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a USDA-ARS Pasture Systems and Watershed Management Research Unit, Building 3702 Curtin Road, University Park, PA 16802
b Department of Horticulture and Crop Science, The Ohio State University, 202 Kottman Hall, 2021 Coffey Road, Columbus OH 43210
c Animal Sciences Section, Department of Agricultural Science, Imperial College London, Wye Campus, Kent, U.K. TN25 5AH
d Department of Crop Science, University of Illinois, 1102 S. Goodwin Avenue, Urbana, IL 61801
e School of Natural Resources, University of Nebraska, Lincoln, NE 68583-0814
* Corresponding author (mas44{at}psu.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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Species diversity is a hot topic in grassland ecology research because the reported benefits of biodiversity appear to contradict the high productivity obtained from relatively few species (through high inputs of fertilizers and chemicals) and from an increasingly narrow genetic diversity seen in current agriculture. Some research results indicate that increased plant species diversity increases primary production in grasslands and benefits other ecosystem functions such as nutrient retention and resistance to weed invasions.
These results and concepts have spilled over into other areas (Brummer, 1998), such as forage and grazing land research, and are beginning to influence management recommendations. For example, Tilman et al. (1999) suggested exploring the concept of high diversity grazing lands for livestock production. Similarly, Minns et al. (2001) extrapolated results from the large BIODEPTH (BIODiversity and Ecosystem Processes in Terrestrial Herbaceous Ecosystems) experiment in Europe to agricultural management. It is not clear, however, how these results and concepts relate to managed forage and grazing lands. Most grassland studies reporting the effects of plant species diversity on ecosystem functioning have used cutting, fire, or in some cases no form of herbage removal, rather than grazing. Results under grazing could be very different because of the capacity of grazers to affect plant community diversity and structure (Bullock and Marriott, 2000; Rook and Tallowin, 2003).
Early research on pasture management seemed to advocate relatively complex mixtures of grasses and legumes (Foster, 1988). During the 1950s, however, the emphasis shifted to monocultures of grasses maintained by N fertilizers or simple mixtures of grasses and legumes (e.g., one of each) and management of those mixtures to maintain the legume component (Blaser et al., 1952; Donald, 1963). Blaser et al. (1952) stated that complex mixtures of grasses and legumes were destined to fail because of seedling competition. Thus, forage and pasture management research since that time focused on monocultures or simple forage mixtures.
Grazing lands are an important component of global land use. Roughly 50% of the earth's terrestrial surface is grazed by large herbivores (Frank et al., 1998). Forage and grazing lands form the backbone of profitable forage–livestock systems and contribute substantially to the agricultural economy. Interest in the management of pastures in temperate regions is moving beyond traditional concerns, such as management to optimize the quality and quantity of herbage for animal production, to encompass a new set of issues. These issues cross traditional boundaries between farming, agronomy, conservation, ecology, and landscape management to include sustainability, reduced inputs of fertilizers and pesticides, soil protection, C sequestration, animal biodiversity, resistance to invasion by alien plants and insects, and the aesthetic value of the landscape (Spellerberg et al., 1991; Watkinson and Ormerod, 2001; Krueger et al., 2002). It is within this context that increased biodiversity may play an important role.
In this paper we review recent concepts of plant diversity and their applicability to the management of pastureland. We first discuss the measurement of species diversity and its interrelationship with scale and then explore the relative diversity of plant species in pastures. Then we briefly summarize the ecological research on the relationship between plant diversity and ecosystem function in grasslands. Finally, we consider the available evidence relating plant diversity to ecosystem function in forage and grazing lands and discuss the potential application of biodiversity concepts to pasture management.
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Diversity: Measurement, Scale, and Interpretation |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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Species richness, the most commonly reported diversity measurement, is relatively easy to interpret. For example, Pasture A with 20 plant species per unit area would be considered more diverse than a Pasture B that has 10 species. Richness, however, does not take into account the distribution of the plant species within the pasture. Although Pasture A has more species, those species may not be evenly distributed across the pasture compared with Pasture B. For example, 50% of the species richness in Pasture A could be associated with a previously disturbed area that accounts for less than 1% of the pasture area. Taking species evenness into account and calculating the Shannon-Wiener index, we would probably see that Pastures A and B have similar levels of diversity even though their species richness differs significantly. This example may be extreme, but illustrates a problem with using only species richness as an index of pasture diversity. More important, the evenness at which plant species are distributed should be closely linked to how that diversity affects ecosystem function (e.g., productivity, nutrient cycling). Recent papers have demonstrated the importance of species evenness in explaining productivity and pest invasion in grasslands (Wilsey and Potvin, 2000; Stirling and Wilsey, 2001; Wilsey and Polley, 2002).
