An Ecological Perspective
D. D. Briske and R. K. Heitschmidt
- Structure of Ecological Systems
- Function of Ecological Systems
- Energy Flow
- Nutrient Cycling
Grazing and Ecological Systems
- Energy Flow
- An Ecological Dilemma
- A Case Study
- Nutrient Cycling
- Grazing Optimization Hypothesis
Humans, Grazing and Ecological Systems
- Livestock Production
- The Management Dilemma
- Management Strategies
List of Figures
Grazing or herbivory is the process by which animals consume plants to acquire energy and nutrients. Grazing management involves the regulation of this consumptive process by humans, primarily through the manipulation of livestock, to meet specific, predetermined production goals. Both the grazing process and associated managerial activities occur within ecological systems and are therefore subject to an identical set of ecological principles which govern system function. These ecological principles impose an upper limit on animal production which cannot be overcome by management. The fact that both the grazing process and efforts to manage it are influenced by a common set of ecological principles justifies the evaluation of grazing management in an ecological context.
Humankind has historically fostered and relied upon livestock grazing for a substantial portion of its livelihood because it is the only process capable of converting the energy in grassland vegetation into an energy source directly consumable by humans. Biochemical constraints determine that herbivores function as "energy brokers" between the solar energy captured by plants in the photosynthetic process and its subsequent use by humans (Southwood 1985). The inability of humans to directly derive caloric value from the 19 billion metric tons of vegetation produced annually in tropical and temperate grasslands and savannas (24 millions km2; Leith 1978) provides the ultimate justification for evaluating grazing as an ecological process.
Ecological processes associated with grazing have probably not changed appreciably since the initial appearance of grasses and grazers in the fossil record some 45 million years ago (Stebbins 1981). However, a rapidly expanding human population, escalating degradation of natural resources, and increasing socio-economic pressures have all increased the complexity associated with the management of grazed systems. Hundreds of experiments have been conducted and thousands of pages written addressing the ecological processes and plant and animal responses within grazed systems. Unfortunately, little attention has been directed toward an interpretative synthesis of this information. The diverse subject matter encompassing several disciplines (e.g., ecology, animal science, hydrology, economics, and systems science), and the difficultly associated with identifying an organizational scheme capable of encompassing this large body of information have undoubtedly contributed to the lack of subject matter synthesis. The absence of a unified conceptual framework has impeded both the study and management of grazed systems.
The objective of this chapter is to identify and evaluate the fundamental processes associated with grazing within the context of ecological systems. We begin by reviewing the structure and function of ecological systems, follow with an evaluation of the major effects of grazing on energy flow and nutrient cycling within ecological systems, and conclude with a discussion of the ability of humans to regulate the ecological processes affecting plant and animal production within grazed systems. The topic is treated in a broad, conceptual manner to provide the organizational framework necessary to evaluate the relative relationships among specific components and processes within grazed systems. Specific subject matter areas are treated in greater detail in subsequent chapters.
Structure of Ecological Systems
Ecological systems or ecosystems are defined as assemblages of living organisms in association with their physical and chemical environment. The ecosystem concept is intended to demonstrate the interrelationship or interdependence among the various components within a system, rather than to delineate a specific set of organisms within a geographic area (Odum 1971, Begon et al. 1986). Consequently, ecosystems are arbitrarily defined depending upon the interest of the investigator.
The living (biotic) component of ecological systems is classified according to the strategy organisms use to acquire energy and nutrients from the nonliving (abiotic) component. The two most basic strategies are autotrophic (self-nourishing), and heterotrophic (other nourishing) (Odum 1971, Begon et al. 1986). Autotrophs acquire energy from solar radiation by photosynthesis, while heterotrophs acquire energy by ingesting other organisms. Autotrophs or producers include all species of green plants, while heterotrophs or consumers encompass all animal species including microorganisms (Fig. 1.1). The abiotic component defines the physical and chemical components of the system. The ultimate energy source, solar energy, and raw materials (e.g., CO2, H2O and nutrients) necessary to convert solar energy into chemical energy are also present within the abiotic component.
Energy Flow. Energy flow within ecological systems may be viewed in terms of an economic analogy (Gosz et al. 1978). Economics, like ecology, is concerned with the movement of valuable commodities through a series of producers and consumers. A viable ecological system depends on the flow of energy just as a viable economy depends on the exchange of currency. An analysis of energy flow provides a balance sheet for monitoring energy inputs and outputs within a system not unlike a ledger of credits and debits. The magnitude and efficiency of energy flow between various feeding levels within systems can also be evaluated. The initial capture of solar radiation by vegetation, the efficiency of vegetation utilization by herbivores and the efficiency with which ingested energy is converted into animal growth, comprise the major energy transfer processes in grazed systems. Equally important, but less obvious, is the influence energy flow exerts on the managerial components of systems including resource availability, supply-demand ratios and price-market structure.
