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close this bookThe Improvement of Tropical and Subtropical Rangelands (BOSTID)
close this folderPart I
close this folderSite evaluation
View the document(introduction...)
View the documentAn ecosystem perspective
View the documentA systems approach to site evaluation
View the documentEvaluation of abiotic and biotic components
View the documentIntegrated evaluations
View the documentReferences


One of the key elements of a range improvement program is the proper selection of sites for demonstration projects. To ensure success, a site must be carefully chosen and must be typical of the particular ecosystem being considered for an extensive program.

The types of information that are needed at the larger scale, more extensive planning level, and the methods by which this information might be gathered and subsequently used, have been described in the preceding chapter. Analysis of this information should help identify specific areas for potential improvement. In many instances, however, a more detailed site evaluation is required before initiating activities. A preliminary site evaluation can be achieved, in part, through an intensive search of the literature. On-site surveys and observations may still be necessary to complete the site assessment and fully evaluate the site's potential for improvement.

An ecosystem perspective

A site for a proposed range improvement project or program is a microcosm of a larger ecosystem. Regardless of how it is delineated, an ecosystem is the basic unit of ecology, typically a complex system, comprising the physical setting, plants, animals, and its human population. Compounding this complexity is the fact that an ecosystem is almost always changing, even in semiarid and arid rangelands.

The natural process of change in the composition of an ecosystem is referred to as succession. Successional changes take place in response to natural or man-made influences in the environment. So-called primary succession happens on newly exposed areas, such as landslides or sand dunes, whereas secondary succession occurs after the previous vegetation has been destroyed or disturbed by fire or agricultural practices, for example. In many areas of Africa and Asia, a disclimax (a climax maintained through disturbance) has been established through savanna burning and heavy use pressure by livestock. In any case, natural ecosystems evolve from essentially bare areas to more or less stabilized types of dominant vegetation through a series of successional stages.

The current successional stage of a site being considered for improvement should be characterized. Some individual plant species grow better when in competition with existing vegetation on sites in the early stages of succession. Other plant species survive and grow with existing vegetation on sites in later stages of succession. Through recognition of the current successional stage, the species to be planted and managed can be better matched with the successional condition of the site, thereby enhancing the probability of continued growth.

Knowledge of successional patterns is gained, in general, from analyses of systematic, long-term observations of cyclical processes by astute ecologists. These analyses are difficult where successional change is orderly, and they are next to impossible where the changes are erratic. Nevertheless, the potential for the improvement of a site is put in a proper ecological perspective when analyses provide approximations of the current successional condition with respect to the range of successional stages that characterize a site.

A systems approach to site evaluation

A systems approach should be adapted in a site evaluation for assessing potentials for range improvements. Otherwise, the effort might simply be a collection of discrete, often incomplete, and generally unrelated exercises in measurement.

A systems approach to problem solving, regardless of its nature, generally involves a holistic study of the interacting elements that function simultaneously for an explicit purpose, emphasizing the connections among the various parts that constitute this whole. The interacting elements of concern function in "driving" the ecosystem processes. By its very nature, a systems approach to designing a comprehensive site evaluation involves several disciplines, including meteorology, soil sciences, and biology.

It may be impractical (or unnecessary) to measure all of the parts of an ecosystem in a particular site evaluation because, in many instances, only a relatively small number of limiting components may be related to the success of the range improvements. With a systems approach, however, the probability of overlooking important, possibly constraining, attributes will be greatly lessened.

Evaluation of abiotic and biotic components

A site evaluation focuses upon two broad sets of components: nonliving (abiotic) components and living (biotic) components. Climate, soil, landform and relief, and water resource are abiotic components; plants and animals of all forms, including humans, are biotic components. The objectives of the proposed range improvement project or program, the complexity of the ecosystem being evaluated, and the completeness of the available relevant knowledge will largely determine the intensity of the effort to be undertaken in evaluating these abiotic and biotic components. Details of measurement and sampling techniques may be found in Avery (1975), Bell and Atterbury (1983), Brown (1954), Cain and de Oliveira (1959), Carmean (1975), Child et al. (1984), Conant et al. (1983), Jones (1969), Lund et al. (1978), Lund et al. (1981), National Research Council (1962) Schemnitz (1980), and Soil Resources Inventory Group (1981).

