Seagrass ecosystem multifunctionality under the rise of a flagship marine megaherbivore

Large grazers (megaherbivores) have a profound impact on ecosystem functioning. However, how ecosystem multifunctionality is affected by changes in megaherbivore populations remains poorly understood. Understanding the total impact on ecosystem multifunctionality requires an integrative ecosystem approach, which is especially challenging to obtain in marine systems. We assessed the effects of experimentally simulated grazing intensity scenarios on ecosystem functions and multifunctionality in a tropical Caribbean seagrass ecosystem. As a model, we selected a key marine megaherbivore, the green turtle, whose ecological role is rapidly unfolding in numerous foraging areas where populations are recovering through conservation after centuries of decline, with an increase in recorded overgrazing episodes. To quantify the effects, we employed a novel integrated index of seagrass ecosystem multifunctionality based upon multiple, well-recognized measures of seagrass ecosystem functions that reflect ecosystem services. Experiments revealed that intermediate turtle grazing resulted in the highest rates of nutrient cycling and carbon storage, while sediment stabilization, decomposition rates, epifauna richness, and fish biomass are highest in the absence of turtle grazing. In contrast, intense grazing resulted in disproportionally large effects on ecosystem functions and a collapse of multifunctionality. These results imply that (i) the return of a megaherbivore can exert strong effects on coastal ecosystem functions and multifunctionality, (ii) conservation efforts that are skewed toward megaherbivores, but ignore their key drivers like predators or habitat, will likely result in overgrazing-induced loss of multifunctionality, and (iii) the multifunctionality index shows great potential as a quantitative tool to assess ecosystem performance. Considerable and rapid alterations in megaherbivore abundance (both through extinction and conservation) cause an imbalance in ecosystem functioning and substantially alter or even compromise ecosystem services that help to negate global change effects. An integrative ecosystem approach in environmental management is urgently required to protect and enhance ecosystem multifunctionality.     

Effects of water dependence on the utilization pattern of woody vegetation by elephants in the Tembe Elephant Park, Maputaland, South Africa

The utilization of vegetation and particularly trees in enclosed small reserves where elephant populations are confined is a contentious conservation issue. In Tembe Elephant Park in Maputaland, the diverse Sand Forest is considered the most valuable feature to conserve; yet it is considered at risk from increasing elephant utilization of the park's vegetation in general. The mean canopy removal by elephants across the park was studied over two periods: a recent period including the twelve months before the study and an older period >12 months earlier. Age of utilization was determined from the degree of decay observed on woody fibres. The relationships between intensity of utilization, vegetation unit selection and distance to water were evaluated. Results show that utilization patterns have shifted in the recent drier period, during which elephants used vegetation communities closer to permanent water. Concurringly, a significant decrease in utilization intensity was correlated to an increasing distance from water points in the park during that period, while this effect was not observed during the wetter old period. We debate that active water availability management may be a way to limit elephant utilization in small fenced reserves.     

Elephants limit aboveground carbon gains in African savannas

Understanding the drivers of vegetation carbon dynamics is essential for climate change mitigation and effective policy formulation. However, most efforts focus on abiotic drivers of plant biomass change, with little consideration for functional roles performed by animals, particularly at landscape scales. We combined repeat airborne Light Detection and Ranging with measurements of elephant densities, abiotic factors, and exclusion experiments to determine the relative importance of drivers of change in aboveground woody vegetation carbon stocks in Kruger National Park, South Africa. Despite a growing elephant population, aboveground carbon density (ACD) increased across most of the landscape over the 6‐year study period, but at fine scales, bull elephant density was the most important factor determining carbon stock change, with ACD losses recorded only where bull densities exceeded 0.5 bulls/km². Effects of bull elephants were, however, spatially restricted and landscape dependent, being especially pronounced along rivers, at mid‐elevations, and on steeper slopes. In contrast, elephant herds and abiotic drivers had a comparatively small influence on the direction or magnitude of carbon stock change. Our findings demonstrate that animals can have a substantive influence on regional‐scale carbon dynamics and warrant consideration in carbon cycling models and policy formulation aimed at carbon management and climate change mitigation.     

Seed dispersal kernel of the largest surviving megaherbivore—the African savanna elephant

Owing to the late Pleistocene extinctions, the megafauna of Europe, Australia and the Americas disappeared, and with them the dispersal service they offered megafaunal fruit. The African savanna elephant, the largest remaining megaherbivore, offers valuable insights into the seed dispersal services provided by extinct megafauna in prehistoric times. Elephant seed dispersal studies have for the most part concentrated on African and Asian forest elephants. African savanna elephants are morphologically distinct from their forest counterparts. Like the forest elephants they consume large quantities of fruit from a large number of tree species. Despite this little is known of the savanna trees that rely on elephants for their dispersal or the spatial scale at which these seeds are dispersed. We combined information from feeding trials conducted on four park elephants with field telemetry data from 38 collared elephants collected over an 8‐year period in APNR/Kruger National Park to assess the seed dispersal service provided by savanna elephants. This study provides the first detailed account of the spatial scale at which African savanna elephants disperse seeds. Our mechanistic model predicts that 50 percent of seeds are carried over 2.5 km, and distances up to 65 km are achievable in maximum gut passage time. These findings suggest the savanna elephant as the longest distance terrestrial vertebrate disperser yet investigated. Maintaining their ecological role as a seed disperser may prove a significant factor in the conservation of large‐fruited tree diversity within the savannas. These results suggest that extinct megafauna offered a functionally unique dispersal service to megafaunal fruit.     

