Richness, not evenness, varies across water availability gradients in grassy biomes on five continents.

We sought to understand the role that water availability (expressed as an aridity index) plays in determining regional and global patterns of richness and evenness, and in turn how these water availability-diversity relationships may result in different richness-evenness relationships at regional and global scales. We examined relationships between water availability, richness and evenness for eight grassy biomes spanning broad water availability gradients on five continents. Our study found that relationships between richness and water availability switched from positive for drier (South Africa, Tibet and USA) vs. negative for wetter (India) biomes, though were not significant for the remaining biomes. In contrast, only the India biome showed a significant relationship between water availability and evenness, which was negative. Globally, the richness-water availability relationship was hump-shaped, however, not significant for evenness. At the regional scale, a positive richness-evenness relationship was found for grassy biomes in India and Inner Mongolia, China. In contrast, this relationship was weakly concave-up globally. These results suggest that different, independent factors are determining patterns of species richness and evenness in grassy biomes, resulting in differing richness-evenness relationships at regional and global scales. As a consequence, richness and evenness may respond very differently across spatial gradients to anthropogenic changes, such as climate change.

Effectiveness of aerial wildlife crossings: Do wildlife use rope bridges more than hazardous structures to cross roads? / Eficacia de los pasos aéreos sobre carretera: ¿Cruza la fauna más por puentes de cuerda que por otras estructuras peligrosas?

Abstract Introduction: Although wildlife crossing structures have proven successful at reducing wildlife-vehicle collisions and linking fragmented habitat, their ability to prevent electrocutions of arboreal wildlife has not been closely examined. Objective: To evaluate the effectiveness of aerial rope bridges in restoring habitat connectivity for arboreal species in Manuel Antonio, Costa Rica, while preventing electrocutions by determining 1) what species are using the rope bridges and 2) whether wildlife prefer to use rope bridges instead of other hazardous structures that cross the roads (such as telephone cables, which are often in close proximity to electric wires). Methods: From January to May 2016, nine rope bridges along the highly-trafficked main road that extends from Quepos to Manuel Antonio, Costa Rica, were monitored using camera traps, and ten rope bridges were observed directly along a paved side road off the main road. Results: A total of 11 species were seen using the bridges, and 1 540 crossings were witnessed via camera traps and observations (1 234 via camera traps, 306 during observations). Results from a paired t-test showed no significant difference in the average number of individuals crossing the road via rope bridges versus telephone cables (t(8) = 1.027, P = 0.334). Conclusions: Rope bridges are used by a variety of arboreal wildlife species with a high degree of frequency; however, due to the equally high usage of telephone cables by arboreal wildlife, they are insufficient to prevent wildlife electrocutions on their own. Rope bridges should be installed in tandem with other methods to prevent electrocutions, such as insulating electric wires, to facilitate the safe passage of wildlife over roads.

Resumen Introducción: Aunque los pasos de fauna han demostrado ser exitosos para reducir las colisiones entre vehículos y vida silvestre y vincular el hábitat fragmentado, su capacidad para prevenir electrocuciones de la vida silvestre arbórea no se ha examinado a fondo. Objetivo: Evaluar la efectividad de los puentes aéreos de cuerdas para restaurar la conectividad del hábitat de las especies arbóreas en Manuel Antonio, Costa Rica y al mismo tiempo prevenir las electrocuciones al determinar 1) qué especies están usando los puentes de cuerda y 2) si la vida silvestre prefiere usar puentes de cuerda en lugar de otras estructuras peligrosas que cruzan las carreteras (como cables telefónicos, que frecuentemente están muy cerca de cables eléctricos). Métodos: De enero a mayo de 2016, se monitorearon nueve puentes de cuerda a lo largo de la carretera principal altamente transitada que se extiende desde Quepos a Manuel Antonio, Costa Rica, utilizando cámaras trampa y la observación directa en diez puentes de cuerda a lo largo de una carretera pavimentada más pequeña fuera de la carretera principal. Resultados: Se observaron un total de 11 especies utilizando los puentes y se presenciaron 1 540 cruces mediante cámaras trampa y observaciones (1 234 mediante cámaras trampa, 306 durante las observaciones). Los resultados de una prueba t pareada no mostraron diferencias significativas en el número promedio de individuos que cruzan la carretera a través de puentes de cuerda versus cables telefónicos, t (8) = 1.027, P = 0.334. Conclusiones: Los puentes de cuerdas son utilizados por una variedad de especies de vida silvestre arbóreas con un alto grado de frecuencia; sin embargo, debido al uso igualmente elevado de cables telefónicos por parte de la vida silvestre arbórea, se considera que son insuficientes para prevenir las electrocuciones de la vida silvestre por sí solas. Los puentes de cuerda deben instalarse junto con otros métodos para evitar electrocuciones, como cables eléctricos aislados, para facilitar el paso seguro de la vida silvestre por las carreteras.

Change in dominance determines herbivore effects on plant biodiversity.

Herbivores alter plant biodiversity (species richness) in many of the world's ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis-that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.

Plant community response to loss of large herbivores differs between North American and South African savanna grasslands.

Herbivory and fire shape plant community structure in grass-dominated ecosystems, but these disturbance regimes are being altered around the world. To assess the consequences of such alterations, we excluded large herbivores for seven years from mesic savanna grasslands sites burned at different frequencies in North America (Konza Prairie Biological Station, Kansas, USA) and South Africa (Kruger National Park). We hypothesized that the removal of a single grass-feeding herbivore from Konza would decrease plant community richness and shift community composition due to increased dominance by grasses. Similarly, we expected grass dominance to increase at Kruger when removing large herbivores, but because large herbivores are more diverse, targeting both grasses and forbs, at this study site, the changes due to herbivore removal would be muted. After seven years of large-herbivore exclusion, richness strongly decreased and community composition changed at Konza, whereas little change was evident at Kruger. We found that this divergence in response was largely due to differences in the traits and numbers of dominant grasses between the study sites rather than the predicted differences in herbivore assemblages. Thus, the diversity of large herbivores lost may be less important in determining plant community dynamics than the functional traits of the grasses that dominate mesic, disturbance-maintained savanna grasslands.

