D.J. PRATT *(*) D.J. Pratt, Principal Scientific Officer, Ministry of Overseas Development Land Resources Division, Tolworth Tower, Surbiton, Surrey, England.
Livestock carrying capacity is a crucial statistic in any category of range evaluation designed to facilitate development or management. Naturally, therefore, there is a tendency for range workers to wish to quantify this statistic. But quantification can be a deceptive lure when dealing with variable and unpredictable data and processes.
The deceptiveness lies partly in the fact that several of the basic parameters of carrying capacity can easily be quantified. There is no problem, for any given rangeland type, in measuring the yield of dry matter or nutrients on offer (1). Nor is it difficult to quantify the dry matter and nutrient requirements and consumption of a given number of animals. The problems arise in judging what allowances should be made for (a) losses through termites, desiccation and wind, fire and trampling, and (b) effects of selective grazing and abnormal climatic conditions. In most cases, the problem lies in the variable nature of these occurrences. In most cases, also, the impact and importance of such factors is greater in arid than in more humid rangelands.
(1) Not, at least, for herbage; browse can be much more difficult.
In practice most workers who seek to quantify carrying capacity allow for losses through termites, fire, etc., by deducting an appropriate percentage from the yield of herbage on offer. Indeed, there is little that can be suggested to improve upon this approach, other than to advise that as much evidence as possible be collected in justification of the "appropriate " percentage that is used. In any event, the greater error is likely to exist in the primary assessment of total yield, and the allowance that is made for abnormal climatic conditions.
The commonest abnormality - so common that the norm hardly exists - is found in rainfall and its distribution. On rainfall, in turn, depends yield, and often the magnitude of the percentage losses experienced through fire and other factors. Most critical are the fluctuations in rainfall that occur in arid areas. Here also are experienced the greatest difficulties in allowing for seasonal variations when assessing carrying capacity.
The easiest approach is to assess carrying capacity on the basis of the average year and average productivity. But what allowance should be made for drought years? Caution requires that carrying capacity be assessed at some point below the average, but how far below? The tendency, when using quantitative methods and mathematical relationships, seems to be to set carrying capacity too high. Too often it is overlooked that as rainfall and yield decrease, so carrying capacity decreases geometrically towards infinity. Where mean annual rainfall is very low, a 50 percent reduction in rainfall could well cause carrying capacity to decrease from, say, 1 livestock unit/5 ha to 1 livestock unit/50 ha. A similar increase in rainfall, on the other hand, might raise carrying capacity only from 5 to 4 ha per livestock unit.
Another limitation of assessing carrying capacity by relating yield to the feed requirements of livestock is that the calculation takes no heed of the ecological effects of grazing pressure. In other words, it may be necessary to consume only half of the calculated available forage if certain of the better species in the pasture are to increase and the ultimate potential of the land be realised.
Against this it can be argued that as often as not we do not know enough of the dynamics of the ecosystem to be able to say which species can, realistically, be encouraged by the manipulation of grazing pressure. But in cases of uncertainty it is always better to err on the conservative side.
Conservatism is also appropriate, for purely practical considerations, when livestock are being introduced into an unused or underutilised area. It is generally no problem to increase livestock numbers, or experience shows this to be justified, but it is not nearly so easy to try to reduce numbers once a scheme has been started.
In days of increasing sophistication in research, it may be regarded as retrograde to advocate taking some of the science out of range evaluation; but with regard to the assessment of carrying capacity in arid rangeland, there does seem to be a case for relying more on a general appreciation of local ecological conditions and less on yield data and chemical analyses. This conference might even address itself to suggesting guidelines for carrying capacity, as determined by rainfall and major ecological conditions. Such guidelines would not be final, but would provide a starting point in the absence of local research results or practical experience.