Synthesizing the effects of large, wild herbivore exclusion on ecosystem function

It is well known that herbivores impact plant communities globally. However, the decade-long effects of herbivores on plant communities remain unclear, due to limited long-term experiments. Using 46-year large herbivore exclosures and 22-year small herbivore exclosures in the salt marsh of the island of Schiermonnikoog, I evaluated the long-term effects of large (cattle) and small herbivores (hares and geese) on plant diversity. I found that long-term management is needed for conserving biodiversity. In addition, using low to moderate densities of large domestic herbivores to conserve plant diversity is sustainable for at least 46 years. However, the effects of large herbivores on biodiversity were more attributed to the non-trophic effects (e.g. trampling, and deposition of urine and dung), particular in the long-term (23 years after). Small wild herbivores (hares and geese) can also slow down plant species decline (for at least 22 years) but only at the early successional stage where their abundance was high. A diverse herbivore community (hares and geese relative to hares alone) had a more positive control of plant communities. On top of that, these ecologically important small herbivores may also have substantial evolutionary effects such as modify local-scale spatial genetic structure of a dominant plant species. Results emphasize the need for the conservation and re-introduction of herbivores, domestic or wild, to sustain long-term grassland plant diversity.

The savannas of Africa are occupied by the earth’s richest and most spectacular large mammal fauna. This fauna was even richer in the distant past and it is reasonable to expect that these large animals have long influenced the plants on which they prey and through them the form of the savannas. This long evolutionary history, the recent widespread disturbance by man and his livestock and the persistence of savannas on other continents without a similar large mammal fauna make it difficult to determine unequivocally the degree to which large wild herbivores have influenced the structure of the African savannas. There is much observational information but, apart from that on grazing systems for livestock, there is little experimentally supported evidence on the effects of large herbivores on savanna structure. The degree to which large herbivore influence is recognized rests largely on the scale and perspective with which one examines the savannas. On a continental scale large herbivore effects pale into insignificance beside that of say climate, but within a particular ecosystem the large herbivores may function as a determinant of savanna structure just as surely as fire, or frost, or man.

The UN declaration of the Decade of Ecosystem Restoration 2021–2030 emphasizes the need for effective measures to restore ecosystems and safeguard biodiversity. Large herbivores regulate many ecosystem processes and functions; yet, their potential as a nature‐based solution to buffer against long‐term temporal declines in biodiversity associated with global change and restore diversity in secondary forests remains unknown. By means of an exclusion experiment, we tested experimentally the buffering effects of large wild herbivores to avert against long‐term biodiversity collapse in old‐growth and secondary tropical forests in the Atlantic Forest of Brazil where sapling abundance and species richness declined c. 20% over the course of 10 years. The experiment comprised 50 large herbivore exclosure‐open control plot pairs (25 at the old‐growth forest and 25 at the secondary forest), where 2 m2 were monitored in every plot during a 10‐year period. Large herbivores were able to decelerate diversity declines and compositional change in the species‐rich old‐growth forest, but only decelerated compositional change in the secondary forest. In contrast, declines in species richness and abundance were unaffected by large herbivores on either forest. The buffering effects of large herbivores were strongly nonlinear and contingent on the initial level of diversity at the patch scale: highly diverse communities suffered the strongest collapse in the absence of large herbivores. Thus, larger buffering effects of large herbivores on the old‐growth forest are the logical consequence of large herbivores buffering the many high diversity plant communities found in this forest. Conversely, as the secondary forest held fewer high diversity patches, buffering effects on the secondary forest were weak. Synthesis and applications. Our study indicates that large herbivores have moderate yet critical effects on slowing down community change and diversity loss of highly diverse plant communities, thus suggesting that the conservation of (and potentially trophic rewilding with) large herbivores is a fundamental nature‐based solution for averting the global collapse of the strongholds of biodiversity. Its buffering effects on biodiversity loss operate at very small spatial scales, are likely contingent on successional stage and are most effective in old‐growth or high diversity secondary forests.

