http://prof-raj.blogspot.co.za/2012/01/carrying-capacity.html

Wednesday, 25 January 2012

In terms of Bailey (1983) “Carrying capacity is the number of animals of a specified quality that a habitat can support while sustaining a specified, but not progressively increasing level of impact on habitat resources.” If management goals do not specify the quality of animals or condition of the habitat then “carrying capacity is simply the number of animals that a habitat can support”.

Essential features of the Carrying capacity concept

1. Carrying capacity is a property of the habitat
2. It is determined by limiting welfare factors. Any combination of welfare factors may be limiting.
3. Carrying Capacity varies as supplies of and animal requirements for limiting factors vary.
4. Carrying capacity is the ability of the habitat to support a number of animals, the quality and productivity of the animals being defined according to management goals.
5. Although habitat condition may be influenced by the number of animals, it is implied that a population at carrying capacity will not cause progressive continuing destruction of the habitat such that management goals cannot be sustained.

The various meanings of carrying capacity

Among various wildlife populations and habitats carrying capacity will be determined by differing sets of limiting factors and can be related to a variety of management goals. No one set of population and habitat conditions will characterize carrying capacity for all types of populations. As such scientists (Dasmann 1964; Caughley 1979; Bailey 1983) find it feasible to distinguish between two types of carrying capacity.

Economic carrying capacity

Economic carrying capacity is defined by management goals for population productivity, animal quality and habitat conditions but is determined by a habitat’s variable and limited ability to sustain achievement of these goals. Economic carrying capacities defined by management goals for population productivity and for population control are termed maximum harvest density and minimum impact density.

Maximum harvest density

The concept is usually applicable to ungulates. It is the number of animals that a habitat will support while producing a maximum sustained harvestable surplus. In terms of the sigmoid model, the population is at or somewhat above the inflection point. The population must be maintained at this level of abundance by harvest. Therefore no lack of welfare factors prohibit the growth of a population

Impact on wildlife populations & its habitat

1. At maximum harvest density, population quality will be very good though not probably the very best possible.
2. Populations at MHD characteristically exhibit a young age structure and high rate of turnover
3. Habitat condition will also be good though not without signs of  use and perhaps retrogressed vegetation

Minimum impact density

Minimum impact density as a goal for wildlife management aims to reduce the impact of wild animals on those of desirable target species. It may be desirable to maintain a population at MID of carrying capacity if

–         The population is considered to be a pest species, one not to be eliminated but to be controlled

–         The predator population depresses the production of livestock or desirable wildlife species

–         Ungulates compete with valuable and perhaps less competitive wildlife species target of a particular management programme

Impact on wildlife population and its habitat

1. Populations maintained at Minimum impact density of carrying capacity have a very low level of ecological density.
2. Reproduction and resistance to natural mortality is generally high in such populations, requiring persistent and abundant harvest of animals to maintain the population at this level.
3. The population habitat should also be in excellent condition, receiving only minor use from the depressed population.

Ecological carrying capacity

“Ecological carrying capacity is a variable habitat characteristic determined by changeful amounts of welfare factors that limit the size and productivity of a species population.”

Sometimes populations are unharvested, or normal levels of harvest do not influence the population size very much. In these cases carrying capacities are determined only by limiting habitat resources, and it is often useful to distinguish which set of limiting resources is important in determining population size. Ecological carrying capacity as determined by limiting amount of forage or interspersion and of space is termed as subsistence density, security density and tolerance density respectively.

Subsistence carrying capacity

It is the size of an unharvested population limited primarily by forage. In terms of the sigmoid model subsistence density occurs at the upper asymptote.

Impact on wildlife population and its habitat

1. At subsistence density, population quality and habitat condition will be comparatively poor because this is the ultimate in ecological density.
2. Reproduction is expected to be low and periodic die-off’s will probably occur in years of severe weather.
3. Subsistence density implies that the primary limitation on reproduction and survival is food

Tolerance density

Tolerance density is the number of animals that a habitat will support when intrinsic behavioral and/or physiological mechanisms are dominant in controlling the population. It is sometimes also called as saturation point density and is especially characteristic of territorial species. In terms of sigmoid model, tolerance density occurs at the upper asymptote. For populations at tolerance density, both spaces as well as intraspecific competition become limiting welfare factors.

Impact on wildlife population and its habitat

1. At tolerance density, all animals may be in good condition or they may be in a hierarchy of condition.
2. The subordinate animals will be in the poorest of condition having low rates of reproduction and survival.
3. Since animals tend to defend resources, there is little or no degradation of limiting factors and therefore the habitat condition is also good.

Security density

Security density as a concept of carrying capacity is the number of animals a habitat will support when welfare factors necessary to alleviate predation are limiting. These welfare factors are escape cover, interspersion and for some animals space. In the sigmoid model security density is at the upper asymptote.

Impact on wildlife population and its habitat

1. At security density, social intolerance may force some animals out of the secure habitat. These animals then suffer high predation losses.
2. Reproduction by dominant animals is high and these animals are in good condition too.
3. Habitat condition should also be good.

Conclusion

Populations in general are found above or below the carrying capacities of their habitats and fluctuate over a continuum of ecological densities. In the long run however, population sizes tend to follow trends in ecological carrying capacities because animal qualities, rates of reproduction and survival and habitat conditions tend to be ecological density dependent. When population is maintained below ecological carrying capacity to maximize harvestable surplus, a precise achievement of this economic goal will depend on understanding the relationship of the population and the habitat to variation in population size and variation in carrying capacity of the habitat. Local data will be needed to achieve this understanding.