Mark E. Ritchie

Effects of herbivores on grassland plant diversity.

The role of herbivores in controlling plant species richness is a critical issue in the conservation and management of grassland biodiversity. Numerous field experiments in grassland plant communities show that herbivores often, but not always, increase plant diversity. Recent work suggests that the mechanisms of these effects involve alteration of local colonization of species from regional species pools or local extinction of species, and recent syntheses and models suggest that herbivore effects on plant diversity should vary across environmental gradients of soil fertility and precipitation.

HERBIVORE EFFECTS ON PLANT AND NITROGEN DYNAMICS IN OAK SAVANNA

Herbivores can often control plant dynamics by mediating positive feedbacks in plant species’ influence on nutrient cycling. In a 7-yr field experiment in a nitrogen-limited Minnesota oak savanna, we tested whether herbivores accelerated or decelerated nitrogen (N) cycling through their effects on plants. We measured effects of excluding insect (primarily Orthoptera and Homoptera) and mammalian herbivores (primarily white-tailed deer, Odocoileus virginianus) on above- and belowground plant biomass, plant species composition, plant tissue nitrogen concentration, available soil N and light, and total N and carbon (C) in different pools (soil, roots, litter, etc.). Herbivore exclusion strongly increased cover and biomass of the legume Lathyrus venosus and a few species of woody plants. These effects were associated with greater aboveground standing crop, reduced belowground standing crop, and reduced light penetration to the ground surface. Herbivore exclusion also modified N cycling through greater N content of live, aboveground plant tissue early in the growing season and of litter and belowground tissue late in the growing season. Herbivore exclusion also increased soil nitrate and total available N concentrations but did not alter total soil or plant N. Our results support the hypothesis that herbivores indirectly decelerate N cycling by decreasing the abundance of plant species with nitrogen-rich tissues. However, other factors, such as disturbance from fire, may mediate herbivore effects on long-term changes in N and C pools. Herbivores may therefore indirectly control productivity, N cycling, and succession by consuming nitrogen-fixing and woody plants that have strong effects on plant resources (e.g., nitrogen and light).

Global environmental controls of diversity in large herbivores

Large mammalian herbivores occupy half of the earths land surface and are important both ecologically and economically, but their diversity is threatened by human activities. We investigated how the diversity of large herbivores changes across gradients of global precipitation and soil fertility. Here we show that more plant-available moisture reduces the nutrient content of plants but increases productivity, whereas more plant-available nutrients increase both of these factors. Because larger herbivore species tolerate lower plant nutrient content but require greater plant abundance, the highest potential herbivore diversity should occur in locations with intermediate moisture and high nutrients. These areas are dry enough to yield high quality plants and support smaller herbivores, but productive enough to support larger herbivores. These predictions fit with observed patterns of body size and diversity for large mammalian herbivores in North America, Africa and Australia, and yield a global map of regions with potentially high herbivore diversity. Thus, gradients of precipitation, temperature and soil fertility might explain the global distribution of large herbivore diversity and help to identify crucial areas for conservation and restoration.

Fractal geometry predicts varying body size scaling relationships for mammal and bird home ranges

Scaling laws that describe complex interactions between organisms and their environment as a function of body size offer exciting potential for synthesis in biology. Home range size, or the area used by individual organisms, is a critical ecological variable that integrates behaviour, physiology and population density and strongly depends on organism size. Here we present a new model of home range–body size scaling based on fractal resource distributions, in which resource encounter rates are a function of body size. The model predicts no universally constant scaling exponent for home range, but defines a possible range of values set by geometric limits to resource density and distribution. The model unifies apparently conflicting earlier results and explains differences in scaling exponents among herbivorous and carnivorous mammals and birds. We apply the model to predict that home range increases with habitat fragmentation, and that the home ranges of larger species should be much more sensitive to habitat fragmentation than those of smaller species.

Nitrogen limitation and trophic vs. abiotic influences on insect herbivores in a temperate grassland

Plant resources, predators, and abiotic conditions represent three major fac- tors that potentially influence insect herbivore abundance in terrestrial ecosystems. In ni- trogen (N)-limited environments the potential for bottom-up (plant resource) control is strong because plant quality may limit herbivore abundance. However, extremes in abiotic conditions, such as temperature and moisture, can mask such effects. I tested these hy- potheses in an 8-yr field experiment that measured responses of plants and grasshoppers (Orthoptera) to N addition and exclusion of bird predators in an N-limited old-field prairie in east-central Minnesota. Plant biomass increased by 150-400%, and plant tissue N in- creased by 78% in response to N addition of 17 g·m 22 ·yr 21 . Total grasshopper density responded positively to N addition following warm years. However, multiple regressions suggested that grasshopper densities were related much more strongly to thermal conditions than to soil N. Bird exclusion yielded weak effects that varied over time and may have been influenced by compensatory responses of other grasshopper predators to bird exclo- sures. Grasshopper feeding guilds differed in the relationship between their in vivo dry- matter digestibility (DMD) and plant tissue N, and this physiological difference explained their drastically different responses to N addition. Plant quality (in vivo DMD) increased with plant tissue N for mixed-feeding grasshoppers, and accordingly, their density was positively correlated with soil N. Plant quality did not change with plant tissue N for grass feeders, and their densities were negatively correlated with soil N. Both guilds responded positively to warmer thermal conditions, but mixed-feeder densities were negatively related to previous years precipitation, and grass-feeder densities were not affected by precipi- tation. These results support the hypothesis that bottom-up influences of insect herbivores can be important in N-limited systems but do not support the hypothesis that more productive environments necessarily support greater top-down influences. Thermal conditions may interact with or eliminate bottom-up effects. Furthermore, different guilds within the her- bivore trophic level may be influenced differently by N addition, predators, and abiotic conditions. These results suggest that exploring the mechanisms of interaction between abiotic and trophic influences within components of food webs is likely to yield many new insights into the regulation of herbivore communities.