Abiotic Components

Reasons for evaluating specific abiotic components of a site are discussed below. Techniques commonly used to quantify these components are briefly described.


Climate can be defined as the total complex of weather conditions and its average characteristics and range of variation over an appreciable area of the earth's surface. Conditions over an extended period of time are usually taken into consideration. weather in turn, comprises a set of atmospheric conditions at a specified point in time and, therefore, refers to events. Climate is basic to an ecosystem because of its significance in soil development and plant productivity.

Climate is difficult to characterize, owing to frequent deficiencies in the length and consistency of necessary meteorological records.

The climate of a site is most easily described from records of the United Nations World Meteorological Organization or from data collected by national weather offices. Unfortunately, many weather stations from which this information is obtained are often poorly distributed, especially in semiarid and arid lands of developing countries.

Precipitation Patterns The amount and distribution of rainfall is important because of its role as a source of soil moisture. Survival and subsequent growth of plants is, of course, closely tied to the availability of water in the soil mantle. Rainfall, in itself, is usually of little direct significance to plants, although there can be some absorption of water through the leaves and, occasionally, the bark.

Although soil moisture is mostly derived from rain, not all of the precipitation that falls on a site is equally effective in raising the soil moisture content. The slower, more gentle a rainfall event, the greater the penetration, or percolation, of water into the soil. However, a series of precipitation events that totals only several millimeters may add little to the soil moisture content, because the individual events are too widely separated and too gentle to have a cumulative effect. The more severe a drought, especially in dry climates, the greater the quantity of rain required subsequently to alleviate the drought.

Reliable measurements of rainfall are most commonly acquired from networks of rain gauges. There are many types (for example, standard, recording, and totalizer), and dimensions of rain gauges, but they all consist essentially of a funnel with a vertical collar that delivers water to a collecting reservoir. Only precipitation records obtained from gauges located away from eddies caused by physical obstructions should be used in a site evaluation. As a general rule, obstructions overhead should be no closer to the gauge than twice the height (from the ground) of the receiver funnel.

Temperature Regimes Heat from solar radiation controls the temperature regimes near the surface of the earth. The temperature at a site is influenced by incoming solar radiation that, in turn, is modified by secondary heat transfers from terrestrial radiation and air movements. Temperatures of either high or low extremes can be detrimental to the establishment and growth of plants. Hot temperatures, in combination with drying winds, can be damaging to recently emerged plants, especially under conditions of minimal soil moisture. Conversely, cold temperatures can delay seed germination and subsequent early growth, placing the survival of plants in jeopardy. Following establishment, temperatures of either extremes can reduce the overall growth performance of most plant species.

For best growth, many plants require nighttime temperatures that are considerably cooler than daytime temperatures. This difference between nighttime and daytime temperatures, termed thermoperiod, is important in the flowering and setting of fruit. In general, plants will become adjusted to regular diurnal fluctuations in temperatures and, as a result, may not exhibit "normal behavior" when grown in foreign environments. Therefore, individual plant species should be selected on the basis of their adaptation to temperature regimes (including mean, maximum, and minimum temperatures) at a site.

Reliable air temperature data are gathered from simple thermometers (for instantaneous determinations), maximum-minimum thermometers (to measure temperature extremes), and thermographs (for a continuous record of temperatures). Thermometers are generally housed in shelters with louvered sides to permit air to circulate freely. The shelters should be located at a distance at least two-thirds the height of any obstructions. Temperature will vary if obtained on steep slopes or in hollow areas.

