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

Meta-analysis shows that wild large herbivores shape ecosystem properties and promote spatial heterogeneity

Megafauna (animals ≥45 kg) have probably shaped the Earth's terrestrial ecosystems for millions of years with pronounced impacts on biogeochemistry, vegetation, ecological communities and evolutionary processes. However, a quantitative global synthesis on the generality of megafauna effects on ecosystems is lacking. Here we conducted a meta-analysis of 297 studies and 5,990 individual observations across six continents to determine how wild herbivorous megafauna influence ecosystem structure, ecological processes and spatial heterogeneity, and whether these impacts depend on body size and environmental factors. Despite large variability in megafauna effects, we show that megafauna significantly alter soil nutrient availability, promote open vegetation structure and reduce the abundance of smaller animals. Other responses (14 out of 26), including, for example, soil carbon, were not significantly affected. Further, megafauna significantly increase ecosystem heterogeneity by affecting spatial heterogeneity in vegetation structure and the abundance and diversity of smaller animals. Given that spatial heterogeneity is considered an important driver of biodiversity across taxonomic groups and scales, these results support the hypothesis that megafauna may promote biodiversity at large scales. Megafauna declined precipitously in diversity and abundance since the late Pleistocene, and our results indicate that their restoration would substantially influence Earth's terrestrial ecosystems.

Functional traits-not nativeness-shape the effects of large mammalian herbivores on plant communities

Large mammalian herbivores (megafauna) have experienced extinctions and declines since prehistory. Introduced megafauna have partly counteracted these losses yet are thought to have unusually negative effects on plants compared with native megafauna. Using a meta-analysis of 3995 plot-scale plant abundance and diversity responses from 221 studies, we found no evidence that megafauna impacts were shaped by nativeness, "invasiveness," "feralness," coevolutionary history, or functional and phylogenetic novelty. Nor was there evidence that introduced megafauna facilitate introduced plants more than native megafauna. Instead, we found strong evidence that functional traits shaped megafauna impacts, with larger-bodied and bulk-feeding megafauna promoting plant diversity. Our work suggests that trait-based ecology provides better insight into interactions between megafauna and plants than do concepts of nativeness.

Weather influences survival probability in two coexisting mammals directly and indirectly via competitive asymmetry

Ecologists have studied the role of interspecific competition in structuring ecological communities for decades. Differential weather effects on animal competitors may be a particularly important factor contributing to the outcome of competitive interactions, though few studies have tested this hypothesis in free-ranging animals. Specifically, weather might influence competitive dynamics by altering competitor densities and/or per-capita competitive effects on demographic vital rates. We used a 9-year data set of marked individuals to test for direct and interactive effects of weather and competitor density on survival probability in two coexisting mammalian congeners: Columbian ground squirrels (Urocitellus columbianus) and northern Idaho ground squirrels (Urocitellus brunneus). Ambient temperature and precipitation influenced survival probability in both species, but the effects of weather differed between the two species. Moreover, density of the larger Columbian ground squirrel negatively impacted survival probability in the smaller northern Idaho ground squirrel (but not vice versa), and the strength of the negative effect was exacerbated by precipitation. That is, cooler, wetter conditions benefited the larger competitor to the detriment of the smaller species. Our results suggest weather-driven environmental variation influences the competitive equilibrium between ecologically similar mammals of differential body size. Whether future climate change leads to the competitive exclusion of either species will likely depend on the mechanism(s) explaining the coexistence of these competing species. Divergent body size and, hence, differences in thermal tolerance and giving up densities offer potential explanations for the weather-dependent competitive asymmetry we documented, especially if the larger species competitively excludes the smaller species from habitat patches of shared preference via interference.

Herbivory limits success of vegetation restoration globally

Restoring vegetation in degraded ecosystems is an increasingly common practice for promoting biodiversity and ecological function, but successful implementation is hampered by an incomplete understanding of the processes that limit restoration success. By synthesizing terrestrial and aquatic studies globally (2594 experimental tests from 610 articles), we reveal substantial herbivore control of vegetation under restoration. Herbivores at restoration sites reduced vegetation abundance more strongly (by 89%, on average) than those at relatively undegraded sites and suppressed, rather than fostered, plant diversity. These effects were particularly pronounced in regions with higher temperatures and lower precipitation. Excluding targeted herbivores temporarily or introducing their predators improved restoration by magnitudes similar to or greater than those achieved by managing plant competition or facilitation. Thus, managing herbivory is a promising strategy for enhancing vegetation restoration efforts.