Solar energy is initially converted into chemical energy by photosynthesis within the chlorophyll-containing cells of plants. The energy captured within plants is subsequently transferred to one of two general categories of heterotrophic organisms (Fig. 1.2). In the absence of herbivores, energy is transferred directly into litter following plant senescence. A series of microorganisms, primarily bacteria and fungi within the soil (decomposers), utilize this organic matter as an energy source eventually releasing heat energy in association with microbial respiration. This pattern of energy flow defines the detrital food chain (Golley 1960, Odum 1971). In the presence of herbivores, a portion of the energy initially captured by plants is consumed and converted into animal tissue. Herbivores, in turn, may be ingested by other consumers at higher feeding levels (i.e., carnivores and humans). Heat energy is released by consumer respiration at each feeding level. This pattern of energy flow through the system defines the grazing food chain. Energy is transferred from the grazing to the detrital food chain in the form of feces and animal tissue following death.
Two thermodynamic laws govern the flow of energy within ecological systems (Golley 1960, Odum 1971). The first law states that energy can be transformed from one form to another (e.g., conversion of solar energy to chemical energy by photosynthesis), but cannot be created or destroyed. The second law establishes that energy transformation processes are not 100% efficient. These laws dictate that a large proportion of the chemical energy, approximately 90%, transferred between feeding (trophic) levels within a system is converted to heat energy which is of limited value to the biotic portion of the system. The energy "loss" between feeding levels results from an inefficient transfer of organic matter (e.g., gaseous, urinary and fecal losses) and the energy required for internal maintenance of organisms (i.e., maintenance energy). For example, only a portion of the solar energy converted into chemical energy by photosynthesis is realized as growth because a portion is utilized in respiration. Similarly, animals utilize a large portion of the total energy ingested for basal metabolism thereby diminishing the amount of energy available for growth or transfer to subsequent feeding levels within the system (see Chapter 2).
The capacity of ecological systems to produce biomass would initially appear limitless given the large, continuous supply of solar energy. However, above-ground primary productivity (plant growth/area/time) is less than 1000 kg/ha/yr in many grasslands (Sala et al. 1988). Primary productivity is limited by two general categories of ecological constraints. The first constraint involves the quality of solar radiation available at the earth's surface. Only about 45% of the solar energy is within the appropriate region of the spectrum to be effective in photosynthesis (Gosz et al. 1978, Begon et al. 1986). The remaining 55% of the spectrum is primarily composed of longwave thermal energy which is unavailable for conversion into chemical energy. However, this energy affects ecological systems, in that it is absorbed as heat energy by the atmosphere, soil and vegetation to generate the thermal environment and power the hydrological and nutrient cycles (e.g., energy required to evaporate water).
The second category of ecological constraints limiting primary productivity involves the occurrence of rigorous abiotic factors which prevent solar energy capture from being maximized. Water, temperature and nutrient limitations frequently prevent a sufficient leaf canopy from developing to intercept the available photosynthetically active radiation (Lewis 1969, Begon et al. 1986). For example, plant canopies may be nonexistent for several months of the year in temperate or arid and semi-arid regions. Similarly, abiotic limitations prevent maximum photosynthetic efficiencies from seldom, if ever, being attained when a plant canopy is present. It is generally estimated that less than 1% of the solar energy at the earth's surface is converted into chemical energy by terrestrial vegetation (Lewis 1969, Begon et al. 1986). This occurs despite the fact that a conversion efficiency of approximately 20% can be realized for individual leaves under ideal environmental conditions before a biochemical limitation is encountered within the photosynthetic process (Lawlor 1987). It is important to note that the amount of solar energy captured in primary production represents the total amount of energy available for utilization by heterotrophic organisms within the system.
Secondary productivity (animal growth/area/time) is also limited by two broad categories of ecological constraints in addition to the availability of primary production. The first involves the inability of herbivores to consistently consume the majority of the primary production produced. Primary production varies widely through time and space making it difficult to balance herbivore density with the fluctuating food resource. That portion of the herbaceous biomass available in excess of current animal demand, senesces within a period of weeks following its production (Parsons et al. 1983, Chapman et al. 1984). Eventually this material enters the decomposer compartment as litter (detrital food chain). In addition, most primary production in grasslands is located below-ground as roots and crowns making it inaccessible to large herbivores (Sims and Singh 1978, Stanton 1988).