Large herbivore communities are changing globally, with populations of wild herbivores generally declining while domestic herbivore populations are increasing, influencing ecosystem function along with the impacts of climate change. Manipulative experiments have rarely captured the interaction between patterns of large herbivore assemblage change and climatic conditions. This interaction may affect the functional traits and functional diversity of herbaceous communities; this requires investigation, as these metrics have been useful proxies for ecosystem function. We used a large herbivore exclosure experiment replicated along a topo-climatic gradient to explore the interaction between climate and herbivore assemblage on community-level functional traits and the functional diversity of herbaceous plant understories. Our findings demonstrate interacting effects between large herbivore assemblages and climate. We found a shift from drought-tolerant traits to drought-avoidant traits with increasing aridity, specifically with regard to plant leaf area and specific leaf area. We also determined that plant community responses to grazing changed from an herbivore avoidance strategy at drier sites to a more herbivore-tolerant strategy at wetter sites. We observed that the effects of herbivores on community-level traits can sometimes counteract those of climate. Finally, we found that cattle and large wild herbivores can differ in the magnitude and direction of effects on functional traits and diversity.

The perpetuation of tree populations is dependent on successful seedling establishment, which is in turn controlled by biotic and climatic factors. California’s endemic oak species face limited recruitment and shrinking ranges due to anthropogenic climate change and land use, both of which can negatively impact germination and seedling survival. In this study, we investigated Quercus seed germination and seedling establishment under three large herbivore regimes (no large herbivores, wild herbivores, and both wild herbivores and cattle) across three sites representing present, near-future, and far-future projections for climate in California’s montane oak woodlands. Seed germination and living seedling abundance were highest in the intermediate and mesic climate levels, where total exclosures resulted in the best outcomes for seedlings. In contrast, in the arid level, the best large herbivore regime for establishment consisted of wild herbivores and no cattle. Seedling mortality was highest in the most arid climate level, and within that level, mortality was reduced by over 25% in both the wildlife and wildlife plus cattle treatments compared to the total exclosures. Our results suggest that increasing aridity will limit oak establishment and modulate the impact of large herbivores on oak seedlings, with wild large herbivores becoming a beneficial factor for seedling abundance, and both wild herbivores and cattle reducing mortality. Dynamic and contextually informed management of large herbivore populations and other restoration actions will be necessary to ensure the sustainability of California’s oak populations.

Plant-plant interactions can be a complex mixture of positive and negative interactions, with the net outcome depending on abiotic and community contexts. In savanna systems, the effects of large herbivores on tree-grass interactions have rarely been studied experimentally, though these herbivores are major players in these systems. In African savannas, trees often become more abundant under heavy cattle grazing but less abundant in wildlife preserves. Woody encroachment where cattle have replaced wild herbivores may be caused by a shift in the competitive balance between trees and grasses. Here we report the results of an experiment designed to quantify the positive, negative, and net effects of grasses, wild herbivores, and cattle on Acacia saplings in a Kenyan savanna. Acacia drepanolobium saplings under four long-term herbivore regimes (wild herbivores, cattle, cattle + wild herbivores, and no large herbivores) were cleared of surrounding grass or left with the surrounding grass intact. After two years, grass-removal saplings exhibited 86% more browse damage than control saplings, suggesting that grass benefited saplings by protecting them from herbivory. However, the negative effect of grass on saplings was far greater; grass-removal trees accrued more than twice the total stem length of control trees. Where wild herbivores were present, saplings were browsed more and produced more new stem growth. Thus, the net effect of wild herbivores was positive, possibly due to the indirect effects of lower competitor tree density in areas accessible to elephants. Additionally, colonization of saplings by symbiotic ants tracked growth patterns, and colonized saplings experienced lower rates of browse damage. These results suggest that savanna tree growth and woody encroachment cannot be predicted by grass cover or herbivore type alone. Rather, tree growth appears to depend on a variety of factors that may be acting together or antagonistically at different stages of the tree's life cycle.