The effect of fire on habitat selection of mammalian herbivores: the role of body size and vegetation characteristics.

Given the role of fire in shaping ecosystems, especially grasslands and savannas, it is important to understand its broader impact on these systems. Post-fire stimulation of plant nutrients is thought to benefit grazing mammals and explain their preference for burned areas. However, fire also reduces vegetation height and increases visibility, thereby potentially reducing predation risk. Consequently, fire may be more beneficial to smaller herbivores, with higher nutritional needs and greater risks of predation. We tested the impacts of burning on different sized herbivores' habitat preference in Serengeti National Park, as mediated by burning's effects on vegetation height, live : dead biomass ratio and leaf nutrients. Burning caused a less than 4 month increase in leaf nitrogen (N), and leaf non-N nutrients [copper (Cu), potassium (K), and magnesium (Mg)] and a decrease in vegetation height and live : dead biomass. During this period, total herbivore counts were higher on burned areas. Generally, smaller herbivores preferred burned areas more strongly than larger herbivores. Unfortunately, it was not possible to determine the vegetation characteristics that explained burned area preference for each of the herbivore species observed. However, total herbivore abundance and impala (Aepyceros melampus) preference for burned areas was due to the increases in non-N nutrients caused by burning. These findings suggest that burned area attractiveness to herbivores is mainly driven by changes to forage quality and not potential decreases in predation risk caused by reductions in vegetation height.

Loss of a large grazer impacts savanna grassland plant communities similarly in North America and South Africa.

Large herbivore grazing is a widespread disturbance in mesic savanna grasslands which increases herbaceous plant community richness and diversity. However, humans are modifying the impacts of grazing on these ecosystems by removing grazers. A more general understanding of how grazer loss will impact these ecosystems is hampered by differences in the diversity of large herbivore assemblages among savanna grasslands, which can affect the way that grazing influences plant communities. To avoid this we used two unique enclosures each containing a single, functionally similar large herbivore species. Specifically, we studied a bison (Bos bison) enclosure at Konza Prairie Biological Station, USA and an African buffalo (Syncerus caffer) enclosure in Kruger National Park, South Africa. Within these enclosures we erected exclosures in annually burned and unburned sites to determine how grazer loss would impact herbaceous plant communities, while controlling for potential fire-grazing interactions. At both sites, removal of the only grazer decreased grass and forb richness, evenness and diversity, over time. However, in Kruger these changes only occurred with burning. At both sites, changes in plant communities were driven by increased dominance with herbivore exclusion. At Konza, this was caused by increased abundance of one grass species, Andropogon gerardii, while at Kruger, three grasses, Themeda triandra, Panicum coloratum, and Digitaria eriantha increased in abundance.

Landscape-scale analyses suggest both nutrient and antipredator advantages to Serengeti herbivore hotspots.

Mechanistic explanations of herbivore spatial distribution have focused largely on either resource-related (bottom-up) or predation-related (top-down) factors. We studied direct and indirect influences on the spatial distributions of Serengeti herbivore hotspots, defined as temporally stable areas inhabited by mixed herds of resident grazers. Remote sensing and variation in landscape features were first used to create a map of the spatial distribution of hotspots, which was tested for accuracy against an independent data set of herbivore observations. Subsequently, we applied structural equation modeling to data on soil fertility and plant quality and quantity across a range of sites. We found that hotspots in Serengeti occur in areas that are relatively flat and located away from rivers, sites where ungulates are less susceptible to predation. Further, hotspots tend to occur in areas where hydrology and rainfall create conditions of relatively low-standing plant biomass, which, coupled with grazing, increases forage quality while decreasing predation risk. Low-standing biomass and higher leaf concentrations of N, Na, and Mg were strong direct predictors of hotspot occurrence. Soil fertility had indirect effects on hotspot occurrence by promoting leaf Na and Mg. The results indicate that landscape features contribute in direct and indirect ways to influence the spatial distribution of hotspots and that the best models incorporated both resource- and predation-related factors. Our study highlights the collective and simultaneous role of bottom-up and top-down factors in determining ungulate spatial distributions.

Forage nutritive quality in the Serengeti ecosystem: the roles of fire and herbivory.

Fire and herbivory are important determinants of nutrient availability in savanna ecosystems. Fire and herbivory effects on the nutritive quality of savanna vegetation can occur directly, independent of changes in the plant community, or indirectly, via effects on the plant community. Indirect effects can be further subdivided into those occurring because of changes in plant species composition or plant abundance (i.e., quality versus quantity). We studied relationships between fire, herbivory, rainfall, soil fertility, and leaf nitrogen (N), phosphorus (P), and sodium (Na) at 30 sites inside and outside of Serengeti National Park. Using structural equation modeling, we asked whether fire and herbivory influences were largely direct or indirect and how their signs and strengths differed within the context of natural savanna processes. Herbivory was associated with enhanced leaf N and P through changes in plant biomass and community composition. Fire was associated with reduced leaf nutrient concentrations through changes in plant community composition. Additionally, fire had direct positive effects on Na and nonlinear direct effects on P that partially mitigated the indirect negative effects. Key mechanisms by which fire reduced plant nutritive quality were through reductions of Na-rich grasses and increased abundance of Themeda triandra, which had below-average leaf nutrients.