Large Herbivore Ecology, Ecosystem Dynamics and Conservation available in Paperback
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- Cambridge University Press
Most large herbivores require some type of management within their habitats. Some populations of large herbivores are at the brink of extinction, some are under discussion for reintroduction, whilst others already occur in dense populations causing conflicts with other land use. Large herbivores are the major drivers for forming the shape and function of terrestrial ecosystems. This 2006 book addresses the scientifically based action plans to manage both the large herbivore populations and their habitats worldwide. It covers the processes by which large herbivores not only affect their environment (e.g. grazing) but are affected by it (e.g. nutrient cycling) and the management strategies required. Also discussed are new modeling techniques, which help assess integration processes in a landscape context, as well as assessing the consequences of new developments in the processes of conservation. This book will be essential reading for all involved in the management of both large herbivores and natural resources.
About the Author
Kjell Danell is Professor of Animal Ecology at the Swedish University of Agricultural Sciences. His main research interests are basic and applied plant-animal interactions, community ecology, invasive species and macroecology.
Roger Bergström is Associate Professor at the Swedish University of Agricultural Sciences.
Patrick Duncan is Director of the UPR 1934, Center of Biological Studies of Chizé, CNRS.
John Pastor is Professor and Senior Research Associate at the Natural Resources Research Institute, University of Minnesota.
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Cambridge University Press
0521830052 - Large Herbivore Ecology, Ecosystem Dynamics and Conservation - by Kjell Danell, Roger Bergström, Patrick Duncan and John Pastor
IntroductionPATRICK DUNCAN, KJELL DANELL, ROGER BERGSTRÖM AND JOHN PASTOR
Biodiversity and productivity vary strongly among ecosystems: understanding the causes of these variations is a primary objective of ecology. To date a few overarching principles have been established. One is the species-area relationship: the species diversity of a system depends principally on its area, and some major mechanisms underlying this principle have been identified (Rosenzweig 2,
Although most direct effects on plants, by grazing or browsing, are negative, indirect effects on seed dispersal, in the gut or on the body, are largely positive. Although some of the seeds do not survive herbivore digestive processes, others require passage through the gut of a herbivore for germination, or at least benefit from it. Further, the effects of large herbivores on seeding establishment are generally positive. Effects of large herbivore activities on plant growth and mortality can of course be strong; these are reviewed in relation to the type of tissue which is affected, the extent and frequency of off-take, and the herbivores involved.
Removal of plant parts above the ground inevitably affects below-ground processes as well. Reallocation of resources, at least in grasses, usually leads to increased shoot growth (i.e. to restoration of root:shoot ratios after damage). Above-ground herbivory can also induce changes in mycorrhizal fungi, thereby affecting nutrient uptake and subsequent growth and survival. Most studies show declines in mycorrhizal colonization as a result of herbivory, which can have powerful effects on the dynamics of the plant communities.
Plant responses to herbivores are reviewed, including defences (physical and chemical) and tolerance. Plants can avoid large herbivores through their spatial location, visibility (apparency), or by producing defence structures such as thorns, hairs or thick cuticles. They may also produce ‘allelochemicals’; several hypotheses have been proposed to explain their ecological and evolutionary occurrence: the merits of these hypotheses, particularly the ‘carbon-nutrient balance’ hypothesis, are reviewed. The conditions determining ‘tolerance’ and ‘compensation’ are reviewed: although plants are most commonly detrimentally affected by herbivory, there is a long-running debate as there are examples of exact- or overcompensation in a considerable number of studies. Most of these, however, were short-term responses and might not accurately reflect long-term fitness – an important distinction.
There is a wealth of literature on the impact of large herbivores on plant diversity (species, structure and genetic), but still much controversy. This is probably due to both the complexity of the subject and a scarcity of long-term controlled studies where all main driving factors are understood. Most studies indicate that herbivores are more likely to increase the diversity and spatial heterogeneity of plant communities. However, the authors of this chapter show that there are exceptions, and that the conditions under which herbivores increase or decrease diversity and heterogeneity, and the mechanisms involved, are still not fully understood.
Some of the most interesting effects of herbivory on plant diversity result from the effects of selective herbivory on the relationships among plant functional types, in particular herbaceous vs. woody plants. Ward focuses on the phenomenon of ‘bush encroachment’, as evidence is accumulating that suggests this trend is a general one in arid and semi-arid savannas throughout the world. He illustrates it from arid regions ranging from the Namib and Kalahari deserts, to the Mitchell grass plains of Australia via the southern Sahara, the Negev and central Asian deserts, and reviews the general explanations that have been proposed for bush encroachment. The first is Walter's two-layer hypothesis, based on tree-grass competition. Later models propose that trees and grasses coexist in a state between that of grassland and forest because the plant communities are ‘pushed back’ into the savanna state by frequent disturbances (human impact, fire, herbivory and drought). Ward then describes the results of experiments to test some of these models, and shows that the results open new perspectives for understanding the fundamental processes and for management of bush encroachment. Under the conventional two-layer competition hypothesis, grazing during years with less than average precipitation should be reduced to a minimum so as not to give the trees a competitive advantage. By contrast, the new results suggest that bush encroachment may not occur when water is limited and consequently such a management protocol would be futile.
Grazing responses in arid and semi-arid rangelands in winter rainfall regions differ from those in summer rainfall regions, and plant height may be a more important factor than palatability, life history or taxonomic affiliation in determining responses to herbivory. Ward argues that the ‘classical’ theory of grassland response to grazing which defines plants as increasers or decreasers has some value in explaining plant responses, but should be replaced by a theory which considers plant size and other relevant traits such as palatability and specific leaf area. More studies on more continents are also needed to tease apart the effects of evolutionary history of grazing and abiotic environmental factors on grazing responses and plant functional traits.