Quantifying the direct and indirect effects of sika deer (Cervus nippon) on the prevalence of infection with Rickettsia in questing Haemaphysalis megaspinosa: A field experimental study

Sika deer (Cervus nippon) are important hosts for all life stages of Haemaphysalis megaspinosa, a suspected Rickettsia vector. Because some Rickettsia are unlikely to be amplified by deer in Japan, the presence of deer may decrease the prevalence of Rickettsia infection in questing H. megaspinosa. As sika deer decrease vegetation cover and height and thereby indirectly cause changes in the abundance of other hosts, including reservoirs of Rickettsia, the prevalence of Rickettsia infection in questing ticks can also change. We investigated these possible effects of deer on the prevalence of infection with Rickettsia in questing ticks in a field experiment in which deer density was manipulated at three fenced sites: a deer enclosure (Deer-enclosed site); a deer enclosure where deer had been present until 2015 and only indirect effects remained (Indirect effect site); and a deer exclosure in place since 2004 (Deer-exclosed site). Density of questing nymphs and the prevalence of infection with Rickettsia sp. 1 in questing nymphs at each site were compared from 2018 to 2020. The nymph density at the Deer-exclosed site did not significantly differ from that at the Indirect effect site, suggesting that the deer herbivory did not affect the nymph density by reducing vegetation and increasing the abundance of other host mammals. However, the prevalence of infection with Rickettsia sp. 1 in questing nymphs was higher at the Deer-exclosed site than at the Deer-enclosed site, possibly because ticks utilized alternative hosts when deer were absent. The difference in Rickettsia sp. 1 prevalence between the Indirect effect and Deer-exclosed sites was comparable to that between the Indirect effect and Deer-enclosed sites, indicating that the indirect effects of deer were as strong as the direct effects. Examining the indirect effects of ecosystem engineers in the study of tick-borne diseases may be more important than previously recognized.

Impacts of large herbivores on terrestrial ecosystems

Large herbivores play unique ecological roles and are disproportionately imperiled by human activity. As many wild populations dwindle towards extinction, and as interest grows in restoring lost biodiversity, research on large herbivores and their ecological impacts has intensified. Yet, results are often conflicting or contingent on local conditions, and new findings have challenged conventional wisdom, making it hard to discern general principles. Here, we review what is known about the ecosystem impacts of large herbivores globally, identify key uncertainties, and suggest priorities to guide research. Many findings are generalizable across ecosystems: large herbivores consistently exert top-down control of plant demography, species composition, and biomass, thereby suppressing fires and the abundance of smaller animals. Other general patterns do not have clearly defined impacts: large herbivores respond to predation risk but the strength of trophic cascades is variable; large herbivores move vast quantities of seeds and nutrients but with poorly understood effects on vegetation and biogeochemistry. Questions of the greatest relevance for conservation and management are among the least certain, including effects on carbon storage and other ecosystem functions and the ability to predict outcomes of extinctions and reintroductions. A unifying theme is the role of body size in regulating ecological impact. Small herbivores cannot fully substitute for large ones, and large-herbivore species are not functionally redundant - losing any, especially the largest, will alter net impact, helping to explain why livestock are poor surrogates for wild species. We advocate leveraging a broad spectrum of techniques to mechanistically explain how large-herbivore traits and environmental context interactively govern the ecological impacts of these animals.

Tropical savanna small mammals respond to loss of cover following disturbance: A global review of field studies

Small-mammal faunas of tropical savannas consist of endemic assemblages of murid rodents, small marsupials, and insectivores on four continents. Small mammals in tropical savannas are understudied compared to other tropical habitats and other taxonomic groups (e.g., Afrotropical megafauna or Neotropical rainforest mammals). Their importance as prey, ecosystem engineers, disease reservoirs, and declining members of endemic biodiversity in tropical savannas compels us to understand the factors that regulate their abundance and diversity. We reviewed field studies published in the last 35 years that examined, mostly experimentally, the effects of varying three primary endogenous disturbances in tropical savanna ecosystems—fire, large mammalian herbivory (LMH), and drought—on abundance and diversity of non-volant small mammals. These disturbances are most likely to affect habitat structure (cover or concealment), food availability, or both, for ground-dwelling small mammalian herbivores, omnivores, and insectivores. Of 63 studies (included in 55 published papers) meeting these criteria from the Afrotropics, Neotropics, and northern Australia (none was found from southern Asia), 29 studies concluded that small mammals responded (mostly negatively) to a loss of cover (mostly from LMH and fire); four found evidence of increased predation on small mammals in lower-cover treatments (e.g., grazed or burned). Eighteen studies concluded a combination of food- and cover-limitation explained small-mammal responses to endogenous disturbances. Only two studies concluded small-mammal declines in response to habitat-altering disturbance were caused by food limitation and not related to cover reduction. Evidence to date indicates that abundance and richness of small savanna mammals, in general (with important exceptions), is enhanced by vegetative cover (especially tall grass, but sometimes shrub cover) as refugia for these prey species amid a “landscape of fear,” particularly for diurnal, non-cursorial, and non-fossorial species. These species have been called “decreasers” in response to cover reduction, whereas a minority of small-mammal species have been shown to be “increasers” or disturbance-tolerant. Complex relationships between endogenous disturbances and small-mammal food resources are important secondary factors, but only six studies manipulated or measured food resources simultaneous to habitat manipulations. While more such studies are needed, designing effective ones for cryptic consumer communities of omnivorous dietary opportunists is a significant challenge.