The second category of constraints limiting secondary productivity is related to the quality of primary production (nutritional value; see Chapter 2) ingested by herbivores. A substantial portion of the total energy ingested by herbivores is lost as methane (in ruminants), urine or feces and a large portion of the metabolizable energy is utilized in basal metabolism (see Chapter 2). It is only the remaining energy, approximately 10% of the total ingested, which is available for animal growth (Dean et al. 1975, Rode et al. 1986).
Plants initially assimilate many of the essential nutrients from the abiotic environment subsequently used by animals. For example, herbivores can only acquire nitrogen by consuming plants, even though the atmosphere contains approximately 80% nitrogen by volume (Odum 1971, Wilkinson and Lowery 1973). However, as with energy, nutrients may be transferred through either the grazing or detrital food chain within the system (Fig. 1.3). Regardless of the food chain in which they are incorporated, nutrients are eventually returned to their inorganic form following organic matter decomposition by microorganisms within the decomposer compartment (Stanton 1988).
Plant consumption by herbivores (i.e., grazing food chain) introduces an additional feeding level between the primary producers and the decomposers. The objective of this section is to examine how grazing influences energy flow and nutrient cycling within ecological systems. A case study is presented to quantitatively illustrate the influence of grazing on energy flow and demonstrate the utility of the ecosystem concept to the investigation of grazed systems.
An Ecological Dilemma. The percentage of annual above-ground primary production utilized by herbivores varies greatly, but estimates generally range between 20 and 50% (Scott et al. 1979, Detling 1988). Although much higher levels of utilization can occur, in excess of 90%, they are generally restricted to specific regions or years. Insects and small mammals may consume as much as 10 - 15% of the annual above-ground production. An even smaller portion of the total annual primary production is utilized by domestic herbivores because approximately 60 - 90% of the production occurs below-ground in grassland systems (Sims and Singh 1978, Stanton 1988). The portion of annual production not utilized by herbivores is eventually consumed by microorganisms in the decomposer compartment while a small portion is stored as organic compounds within a long-term carbon pool in the soil (Clark 1977).
The fundamental ecological dilemma encountered in grazed systems is the inability to simultaneously optimize the interception and conversion of solar energy into primary production and the efficient harvest of primary production by herbivores (Parsons et al. 1983). Severe grazing ensures that available production is efficiently harvested, but eventually reduces production by minimizing the subsequent capture of solar energy. Alternatively, lenient grazing maximizes primary production, but a large percentage of the production is incorporated into the decomposer compartment without being consumed by herbivores.
Contrasting patterns of energy flow between the grazing and detrital food chains are clearly illustrated in an experiment conducted with two perennial ryegrass pastures grazed by sheep (Fig. 1.4). One of the pastures was grazed leniently (24 sheep/ha) to maintain a leaf area index of approximately three (leaf area:ground area ratio of three) and the other was grazed severely (47 sheep/ha) to maintain a leaf area index of one. The total amount of energy captured by photosynthesis was 43% greater in the leniently grazed pasture as opposed to the severely grazed pasture because a greater percentage of the available solar radiation was intercepted by the plant canopy. However, animal consumption was 40% greater in the severely grazed pasture than in the leniently grazed pasture, even though production was less, because of greater livestock numbers per hectare. The end result was that 42% of the energy captured by ryegrass in the leniently grazed pasture entered the detrital food chain while only 13% was consumed by livestock. By contrast, only 26% of the energy captured by ryegrass in the severely grazed pasture entered the detrital food chain, while 25% entered the grazing food chain. The remainder of the energy captured by photosynthesis (49%) was either allocated to root growth or used in plant respiration. These data illustrate that primary production and efficient biomass utilization cannot be maximized simultaneously because of the contribution of leaf area to both processes.
These production values can easily be converted into energy values by virtue of the fact that 1 kg of plant and animal tissue contain approximately 19.7 and 23.5 MJ (mega [million] joules), respectively (Golley 1961, Odum 1971) (A joule [J] is the expression for energy in the International System of Units; 1 calorie = 4.19 J; 1 J = 0.24 calorie.). The total amount of solar energy received annually at the site provides a convenient reference point with which to compare energy values at various locations within the system. The total input of solar energy at the latitude of the Experimental Ranch (33 20'N) is approximately 63,000,000 MJ/ha/yr (Cinguemani et al. 1978). In this system, approximately 62,705 MJ/ha/yr is captured in herbaceous above-ground vegetation, 313,525 MJ/ha/yr is captured in total (above- and below-ground) herbaceous vegetation (assuming 80% of the primary production is below-ground; Stanton 1988) and 1,257 MJ/ha/yr is transferred into livestock gains (Fig. 1.5). Decreasing energy values from the initial input of solar energy to vegetation and finally to livestock gains clearly demonstrate the inefficiency of energy transfer within the system.