The air within plant leaves is usually saturated with moisture under growing conditions, and vapor therefore will move from the leaves into the surrounding atmosphere, cooling the atmosphere in the process; this is transpiration. The rate of transpiration in plants depends in part on the amount of atmospheric moisture present; the drier the atmosphere, the higher the rate of water loss. Transpiration is the dominant process in the water balance of plants and can cause water deficits to occur. Under conditions of limited soil moisture, these water deficits may be responsible for growth reductions or death.

To characterize atmospheric moisture at a site in a given period of time, relative humidity is often measured. With summaries of relative humidity regimes over a growing season, it may be possible to determine the changes of transpiration in plants resulting in water deficits. Because certain plant species are better able to withstand the stresses of water deficits, this knowledge can be useful in evaluating the value of plant species for revegetative purposes at a particular site.

Instantaneous measures of relative humidity are obtained from manual observations of dry- and wet-bulb thermometers on a sling psychrometer. Hydrographs, of which several types exist, are used to record relative humidity on a continuous basis. These instruments should be housed in the shelters containing thermometers.

Wind The effect of wind on evapotranspiration, the total moisture loss from soil by evaporation and plants by transpiration, can be critical, particularly in dry climates. When a plant is exposed to drying winds and hot temperatures, water deficits in its leaves are likely to occur; this situation is compounded under minimum soil moisture conditions. The desiccating impact of wind on plants is demonstrated by low survival rates, stunted growth, and frequently death in many plant communities of semiarid and arid lands.

Data on wind patterns (prevailing direction, velocities, and seasonal fluctuations, for example), characterizing a particular site, are uncommon in many nonindustrialized countries . When this information is available, it has generally been obtained by using an anemometer during short-term site visits.

Light Another climatic factor that affects the growth of plants - an important factor that is seldom measured extensively - is light. Solar radiation in the visible bands of the spectrum controls photosynthesis. At very low light intensities, photosynthesis may take place at such a slow rate that all of the carbon dioxide evolved by respiration is not used; with these conditions, carbon dioxide is given off by the plant, not absorbed by the plant from the atmosphere. On the other hand, high light intensities promote rapid transpiration, which can often have detrimental effects. In general, individual plant species differ in their relative tolerances to either low or high intensities of light.

The photoperiodism of plants also differs among species. Some plants require long photoperiods (that is, length of day) to grow and develop, while other plants do better with shorter photoperiods. Photoperiods can be easily measured by the length of daylight at a site.


The word soil refers, in general, to the natural surface layer of the earth's crust in which plants grow. It is a porous medium, comprising minerals and organic materials. Living organisms, water, and gases are other constituents of soil. Whether climate or soil is more important in governing plant growth is immaterial, since both are necessary.

A site evaluation is normally incomplete without some kind of soil inventory, classification, or assessment. The evaluation of soil resources is conducted to determine the capacity of a particular site for a prescribed range improvement project or program, specifically in terms of supporting individual plant species or groups of plant species. More general information may be available from in-country files and experiment stations. Some international organizations, such as the Food and Agriculture Organization of the United Nations, can also provide general information (Dudal, 1970).

Evaluations of soil resources are made to provide adequate information for decision makers. Herein, the decision makers are concerned principally with the improvement of semiarid and arid rangelands. The kinds of decisions that these individuals will make must be known before a soil survey begins. Specific needs will largely determine which soil parameters should be measured and the procedures to be used in evaluating the soil resources.

It is beyond the scope of this report to describe the many techniques of conducting a soil evaluation. This information is available in numerous references; see, for example, Conant et al. (1983), Lutz and Chandler (1946), and the Soil Resources Inventory Group (1981). Instead, a "checklist" has been prepared to indicate many of the attributes that may be included in an evaluation of the soil resources on a site; each item is briefly discussed below. Obviously, the factors that are ultimately included in a particular soil survey must be those that relate to decisions made relevant to the particular range improvement project or program. Emphasis should be placed upon those factors that are "limiting" to the growth and development of the plant species.