Spatial scale strongly influences biodiversity (Crawley and Harral, 2001; Symstad et al., 2003). Two important principles are that (i) species richness increases with the area sampled and (ii) that small-scale (
) diversity varies independently from large-scale (ß) diversity.
At the smallest scale (<1 cm2), species diversity is low because of the physical limitation of the space occupied by a single individual. As the scale or area measured increases, species richness increases, but at a declining rate. Although this positive relationship results partly from the greater variability encountered with larger sampling areas, it also occurs within uniform environments. Because species richness (and biodiversity) is related to the size of area measured, all studies on plant species diversity must report the scale at which observations are made.
Alpha diversity refers to diversity within a close spatial scale or within plant communities. Beta diversity refers to variation in species composition at large spatial scales or between plant communities. Typically in agriculture there is both low diversity resulting from fields of relatively few plant species, and also low ß diversity from those same species being used in adjacent fields, counties and states. At large (farm and region) scales we have the practical option of using small-scale () diversity within fields, and also larger-scale (ß) diversity of using forage seed mixtures customized for the environmental, topographic and management conditions of each field or region. Such specificity might include both the number and identity of species used.
Thus, measuring and evaluating species diversity on grazing lands is more than simply counting or listing the number of species encountered. Evaluating species richness without taking into account evenness and spatial scale effects could underestimate the importance of diversity in shaping the function of pasture ecosystems.
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Extent of Plant Diversity in Pastureland |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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In the early 20th century, there were several surveys of the botanical composition of grazing lands in the northeastern USA (e.g., Sprague and Reuszer, 1928; Johnstone-Wallace, 1933; Pierre et al., 1937). Although these surveys did not address plant species diversity per se, they are instructive in comparing trends. The predominant species found in earlier surveys included Poa, Agrostis, and white clover and the weedy grasses sweet vernalgrass (Anthoxanthum odoratum L.) and poverty grass (Danthonia spicata L.). Frequently occurring weedy forbs included yarrow (Achillea milleflorum L.), hawkweed (Hieracium canadense L.), and buttercup (Ranunculus acris L.). Most of the weedy grasses and forbs were typical of low soil fertility conditions (Cooper et al., 1929). A recent survey also showed dominance by Poa and white clover in northeastern grazing lands (Tracy and Sanderson, 2000); however, the dominant weedy species included quackgrass [Elytrigia repens (L.) Desv. Ex Nevski], broadleaf plantain (Plantago major L.), and dandelion (Taraxacum officinale Webber in Wiggers). The weedy species in the recent survey are more typical of higher soil fertility conditions. The data indicated that changes in grazing land management including new forage species and cultivars along with improvements in soil fertility management likely contributed to the changes in botanical composition during the 20th century (Sanderson et al., 2001). Even in cases where management has not changed, the species composition and diversity of grazing lands in northern Europe and Great Britain have changed in the last few decades because of chronic nitrogen loading from air pollution (Bobbink, 1991; Smith et al., 1999) For example, regional atmospheric N deposition rates in the Netherlands have exceeded 50 kg N ha–1 since the 1980s. Rates of N deposition in the North American Midwest and Northeast, although not as high as those seen in Europe, may have similarly affected the species composition and diversity of managed and natural grasslands (Wedin and Tilman, 1996).