An important aspect of energy flow analysis is that transferefficiencies can be calculated by dividing the amount of energy captured within one level of the system by the amount of energy in a preceding level (Golley 1960, Pimm 1988). Above-ground herbaceous vegetation captured 0.10 and 0.22% of the total solar radiation and photosynthetically active radiation/ha/yr, respectively (Fig. 1.5). A slightly greater conversion efficiency, 0.50%, is observed when total (above- and below-ground) herbaceous annual production is considered. Conversion efficiencies decrease even further when energy transfer into livestock production is calculated because of the energy loss which occurs between feeding levels. Approximately 0.002% of the energy available in total annual solar radiation is transferred into livestock gains in comparison with 2.0% of the energy available in above-ground production. These minimal conversion efficiencies, which progressively decrease with the incorporation of additional feeding levels, are similar to those estimated in other grassland systems (Macfadyen 1964, Coleman et al. 1976, Snayden 1981, Akiyama et al. 1984).
The intrinsic inefficiencies of energy flow in grazed systems should not be interpreted to imply that these systems possess an insignificant potential for secondary production. Grazing could potentially yield an estimated 7.5 trillion MJ of animal production annually from grasslands and savannas of the world assuming that large herbivores consume 20% of the above-ground production and possess a conversion efficiency of 2.0% (Fig. 1.5). These estimates demonstrate the tremendous importance of grazing to the human food supply on a global basis. Substantial increases in secondary production can be attained with only modest increases in ecological efficiencies resulting from effective management strategies. A combined increase of only 0.01% in harvest and conversion efficiencies would potentially increase secondary production by 75 billion MJ.
Grazing may also modify the rate and pattern of energy flow in ecological systems by influencing nutrient availability. Nutrient availability, in turn, governs the efficiency with which organisms acquire and process energy. Grazing affects nutrient cycling by accelerating the rate of nutrient conversion from an organic (amino acids and proteins) to an inorganic form (nitrate and ammonium). This process, termed mineralization, is critical to grassland production because a large proportion of the essential nutrients are bound in organic matter within the soil (Wilkinson and Lowrey 1973, Woodmansee et al. 1978). However, only those nutrients in specific inorganic forms are available for plant absorption.
Grazing increases mineralization by reducing the particle size of plant material (e.g., chewing and rumination) and providing a favorable environment for microbial activity (e.g., high body temperatures; Wilkinson and Lowrey 1973, Floate 1981). Yet, herbivores retain only a small portion of the nutrients consumed, thereby rapidly returning most nutrients to the system in urine and feces. Nutrients excreted in the urine, primarily nitrogen, potassium, magnesium and sulfur, are in the inorganic form and therefore, immediately available for plant absorption (Wilkinson and Lowrey 1973). In contrast, a greater proportion of nutrients in fecal material and ungrazed plant material than in urine are bound in organic compounds and must be mineralized by decomposers prior to plant absorption. Consequently, a portion of the nutrients incorporated into primary production become available for reabsorption more rapidly when transferred through the grazing food chain than when transferred directly into the decomposer compartment (Wilkinson and Lowrey 1973, Floate 1981). Estimates of higher nutrient concentrations in vegetation of grazed than of ungrazed systems support the assumption of increased rates of nutrient cycling (Detling 1988).
Nutrient transfer through the grazing food chain potentially increases the rate of cycling, but in so doing, it also increases the potential for nutrient losses from the system (Woodmansee 1978, Floate 1981). Nutrient losses from grazed systems primarily occur as volatilization, leaching, soil erosion and livestock removal from the system (Wilkinson and Lowrey 1973, Woodmansee et al. 1981). Nutrient losses are highly variable and influenced by a large number of environmental variables including nutrient solubility, soil morphology and chemistry, climate, and topography (see Chapter 5). Nutrient losses associated with herbivore removal from the system are minimized by the limited productivity of many grasslands and the digestive physiology of herbivores (Floate 1981). Nutrient availability is frequently limited by low above-ground productivity containing low nutrient concentrations, 1.5 - 2.0% nitrogen in live and less then 1.0% in dead grassland vegetation (see Chapter 2). In addition, most nutrients ingested by herbivores are voided as urine or feces, thus leaving only a relatively small proportion to be removed as animal products (Wilkinson and Lowrey 1973).