Parent Material The underlying parent material from which soil develops has an important influence on the type of plants that a site will support. When the growth of an individual plant species is good on one site but poor on an adjacent site, investigation will often disclose that the two sites are characterized by geologic material of differing mineralogical composition and origin. In general, the soil is derived from the underlying rock. In some instances, however, the parent material may have been transported to the site by gravity, water, or wind.

Parent material, a descriptive parameter, is usually determined by field observations by a competent soil scientist. Geologic maps, if available, are also helpful in delineating the extent of soil that has been developed from a parent material.

Depth If soil depth is limited, the development of roots can be restricted. Soil depth is measured by exposing the soil profile and measuring the thickness of the separate layers. Many basic soil properties are characterized by horizon. The number of soil profiles taken at a site depends largely on the inherent variabilities of the individual properties.

Texture and Structure Two important physical properties of soil that greatly influence plant growth and development are texture and structure. Texture refers to the size and distribution of the soil particles (sand, silt, clay, and mixtures of them in various proportions); structure refers to the grouping of these particles into aggregates. Texture can affect and may restrict the development of roots, primarily through its influence on nutrient retention and aeration. Structure, which is most important in soils high in silt and clay particles, affects the percolation of water and air. The success of individual plant species in revegetation is dictated, in many respects, by texture and structure of the soil at a site.

Both texture and structure are descriptive measures, most commonly taken by horizon in the soil profile. Care must be exercised to ensure that samples are representative of the site.

Soil pH Different species of plants generally exhibit a preference for a degree of acidity or alcalinity in the soil, and have their own optimum pH values. Because pH may vary from one site to another, it should be included in a soil survey to maximize the returns from revegetative efforts. The pH of soil can be determined by using inexpensive but accurate field colorimetric sets.

Water-Holding Capacity As mentioned above, the survival and growth of plants is dependent on the availability of water in the soil. Waterholding capacity is a soil parameter of considerable utility. After saturated soil has been drained of gravitational water, it is (by definition) at field capacity. Field capacity is often determined in the laboratory, although approximations can also be made in the field by using a tensiometer. If desired, field capacity can be measured by horizon.

Organic Material The accumulation of dead organic material on a soil surface is significant to the "well-being" of plants in various ways. Organic material is, in time, a primary source of mineral nutrients. The organic increment of a soil profile is also a source of food for soil organisms that, in turn, are the chief causes of decay of the organic material; this process is critical in the nutrient cycles of a site. Organic material is colloidal, and thus, its waterholding capacity is relatively high.

Organic material content is minimal on many semiarid and arid lands. The sparse vegetation and year-round high temperatures favorable for rapid decomposition do not allow the accumulation of organic matter in appreciative amounts. The water-holding capacity of these soils is also frequently low. In general, the presence of organic litter may be quantified through visual inspection at a site, but differentiation into other compounds may not be possible except under laboratory conditions.

Salinity Salinity is often a constraint in revegetative activities. Saline and alkaline soils are commonly found in the valley bottoms of semiarid and arid lands. These soils create specific difficulties in selecting appropriate species for planting, and only plant species that are adapted to these sites should be used. Also, high levels of saline in soil reduces the amount of water available to plants and, therefore, can accentuate physiological drought.

Soil salinity is frequently measured with the aid of a Wheatstone bridge (an electrical device which measures conductivity). However, as is the case in surveying many soil attributes, these measurements are based on "point samples," which limit their extrapolation because of site variability.

Fertility Individual plant species have their own nutrient requirements for growth and development. When the soil lacks these nutrients, certain plant species may not be suitable for revegetation. In that case, it may be necessary to apply a fertilizer, although this practice may be uneconomical in an extensive revegetation project or program except for establishment. To the extent possible, the natural fertility of soil should be ascertained by chemical analyses. A practical approach to "measuring" soil fertility is to employ native plants as indicators of fertility ranges. Knowledge of the ecosystem and successional cycles of a site is necessary for this technique, however. Quantitative expressions of soil fertility are obtained with soil-testing kits.

