Grazing, Effects of structure and species composition This is especially true in attempts to prevent exotics and woody species from altering grasslands Although fire may be a necessary disturbance in some systems, such as tallgrass prairie, fire alone is often not sufficient for restoration of biodiversity After years, in annually burned tallgrass prairie with nitrogen-addition, species diversity was 48% lower than it was years before burning started and 66% lower than in unburned plots In contrast, mowed, annually burned, nitrogen-addition plots had more than twice the species diversity than similar unmowed plots Therefore, mowing prevented a decrease in species diversity that would have occurred with only fire and nutrient addition When bison grazing replaced mowing, plots that were grazed and burned had the highest species diversity of all plots Similarly, in a Mediterranean grassland study, cattle grazing led to an increase in species diversity, whereas burning resulted in no significant difference Forests Many tropical forest trees produce large fruits that either fall near the parent tree or are dispersed by birds, primates, and other large mammals to sites remote from the adult tree Dispersal is so critical for some of these species that their seeds require gut passage through dispersal agents before they can germinate Because of the reliance on dispersers and the heavy predation on seeds near the parent canopy, it has been suggested that seed predators and dispersers are critical for maintaining tree species diversity in tropical forests By selectively preying on seeds that fall near parent trees, specialized seed predators may decrease the survivorship of seeds near parents, preventing the occurrence of monospecific patches of forest trees and facilitating a more even and diverse assemblage of species Because larger animals disperse the seeds of more than half of woody plant species, they play a critical role in removing seeds from sites of high predation and in introducing new species into different patches within a forest The hunting, poaching, and habitat modifications that have left many forests depauperate in these animal dispersers have been suggested to be a cause of lowered plant diversity in tropical forests If this is correct, then effective management of tropical forests may require the replenishment of large-bodied seed dispersers In addition to grazing by larger animals, plants are also attacked by a diverse set of microbial pathogens and parasites Recent investigations have shown that soil microbes may also play important roles in preventing monospecific patches If microbes that attack a common plant build up in soils near that plant, they can lower the probability of that species’ seedlings being successful in that site and thus promote diversity by favoring unrelated plant species in the vicinity of a long-established adult of a different species Livestock introduced to serve as seed dispersers can partially replace native dispersers that disappeared during the Pleistocene megafaunal extinction The feeding activities of these introduced species can increase the range of some plants that produce large, fleshy fruits In a lowland deciduous forest in Costa Rica, introduced horses and cattle feed on the fruits of jicaro (Crescentia alata) and disperse their seeds In areas 15 with livestock, jicaro are common In areas where livestock are absent, jicaro is relatively rare and occurs primarily in small, spatially restricted patches Many seed dispersers subsist exclusively on fruit for at least part of each year This suggests that removal of seed dispersers by anthropogenic activities could have several repercussions First, decreased seed dispersal could eventually lead to a decrease in the number of mature seed-producing parent plants This would in turn lead to a decrease of important wildlife food resources, which could then lead to even lower numbers of seed dispersers This situation can be especially critical if a particular tree species is pivotal in maintaining the health of dispersers during food-limiting seasons For example, Casearia in tropical rain forests produce rich fruit during seasons of relatively low fruit abundance and likely supply much of the diet of local fruit-eating dispersers In addition, this species supports frugivorous birds that are important dispersers of many other tree species during seasons when fruit is more abundant The disappearance of animals that feed on Casearia fruits, leading to decreased recruitment of Casearia, could have far-reaching effects on the forest community This scenario is thought to have occurred with jicaro Frugivorous bats also feed on jicaro, and it is thought that a decrease in jicaro following the loss of Pleistocene seed dispersers led to a decrease in bats and then to a corresponding decrease in other fruitproducing plant species on which bats feed Lakes Attempts to regulate the community composition of lakes have produced a mature science in which the separate and interactive effects of both physiochemical and biotic interactions are reasonably well understood and used in novel ways to manage lake ecosystems Success in these systems has been achieved by fusing an understanding of biotic processes such as competition and predation (traditionally studied by population and community ecologists) with an understanding of the role of physiochemical processes (traditionally studied by limnologists and ecosystem ecologists) Investigations by Steven Carpenter and colleagues have been especially important in stimulating this field Problems of harmful algal blooms, fish kills, and general eutrophication of lakes have become increasingly apparent Initial attempts to explain these problems, and lake productivity in general, as a function of nutrient levels revealed that nutrients could vary considerably in lakes with similar biotic communities and that nutrient levels (also known as bottom-up effects) explained only approximately 50% of the variability in lake productivity This finding prompted lake ecologists to investigate the effects of trophic cascades (or top-down effects) in structuring these ecosystems By understanding and manipulating both the bottom-up effects of nutrients and the top-down effects of consumers, lake ecologists have been remarkably successful at altering biotic communities and fundamental processes (e.g., productivity and nutrient cycling) occurring in lakes Because lake communities are structured by interactions between physiochemical conditions and biotic processes, initial attempts to manage lakes by managing nutrient input met