In one of the few recent studies available on the degree of plant diversity in pastures, total plant species richness of northeastern grazing lands ranged from 16 to 49 species 1000 m–2 with an average of 32 species 1000 m–2 (Tracy and Sanderson, 2000; Sanderson et al., 2002). The Shannon-Weiner index of northeastern grazing lands ranged from 0 to 3.0 with an average of 1.6 (based on 1-m2 quadrats) and evenness (J) ranged from 0.13 to 1.00 with an average of 0.75. Perennial and annual forbs, along with perennial grasses, dominated the above ground species pool, whereas bluegrass (Poa pratensis L.) and white clover dominated the soil seed bank (Tracy and Sanderson, 1999). In comparison, Stohlgren et al. (1999) reported a range of nine to 50 native plant species and one to eight exotic plant species 1000 m–2 at several sites in Rocky Mountain grasslands. Species evenness in that study tended to be greater than reported for managed pastures in the northeastern USA. Species richness in traditionally managed grasslands (i.e., species-rich ancient grazing lands such as chalk grassland and heathland) in northwest Europe ranged from 50 to 60 species 100 m–2, whereas more intensively managed grasslands contained 10 to 20 species 100 m–2 (Peeters and Janssens, 1998).
In North America, managed grazing lands are often assumed to be less diverse than natural grassland ecosystems. The European experience with traditional grazing lands calls this generalization into question, however. Rangeland research also emphasizes the role of grazers, domestic or native, in maintaining grassland diversity (Collins et al., 1998; Milchunas et al., 1998). Information is lacking, however, on the level of plant diversity that actually occurs on farms. Producers frequently manage several different forage species in pastures whether grown in complex mixtures or spatially distributed among different pastures.
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Plant Diversity and Ecosystem Function |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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The proposed mechanisms behind the observed responses to plant diversity in grasslands include (i) the "sampling effect," resulting from the greater chance of including more productive species in highly diverse plant communities (Huston, 1997; Wardle, 1999); (ii) facilitation, whereby the presence of one species increases the growth or survival of another species (Callaway and Walker, 1997; Brooker and Callaghan, 1998; Callaway, 1998); (iii) niche differentiation–niche separation, or greater coverage of habitat caused by a wider range of species traits in a more diverse community; and (iv) the "insurance effect," where a highly diverse plant community is buffered from environmental extremes by having some species that are tolerant of different stresses and thereby stabilize productivity (McNaughton, 1977; Ives et al., 2000; Yachi and Loreau, 2001). Physiological and phenological diversity in complex plant communities may allow for complementarity among species and result in more efficient use of soil, water, air, and light resources compared with simple plant communities (Hector, 1998).
The ecological literature, although reporting on relatively recently planted experimental grassland plots, has emphasized long-term benefits of diversity, perhaps in an attempt to relate the results to permanent, natural ecosystems. An obvious and common example of species diversity in grassland establishment would be the use of cover or nurse species, such as annual grains, to facilitate slow growing perennial grasses and legumes. This phenomenon is at the center of some of the controversy in the ecological literature regarding the interpretation of results from diversity experiments. Huston (1997) argued that the increased productivity of high diversity experimental plots reported by Tilman et al. (1996) was simply a consequence of those plots having fast growing, weedy species, particularly yarrow and black-eyed susan (Rudbeckia hirta L.). Thus, the mechanism responsible for the diversity effect was the sampling effect, and the effect would be short-lived. Later analyses by Tilman et al. (2001) indicated that the diversity effect observed initially in their experiment was largely a sampling effect with niche differentiation becoming more important when the plantings were several years old. Thus, it is likely that the observed effect of diversity on productivity and the mechanism responsible will change with time, especially for relatively young or newly planted grasslands. Even if the positive effects of diversity turn out to be short-lived in managed grasslands, the "nurse crop" effect may be important considering the effort and expense involved with grassland establishment.
Although there is a general consensus that diversity benefits ecosystem function, there are reports that indicate no general benefit of increased plant diversity (e.g., see Huston et al., 2000; Wardle et al., 2000). Frequently, highly productive agricultural systems rely on low plant species diversity [e.g., row-crop production; alfalfa (Medicago sativa L.) hay, etc., Huston, (1994)]. Others, however, have argued that the benefits of plant species diversity depend on the environment and the spatial scale considered (Fridley, 2001).