Soil Classification The classification of soil is an attempt to group soils into categories that, in general, are useful in understanding the dynamics of an ecosystem. It is based, regardless of the system, on an examination of the "typical" soils in an area. In the process of classification, a number of soil attributes may be considered, including (but not exclusively) many of those described above. Soil classification is time-consuming and generally expensive, but it can provide decision makers with necessary information for the effective planning of range improvement activities. Fortunately, many countries have soil survey departments that can furnish technical assistance for this purpose.

The selection of a soil classification system to be used in a resource evaluation program is an important consideration. For instance, the needs of a project with one objective may be met by a specificpurpose, site-specific soil classification, whereas an integrated rural development program requires the use of a general purpose soil classification. An example of the former is an irrigation project aimed at increasing the production of bananas. In this case, soil properties important to water management considerations must be used as differentiating criteria in the development of a specific purpose soil classification. On the other hand, a multifaceted rural development program requires a general-purpose soil classification to assess the suitability of soils for a variety of uses. In this case, soil classes must be defined by attributes relevant to a wide spectrum of management goals.

Several general-purpose soil classification schemes have been developed by different countries to meet their needs. As Smith (1963) noted, a soil classification scheme developed in a particular country is biased by the accidents of geology, climate, and the evolution of life in that country. Its application in other countries can be problematic.

The FAO/Unesco soil-classification system (Dudal, 1968) and the U.S Comprehensive Soil Classification System (Soil Survey Staff, 1975) are now used in many nonindustrialized countries (Conant et al., 1983). The FAO/Unesco system attempts to group the soils of the world. Because of the wide spectrum of soil-forming environments, groups in this system include considerable variability. On the other hand, soil taxonomy was developed to facilitate soil survey in the United States (Smith, 1963). To avoid ambiguity, soil classes are precisely delimited by chemical and morphological properties. The rigidity of class boundaries and the need for laboratory analysis hamper the successful application of soil taxonomy in nonindustrialized countries. Furthermore, since the current version of soil taxonomy was based primarily on soils from temperate regions, its use in tropical areas may be problematic.

In addition to the FAO/Unesco soil grouping and soil taxonomy, several other soil classification schemes are in use in nonindustrialized countries. The relationships and main features of several of these are outlined by Beinroth (1975), Buol et al. (1980), Butler (1980), Camargo and Palmieri (1979), Conant et al. (1983), Jacomine (1979), and The Soil Survey Staff (1975).

Landform and Relief

Characterizations of landform and relief are necessary to the evaluation of a site because of their influence on climate and soil conditions. In many instances, the relation of plant survival to landform and relief is very close. At a minimum, general landform and relief should be quantified at a macro level.

In general, an area should be divided into "warm" and "cool" sites on the basis of aspect and slope combinations. So-called warm sites in the northern hemisphere are oriented, in a clockwise direction, from southeast to northwest, while "cool" sites are oriented from northwest to southeast. Of course, this situation is reversed in the southern hemisphere. Within a particular aspect class at a given latitude, slope is important when orienting a site to the sun. More gradual and steeper slopes receive less intensive sunlight than do "intermediate" slopes; the hottest and often the driest sites are those that most directly face the sun on a summer day. The amount of solar radiation received on a site is closely related to other factors (for example, precipitation, temperature, and soil moisture) that, individually or collectively, influence the choice of species, establishment, and the growth of plants.

The position of a site on a slope can also determine the growth potential of an individual plant species. High, convex surfaces, which are frequently subject to wind erosion and weathering, tend to be drier and (in dry climates) warmer than is average for an area. Conversely, low, concave surfaces, on which soil tends to accumulate rather than erode, are generally moister and cooler than average. Midslopes are typically intermediate in these characteristics.