Drivers of megaherbivore demographic fluctuations: inference from elephants

Environmentally induced variation in survival and fecundity generates demographic fluctuations that affect population growth rate. However, a general pattern of the comparative influence of variation in fecundity and juvenile survival on elephant population dynamics has not been investigated at a broad scale. We evaluated the relative importance of conception, gestation, first year survival and subsequent survivorship for controlling demographic variation by exploring the relationship between past environmental conditions determined by integrated normalized difference vegetation index (INDVI) and the shape of age distributions at 17 sites across Africa. We showed that, generally, INDVI during gestation best explained anomalies in age structure. However, in areas with low mean annual rainfall, INDVI during the first year of life was critical. The results challenge Eberhardt's paradigm for population analysis that suggests that populations respond to limited resource availability through a sequential decrease in juvenile survival, reproductive rate and adult survival. Contrastingly, elephants appear to respond first through a reduction in reproductive rate. We conclude that this discrepancy is likely due to the evolutionary significance of extremely large body size – an adaptation that increases survival rate but decreases reproductive potential. Other megaherbivores may respond similarly to resource limitation due to similarities in population dynamics. Knowing how vital rates vary with changing environmental conditions will permit better forecasts of the trajectories of megaherbivore populations.     

Evaluating the potential for reintroducing the endangered wild water buffalo (Bubalus arnee) in Kanha National Park,central India

Reintroducing megafauna to their historic range is an effective strategy to halt their extinctions and restore ecosystems. Wild water buffalo (Bubalus arnee) is an endangered megaherbivore that is lost from 95% of its range. About 90% of its global population (less than 4000) resides within India, in two isolated populations: northeast and central India. The central Indian population is on the verge of extinction and warrants urgent conservation interventions. We assess the potential and provide a strategy for reintroducing buffaloes in Kanha National Park, India. Habitat suitability using the global occurrence of buffalo revealed low-lying grasslands with least human pressure found in Kanha (390 km²) to be suitable. Within this suitable range, we evaluated vegetation composition, forage biomass, and potential carrying capacity. Multidimensional ordination classified these suitable sites into moist and dry grassland clusters. Moist grasslands were found to better suit reintroduction due to lower grazing pressure, higher productivity, and availability of perennial water sources. Distinct matriarchal clades of swamp and river buffalo were observed. Within the river buffalo clade, Indian wild buffaloes formed a distinct cluster with close proximity between northeast and central Indian samples, suggesting northeast buffaloes could be sourced for a founding population in Kanha. Following IUCN guidelines, we discuss the reintroduction strategy that could sustain approximately 200 buffaloes in the area and subsequently expand to other suitable habitats in central India. If implemented as proposed, recovery of this lost ecosystem engineer will help to restore grasslands, and swamp habitats, as well as contribute to its global conservation efforts.     

Continued decline in tree euphorbias (Euphorbia tetragona and E. triangularis) on the Great Fish River Reserve,Eastern Cape,South Africa

A dramatic decline averaging 43% over a 4‐year period has occurred in tree Euphorbia (Euphorbia tetragona and Euphorbia triangularis) populations on the Great Fish River Reserve, South Africa. These changes are evident from data gathered by general vegetation monitoring methods as well as from a focused study of four tree Euphorbia populations. The decline from 2003 to 2007 was more marked for E. triangularis than for E. tetragona and was accompanied by a general absence of seedlings and a reduced presence of younger age classes of both species, decreasing the proportion of younger trees in the populations. The role of megaherbivores, specifically the black rhinoceros, in these changes is well established. However, the impact of baboon activity, leading to damage to tree crowns and upper branches, is also substantial, especially on E. triangularis populations. Damaged crowns were recorded significantly more often for E. triangularis than for E. tetragona, and the damage frequency increased with decreasing tree height. Thus, our work provides the first evidence that these two closely related Euphorbia species may be affected differently by herbivory.     

