Todd M. Palmer

Large mammalian herbivores (LMH) play complex roles in shaping tree cover and composition in savannas by suppressing trees through browsing and reducing competition from neighbouring trees and understory vegetation. Where LMH are declining, release from browsing and intensification of competition has unpredictable physiological consequences. To test how LMH removal affects tree growth and water stress, we combined 16 years of growth data with measurements of leaf water potential and gas exchange from a herbivore‐exclusion experiment in Kenya. Across tree species, LMH exclusion increased growth but elevated water stress, particularly under dry conditions. Moreover, the tree species with the most acquisitive water‐use strategy was most susceptible to increased water stress and reduced carbon assimilation under exclusion. These findings indicate that although LMH limit tree growth, they also reduce water stress. Ultimately, interspecific differences in tree physiology will affect how savanna communities respond to interacting pressures from climate change and LMH declines.

Pollination is a key ecological process sustaining biodiversity and food security, yet global patterns of plant–pollinator specialization remain unresolved. Using a global dataset of quantitative plant–pollinator networks (>3,400 networks, >110,000 interactions), we show that specialization is latitudinally structured, but not as a single directional pattern. We find no consistent monotonic increase or decrease in specialization towards the Equator. Instead, network-level and pollinator specialization peak at low northern latitudes near the tropical–subtropical boundary, whereas plant specialization is hemispherically asymmetric. Climate is most often the best-supported predictor of specialization compared with latitude, biodiversity and environmental productivity. Network-level specialization declines with increasing mean annual temperature, with strong group-specific differences. Precipitation exerts nonlinear effects: plant specialization peaks at intermediate precipitation but is lower at the wettest sites, whereas pollinator specialization is lower at intermediate precipitation, although often in interaction with temperature. Pollinator groups diverge strongly, especially in contrasting precipitation-related patterns between birds and insects. These findings show that global plant–pollinator specialization is structured by latitude and climate in complex and taxon-specific ways, with implications for uneven responses of pollination networks to future climatic change.

Questions: Facilitation is a common phenomenon that underlies the local abundance and distribution of many plants. Facilitation can arise via multiple mechanisms, and classes of mechanistic effects have the potential to modulate one another's effects. We quantified two mechanisms by which trees may facilitate understory plants: amelioration of abiotic conditions and herbivory reduction. Location: Kenya. Methods: We quantified the spatial association of a common forb, Hibiscus meyeri, with tree canopies in an East African savanna in the presence and absence of large mammalian browsers across an aridity gradient. Namely, we compared the observed proportion of H. meyeri under a tree canopy and their distance to the tree canopy to null distributions derived from simulated landscapes. We also used a spatially naïve linear model approach to test for site and exclosure treatment effects on the probability of being under a tree canopy and on distance to canopy. Results: Both our randomization and linear model approach suggested that both aridity and herbivore presence increased H. meyeri individuals' association with the tree canopy, suggesting trees facilitate H. meyeri via both abiotic habitat amelioration and herbivory reduction. In addition, herbivore presence more strongly increased H. meyeri 's association with the tree canopy at the arid site, suggesting that facilitative effects of abiotic habitat amelioration could be modulating those of herbivory reduction. Conclusions: We found that trees facilitate H. meyeri via multiple facilitative mechanisms. Our results suggest that the strength of one facilitative mechanism can depend on whether other facilitative mechanisms are also operating.

Ecologists theorize that removing highly connected species from ecological networks will trigger waves of coextinction, but empirical evidence is scant. We show that elephants are central to a generalized network of interactions between dung beetles and large herbivores in East Africa. Computer simulations based on this network predicted disproportionate dung beetle losses if elephants go extinct, and a 15-year experiment supported this prediction: Excluding elephants diminished dung beetle abundance, diversity, and ecosystem functions, whereas excluding smaller ungulates had little added effect. Dung beetle abundance was depressed in nearby landscapes where livestock had displaced elephants, corroborating the experimental results. Our findings affirm the long-standing idea that large animals—which are inordinately extinction-prone—act as keystone species by provisioning many linked consumers with essential resources, thereby sustaining biodiversity and ecosystem services.