Knowledge of the terrain of a site may be helpful in selecting the most appropriate method of revegetation. For example, level to gently rolling lands are preferred in many instances because ground preparation, if necessary, can be more effectively accomplished with machinery at less cost. Investigation has shown that the cost of ground preparation rises sharply on slopes that exceed 20 percent and is generally uneconomical.

Measures of landform and relief of a site can be obtained from topographic maps, if they are available. On site, aspect is normally determined with a compass, and slope is measured with an Abney hand level (or pocket altimeter). Aspect and slope can be "integrated" into a single measure of site orientation through use of daily solar radiation values for explicit combinations of aspect, slope, and latitude (Buffo et al., 1972; Frank and Lee, 1966).

Water Resources

Range improvement projects or programs on semiarid and arid lands are highly dependent on the distribution and availability of water to me et the water requirements of individual plant and animal species. The seasonal availability of surface water resources must be inventoried in terms of surface flows and impoundments, and seeps and springs. Locations of existing wells and promising groundwater aquifers (for subsequent development) should also be studied. In general, estimates of potential yields and (as mentioned below) quality of water resources may be necessary in comprehensive range improvement.

Water quality, both physical and chemical, must be considered in the evaluation of a site if, as part of a revegetation effort, artificial watering is required. Individual plant species possess their own "tolerance" to the physical and chemical properties of water. Therefore, when water is to be applied, these properties should be known to maximize the benefits and minimize the detriments of the watering.

Sampling and analytical techniques of assessing water quality are numerous. However, to ensure high-quality results, a prerequisite to the extrapolation of water quality information, only "standardized" methods should be employed. These methods are detailed in Conant et al. (1983), Dunne and Leopold (1978), and Wisler and Brater (1965).

Biotic Components

Plants, animals, and humans comprise the biotic components of a site. The importance of these components and ways of measuring them are discussed below.


The native plants that are growing on a site, if any, can be helpful in describing the inherent productivity of the site and, from this knowledge, the chances for a successful range improvement activity. The occurrence of “key" plants can often be used to indicate site quality. Also, knowledge of the productivity levels of native plants can “index" levels of production that might be expected from subsequent range improvement activities. Observations of plants that can be important in the evaluation of a site include, but are not limited to, identification of the individual plant species (taxonomy), properties of the individual plant species (for example, chemical composition and particularly, the traditional uses of the plants which indicate important properties), groupings of the individual plant species into communities, and vegetation-soil-terrain relations.

Of course, interpretations of individual plants and communities of plants must be undertaken in light of the on-site land-use patterns. Use of plant resources as described above can be hampered by land management practices that result in excessive utilization of the plants on a site. Because previous and current land uses may tend to cloud the picture, the ecological impacts of these previous or existing land use patterns on the plant resources must be well known and thoroughly understood.

Plant Indicators Various key plants may be useful in analyzing the capacity of a site for range improvement. To a large extent, the presence, abundance, and size of these plants will often reflect the nature of the ecosystem of which they are a part and, therefore, may serve as indicators of site quality. However, the correlations between “key" plants and associated site quality, which are generally based on detailed ecological investigation, may not always be apparent. Effects of competition among individual plant species, events in the history of plant development (such as drought, fire, and outbreaks of insects), and land management practices can weaken a plant association to the point that it has little predictive value. Nevertheless, in many situations, site quality is sufficiently reflected by plant indicators to make use of the latter in an evaluation of a site for range improvement.

Sometimes, the occurrence of plant indicators is combined with abiotic components of the environment (for example, climate, soil, and topography ) in an attempt to describe more accurately the quality of a site. The more factors that are taken into consideration, the better is the estimate of site quality and, consequently, the understanding of the potential of a site for improvement practices. Comprehensive reviews and comparisons of site evaluation, including its history, methods, and applications, have been prepared by Jones (1969) and Carmean (1975).