The limited amount of information available indicates that grazing does not increase nutrient losses from the system thereby creating a negative nutrient balance, but this potential does exist (Wilkinson and Lowrey 1973, Woodmansee 1978, Floate 1981). Atmospheric nutrient inputs, 0.3 - 1.0 kg/ha/yr in the case of nitrogen, which represent the largest input into nonfertilized systems, in conjunction with the increased cycling rates, appear sufficient to offset grazing-induced losses from most systems. The large nutrient pool within the organic component of the soil may also buffer nutrient losses in the short-term (Woodmansee 1978). However, additional research is required to more definitively assess the long-term consequences of grazing on the cycling of essential nutrients within grazed systems.
Grazing has traditionally been viewed as having a negative impact on the subsequent rate of energy capture and primary production within grazed systems through a series of direct (see Chapter 4) and indirect affects on plant growth (see Chapter 5 & Chapter 6). However, the grazing intensity necessary to induce a decrease in primary production is difficult to establish definitively. The "grazing optimization hypothesis" suggests that an optimal grazing intensity can potentially increase primary production over that of an ungrazed system (Fig. 1.6). A limited amount of evidence exists to support the grazing optimization hypothesis (Dyer and Bokhari 1976, McNaughton 1979, Hart and Balla 1982, Paige and Whitham 1987), but it does not appear to be a significant ecological process operating on a regular basis in grassland systems (Belsky 1986, Heitschmidt 1990). Illustrations of the grazing optimization hypothesis tend to exaggerate the potential increases in primary production resulting from an optimal level of grazing relative to the potential decreases which may occur in response to severe grazing, that is, the potential increase in production is shown to be equivalent to the potential decrease.
It is important to recognize that much of the data collected in support of the grazing optimization hypothesis were derived from grazed systems where herbivore density and movement were not directly regulated by humans. In these systems, primary production and herbivore density fluctuate widely in a series of continuous feedback loops in response to climatic variation (Sinclair 1975, Walker et al. 1987). Conversely, herbivore density and movement are rigidly restricted in managed systems and precautions are taken to minimize deleterious consequences on animal production. Consequently, the grazing intensity in many, if not all, managed systems may frequently exceed the intensity required to consistently stimulate primary production as indicated by this hypothesis (Heitschmidt 1990). This difference likely explains why the hypothesis originated with researchers working in natural rather than managed systems and why the hypothesis receives limited support from natural resource managers.
Grazed systems are manipulated by humans to meet a diverse set of personal- and/or firm-level production goals (see Chapter 9). The most pervasive of these goals is the maximization of livestock production or profitability on a sustainable basis. The strategies used to attain the desired production goals vary along a continuum of managerial involvement that can be categorized as either extensive or intensive. However, regardless of the managerial strategies employed, livestock production is limited by several constraints intrinsic to ecological systems. The objective of this section is to briefly examine the degree to which managerial strategies can influence the function of ecological systems thereby increasing livestock production within grazed systems.
The inverse relationship between animal production per individual and production per unit land area with increasing grazing intensity is a fundamental production response within all grazed systems (see Chapter 7). This response originates from the combined effects of the following processes: 1) decreasing efficiency of solar energy capture, 2) increasing efficiency of forage harvest, and 3) decreasing conversion efficiency (i.e., the efficiency with which ingested energy is converted into animal products) as grazing intensity increases (Fig. 1.7). Primary production decreases because of a reduction in the availability of leaf area to intercept solar energy. Harvest efficiency increases as an increasing number of animals per unit land area consume plant material before it senesces and is transferred to litter. Conversion efficiency decreases as forage intake restrictions per individual animal limit nutrient and energy availability for growth (Van Soest 1982). The end result is that production per animal decreases as grazing intensity increases while production per unit land area increases. Livestock production per unit area continues to increase with grazing intensity because it is dependent upon both individual animal performance and the total number of animals. Eventually, production per unit area decreases rapidly with increasing grazing intensity because increasing livestock numbers are no longer able to compensate for the limited production per individual animal. Therefore, the grazing intensity which maximizes sustainable animal production per unit area, is that which optimizes the processes of solar energy capture, harvest efficiency, and conversion efficiency within a system.
The primary ecological constraints limiting the magnitude and efficiency of animal production within grazed systems are summarized as follows:
1) The inefficient capture and conversion of solar energy into primary production, frequently less than 1% per yr (Leith 1978, Begon et al. 1986),
2) The limited proportion of total primary production consumed by livestock, less than 20%, considering that 60 - 90% is below-ground (Stanton 1988) and that approximately 50% of the above-ground proportion is grazed, and
3) The inefficient conversion of ingested energy into animal gains, approximately 10% (Dean et al. 1975, Rode et al. 1986).