1. Savanna ecosystems are defined largely by tree-grass mixtures, and tree establishment is a key driver of community structure and ecosystem function in these systems. The factors controlling savanna tree establishment are understudied, but likely involve some combination of seed, microsite and predator/fire limitation. In African savannas, suppression and killing of adult trees by large mammals like elephants (Loxodonta africana Blumenbach, 1797) and giraffes (Giraffa camelopardalis Linnaeus, 1758) can maintain tree-grass co-dominance, although the impacts of even these conspicuous herbivores on tree establishment also are poorly understood. 2. We combined seed addition and predator exclusion experiments with a large-scale, long-term field manipulation of large herbivores to investigate the relative importance of seeds, microsites and predators in limiting establishment of a monodominant tree (Acacia drepanolobium Sjostedt) in a Kenyan savanna. 3. Both wild and domestic (i.e. cattle; Bos taurus Linnaeus, 1758) large herbivores facilitated tree establishment by suppressing abundances of rodents, the most important seed and seedling predators. However, this indirect, positive effect of wild herbivores was negated by wild herbivores' suppression of seed production. Cattle did not have this direct, negative impact; rather, they further assisted tree establishment by reducing cover of understorey grasses. Thus, the impacts of both groups of large herbivores on tree establishment were largely routed through other taxa, with a negligible net effect of wild herbivores and a positive net effect of cattle on tree establishment. 4. The distinction between the (positive) net effect of cattle and (neutral) net effect of wild herbivores is due to the inclusion of browsers and mixed feeders within the assemblage of wild herbivores. Browsing by wild herbivores limited seed production, which reduced tree recruitment; grazing by cattle was more pronounced than that by wild herbivores, and thus promoted germination and subsequent establishment of small trees. 5. Our study is the first to link seed fates to tree establishment in savanna ecosystems in experimentally-manipulated herbivore communities. Further, our results highlight how large herbivores can modify a suite of independent factors - seed production, competition with understorey species, and seed and seedling predation - to collectively drive tree establishment.

Indirect interactions driven by livestock and wild herbivores are increasingly recognized as important aspects of community dynamics in savannas and rangelands. Large ungulate herbivores can both directly and indirectly impact the reproductive structures of plants, which in turn can affect the pollinators of those plants. We examined how wild herbivores and cattle each indirectly affect the abundance of a common pollinator butterfly taxon, Colotis spp., at a set of long-term, large herbivore exclosure plots in a semiarid savanna in central Kenya. We also examined effects of herbivore exclusion on the main food plant of Colotis spp., which was also the most common flowering species in our plots: the shrub Cadaba farinosa. The study was conducted in four types of experimental plots: cattle-only, wildlife-only, cattle and wildlife (all large herbivores), and no large herbivores. Across all plots, Colotis spp. abundances were positively correlated with both Cadaba flower numbers (adult food resources) and total Cadaba canopy area (larval food resources). Structural equation modeling (SEM) revealed that floral resources drove the abundance of Colotis butterflies. Excluding browsing wildlife increased the abundances of both Cadaba flowers and Colotis butterflies. However, flower numbers and Colotis spp. abundances were greater in plots with cattle herbivory than in plots that excluded all large herbivores. Our results suggest that wild browsing herbivores can suppress pollinator species whereas well-managed cattle use may benefit important pollinators and the plants that depend on them. This study documents a novel set of ecological interactions that demonstrate how both conservation and livelihood goals can be met in a working landscape with abundant wildlife and livestock.

Large wild herbivores are important and natural components of forest ecosystems, but through their browsing activities have the potential to influence the structure and composition of forest communities, thus timber production and ecosystem dynamics. To examine the effects of browsing by wild herbivores on a young post-disturbance forest in the Kysuce region of northwestern Slovakia, we established two sets of 2 m radius plots, 15 within a fenced area (5.12 ha) that excluded large wild herbivores, and 15 within an adjacent unfenced area. In each plot, we recorded the species, tree height, stem base diameter, and mutual geographic positions of trees. When we compared tree community characteristics between the unfenced and fenced plots, we found fewer and smaller broadleaved tree species, except silver birch (Betula pendula Roth.) in the unfenced plots. Although common rowan (Sorbus aucuparia L.) was the dominant species within fenced plots, where some individuals were over 6.0 m tall, this species was rare outside the fenced area and usually did not exceed 1.5 m. In contrast, Norway spruce (Picea abies Karts L.) was more abundant and taller within the unfenced area, likely released from competition by suppression of broadleaved trees by herbivores. In addition, fenced plots also showed twice the tree species richness (Shannon index) of unfenced ones. Despite changes in tree communities, total aboveground biomass stock was only slightly but significantly lower in the unfenced than the fenced plots (29.6 kg per 10 m2 vs. 33.5 kg per 10 m2). Our study suggested that browsing pressure by large wild herbivores that focused on most broadleaved trees weakened interspecies competition and allowed the expansion of Norway spruce. As a consequence, converting spruce monocultures to mixed species stands is likely unrealistic when faced with heavy browsing pressure by wild large herbivores.