Productivity Levels Knowledge of the productivity levels (that is, amounts of plant material present) of plants growing on a site can provide insight into what might be expected from any range improvement practice. Information regarding the total production of all herbaceous plants, taking into account the loss of plant material to utilization, is often used as a "threshold" productivity value. In other words, improvement should be expected to exceed the existing productivity levels. If excessive utilization of the plants has occurred, the measures of existing production may be biased downward.

Volumetric measurements of plants are seldom made to quantify productivity levels. Instead, weights are used to measure the biomass of the plant material present. The weights of plants are most precisely obtained by the clipping of sample plots. But, since clipping is time-consuming and costly, a double sampling procedure is frequently employed to measure productivity on an extensive basis; weights of plants are estimated on all plots, with only a few plots clipped to derive a factor to correct the estimates, if necessary.

Whenever feasible, productivity levels should be obtained on the basis of individual plant species to allow subsequent groupings into plantform categories or grazing value classes for decision-making purposes.

Plant Cover In addition to the productivity or biomass available for utilization, the ability of the plant community to stabilize the site and arrest the soil erosion process should also be determined. Productivity information alone does not provide the manager with this knowledge. The percentage of the soil surface that is covered by plants, either only by the base of the plant (basal cover) or by all above-ground plant parts when viewed from above the canopy (canopy cover), indicates both the susceptibility of the site to erosion and the established dominance of one plant species over another.

Plant Number A plant community might be dominated, in terms of productivity and cover, by one or two plant species, the individuals of which are old and decadent. As these individuals die, they will be replaced by the same or new species. Data on number of plants of each species may give the land manager an indication of the health (vigor) and reproductive status of species in the community and, therefore, insight as to which species are likely to increase or decrease. Knowledge of this sort will guide the selection of the appropriate range improvement practice.

For more detailed information concerning techniques for vegetation analyses, the reader is referred to Conant et al. (1983), a publication prepared specifically for efforts in nonindustrialized countries.

Other Measurements and Observations To help in the selection of individual plant species for revegetation projects and programs, there may well be other kinds of on-site surveys or observations of plant resources that should be taken. Depending on the goals of the improvement practice, these may include growth forms and plant community structures (that is, vertical layerings); seasonal growth, development, and maturity patterns, including the differences among grasses and forbs, shrubs, and trees; and ecological conditions and trends (Pratt and Gwynne, 1977), as these parameters are influenced by successional cycles and degree of site deterioration.


Individually and collectively, animals have a major impact on the physical environments and the plant communities with which they are associated, and, as a result, can affect range improvement activities in diverse ways. Depending on the activity, these impacts can be beneficial, detrimental, or both. Some animals greatly influence the ecosystem processes that are basic requirements for plant growth and development, such as nutrient and water cycling. Successional patterns are affected by other animals, by regulating competition, development, and productivity among individual plant species and communities of plants.

A major influence on the success of range improvement activities is the grazing activity of larger ruminant herbivores. However, both the positive and the negative roles that animals play in an ecosystem should be considered in a site evaluation.

Many semiarid and arid lands furnish habitats for wild animals and domestic livestock. Therefore, knowledge of animal types that occur on a site, their distribution and routes of migration, and their ownership or legal status can dictate, to some degree, the options for range improvement.

Animal Types All types of avian and terrestrial fauna (including soil biota) are part of an ecosystem. Difficulties arise, however, in defining the geographic boundaries when more-mobile animals are evaluated. In practice, ecosystems are commonly based on plant communities, soil classification units, or other abiotic features, or combinations thereof, and animals are then “incorporated” into the delineated ecosystems as consumers and secondary users. Mobile animals generally roam over several ecosystems.