There is evidence for the simultaneous existence of all stages of this cycle, and there is no reason to suggest that the rates of regeneration and senescence will be balanced. Rates of tree regeneration and damage by large herbivores can be highly variable. Facilitation by thorny plants will depend on the suitability of the site for the nurse species. Each species of herbivore has a unique pattern of habitat and diet selection. As a result, the impact of large herbivores can lead to dominance of either grassland or of closed-canopy woodland. The effect of large herbivores on nutrient flows can bring about enduring changes in vegetation composition. Since the amount of food for herbivores can be sharply reduced by shade, animal populations will decline if trees grow dense enough to form closed-canopy woodland over an extensive areas, which then limits the extent to which herbivores can maintain openings. In the savanna regions of Africa, switching between woodland and grassland states can occur: as a result of a combination of grazing, elephants and fire, woodlands in the Serengeti-Mara region were opened up in the 1960s, but began to recover again in part of the region during the 1980s, when elephant numbers were severely reduced. These changes suggest that savanna ecosystems may be unstable, or have alternative stable states, and events affecting herbivore numbers or grazing pressure can prompt major changes in vegetation structure.
Evidence from exclosures suggests that the selective browsing by deer tends to reduce tree species diversity. Unfortunately there is insufficient information to generalize for other herbivores in forest habitats, although a study of the impact of elephants found that diversity of trees and shrubs were reduced, but diversity of plants near ground level was increased. A similar result was reported for moose browsing, where diversity of the smallest trees increased, but apparently not in older trees.
In Chapter 7,
These ‘wood pastures’ have an extremely high diversity of plant and animal species, because of the structural diversity of the vegetation. The oak has a special place as a host for insects, since no other species of tree is associated with so many species of insects: more than 50% of all insect species found in Great Britain live in the 20000 hectares of wood pasture in the New Forest alone, and this landscape is habitat for a great variety of bird species, especially songbirds. These observations are clearly highly relevant to current issues of nature management, at the reserve and the landscape scales.
In Chapter 8,
Work in the Negev on the ecologically important and complex interaction between Acacia trees, bruchid beetles and ungulates (which are both important seed predators) shows that, as expected, when ungulates are present, seed dispersal increases, thus reducing competition for the seedlings from the parent trees. However, contrary to the idea that large mammalian herbivores reduce the impact of seed predation by consuming seed pods before they can be infested, the results of this study indicate that ungulate activity does not reduce the impact of the bruchid beetles on seeds. Very few seeds eaten by bruchids germinate, and many eaten by ungulates do: interestingly the germination rate of Acacia seeds eaten by herbivores increases with the body size of the herbivores. The dispersal of Acacia seeds by large mammalian herbivores seems to have affected and, perhaps controls, the distribution of different ecotypes of Acacia trees on a large geographic scale in the Middle East. In conclusion, browsing by ungulates at high densities, reduced the growth rates of the young Acacia, but did not inhibit juvenile Acacia escaping above the browsing level. Negative effects of browsing on juvenile trees may not translate into changes in tree demography because of the enhancement of seed viability and germination by mammalian herbivores.
Patches of faeces and urine can also be viewed as disturbances since they represent very abrupt changes in nutrient availability from the surrounding areas, a point which echoes that made by Frank in Chapter 11. Approximately 2% of an animal's home range is in urine patches at any one time, but if the animal urinates in certain types of sites more often than at random, then, like trampling, the proportion of actually grazed area affected by urination is much higher than expected at random. Hobbs presents equations demonstrating that faecal N deposition increases linearly with plant N concentration and body mass while urine N deposition increases quadratically with body mass and plant N. Therefore, very large herbivores which graze high N forage will return most of their N to the soil as urine, which is more readily available to plants than faecal material since the latter needs to be decomposed by soil microflora. This in turn results in higher plant N, higher productivity, and a higher probability that the herbivore will subsequently graze the urine patch and surrounding area.
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Table of Contents
Preface; Introduction; 1. Large herbivores across biomes Hervé Fritz and Anne Loison; 2. Living in a seasonal environment Jon Moen, Reidar Andersen and Andrew W. Illius; 3. Linking functional responses and foraging behaviour to population dynamics Andrew W. Illius; 4. Impacts of large herbivores on plant community structure and dynamics Alison J. Hester, Margareta Bergman, Glenn R. Iason and Jon Moen; 5. Long-term effects of herbivory on plant diversity and functional types in arid ecosystems David Ward; 6. The influence of large herbivores on tree recruitment and forest dynamics Robin Gill; 7. Large herbivores: missing partners of western-European light-demanding tree and shrub species? Frans W. M. Vera, Elisabeth S. Bakker and Han Olff; 8. Frugivory in large mammalian herbivores Richard Bodmer and David Ward; 9. Large herbivores as sources of disturbance in ecosystems N. Thompson Hobbs; 10. The roles of large herbivores in ecosystem nutrient cycles John P. Pastor, Yosef Cohen and N. Thompson Hobbs; 11. Large herbivores in heterogeneous grassland ecosystems Douglas A. Frank; 12. Modelling of large herbivore - vegetation interactions in a landscape context Peter J. Weisberg, Michael B. Coughenour and Harald Bugmann; 13. Effects of large herbivores on other fauna Otso Suominen and Kjell Danell; 14. The future role of large carnivores on terrestrial trophic interactions: the northern temperate view Reidar Andersen, John D. C. Linnell and Erling J. Solberg; 15. Restoring the functions of grazed ecosystems Ian J. Gordon; 16. Themes and future directions in herbivore-ecosystem interactions and conservation John Pastor, Kjell Danell, Roger Bergström and Patrick Duncan; References; Index.