The aim of this chapter is to discuss how large herbivores influence competition between plants and thereby may drive or change succession of vegetation in savannah rangelands and thus influence the structure and function of ecosystems. Large herbivores in savannah rangelands utilize both herbaceous and woody plants, and impact by herbivory on both these vegetation components will be discussed. To understand how competition between plants is affected by grazing and browsing animals, the traits and strategies that plants adopt to obtain or maintain competitive power and fitness in a savannah environment with large herbivores will be considered in the section 'adaptations by plants to herbivory'. Thereafter, in the section 'large herbivores and plant communities', ways in which the competitive success of plants with different characteristics may influence vegetation succession and the composition of plant communities will be discussed. The section 'Herbivory and plant competition on an ecosystem scale' will consider how large herbivores interact directly and indirectly with ecosystem processes and thereby change the system within which plants compete for resources. Emphasis will be on African savannahs but examples from other regions will be included.

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.

Studies addressing the role of large herbivores on nitrogen cycling in grasslands have suggested that the direction of effects depends on soil fertility. Via selection for high quality plant species and input of dung and urine, large herbivores have been shown to speed up nitrogen cycling in fertile grassland soils while slowing down nitrogen cycling in unfertile soils. However, recent studies show that large herbivores can reduce nitrogen mineralization in some temperate fertile soils, but not in others. To explain this, we hypothesize that large herbivores can reduce nitrogen mineralization in loamy or clay soils through soil compaction, but not in sandy soils. Especially under wet conditions, strong compaction in clay soils can lead to periods of soil anoxia, which reduces decomposition of soil organic matter and, hence, N mineralization. In this study, we use a long-term (37-year) field experiment on a salt marsh to investigate the hypothesis that the effect of large herbivores on nitrogen mineralization depends on soil texture. Our results confirm that the presence of large herbivores decreased nitrogen mineralization rate in a clay soil, but not in a sandy soil. By comparing a hand-mown treatment with a herbivore-grazed treatment, we show that these differences can be attributed to herbivore-induced changes in soil physical properties rather than to above-ground biomass removal. On clay soil, we find that large herbivores increase the soil water-filled porosity, induce more negative soil redox potentials, reduce soil macrofauna abundance, and reduce decomposition activity. On sandy soil, we observe no changes in these variables in response to grazing. We conclude that effects of large herbivores on nitrogen mineralization cannot be understood without taking soil texture, soil moisture, and feedbacks through soil macrofauna into account.

Re-framing deer herbivory as a natural disturbance regime with ecological and socioeconomic outcomes in the eastern United States

Summary Wild large herbivores provide goods and income to rural communities, have major impacts on land use and habitats of conservation importance and, in some cases, face local or global extinction. As a result, substantial effort is applied to their management across the globe. To be effective, however, management has to be science‐based. We reviewed recent fundamental and applied studies of large herbivores with particular emphasis on the relationship between the spatial and temporal scales of ecosystem response, management decision and implementation. Long‐term population dynamics research has revealed fundamental differences in how sex/age classes are affected by changes in density and weather. Consequently, management must be tailored to the age and sex structure of the population, rather than to simple population counts. Herbivory by large ungulates shapes the structure, diversity and functioning of most terrestrial ecosystems. Recent research has shown that fundamental herbivore/vegetation interactions driving landscape change are localized, often at scales of a few metres. For example, sheep and deer will selectively browse heather Calluna vulgaris at the edge of preferred grass patches in heather moorland. As heather is vulnerable to heavy defoliation, in the long term this can lead to loss of heather cover despite the average utilization rate of heather in a management area being low. Therefore, while herbivore population management requires a large‐scale approach, management of herbivore impacts on vegetation may require a much more flexible and site‐specific approach. Localized impacts on vegetation have cascading effects on biodiversity, because changes in vegetation structure and composition, induced by large herbivores affect habitat suitability for many other species. As such, grazing should be considered as a tool for broader biodiversity management requiring a more sophisticated approach than just, for example, eliminating grazing from conservation areas through the use of exclosures. Synthesis and applications. The management of wild large herbivores must consider different spatial scales, from small patches of vegetation to boundaries of an animal population. It also requires long‐term planning based on a deep understanding of how population processes, such a birth rate, death rate and age structure, are affected by changes in land use and climate and how these affect localized herbivore impacts. Because wild herbivores do not observe administrative or political boundaries, adjusting their management to socio‐political realities can present a challenge. Many developing countries have established co‐operative management groups that allow all interested parties to be involved in the development of management plans; developed countries have a lot to learn from the developing world's example.

Large herbivore populations outside protected areas in the human-dominated Western Ghats, India