Wildlife populations all have the capacity to grow given the right circumstances. What limits this inherent ability to grow includes the availability of resources (food, water, shelter) and the loss of individuals through predators and or disease. Rare and threatened species are in low numbers either because there are insufficient key resources that the species need or the loss of individuals to predators and or disease is greater than the population’s ability to grow. Sometimes it is the combined effect of several factors impacting on the species in a negative way. Overabundant species (often considered pests) usually have an unlimited supply of resources and insufficient predators and or disease to keep populations in check. Under these circumstances the pest species’ ability to grow is greater than the loss of individuals through predation and or disease.
Most of the world’s wildlife are somewhere between these two extremes and are able to find sufficient resources most of the time to keep breeding year to year. These populations are in turn a key food resource to other wildlife. If predators and disease do not keep the rate of population increase in check then eventually the population will double and then double again. If this rate of increase occurs over a set time period it is known as exponential growth which is illustrated in Graph 1. The doubling of the size of the population has little overall effect on key resources when the size of the population is small. However as Graph 1 shows the effect of doubling in population size becomes very significant as the population grows. For species like mice or rabbits exponential growth can be seen to happen in a relatively short time. For species like deer exponential growth can take a long time before it is apparent but can be very dramatic when it does occur!
Eventually populations growing exponentially will reduce the available resources and the growth will slow down. This form of growth pattern is known as logistic growth and is illustrated in Graph 2.
Understanding the concept of exponential population growth and logistic population growth is important when managing a game resource and setting some kind of a harvest limit. Harvesting of wild game is another form of predation. Sometimes harvesting occurs where there are natural predators and sometimes it is the sole predatory effect on the population. In either case the amount of harvesting can and does affect how the population will grow. When a population is somewhere near the top of the logistic growth curve then the availability of resources is limiting the population growth. Should there be an increase in predation (including harvesting) then the population will decrease in size but increase its potential to grow. This happens because the decreased population now has more resources per individual.
Back in 1933 Aldo Leopold defined ‘productivity’ of game as ‘the rate at which breeding stock produces a removable crop or additional breeding stock’. Productivity to Aldo was synonymous with annual yield. Aldo was a forester by training but applied his knowledge to game management at a time when this science was just beginning in North America. Around the same time fisheries scientists around the world based fisheries management on the concept of Maximum Sustainable Yield. To a forester, fisheries scientist and Aldo Leopold, back then the yield of a population represented the maximum amount of wild resource that could be harvested sustainably in theory. This could be measured in number of individuals or weight of material be it protein or timber.
The concept of Maximum Sustained Yield is simple enough in principle. A species population will grow each year and produce a ‘harvestable crop’. By removing this harvestable crop from the population there are more resources available to the remaining individuals who will be able to reproduce at their maximum rate. If we look at the logistic growth curve in Graph 2 we can think of harvesting as maintaining the population at a point where it is growing at the maximum rate of increase before the curve flattens out.
In theory there are two levels where harvesting the same rate of animals can occur when representing the relationship between growth and harvest. Let’s imagine a population of deer where a harvest occurs at 150 animals per year. If the population size is 500 individuals then the harvest may stimulate the population to grow at its maximum potential because no resources are limiting. If however the population size is 1,500 deer then the harvest may well have little effect on growth potential because resources are still limiting. The population could be 1,000 and the harvest might be 200 animals. This rate of harvesting may provide the point where population growth is stimulated with each harvested animal but only just. These three examples are known as the lower, upper and maximum sustained yields. Graph 3 illustrates these yields on a theoretical population. The lower sustained yield occurs where the population is still growing. Harvesting at this proportion would be considered safe in terms of protecting the renewable resource. The maximum sustained yield occurs at the top of the population curve and is the maximum amount of animals that can be harvested before the population begins to decline due to the harvest. Harvesting at the upper sustained yield level is riskier than harvesting at the lower sustained level as the balance between decreasing the population size and growth is trending down.
The theory of maximum sustained yield harvesting is still valid today. It is no longer the cornerstone of fisheries management as there are several examples where it failed to protect the resource from decline as it was meant to do. Whilst the theory was solid the ability to actually count fish stocks accurately was not. Big game managers in the United States moved on from Aldo’s early game theory as it was realised that the best measure of yield was not in terms of numbers of individuals harvested but quality. From the 1970s on the goal of deer managers was to increase the hunting experience for hunters. In many instances this meant improving the trophy potential. For bird hunters where the yield is measured in numbers alone the theory has always been sound even if it is sometimes difficult to implement. For many other terrestrial vertebrates proportional harvesting is considered a far safer approach to protecting the resource.
The theory of maximum sustained yield explained here assumes that the harvest is 50 per cent female. However as discussed in previous articles the proportion of female harvest can substantially change a populations potential to grow. Wherever deer numbers are considered overabundant and causing negative impacts to some stakeholders the theory is a good place to start considering the effects of the current harvest strategy. In Australia for example there are very few natural predators of wild deer populations. Whilst wild dogs, foxes and eagles surely account for some mortality there are no tigers, wolves or bears. Predation by humans is likely to be the most significant factor affecting population size and growth in many locations. The number of deer harvested from a population annually may have differing effects depending on the proportion of population it represents.