How many is too many, or not enough?

Kaimanawa feral horses on spring-green grasslands southeast of Mount Ruapehu. Photo credit: Tarmo Poldmaa

Debates about how many animals the landscape can support are common when managing or conserving wildlife. Some will claim there are too many because the population exceeds the capacity of the landscape’s resources to support them. Others will claim the landscape can support more. In this post I will introduce the concept of carrying capacity – a concept central to ecological science and the management and conservation of, for example, horses and rhinoceros.

The science to understand how many wild animals a landscape can support is the progeny of our agricultural heritage. Agriculture – farming plants and animals – began after the end of the last ice age about 13,000 years ago. The first planting-eating mammals domesticated were sheep and goats.

Since the beginnings of agriculture it has been important for farmers to understand how many animals the land can support. The question is important because if a farmer has too much stock then they risk the health and productivity of the animals and pasture on which their livelihood depends.

Overgrazed rangeland by sheep in the steppes of Inner Mongolia. Photo credit: J.R. Conner

Stock will compete amongst themselves for resources – shelter, water, and food. If there are too many stock some animals might not get enough of a resource, and not grow or reproduce well. Too much stock might also damage the ability of the landscape to produce food-plants. If grazing or browsing is too intense, grasses and shrubs may also not grow or reproduce.

Too little stock, however, and the food plants might also deteriorate. Some feeding can facilitate the growth and nutritional value of a plants leaves. For example, the food-quality of rank grass will not be as nutritious as cropped grass. The farmer who under-stocks also risks missing opportunities to reap additional product or income, especially when conditions are good. Somewhere between having too few and too many stock is an optimum number that a landscape can support indefinitely. That optimum number is called the land’s carrying capacity.

A farmer with fenced paddocks can know the carrying capacity of their fields and farm. The quantity and quality of grass can be measured and the nutritional needs of their stock known. But it is much more difficult to know the amount of food in wilderness and the nutritional needs of wildlife. The grasses in a paddock can be cultivated to provide just the few species best for feeding stock and even watered to promote growth. The community of plants in wilderness, however, is much more complex and varies over distance and time. There will be good and bad habitat, good and bad years.

grazing good and poor pasture

Grazing on pastures that have been improved by cultivation and in wilderness where the community of plants and animals is much more complex.

In wildlife ecology, carrying capacity is difficult to measure and apply, largely because it is harder to define the limits of wild landscapes and the climatic conditions, plants, and animals feeding are more variable than those on farms. Not only is the landscape more complex but many more animal species will be living there. Some of those will compete with each other, as well as with members of their own species, for resources. The diet of horses, for example, is similar to the diet of cattle and sheep – they all largely eat grass. It is also possible that some species may improve habitat for others. For example, animals that eat and suppress shrubs may open up habitat for grasses and more grass-eating animals. Estimating the carrying capacity of landscapes for wildlife, therefore, is a Herculean task – difficult, time-consuming, and expensive.

text box insertIt is very rare in debates about wildlife for an estimate of carrying capacity to be available and, even if one is, it will have a large measure of uncertainty. It is acknowledged, however, that the concept of carrying capacity has, at least, heuristic value. Carrying capacity cannot be exactly measured, but we know that a carrying capacity probably exists and we can measure for evidence that a population is under- or over-capacity.

In future posts I will examine what influences the distribution of Perissodactyla, like horses and rhinoceros, on landscapes, what are their important resources, how we might detect that a population is over- or under-its carrying capacity, and the implications that understanding has for the conservation and management of our odd-toed friends.

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