Trenbath (1974) reviewed several studies involving species mixtures in agricultural systems and suggested that a true benefit from increased diversity occurred only when mixtures yielded more than the most productive species in monoculture. This stricter standard is appropriate for agriculture where the best performing species or mixtures are selected and managed for maximum productivity. Total yield, however, is not the only criterion for evaluating the potential benefits of increased biodiversity in grazing lands. Seasonal distribution of yield, amount and costs of inputs needed to achieve maximum yield (Ruz-Jerez et al., 1991; Kanneganti et al., 1998), as well as nonagronomic benefits (e.g., improved wildlife habitat) from increased biodiversity must also be considered (Table 1).
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Yield and Diversity—A Question of Cause or Effect?
Much of the ecological literature has focused on the relationship between diversity and yield, with yield the independent variable and diversity the dependent variable (Tilman and Pacala, 1993; Schmid, 2002). In the debate on the relationship between diversity and ecosystem function, the discussion currently focuses on productivity and its response to diversity (i.e., yield is the dependent variable and diversity the independent variable; e.g., Tilman et al., 1996; Hector et al., 1999). We focus on the yield vs. diversity relationship, with diversity the independent variable and yield the dependent variable. This relationship has a mechanistic interpretation because the species present in the vegetation are the actual components of the resultant yield. Where positive species interactions occur (such as with nitrogen fixation, species sheltering other species, or synergistic benefits of water uplift or rooting depth), we might see positive responses of diversity and yield. Where simple replacement occurs, we might see no response of yield to diversity. Where competition occurs (e.g., competitive suppression, allelochemical effects), we might see negative effects of diversity on yield.
On occasions where there is a positive response between diversity and yield, the debate between diversity vs. yield or yield vs. diversity might be one of semantics, because the essential relationship is identical. However, where nonlinear responses (e.g., the unimodal response proposed by Waide et al., 1999) are proposed, the debate between diversity vs. yield or yield vs. diversity is significant because these responses and the causative biology are not the same.
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Evidence for Diversity Effects in Pastureland |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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Morphological and Physiological Interactions in Diverse Forage Plant Communities
Diversity theory suggests that greater plant diversity buffers plant communities from environmental extremes. For example, the productivity of grazing lands during summer drought could be improved by sowing a percentage of pastures to warm-season grasses (Skinner et al., 2002) or by planting multispecies mixtures that include some of the more drought resistant cool-season grasses and forbs (Lucero et al., 1999). Interactions among species within complex mixtures may also improve the ability of normally drought-sensitive species to maintain production under stressful conditions.
Some evidence suggests that growing drought sensitive species in complex mixtures rather than as monocultures might improve their productivity during dry years (Lucero et al., 1999; Trenbath, 1974). Many deep-rooted rangeland species can transfer water from relatively moist, deep soil layers to dry layers near the soil surface (Caldwell et al., 1998) by a process known as hydraulic lift (Richards and Caldwell, 1987). Neighbors near species that lift water hydraulically can use a significant proportion of that water resource, effectively ameliorating the adverse effects of drought (Dawson, 1993). Through niche separation (Tilman, 1999), deep-rooted species might also preferentially use water from lower soil levels, leaving more surface water available for shallow-rooted species. Similarly, Berendse (1982) suggested that the association of deep- and shallow-rooted grassland species in mixture could cause greater nutrient extraction from deeper soil layers by the deep-rooted species than would normally be observed in monoculture. Nutrients taken up by deep-rooted species can also be deposited on the soil surface via litter fall and thus made available to shallow-rooted species (Callaway, 1995).
Neighboring plants can also favorably alter other environmental conditions. For example, shading by larger plants can lower soil temperature, reducing heat stress effects while also reducing evapotranspiration leading to improved leaf water relations of smaller neighboring species (Wilson, 1996; Shumway, 2000; Carrillo-Garcia et al., 2000). In a New Zealand study, deferred grazing of perennial ryegrass (Lolium perenne L.)–white clover pastures during the summer resulted in increased herbage accumulation which, in turn, led to lower soil temperatures, increased soil moisture, and improved white clover survival compared with conventionally grazed pastures (Harris et al., 1999). Sheltered plants can also benefit in cold environments where shade from neighbors provides protection from the direct effects of low temperature due to radiation frost as well as protection from photoinhibition resulting from the combined effects of low temperature and high irradiation (Egerton et al., 2000). Shading by salt marsh plants that limits surface evaporation can also reduce the accumulation of soil salts (Bertness and Hacker, 1994). In addition, oxygenation of submerged soils by aerenchymous plants can enhance nutrient availability and increase survival of nonaerenchymous neighbors (Callaway, 1995).