Introduced ecological engineers drive behavioral changes of grasshoppers, consequently linking to its abundance in two grassland plant communities

Introduced ecosystem engineers are expected to have extensive ecological impacts on a broad range of resident biota by altering the physical-chemical structure of ecosystems. Livestock that are potentially important introduced ecosystem engineers in grassland systems could create and/or modify habitats for native plant-dwelling insects. Yet, there is little knowledge of how insects respond to engineering effects of introduced livestock. To bridge this gap, we tested how domestic sheep affects the behavior and abundance of a native grasshopper Euchorthippus unicolor at both low (11.8 ± 0.4 plant species per plot) and high (19.8 ± 0.5 plant species per plot) diversity sites. Results found grasshoppers shifted their resting and feeding locations from the upper to the intermediate or low layers of vegetation, and fed on more plants species following livestock engineering effects. In the low plant diversity habitats, grazing caused grasshoppers to increase switching frequency, spend more time searching for host plants, and reduce time spent feeding, but had opposite effects on all the three behaviors in the high-diversity habitats. Moreover, grazing engineering effects on behavioral changes of grasshoppers were potentially related to their abundance. Overall, this study highlights native insect species' behavior and abundance in responses to introduced ecological engineers, and suggests that ecosystem engineers of non-native species have strong and important impacts extending beyond their often most obvious and frequently documented direct ecological effects.

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

Multiple consumer species frequently co-occur in the same landscape and, through effects on surrounding environments, can interact in direct and indirect ways. These interactions can vary in occurrence and importance, and focusing on this variation is critical for understanding the dynamics of interactions among consumers. Large mammalian herbivores are important engineers of ecosystems worldwide, have substantial impacts on vegetation, and can indirectly affect small-mammal populations. However, the degree to which such indirect effects vary within the same system has received minimal attention. We used a 16-year-old exclosure experiment, stratified across a heterogeneous landscape, to evaluate the importance of context-dependent interactions between tule elk (Cervus canadensis nannodes) and small mammals (deer mice [Peromyscus maniculatus], meadow voles [Microtus californicus], and harvest mice [Reithrodontymys megalotis]) in a coastal grassland in California. Effects of elk on voles varied among habitats and seasons: In open grasslands, elk reduced vole numbers during fall 2013 but not summer 2014; in Lupinus-dominated grasslands, elk reduced vole numbers during summer 2014 but not fall 2013; and in Baccharis-dominated grasslands, elk had no effect on vole numbers in either season. Effects of elk on the two mice species also varied among habitats and seasons, but often in different ways from voles and each other. In fall 2013, elk decreased mice abundances in Lupinus-dominated grasslands, but not in Baccharis-dominated or open grasslands. In summer 2014, elk decreased the abundance of harvest mice consistently across habitat types. In contrast, elk increased deer-mice numbers in open grasslands but not other habitats. Within the same heterogenous study system, the influence of elk on small mammals was strongly context-dependent, varying among habitats, mammal species, and seasons. We hypothesize that such variability is common in nature and that failure to consider it may yield inaccurate findings and limit our understanding of interactions among co-occurring consumers.

Interacting effects of wildlife loss and climate on ticks and tick-borne disease

Both large-wildlife loss and climatic changes can independently influence the prevalence and distribution of zoonotic disease. Given growing evidence that wildlife loss often has stronger community-level effects in low-productivity areas, we hypothesized that these perturbations would have interactive effects on disease risk. We experimentally tested this hypothesis by measuring tick abundance and the prevalence of tick-borne pathogens (Coxiella burnetii and Rickettsia spp.) within long-term, size-selective, large-herbivore exclosures replicated across a precipitation gradient in East Africa. Total wildlife exclusion increased total tick abundance by 130% (mesic sites) to 225% (dry, low-productivity sites), demonstrating a significant interaction of defaunation and aridity on tick abundance. When differing degrees of exclusion were tested for a subset of months, total tick abundance increased from 170% (only mega-herbivores excluded) to 360% (all large wildlife excluded). Wildlife exclusion differentially affected the abundance of the three dominant tick species, and this effect varied strongly over time, likely due to differences among species in their host associations, seasonality, and other ecological characteristics. Pathogen prevalence did not differ across wildlife exclusion treatments, rainfall levels, or tick species, suggesting that exposure risk will respond to defaunation and climate change in proportion to total tick abundance. These findings demonstrate interacting effects of defaunation and aridity that increase disease risk, and they highlight the need to incorporate ecological context when predicting effects of wildlife loss on zoonotic disease dynamics.