A disease-mediated trophic cascade in the Serengeti and its implications for ecosystem C.

The removal of rinderpest had cascading effects on herbivore populations, fire, tree density, and even ecosystem carbon in the Serengeti ecosystem of East Africa.

Fragmented nature: consequences for biodiversity

We discuss how fragmentation of resources and habitat operate differently on species diversity across spatial scales, ranging from positive effects on local species coexistence to negative effect on intermediate spatial scales, to again positive effects on large spatial and temporal scales. Species with different size and mobility can be regulated by different processes at the same spatial scale, a principle that may contribute to diversity. Differences in species richness between local communities may be differentially regulated at larger spatial scales. This causal connection between local and regional processes has several practical conservation implications. We furthermore show that fractal geometry can be a valuable tool in the separation of the effects of habitat loss (percentage cover), habitat fragmentation (contiguity) and habitat (texture). Especially important may be the notion that the same effective degree of fragmentation can exist at in a very aggregated habitat (one big patch) and a very sparse, random landscape (many small, nearby patches). The geometric behaviour and covariance of these three basic parameters of spatial configuration needs further investigation. The fractal approach is tested using data on heathland habitat configuration and biodiversity in 36 Dutch landscapes of each 9 km � 9 km. Fractal geometry was indeed successful in separating the patterns of habitat loss from habitat fragmentation in a subset of the landscapes, despite covariance across all landscapes. Habitat loss and habitat fragmentation both had a negative effect on diversity of heathland breeding birds, while only habitat loss seemed to affect butterfly species richness. We conclude that fractal geometry seems to be a promising approach for linking population and community processes to landscape spatial structure. # 2002 Elsevier Science B.V. All rights reserved.

Responses of Legumes to Herbivores and Nutrients During Succession on a Nitrogen-Poor Soil

We measured legume abundance following 13- and 5-yr experiments testing for the effects of mammalian herbivores, nutrients, and climate on plant communities in an old field and a savanna in east central Minnesota. Total legume abundance was signif- icantly greater in plots with herbivores excluded. Within herbivore exclosures, legumes were more abundant in plots to which P, K, S, Mg, Mn, Ca, Na, and trace minerals had been added. Legumes were significantly more abundant in the savanna than in the old field. Lathyrus venosus, a rapidly growing early-maturing species, was largely responsible for these results. Two other common legumes at our study site, Amnorpha canescens and Les- pedeza capitata, were not significantly affected by herbivore exclosures. However, within herbivore exclosures, addition of nutrients significantly reduced Lespedeza. Late summer total legume biomass within herbivore exclosures increased strongly following exclosure establishment in 1982, declined dramatically following a severe drought in 1988, and then increased again following the drought. This trend suggested that herbivore effects we measured in 1994 restUlted from long-term accumulation of legumes following the estab- lishment of exclosures. Our results suggest that herbivores and nutrients other than nitrogen can dramatically limit the abundance of some legume species that might otherwise dominate grassland plant communities on nitrogen-poor soils. Limitation of legumes by colonization and drought may also be important. Thus, herbivores and nutrients other than nitrogen may be critical in structuring grassland plant communities and influencing succession, even on nitrogen-poor soils.

Introduced grazers can restrict potential soil carbon sequestration through impacts on plant community composition.

Grazing occurs over a third of the earths land surface and may potentially influence the storage of 10(9) Mg year(-1) of greenhouse gases as soil C. Displacement of native herbivores by high densities of livestock has often led to overgrazing and soil C loss. However, it remains unknown whether matching livestock densities to those of native herbivores can yield equivalent soil C sequestration. In the Trans-Himalayas we found that, despite comparable grazing intensities, watersheds converted to pastoralism had 49% lower soil C than watersheds which retain native herbivores. Experimental grazer-exclusion within each watershed type, show that this difference appears to be driven by indirect effects of livestock diet selection, leading to vegetation shifts that lower plant production and reduce likely soil C inputs from vegetation by c. 25 gC m(-2) year(-1). Our results suggest that while accounting for direct impacts (stocking density) is a major step, managing indirect impacts on vegetation composition are equally important in influencing soil C sequestration in grazing ecosystems.

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.