These constraints are absolute and defy the best intended and designed managerial strategies. Therefore, managerial strategies must be designed to work within, rather than attempt to overcome or circumvent, these ecological constraints.
Many of the problems encountered in grazing management arise from the attempt of humans to sustain high levels of animal production on a continuous basis. The concepts of overgrazing and undergrazing, for example, address managerial or economic considerations to a greater extent than ecological processes associated with grazing (Crawley 1983). Overgrazing refers to situations where improper managerial decisions (e.g., stocking rate) reduce potential livestock production per unit land area by limiting the amount of solar energy captured by species of high nutritive value (e.g., limited leaf area). Similarly, undergrazing refers to situations where inappropriate managerial decisions prevent livestock production from being maximized per unit land area because species of high nutritive value are not fully utilized within the limits of sustainable production.
In systems where herbivore movement and survival are not regulated by humans, the concepts of overgrazing and undergrazing are merely points at which primary production and herbivore density show the greatest oscillation within a dynamic equilibrium. Climatically induced periods of limited primary production function as feedback mechanisms to influence herbivore growth, reproduction and survival (Sinclair 1975, Walker et al. 1987). The plant community is provided with a period for recovery during the interval following the return of normal precipitation, but prior to an increase in herbivore density to predrought levels. Herbivore production is determined by climatically induced variation in primary production within these systems, not vice versa as is frequently the case in managed systems (Pieper and Heitschmidt 1988). The period required for vegetation recovery from grazing is frequently eliminated in managed systems by maintaining high livestock density with supplemental feeding during periods of limited primary production (i.e., feed bag syndrome) or the rapid replacement of livestock immediately following the return of favorable environmental conditions (i.e., sale barn syndrome). For this reason, the potential for resource degradation is often greater in managed than in naturally grazed systems.
The managerial intent to maximize livestock production on a sustainable basis magnifies the associated problems of climatic variation and selective grazing. Climatic variation determines that the optimal grazing intensity to maximize livestock production is variable in both time and space. For example, in the case study presented previously, above-ground herbaceous production averaged 3183 kg/ha/yr, but ranged from approximately 1500 to 4925 kg/ha/yr within a 4-year period. Similarly, harvest efficiency in the set-stocked treatment averaged 42%, but ranged from 20 to 64%. These data demonstrate that flexible stocking is essential for maximizing livestock production from year to year in most grazed systems. Variability in the magnitude and distribution of precipitation among years is typical of most grazed systems, particularly those in arid and semi-arid environments (Sala et al. 1988).
Selective grazing is displayed to various degrees by all wild (see Chapter 8) and domestic herbivores (see Chapter 2 and Chapter 3). Selective utilization of plant species and parts by herbivores in grazed systems frequently decreases harvest efficiency, energy flow within the grazing food chain and ultimately secondary production. For example, in the case study previously presented, only four of the five most abundant herbaceous species were utilized by cattle and the most common shrub species, honey mesquite, was never utilized appreciably (Walker et al. 1989). Consequently, only 17% of herbaceous above-ground production was consumed by livestock while only about 2.0% of the total primary production (i.e., honey mesquite plus total below-ground production) was utilized by livestock. Although stocking rate, animal distribution, season of grazing and mixed-species grazing may minimize selective grazing in some systems, it is impossible to eliminate this problem within most multispecies systems.
Climatic variation and selective grazing frequently interact to affect the magnitude and efficiency of livestock production on both a short- and long-term basis. In the short-term, the amount and relative proportion of primary production by species at a particular location is dependent upon the prevailing environmental conditions and unique physiological requirements of the species (e.g., warm- versus cool-season species). Consequently, the availability and utilization of species varies both seasonally and annually in response to climatic variation and animal preference. Therefore, the appropriate stocking rate to attain optimal harvest and conversion efficiencies also varies within and among years. Stocking rate decisions are difficult to make in a timely manner because the optimal grazing intensity at any given time is dependent upon the occurrence of future climatic conditions. In the long-term, the interaction between climatic variation and selective grazing has a pronounced affect on the rate and direction of ecological succession in grazed systems (see Chapter 4, Chapter 5 and Chapter 6). Grazing-induced modifications of species composition can greatly affect livestock production depending on the specific plant community and management goals.