The Dominance of Introduced Plant Species in the Diets of Migratory Galapagos Tortoises Increases with Elevation on a Human‐Occupied Island

The distribution of resources and food selection are fundamental to the ecology, life history, physiology, population dynamics, and conservation of animals. Introduced plants are changing foraging dynamics of herbivores in many ecosystems often with unknown consequences. Galapagos tortoises, like many herbivores, undertake migrations along elevation gradients driven by variability in vegetation productivity which take them into upland areas dominated by introduced plants. We sought to characterize diet composition of two species of Galapagos tortoises, focussing on how the role of introduced forage species changes over space and the implications for tortoise conservation. We quantified the distribution of tortoises with elevation using GPS telemetry. Along the elevation gradient, we quantified the abundance of introduced and native plant species, estimated diet composition by recording foods consumed by tortoises, and assessed tortoise physical condition from body weights and blood parameter values. Tortoises ranged between 0 and 429 m in elevation over which they consumed at least 64 plant species from 26 families, 44 percent of which were introduced species. Cover of introduced species and the proportion of introduced species in tortoise diets increased with elevation. Introduced species were positively selected for by tortoises at all elevations. Tortoise physical condition was either consistent or increased with elevation at the least biologically productive season on Galapagos. Santa Cruz tortoises are generalist herbivores that have adapted their feeding behavior to consume many introduced plant species that has likely made a positive contribution to tortoise nutrition. Some transformed habitats that contain an abundance of introduced forage species are compatible with tortoise conservation.     

Seed dispersal by Galápagos tortoises

Aim Large‐bodied vertebrates often have a dramatic role in ecosystem function through herbivory, trampling, seed dispersal and nutrient cycling. The iconic Galápagos tortoises (Chelonoidis nigra) are the largest extant terrestrial ectotherms, yet their ecology is poorly known. Large body size should confer a generalist diet, benign digestive processes and long‐distance ranging ability, rendering giant tortoises adept seed dispersers. We sought to determine the extent of seed dispersal by Galápagos tortoises and their impact on seed germination for selected species, and to assess potential impacts of tortoise dispersal on the vegetation dynamics of the Galápagos. Location Galápagos, Ecuador. Methods To determine the number of seeds dispersed we identified and counted intact seeds from 120 fresh dung piles in both agricultural and national park land. To estimate the distance over which tortoises move seeds we used estimated digesta retention times from captive tortoises as a proxy for retention times of wild tortoises and tortoise movement data obtained from GPS telemetry. We conducted germination trials for five plant species to determine whether tortoise processing influenced germination success. Results In our dung sample, we found intact seeds from > 45 plant species, of which 11 were from introduced species. Tortoises defecated, on average, 464 (SE 95) seeds and 2.8 (SE 0.2) species per dung pile. Seed numbers were dominated by introduced species, particularly in agricultural land. Tortoises frequently moved seeds over long distances; during mean digesta retention times (12 days) tortoises moved an average of 394 m (SE 34) and a maximum of 4355 m over the longest recorded retention time (28 days). We did not find evidence that tortoise ingestion or the presence of dung influenced seed germination success. Main conclusions Galápagos tortoises are prodigious seed dispersers, regularly moving large quantities of seeds over long distances. This may confer important advantages to tortoise‐dispersed species, including transport of seeds away from the parent plants into sites favourable for germination. More extensive research is needed to quantify germination success, recruitment to adulthood and demography of plants under natural conditions, with and without tortoise dispersal, to determine the seed dispersal effectiveness of Galápagos tortoises.     

Ecology of grazing lawns in Africa

Grazing lawns are a distinct grassland community type, characterised by short‐stature and with their persistence and spread promoted by grazing. In Africa, they reveal a long co‐evolutionary history of grasses and large mammal grazers. The attractiveness to grazers of a low‐biomass sward lies in the relatively high quality of forage, largely due to the low proportion of stem material in the sward; this encourages repeat grazing that concomitantly suppresses tall‐grass growth forms that would otherwise outcompete lawn species for light. Regular grazing that prevents shading and maintains sward quality is thus the cornerstone of grazing lawn dynamics. The strong interplay between abiotic conditions and disturbance factors, which are central to grazing lawn existence, can also cause these systems to be highly dynamic. Here we identify differences in growth form among grazing lawn grass species, and assess how compositional differences among lawn types, as well as environmental variables, influence their maintenance requirements (i.e. grazing frequency) and vulnerability to degradation. We also make a clear distinction between the processes of lawn establishment and lawn maintenance. Rainfall, soil nutrient status, grazer community composition and fire regime have strong and interactive influences on both processes. However, factors that concentrate grazing pressure (e.g. nutrient hotspots and sodic sites) have more bearing on where lawns establish. Similarly, we discuss the relevance of enhanced rates of nitrogen cycling and of sodium levels to lawn maintenance. Grazer community composition and density has considerable significance to grazing lawn dynamics; not all grazers are adapted to foraging on short‐grass swards, and differences in body size and relative mouth dimensions determine which species are able to convert tall‐grass swards into grazing lawns under different conditions. Hence, we evaluate the roles of different grazers in lawn dynamics, as well as the benefits that grazer populations derive from having access to grazing lawns. The effects of grazing lawns can extend well beyond their borders, due to their influence on grazer densities, behaviour and movements as well as fire spread, intensity and frequency. Variation in the area and proportion of a landscape that is grazing lawn can thus have a profound impact on system dynamics. We provide a conceptual model that summarises grazing lawn dynamics, and identify a rainfall range where we predict grazing lawns to be most prevalent. We also examine the biodiversity associated with grazing lawn systems, and consider their functional contribution to the conservation of this biodiversity. Finally, we assess the utility of grazing lawns as a resource in a rangeland context.