Wild animals have varying effects on the ecosystem. Earthworms, arthropods, and ground-dwelling mammals play major roles in the decomposition of organic material. In their absence, the nutrient cycles of a site can be adversely disrupted. Birds are important agents of seed dispersal for many individual plant species. This dispersal activity can be beneficial or harmful to reproductive strategies. Mammals, especially rodents, can also be important agents of seed dispersal in many plant communities.

The grazing activities of the larger ruminant herbivores, both wild species and domestic livestock, have already been mentioned, and are covered in more detail in chapter 6. Grazing is commonly considered destructive, although it can benefit the desired vegetation on a site by removing competitive plants that otherwise may use limiting water and nutrient resources. Grazing activities also prevent the buildup of coarse, unpalatable plant parts and stimulate the growth and tillering of more plant materials.

Techniques of enumerating animal types and their respective numbers, including the censusing or sampling of animal populations, are described in Child et al. (1984), Conant et al. (1983), and Schemnitz (1980).

Distribution and Migration Patterns In addition to knowing what types of animals occur, knowledge of their distribution and patterns of migration can also be important in site evaluation. Uniformly distributed animal populations generally tend to exert uniform effects on a site. On the other hand, a population of animals that is unevenly distributed will frequently have uneven effects upon a site (for example, animals clustered around a wellhead). The distribution of animals is also influenced by their migratory patterns. In general, migration (whether seasonal, yearly, or indeterminate in response to unknown stimuli) can result in cycles of impacts that should be known when planning a range improvement program.

Information regarding the distribution of animals can often be obtained while their kind and number are being enumerated. Migratory patterns, which are descriptive measures, can be determined only through repeated observations of the animals on a site.

Ownership Status The ownership of animals can be important to a range improvement activity, particularly in situations in which the control of the animals is necessary to the success of the venture. In general, the ownership of wild animals, if specified by law, lies with the state, which may assume responsibility for their regulation. However, domestic livestock are often privately owned by individuals or groups of individuals that operate in a cooperative. The ownership of animals must be established, and the responsible parties contacted and their cooperation obtained, if manipulation of the animal populations is considered necessary to ensure the success of an improvement project or program.


It is necessary to assemble and analyze what is known about the human users of the land and, of equal importance, what values people attach to the natural resources that will be affected by a range improvement activity. Deficiencies of information in this area can, and in most cases do, constrain the effectiveness of a project or program. Serious conflicts often exist between traditional land management practices and what might be proposed in a range improvement plan. Therefore, it is imperative that proponents of range improvement understand the people who will be affected, and the reasons why the people do what they do (see National Research Council, 1986).

Information regarding human activities, land use in the area of concern, population densities and community structures, and the migration of families and family groups is a minimum baseline for the evaluation of humans. Rural sociologists, ethnologists, cultural geographers, and ethnic botanists, all working at the local level, should be involved in this work.

Integrated evaluations

Two basic approaches are employed in the evaluation of a site, regardless of the purpose: an integrated approach and a component approach, as explained in chapter 4. Abiotic components and biotic components that are important in evaluating a site for range improvement have been discussed in the preceding sections, component by component. Most on-site surveys and observations of natural resources are based on a component approach, although the individual components being evaluated are not necessarily considered in isolation. Within a systems framework, the components that are related to the improvement of a site must be evaluated in an integrated manner. By doing so, all of the elements of an ecosystem, both abiotic and biotic, will be studied to form as complete a picture as possible of a site being considered for improvement.

One final point: many site evaluations fail to include a provision to monitor the changes in an ecosystem that could, subsequently, have an influence on the success of a range improvement effort. Therefore, to the extent possible, monitoring should be provided to be sure that temporal changes, both positive and negative, are recorded and identified for possible changes in past project management as well as for use in future project planning.

In the preceding pages, much has been said about the various factors that need to be measured and evaluated. How these components are used in evaluation has not been discussed. A computer model is not generally the answer. Some limits may be needed to delineate the range of acceptable conditions.


Avery, T. E. 1975. Natural Resources Measurements. McGraw-Hill, New York, New York, USA.

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