Pollination is a key ecological process sustaining biodiversity and food security, yet global patterns of plant–pollinator specialisation have remained unresolved. Using the largest global dataset of quantitative networks (>3,400 networks, >110,000 interactions), we show that the latitudinal specialisation gradient (LSG) exists, but it is non-linear, hemispherically asymmetric, and strongly taxon-dependent. Network-level and pollinator specialisation were lowest in the tropics and peaked at northern mid-latitudes, whereas plants tended to become more specialised toward higher latitudes. Climate consistently outperformed latitude, species richness, and environmental productivity as a predictor of these patterns. Specialisation declined with increasing temperature, rose with moderate rainfall before declining at the wettest sites, and increased with temperature seasonality, but plants and pollinators responded differently to these drivers. Functional groups diverged strongly: ectothermic insects were most specialised in cooler, seasonal climates, while birds showed weaker links to latitude but reduced specialisation in wetter regions. These findings demonstrate that climate, rather than latitude or species richness, structures global variation in specialisation. Because warmer and less seasonal climates promote generalisation, climate change is likely to disrupt the most specialised pollination systems, unevenly across taxa and regions, with important consequences for biodiversity and ecosystem stability.

Summary: Many plants are defended from herbivory by costly insect mutualists. Understanding positive associations between plants and mutualists requires a whole‐plant perspective including roots. We hypothesized that root surface area increases with mutualist activity (to a saturation threshold) and recent rainfall but that this relationship shifts when herbivores are excluded. We also hypothesized that invasive ants limit root surface area and that mutualism breakdown driven by invaders blunts root responses to rainfall and herbivore exclusion. Using minirhizotrons (est. 2021), we surveyed root surface area of ant‐acacias during a dry (2022) and then a wet (2023) season. Study plots either excluded or permitted vertebrate browsers, within a natural experiment comparing mutualist‐defended ant‐acacias to those invaded by a mutualism‐disrupting ant. Root area increased with mutualist activity to a threshold, but this positive association was less apparent during rainy periods. Megabrowser exclusion increased overall root area but reduced the threshold for a positive association with mutualist activity and reduced the steepness of the root area–rainfall correlation. Ant‐invaded acacias had smaller root areas that correlated less steeply with rainfall. Positive associations between insect defense and root area were thus contingent on rainfall, herbivory, and biotic invasion, drivers that are shifting under global change.

Plasticity in resource allocation can be beneficial for plants under stress. In savannas, tree-grass competition forces tree saplings growing in the grass layer to compete for water, nutrients, and light. Savanna tree saplings are also vulnerable to fire and herbivory, which may favour investment in storage belowground to support regrowth aboveground. It is unclear whether carbon (C) limitation from grass shading similarly favours allocation belowground. Further, investigating how light reduction changes allocation by juvenile trees to above- and belowground biomass, storage, and defence can help us understand juvenile tree allocation strategies during ubiquitous C limitation. Using a screenhouse experiment, we evaluated the effects of shade on carbon allocation and leaf physiology in saplings of a dominant ant-acacia, Acacia (Vachellia) drepanolobium. We hypothesized that shade would induce greater belowground allocation by saplings to root growth and storage. Indeed, we found that shaded saplings had higher root mass fractions and higher concentrations of starch in their roots than plants in full sunlight. Plants in full sunlight, meanwhile, invested more in aboveground growth, with higher stem mass fractions than shaded plants. Shade did not affect leaf mass fraction, but plants in the shade had a lower leaf mass per area, higher stomatal conductance, and a higher maximum photosynthetic rate, indicating leaf-level adjustments that increased carbon capture under light limitation. These responses are consistent with possible adaptive allocation strategies that buffer the impacts of fire and herbivory, underscoring the essential role of belowground reserves for regrowth.

Mutualisms often define ecosystems, but they are susceptible to human activities. Combining experiments, animal tracking, and mortality investigations, we show that the invasive big-headed ant ( Pheidole megacephala ) makes lions ( Panthera leo ) less effective at killing their primary prey, plains zebra ( Equus quagga ). Big-headed ants disrupted the mutualism between native ants ( Crematogaster spp.) and the dominant whistling-thorn tree ( Vachellia drepanolobium ), rendering trees vulnerable to elephant ( Loxodonta africana ) browsing and resulting in landscapes with higher visibility. Although zebra kills were significantly less likely to occur in higher-visibility, invaded areas, lion numbers did not decline since the onset of the invasion, likely because of prey-switching to African buffalo ( Syncerus caffer ). We show that by controlling biophysical structure across landscapes, a tiny invader reconfigured predator-prey dynamics among iconic species.

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.