Carrying capacity - Game Management

You may have heard the term Carrying Capacity used in a number of contexts. Manufacturers of commercial vehicles and utes often describe a cargo payload as carrying capacity. The ability to legally carry a certain weight in kilograms rates the vehicle against similar competitors. The greater the carrying capacity the more load the vehicle can handle. Pastoralists define the ability of land or pastures to produce livestock by this term. Most farm improvements aim to increase the carrying capacity of land to increase profitability for the farm.

Some people concerned about the global human population use carrying capacity also. In this context people are concerned about the impact of too many people might have on the planet and attempt to define a maximum population size the planet can cope with. This article is about the concept of carrying capacity as it is used in ecology or population biology. Of particular importance how carrying capacity might be used in relation to wild deer in an Australian context.

Carrying capacity is sometimes used in media articles or wildlife documentaries as if it is a definitive number. If you Google carrying capacity and ecology you will find some clear definitions such as;

· The maximum population size the environment can sustain indefinitely (Wikipedia),

· The population size at which population growth equals zero (Encyclopaedia of Earth),

· The maximum number of individuals that a given environment can support without detrimental effects (The free dictionary).

Graph 1 accompanying this article gives a graphical representation of the ecological concept of carrying capacity. There have been many documented cases around the world where wild populations of animals have mirrored the theoretical population in the graph. Imagine a herd of deer being introduced to a new environment where there is good soil, ample rainfall, lots of grass and no predators. Initially the deer encounter unlimited resources per individual. The herd grows exponentially until such time as the amount of resources per individual becomes limited. Eventually the habitat does not have enough food for every deer and many die of starvation or disease brought on by poor condition. Hinds can’t maintain sufficient body weight to get pregnant and many of those that do can’t produce enough milk for their calves.

The size of the population decreases through intra-specific competition almost as quickly as it initially grew. As the herd decreases less of the vegetation is eaten and the plant communities are eventually able to recover. After a few years the food resources of the habitat are sufficient for the deer herd to once again grow. The graph demonstrates a population expanding and contracting according to the available resources over the course of a long period of time. In terms of a deer population a century would be an appropriate length of time for this pattern to stabilise. The carrying capacity of the habitat is an imaginary line somewhere between the highs and the lows of the fluctuating population.

This explanation describes the maximum carrying capacity that exists for a particular species and habitat. For every different species there is a different carrying capacity for every different habitat. When other species are introduced or removed from the habitat the carrying capacity changes. If the environmental conditions change over time (such as average rainfall or average temperature) the carrying capacity also changes. For many species available food resources is not the limiting factor. Many species of wildlife have social structures that limit the population long before food runs out. For example lions will only ever reach a maximum density regardless of how much prey is available. Above this density lions will kill or chase off all other lions! The important point here is, in ecological terms, carry capacity is a concept that helps define and describe the relationship between a species and habitat. It is never an exact number and is always subject to change.

Because most habitat and deer relationships are complex and involve other wildlife species the concept of carrying capacity is anything but straight forward. In practice the use of carrying capacity as a management tool is heavily influenced by the goals of the person or group using it. For instance a particular area may be able to support 1,000 deer year after year with little detriment to the breeding capacity of the herd. Increases in deer numbers over a couple of years are offset by higher mortality which brings the herd back to around 1,000 before long. In this situation the herd is at maximum carrying capacity for the habitat in question as demonstrated in graph 1.

The maximum carrying capacity may not be the optimal density of deer in terms of the harvestable amount of venison the herd could produce in this habitat. It is possible that in a herd of 800 deer in the same habitat each individual will have greater access to all resources and therefore individuals will gain weight faster each spring and maintain condition better. This carrying capacity is based on kilograms of meat and exists at a lower herd size than the maximum carrying capacity. The term Maximum Sustained Yield (MSY) relates to this type of carrying capacity and is often used when talking about species managed for a harvestable offtake.

The habitat in question may be under a Quality Deer Management plan. One of the central goals of the plan is to produce 50 adult male deer each year with ‘trophy’ sized antlers (however trophy is defined). This objective may be best achieved at a herd size of 600 animals where 50 per cent are male. If the age classes are even within the herd then only 1/6 of the males need be older than five years and considered a trophy. If the herd size grows above this number then individual males may actually grow smaller antlers each year due to less minerals being naturally available. This carrying capacity is based on the trophy score of mature adult males and will be less than the meat carrying capacity or MSY.

It is also possible that this imaginary herd exists in a forest next to a farm where crops are grown. When the herd size is less than 200 individuals for the entire forest the farmer rarely sees a deer and doesn’t begrudge those seen a bit of feed now and then. There may be some economic loss but it is hardly noticeable compared to factors such as rainfall and the value of the Aussie dollar. However above this number the farmer sees groups of deer regularly on the crops and knows that they must be reducing the financial return of the farm. This carrying capacity is a financial carrying capacity and is less than any of those previously mentioned.

In all the carrying capacities discussed there is a direct relationship between the number of deer and the amount of vegetation growing in the habitat. More deer will result in less vegetation with different plant species more susceptible to deer numbers than others. If we consider carrying capacity in terms of a very rare plant species (that deer find incredibly delicious) it may be that no deer is the optimal carrying capacity. Graph 2 accompanying this article provides a simplified representation of the various carrying capacities different stakeholders may desire.

Human wildlife management is based on value judgements. All the different carrying capacities discussed here are very real and currently exist around the world wherever deer are subject to human management. When a particular deer species is native and rare carrying capacity is usually discussed in terms of the maximum amount of breeding that can be achieved for the species. Where deer are a resource then the carrying capacity is relative to the amount of resource that can be sustainably harvested or produced. Wherever deer are believe to be causing negative financial impacts you can be sure the perceived carrying capacity is related to any loss in dollars. When the loss in dollars is minimal then the carrying capacity is optimal. Of particular importance in an Australian context is any relationship between native vegetation and an introduced species. It is possible for there to be more than one carrying capacity for any deer species in any given habitat. The challenge for resource managers is identifying the various goals of stakeholders and achieving an appropriate carrying capacity accordingly.

References

A.R.E. Sinclair. 1997. Carrying Capacity and the Overabundance of Deer: A Framework for Management. In “The Science of Overabundance, Deer Ecology and Population Management”. McShea W.J., Underwood H.B. and J.H. Rappole. (eds.) Smithsonian Books.