Most examples of facilitative interactions in the literature are from extreme environments such as desert and alpine ecosystems. However, as Harris et al. (1999) observed, amelioration of less extreme environments can also occur. Additional research is needed to determine the importance of positive plant-plant interactions in temperate pasture systems.
Diversity and Nutrient Cycling in Forage Plant Communities
Aboveground plant composition can strongly affect nutrient cycling rates in pastures. Probably the best example is the well documented effects of legume addition to grass mixtures and subsequent increases in soil nitrogen availability because of N fixation (Haynes and Williams, 1993). Beyond this simple fertilization effect, aboveground plant diversity may also influence nutrient cycling through microbial decomposition of plant litter.
Most studies on decomposition of plant litter have evaluated decomposition dynamics using single species assays (e.g., Cornelissen, 1996; Kalburtji et al., 1998; Koukoura, 1998). But in multispecies plant communities, like pasture, litters of different plant species usually decompose mixed together. In such a situation, litters that differ in quality (e.g., N concentration) may interact antagonistically or synergistically to produce decomposition dynamics that differ from those predicted from species decomposing alone. Plant diversity may also modify the microenvironment for decomposition (Hector et al., 2000). Contrasting results concerning plant diversity effects on litter decomposition in experimental systems [e.g., positive effects (Bardgett and Shine, 1999); no effects (Wardle et al., 1997), or mixed results (Hector et al., 2000; Knops et al., 2001)] demonstrate that much more remains to be done to clarify how aboveground plant diversity affects belowground processes in grazing land (Wardle, 2002).
Increasing the diversity of grassland plant communities may increase nutrient retention. Soil nitrate levels, both within and below the rooting zone, were reduced as the number of plant species increased in growth chamber studies and in tallgrass prairie communities (Naeem et al., 1994; Tilman et al., 1996). On serpentine grasslands, diverse mixtures of plants used total resources more completely compared with simpler plant communities (Hooper and Vitousek, 1998). Whether reductions in soil nitrate were caused by true complementarity among different species (i.e., a diversity effect), reduced soil N mineralization (Reich et al., 2001), or by one deep rooted, species with high N uptake rates is not clear. In pastureland, deep rooted species like alfalfa have been shown to lower water tables and reduce nitrate losses when seeded into grass mixtures (Cransberg and MacFarlane, 1994; Owens et al., 1994), so it is possible that one species could have a dominant effect on nitrate uptake in some communities. For example, increasing the diversity of nonleguminous species grown with legumes in grassland mixtures could help reduce nitrate leaching species while still benefiting production through N fertilization (Scherer-Lorenzen et al., 2003). Given the potential for environmental problems associated with nitrate losses in pasture-based systems, more tests of this concept are needed under realistic grazing conditions and backed up with experiments that evaluate nitrate losses from mixtures and monoculture treatments (Hooper and Vitousek, 1998).
Forage Productivity Trials with Diverse Mixtures
Much of the early applied research on complex forage mixtures was done in clipping studies to screen various combinations of forages. For example, early research in Connecticut compared 50 different single and multiple species combinations of grasses and legumes (1, 2, 3, 4, or 7 species) for yield under clipping (Brown and Munsell, 1936). The range in herbage yield was large for the single species and two species plots, but there was no significant trend in yield with increasing seeded species richness (Fig. 1)
. The yields of the complex mixtures were mid range of the two-species mixtures. The highest yielding two-species mixture included a large-leaved (Ladino type) white clover cultivar, which is more adapted to infrequent cutting. In most of the mixtures, the legume component had nearly disappeared by the third year probably as a result of the relatively lax cutting interval.
