Ecosys1

RESEARCH, MANAGEMENT, AND ADMINISTRATION OF THE
RANGELAND ECOSYSTEM

-by C. Wayne Cook

A NEW approach to rangeland management concerns the synthesis and use of information dealing with the structure and function of the rangeland ecosystem. Management and administrative decisions can be made more logically if the entire ecosystem is understood. In addition, voids in information are more readily discovered through this approach. Thus, the need for and the role of research become more obvious.

Many flow charts have illustrated the function of the natural and the agricultural ecosystems. The rangeland ecosystem, since it is concerned largely with native vegetation, more nearly resembles the natural ecosystem. However, since livestock are introduced and manipulation procedures are frequently involved, it partly resembles the agricultural ecosystem.

The rangeland ecosystem differs from a natural ecosystem in that it will be purposely manipulated for more effective production of salable produce. It differs from the domestic crop ecosystem in that the total standing crop does not represent the exported product.

All physical features, along with expressions of flora and fauna, characterize the structure of the ecosystem. Structure of the ecosystem is concerned with the whole community complex of plant composition, plant aggregation, life forms, layering, density, crown cover, leaf form, age classes, herbage yield, constancy, fidelity, etc. Measurements describing the structure of the ecosystem involve vegetation patterns, dispersion, heterogeneity, population dynamics: in general, the synecology of each community and the autoecology of each species in both above- and belowground environments.

Because both plant and soil development on any site is the product of environment, both micro and macro climate, along with latitude, altitude, and exposure, should be described.

The status of animal life is an important characteristic of the ecosystem. This segment of the biotic community represents the consumers in the food web. Structure of the ecosystem concerns the identification of dominant species of fauna, their population dynamics, dietary habits, and behavioral patterns.

New statistical methodology allows for the evaluation of the influence of each independent factor upon any one of the several dependent factors in the ecosystem. Such programming in ecosystem analysis has been rather meager to date, but in the future such an approach shows great promise. By such analysis, the effect of environmental factors on yield, frequency, soil nutrients, plant chemical content, and many others can be assessed. The effect of each environmental factor can be evaluated when operating separately or when working together with all or any of the other designated environmental factors.

The function of the ecosystem is rather complicated and involves many biotic and abiotic interactions. These phenomena are not as well understood as the structure of the ecosystem because of the lack of research in this area. The research concerning functions of the ecosystem has been fragmented, and more work remains to adequately explain the complete operations of the ecosystem.

The importance of soil organisms as decomposers and contributors to soil fertility is becoming more evident. The addition of organic matter by fungi is believed to be important to primary productivity on desert biomes. Soil crust algae and other cryptogams may be major nitrogen fixers. Population levels of soil bacteria have been identified with high soil fertility and optimum structure. Transfer and conversion of soil nutrients for absorption by plants are of paramount importance to nutrient cycling in the ecosystem. However, in arid range soils their role has not been adequately studied or evaluated to the degree that explains their full importance.

A primary consideration in the function of an ecosystem is the efficiency of the conversion of solar energy to plant material by the primary producers. This is the first step in evaluating efficiency of turnover of energy by the various trophic levels and the efficiency of energy export from the ecosystem.

The conversion of solar energy through photosynthesis by plants is governed largely by light, water, and nutrients. On range ecosystems there is usually unlimited sunlight, and the leaf area index is far from complete coverage of the surface area. Therefore, there is little correlation between primary productivity and incident solar radiation. Plant life may be dormant many months of the year because low temperature, limited soil nutrients and inadequate moisture reduce photosynthetic activity. Even at best, less than 2 percent of the solar energy is converted to chemical energy. However, energy conversion and transfer among different trophic levels in the rangeland ecosystem is an important principle that can serve as a guide to management activities for efficient use of all cycling nutrients in the system.

Duncan proposed a model for predicting or evaluating efficiency of solar energy conversion by plant foliage by considering the following canopy architecture: leaf area, leaf angle, leaf position, reflectivity, transmissivity, brightness, and position of sun. It does not appear that this model would optimize energy conversion in rangeland ecosystems unless it included other limiting factors such as soil nutrients and moisture, either of which should be as important or more important than the leaf characteristics themselves.

Since the function of the ecosystem actually starts with the process of photosynthesis, it is important to understand this physiological phenomenon. Age of leaves is important in the effectiveness of conversion of energy. As would be expected, the more mature leaves are less efficient than the younger ones. Life forms and species differ in the effectiveness of their chlorophyll-bearing tissue in converting radiant energy to chemical energy.

Defoliation by animals, infestation by parasitic insects that suck the life-giving saps from the plants, and diseases that cause malfunction of the plant tissue all decrease the efficiency of food manufactured by the plants.

In natural native ecosystems the herbivore population is generally proportional to food produced or food available for consumption. In any event, the entire consumer population is directly or indirectly related to the ability of the plant population to furnish food. Animal population dynamics, of course, depend on more than food supply, because diseases, catastrophe, or social behavior may also be regulatory.

Efficiency of solar energy conversion within the ecosystem has received much attention. This is of paramount importance because of the transfer of solar energy for sustenance of life in the ecosystem. The existence of the ecosystem itself is dependent upon this phenomenon.

The herbivore population represents the first major turnover in the use of fixed photochemical energy in plants. This involves exchanges between plants and animals and exchanges among animals with each other. The first interaction is obvious since it concerns animal response from consuming plants and plant response from being defoliated by animals. In this case, the stability of plant cover and animal response to consumption of the primary producers is a measure of efficiency of energy turnover.