Grazing managers frequently overlook the fundamental ecological basis for implementing various management strategies. Grazing management strategies are intended to increase livestock production by minimizing the detrimental consequences of inherent ecological constraints on the magnitude and efficiency of energy flow within the system (Williams 1966, Lewis 1969). The managerial strategies used to affect the magnitude and efficiency of energy flow can be categorized as extensive or intensive. Extensive management strategies are primarily implemented on rangelands characterized by low and/or highly variable production. These strategies focus on the temporal and spatial distribution of various species and numbers of herbivores (see Chapter 7). For example, increased stocking rate and mixed species grazing are strategies commonly employed to increase harvest efficiency and forage quality, thereby increasing the amount of energy and nutrients transferred into the grazing food chain.
Intensive managerial strategies rely upon the direct incorporation of energy inputs into a system beyond those associated with extensive management. Examples of energy-intensive inputs include irrigation, fertilization, introduction of improved forage species, and a variety of vegetation manipulation procedures (Pimentel et al. 1980, Klopatek and Risser 1982). This level of managerial involvement exceeds that associated with grazing management perse which is described as the manipulation of livestock in time and space. Although intensive management can substantially increase primary production over that of extensively managed systems, intensive management does not overcome the ecological constraints limiting energy flow efficiencies previously described (Klopatek and Risser 1982).
The introduction of plant species into a system frequently increases primary production by eliminating competing vegetation and replacing it with genetic material selected for high productivity (Pimentel et al. 1980). Species introduction may also minimize selective grazing by replacing multi-species systems with monocultures. Therefore, species introduction affects the function of ecological systems by potentially increasing the efficiency of solar energy capture and the amount and proportion of energy transferred through the grazing food chain. This example demonstrates that management strategies must affect the magnitude and/or efficiency of energy flow if they are to increase livestock production in ecological systems.
Similarly, vegetation manipulation (e.g., noxious plant control through chemical, mechanical or biological means) may increase the grazeable proportion, but not necessarily the total amount of primary production. An increase in total above-ground production following removal of undesirable species requires that productivity of the existing desirable species exceed that of the undesirable species. It is unlikely that this will occur in most systems without the use of additional cultural practices (i.e., seedbed preparation, plant species introduction, etc.). Yet, if total primary production is not increased by the removal of undesirable species, the proportion available for livestock may be greater, thereby increasing the magnitude of energy flow through the grazing food chain.
Intensive management strategies directly incorporate energy into the system in the form of fossil fuels (Pimentel and Burgess 1980, Klopatek and Risser 1982). For example, large amounts of fossil fuels are required for the industrial production of nitrogen fertilizer and its subsequent application. This is also the case for other intensive management practices, including vegetation manipulation, species introduction, etc. An accurate assessment of net energy output from intensively managed systems requires that the fossil fuel input be subtracted from the total system output. When intensive management strategies are evaluated in this manner, they generally display lower ratios of energy output per unit of energy input than do extensively managed systems (Table 1.1). Evaluation in this manner demonstrates that increased production does not result from increased production efficiency within intensively managed systems, but rather from the direct incorporation of energy subsidies into the system (Odum 1971, Klopatek and Risser 1982). Energy subsidies also increase the efficiency of solar energy capture and conversion into primary production by partially overcoming biotic and abiotic limitations to the development of plant canopies and the occurrence of optimal photosynthetic rates (Table 1.1). Many techniques associated with industrialized agricultural are based upon the practice of trading calories of fossil fuel energy for calories of food energy.
The absolute increase in plant and animal production realized from energy incorporated into intensively managed systems minimizes the affect of energy transfer efficiencies on the human food supply. A greater amount of energy is available for human consumption from digestible plant products than from animal products (Odum 1971, Pimentel et al. 1980). For example, if 1000 MJ of energy are available to humans functioning as herbivores by consuming grain, then only approximately 100 MJ are available to humans functioning as carnivores by consuming grain-fed animals. The amount of available energy decreases by a factor of approximately 10 because of the inherent inefficiency of energy transfer between feeding levels, as described by the second law of thermodynamics. In industrial agriculture the conversion of fossil fuel energy to food energy has proven, and will prove economically sound, as long as a relatively inexpensive energy supply is available for its continuation. On the other hand, it must be recognized that a large portion of the primary production consumed by livestock in grazed systems is neither digestible by humans nor dependent upon large energy subsidies (Pimentel et al. 1980). Therefore, grazing functions as an essential intermediary process between solar energy capture by vegetation and energy consumption by humans.
Grazing management can be realistically and profitably evaluated within the context of an ecological system because both the grazing process and efforts to manage it are influenced by a common set of ecological principles. An ecological perspective requires that the ecological processes associated with grazing be identified and organized within the structure and function of ecological systems. Grazing management is intended to minimize the detrimental consequences of several intrinsic ecological constraints on animal and, to a lesser extent, plant production within grazed systems. Management strategies must affect the magnitude and/or efficiency of energy flow if they are to increase livestock production within ecological systems.
