to our knowledge, no studies have tested for interactive effects between large wild herbivores and livestock at varying stocking rates on small‐bodied wild vertebrates.
We investigated the effects of cattle stocking rates (none/moderate/high) on the diversity of wildlife 0.05–1,000 kg using camera traps at a long‐term exclosure experiment within a semi‐arid savanna ecosystem in central Kenya. In addition, by selectively excluding wild ‘mesoherbivores’ (50–1,000 kg) and ‘megaherbivores’ (>1,000 kg; elephant and giraffe), we tested whether the presence of these two wild herbivore guilds (collectively, ‘larger wild herbivores’) mediates the effect of cattle stocking rate on habitat use and diversity of ‘smaller wildlife’ (mammals ranging between 10 and 70 cm shoulder height and birds).
Our results show that cattle enhance alpha diversity of smaller wildlife (with or without larger wild herbivore presence) and of all wildlife 0.05–1,000 kg (with or without megaherbivore presence), by altering vegetation structure. However, for smaller wildlife, this effect is less pronounced in the presence of larger wild herbivores, which also shorten grass. In the absence of cattle, mesoherbivore‐accessible sites showed higher alpha diversity of smaller wildlife than sites excluding mesoherbivores.
Smaller wildlife habitat use was increased by high cattle stocking rates and wild mesoherbivores more in the presence of the other.
Synthesis and applications . Our findings imply that grazing, whether by livestock or wildlife, can enhance local savanna wildlife diversity. The biodiversity benefits of localised increases in herbivory are likely to be due to shortened grass and associated visibility improvements (for predator avoidance/foraging). This suggests that land managers can increase local biodiversity by shortening grass, with wild or domestic herbivores (or both), at least in patches within a taller grass matrix. -
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Abstract The extinction of 80% of megaherbivore (>1,000 kg) species towards the end of the Pleistocene altered vegetation structure, fire dynamics and nutrient cycling world‐wide. Ecologists have proposed (re)introducing megaherbivores or their ecological analogues to restore lost ecosystem functions and reinforce extant but declining megaherbivore populations. However, the effects of megaherbivores on smaller herbivores are poorly understood.
We used long‐term exclusion experiments and multispecies hierarchical models fitted to dung counts to test (a) the effect of megaherbivores (elephant and giraffe) on the occurrence (dung presence) and use intensity (dung pile density) of mesoherbivores (2–1,000 kg), and (b) the extent to which the responses of each mesoherbivore species was predictable based on their traits (diet and shoulder height) and phylogenetic relatedness.
Megaherbivores increased the predicted occurrence and use intensity of zebras but reduced the occurrence and use intensity of several other mesoherbivore species. The negative effect of megaherbivores on mesoherbivore occurrence was stronger for shorter species, regardless of diet or relatedness.
Megaherbivores substantially reduced the expected total use intensity (i.e. cumulative dung density of all species) of mesoherbivores, but only minimally reduced the expected species richness (i.e. cumulative predicted occurrence probabilities of all species) of mesoherbivores (by <1 species).
Simulated extirpation of megaherbivores altered use intensity by mesoherbivores, which should be considered during (re)introductions of megaherbivores or their ecological proxies. Species' traits (in this case shoulder height) may be more reliable predictors of mesoherbivores' responses to megaherbivores than phylogenetic relatedness, and may be useful for predicting responses of data‐limited species.
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Abstract Wild large herbivores are declining worldwide. Despite extensive use of exclosure experiments to investigate herbivore impacts, there is little consensus on the effects of wild large herbivores on ecosystem function.
Of the ecosystem functions likely impacted, we reviewed the five most‐studied in exclosure experiments: ecosystem resilience/resistance to disturbance, nutrient cycling, carbon cycling, plant regeneration, and primary productivity.
Experimental data on large wild herbivores' effects on ecosystem functions were predominately derived from temperate grasslands (50% grasslands, 75% temperate zones). Additionally, data were from experiments that may not be of adequate size (median size 400 m2despite excluding all experiments below 25 m2) or duration (median duration 6 years) to capture ecosystem‐scale responses to these low‐density and wide‐ranging taxa.
Wild herbivore removal frequently impacted ecosystem functions; for example, net carbon uptake increased by three times in some instances. However, the magnitude and direction of effects, even within a single function, were highly variable.
A focus on carbon cycling highlighted challenges in interpreting effects on a single function. While the effect of large herbivore exclusion on carbon cycling was slightly positive when its components (e.g. pools vs. fluxes of carbon) were aggregated, effects on individual components were variable and sometimes opposed.
Given modern declines in large wild herbivores, it is critical to understand their effects on ecosystem function. However, this synthesis highlights strong variability in direction, magnitude, and modifiers of these effects. Some variation is likely due to disparity in what components are used to describe a given function. For example, for the carbon cycle we identified eight distinctly meaningful components, which are not easily combined yet are potentially misrepresentative of the larger cycle when considered alone. However, much of the observed difference in responses likely reflects real ecological variability across complex systems.
To move towards a general predictive framework we must identify where variation in effect is due to methodological differences and where due to ecosystem context. Two critical steps forward are (a) additional quantitative synthetic analyses of large herbivores' effects on individual functions, and (b) improved, increased systematic exclosure research focusing on effects of large herbivores' exclusion on functions.
A free
Plain Language Summary can be found within the Supporting Information of this article. -
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Abstract Herbivores form an important link in the transfer of energy within a food web and are strongly influenced by bottom‐up trophic cascades. Current hypotheses suggest that herbivore consumption and impact on plants should scale positively with plant resource availability. However, depending on the effect of resources on plant quantity and quality, herbivore impact may vary with different types of resources.
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Our data supported the hypothesis that herbivore impact is controlled by plant quality, in this case driven by plant P, as herbivore effects on biomass decreased with higher rainfall but increased with greater plant P, but not N content. To our knowledge, this is the first experimental study to indicate that wild mammalian herbivory is associated with P availability rather than N.
Synthesis . Our results suggest that P, in addition to water and N, may play a more important role in driving trophic interactions in terrestrial systems than previously realized. Given the uncertainties in rainfall due to climate change and increasing anthropogenic manipulations of global N and P cycles, our findings emphasize the need to consider multiple resources for understanding how trophic interactions might be influenced by environmental variables.