In evaluating consumer response, it is necessary to consider more than gross energy content of the ingested forage. For instance, according to some studies, maximum digestible energy (TDN X 2000) per unit area was produced by plants that were harvested only after they completed their annual growth cycle; but maximum digestible protein was produced when plants were harvested some time during mid-growth and, again, at the end of the growing season. Livestock responses on grassland ranges were satisfactory only when phosphorus and digestible protein were high or added to the diet. This was true even though digestible energy remained more than adequate.

It would seem that the biological efficiency of the rangeland ecosystem could be based upon: 1) the amount of photochemical energy or nutrients produced per unit of land surface area, and 2) the amount of this material that is transported from the ecosystem in eatable produce, services, or wealth. The amount of solar energy converted to chemical energy or nutrients and then to the consumers serves as a measure of efficiency of the ecosystem, provided limiting factors are corrected through manipulation consistent with all uses to be made of the area and with reasonable cost and return relations.

It is generally acknowledged that as energy passes down the food chain from producer to consumer, the efficiency of use of solar energy diminishes. In each case, however, management can increase the efficiency of energy utilization in the transfer from one trophic level to another.

The total gross energy produced by plants has already been identified as a measure of efficiency of the primary producers, but the transfer of this energy to the consumers would, to a large degree, depend upon the chemical form of the energy-furnishing constituents. For instance, many native browse and forb species produce rather high quantities of resins, waxes, and essential oils that represent chemical energy. However, energy in these forms is not available to most consumers. In studies by Cook, it was found that grass species produced approximately 2050 kcal of gross energy per pound of dry matter during early growth, and browse species produced about 2322 kcal. The browse yielded only 1205 kcal of metabolizable energy for animal use, compared to 1324 kcal of metabolizable energy per pound of dry matter in grasses. Later in the season, when plants were mature, the grasses contained 1859 kcal, and browse contained 1922 kcal of gross energy per pound of dry matter. The grasses netted 42.9 percent usable energy, but the browse netted only 33.4 percent usable energy. This was because the essential oils in browse are not used by animals.

Nutritional deficiency among animals is commonly a limiting factor in the efficiency of energy transfer from the primary to the secondary producers. In most rangeland areas of the world, the nutritional value of forage can be assessed on the basis of digestible protein, phosphorus, vitamin A, and energy-furnishing constituents. In local areas of higher precipitation and sandy soils, cobalt and copper may be deficient in the forage. Effective conversion or transfer of energy in the food chain is regulated by more than the mere supply of energy in plant material.

It should also be noted that the requirements of range livestock are different for various physiological functions of the animal. For instance, the requirements for lactation are about 25 to 30 percent higher than for gestation. For gestation they are slightly higher than for maintenance; and for early lactation, somewhat higher than for late lactation (Table 1).

Providing proper nutrition through the conjunctive use of seeded range with native range for lactating animals in the Intermountain area of the western United States has shown that more pounds of lamb and calf at weaning time per breeding female can be obtained. In similar fashion, females in gestation that were supplemented to correct protein and phosphorus deficiencies produced more pounds of lamb and calf at weaning time compared with unsupplemented females. Thus, it is demonstrated that a nutritionally balanced diet can materially increase the efficiency of energy turnover in the ecosystem. This would be true for all animal life in the ecosystem.

Supplements not only furnish the deficient nutrient in the diet but, in many cases, they enhance the value of the range forage by increasing daily intake and digestibility of ingested range forage. In trials by Cook and Harris, protein supplements consisting of cottonseed meal or soybean meal increased digestibility of protein and cellulose in the range forage on desert ranges used during the winter. Intake of digestible protein and metabolizable energy from the range forage was increased by the protein supplements fed to range sheep at either 0.5 or 0.33 pounds per day. Cottonseed meal supplements, in most cases, increased daily intake of range forage. Energy supplements consisting of barley or corn reduced the digestibility of protein and cellulose in the range forage and in some cases decreased daily intake of range forage (Table 2).

Proper supplements fed to sheep on winter range during gestation have given as much as a 15 percent increase in lamb crop, 8 pounds more lamb per ewe, and a pound more wool per animal. Replacement heifers, when supplemented on winter range, gained 20 pounds more than the controls, and supplemented cows weaned calves weighing 23 pounds more than unsupplemented cows.

Thus, it is shown that a protein supplement, when supplied to grazing animals on dormant desert ranges, actually increases the efficiency of energy turnover in the ecosystem, even though protein may not be deficient in the forage according to common recommendations.

It is generally acknowledged that sheep are slightly more effective in converting energy into grain and milk than cattle. There is, however, a rather marked difference in the efficiency of energy conversion from range forage into gain of young weaned lambs and steers compared with gain from lambs and calves receiving milk during lactation. As shown in Table 3, about 40 percent more gross energy is required per pound gain in sucking calves compared with steers. By knowing the degree of utilization and the response of the grazing animal, it is possible to calculate the energy production of the ecosystem by the indirect method. Calculations for energy conversion in the range ecosystem (Table 3) with respect to species of animal and physiological function involved the following data:

Even if we assume that sheep and cattle convert plant nutrients with similar efficiency, we still have marked differences in dietary habits between the two species on most rangeland ecosystems. This is a result of the difference between the two in use of plant species and varying topography. Some rangeland ecosystems are better suited to cattle, and others are better suited to sheep.

In Table 4 it is shown that a mountain rangeland was slightly better suited to sheep than cattle when comparing the quantity of energy consumed. However, the two animal species using it in common were more effective from the standpoint of both quantity of herbage consumed and efficiency of transferring energy into meat produced in the ecosystem.