The primary constraints limiting production efficiency in grazed systems are summarized as follows:
1) The inefficient capture of solar energy in primary production, frequently less than 1% per year
2) The limited proportion of total primary production consumed by livestock, less than 20%, and
3) The relatively inefficient conversion of ingested energy in secondary production, approximately 10% of the consumed energy.
These constraints are absolute and defy even well intended and effectively designed managerial strategies. Managerial strategies must be designed, therefore, to work within the limits of these constraints, rather than attempt to overcome or circumvent them. Modest increases in the efficiency of energy flow within the limits established by these intrinsic ecological constraints can substantially increase secondary production. An increase in the efficiency of energy transfer from primary to secondary production of only 0.01% (Fig. 1.5) could potentially increase secondary production by 75 billion MJ in grasslands and savannas globally.
The fundamental ecological dilemma encountered in grazing management is the inability to simultaneously optimize the interception and conversion of solar energy into primary production and the efficient harvest of primary production by herbivores. Severe grazing ensures that available production is efficiently harvested, but may eventually reduce production by minimizing leaf area for the subsequent capture of solar energy. Alternatively, lenient grazing maximizes primary production, but a large percentage of the production is incorporated into litter without being consumed by livestock. Grazing management involves the manipulation of kinds and classes of livestock, stocking rate, grazing season and grazing intensity, as implemented through grazing systems, to optimize these two opposing processes and so maximize livestock production per unit land area on a sustainable basis. The managerial task of optimizing primary production and efficient forage harvest is further complicated by climatically induced variation in plant production and the selective grazing typical of various herbivore species.
A thorough evaluation of the components and processes within grazed
systems requires a multidisciplinary effort integrating information from
several disciplines. The nutritional requirements of grazing animals (see
2) and foraging behavior employed to acquire energy and nutrients (see
must be understood to accurately evaluate animal production. Insight into
the effects of grazing on individual plant growth and function, plant population
Chapter 4), structure and function of communities and ecosystems (see
5), and hydrological considerations (see Chapter
6) is necessary to evaluate the influence of grazing on system integrity
and sustainable production. Finally, an understanding of the integrated
effect of these processes on livestock and wildlife production (see Chapter
7 and Chapter
8) and economic considerations (see Chapter
9) is essential for the development of ecologically sound management
strategies within a complex decision environment (see Chapter
Figure 1.1. Generalized description of the structure of ecological systems. The abiotic (non-living) component comprises the physical and chemical environment of the biotic (living) component.
Figure 1.2. Simplified illustration of energy flow through ecological systems. Solar energy is initially captured by primary producers and transferred through at least one consumer feeding level to form the grazing food chain or directly into the decomposer compartment to form the detrital food chain (after Whittaker 1972).
Figure 1.3. Simplified illustration of nutrient cycling within ecological systems. Nutrients move from their respective reservoirs within the abiotic component of the system, into the biotic component, and back into the environment in a cyclic pattern. (after Wilkinson and Lowery 1973).
Figure 1.4. Energy capture and flow (kg carbon/ha/day) within (a) leniently and (b) severely grazed perennial ryegrass pasture. A greater amount of solar energy is converted into ryegrass production in the leniently grazed pasture, but this grazing regime reduces the relative amount of energy consumed by livestock and increases the relative amount of energy transferred into the decomposer compartment in comparison with the severely grazed pasture. (after Parsons et al. 1983).
Figure 1.5. Energy content (MJ/ha/yr) and transfer efficiencies (%) for primary and secondary production in relation to total and photosynthetically active solar radiation at the Texas Experimental Ranch (from Heitschmidt et al. 1987, 1990). Energy values are calculated by multiplying primary and secondary production values by 19.7 and 23.5 MJ/kg, respectively. Conversion efficiencies represent the quotient of two energy values at specified locations within the system multiplied by 100.
Figure 1.6. Three potential responses of primary production to increasing grazing intensity as indicated by the grazing optimization hypothesis. Primary production may: (A) decrease with increasing grazing intensity, (B) remain unaffected until intermediate levels of grazing intensity are attained and then decrease, or (C) increase with increasing grazing intensity to an optimal level and then decrease. (From Detling 1988).
Figure 1.7. Livestock production per individual and per unit area originate from the combined effects of efficient solar energy capture (i.e., primary production), forage harvest efficiency and conversion efficiency in response to grazing intensity (see text for explanation).