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Simple and complex mixtures (1 to 12 species) of grasses, legumes, and a forb were compared for herbage yield under grazing in a multilocation trial in Ohio and Pennsylvania (Barker et al., 2003). In all seasons and at all sites, there was a positive relationship between the number of species sown and forage production (Fig. 5) . In Ohio, where summer–fall production was more severely affected than in Pennsylvania, this relationship was less pronounced than in other instances. The highest yielding treatment in most seasons and sites had only one or two sown species; however, in most instances, this was not significantly different from the nine-species treatment. The identity of the highest yielding treatments varied among sites and seasons. It was concluded that although maximum forage yield might occur for monocultures or two-species mixtures of the best adapted species, the difficulty in predicting which species to use, and variation in the best species between spring and summer, suggests forage production might be most consistently maximized from planting complex mixtures.
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Animal Productivity Trials with Diverse Mixtures
Grazing animals have a key role in affecting plant species diversity in grazing lands (Rook and Tallowin, 2003); however, there are few data on how biodiversity affects animal performance. The paucity of studies probably reflects the large spatial scale and inputs required to conduct replicated field experiments where animal performance is measured from pastures sown to different diversity. Animal productivity in a grazing system is a function of the output per animal (e.g., milk per cow, gain per head; a measure of forage quality) and the number of animals that a unit of grazing land will support (Mott and Moore, 1985). Voluntary dry matter intake and stocking rate are key determinants of animal performance on pasture. The botanical composition and population of grazing land along with the morphology and structure of the sward affect the amount of herbage grown and consumed (Hodgson, 1990).
Research on New Zealand high-country grazing lands showed that species richness and evenness were weakly associated with sheep carrying capacity or stability of production (coefficient of variation in annual carrying capacity; Scott, 2001). Grazing research with lactating dairy cows indicated that there was no benefit to planting a complex mixture of grasses and legumes for grazing (Wedin et al., 1965). Dairy cows grazed on replicated pastures of N-fertilized grass, a simple (four species) grass–legume mixture, and a complex (eight species) grass–legume mixture. Nitrogen-fertilized grass was the most productive (greatest carrying capacity), whereas individual cow milk production was similar among treatments (Table 2). Both the simple and complex mixtures were primarily dominated by alfalfa-smooth bromegrass (Bromus inermis Leyss.), with less than 10% of the other seeded species present.
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More animal production studies are needed to ascertain the effects of species diversity on per head and per hectare production along with effects on the composition of animal food products (e.g., Carpino et al., 2003). Such studies must be long term to consider whether the effects of species diversity on the seasonal supply of herbage (Daly et al., 1996) contribute to improved animal performance. They must also be long term to consider the dynamic interaction between grazing and plant species diversity (Provenza et al., 2003), in particular how changes in species diversity under grazing (e.g., abundance of preferred/non preferred herbage) affects animal performance.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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Despite the limited research, we believe that enough evidence exists to support some general recommendations. First, diversity is not simply a numbers game. The proportional abundance of species, their unique attributes, and their spatial distribution across the landscape are critical features in pasturelands. Thus, a highly diverse system may not be appropriate for a highly productive, stable environment where the objective is simply maximum forage production. Most temperate grazing lands, however, are highly heterogeneous in soil resources, climate, and landscape and often fulfill multiple functions for producers (e.g., animal production, resource protection, and wildlife enhancement). It is in these situations where greater plant diversity may be most beneficial.
Second, research on plant diversity in temperate pastures must move beyond small-scale experiments dealing mainly with diversity and primary production. More data are needed from grazing trials that measure animal productivity, behavior, and selection on a range of species mixtures at relevant scales so that practical recommendations can be made for grazing management. In particular, we must determine how changes in diversity associated with grazing may affect the benefits of planting species-rich mixtures. Vegetation has pronounced spatial patterns; however, we do not fully understand the significance and practical benefits of this variability. Systems research is needed on using diversity at the farm scale where combinations of simple forage mixtures or monocultures are used in several pastures to complement work on complex intimate mixtures of forages in single fields. Finally, embracing a multifunctional view of grazing lands to include environmental benefits as well as productivity opens the door for greater use of biodiversity in sustainable grazing land management.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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Received for publication June 9, 2003.
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TOPNOTESABSTRACTINTRODUCTIONDiversity: Measurement, Scale,...Extent of Plant Diversity...Plant Diversity and Ecosystem...Evidence for Diversity Effects...CONCLUSIONSREFERENCES |
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