Effects on Populations

The fundamental effect of exploitation on natural populations is to increase their mortality rate. Increased mortality reduces the density and biomass of exploited populations (Figure 1). This reduction in density, however, can potentially free up resources such as space or food, which may increase growth and reproductive success in the

Table 1 Yearly estimates of total global exploitation rates for major natural resources

Realm

Product

Exploitation

Unit

Source

Forests Oceans Freshwater Terrestrial

Wood

Fish

Fish

80-120a

1.1-3.5b

Million m3yr_1 Million t yr-1 Milliont t yr-1 Million t yr-1

UN Food and Agriculture Organization UN Food and Agriculture Organization UN Food and Agriculture Organization SCB Wild Meat Group

aHigh figure includes discards, illegal and unreported catch. ^Estimates available for Central Africa and Brazil only.

aHigh figure includes discards, illegal and unreported catch. ^Estimates available for Central Africa and Brazil only.

195Cs (6232 kg)

Blue shark

Yellowfin tuna

Silky shark Bigeye tuna

100 50 0 50 100

Mean mass (kg)

Figure 1 Effects of exploitation on an oceanic fish community from the 1950s to 1990s. Changes in total biomass, species composition, mean abundance, and mean mass per species in the Central Pacific are shown. Exploitation caused a 87% reduction in total biomass, and the near loss of sharks and other top predators from the system. Adapted from Ward P and Myers RA (2005) Shifts in open ocean fish communities coinciding with the commencement of commercial fishing. Ecology 86: 835-847.

remaining individuals. Such density-dependent feedbacks (reduced density ! increased resources ! higher population growth) may compensate moderate exploitation mortality. Thus, the longer-term effects of exploitation on populations depend on the dynamic balance between births, deaths, and growth. Every population is capable of net population growth under favorable conditions. For example, every single wheat seed can produce a plant that yields many seeds. When a few seeds per plant are retained for seeding next year's crop, the remainder is the reproductive surplus that can be harvested on a sustained basis. Similarly, a few seeds from mature trees can be grown in nurseries and planted to regenerate forests, or seeds from uncut trees can be a source of natural regeneration. The same principles have been shown to apply to many mammal and fish populations. The problem is that the reproductive surplus is not constant. It is often a function of population density, but also strongly influenced by variability in climate, habitat quality, disease, and other factors. Failure to account for this variation can lead to overexploitation and subsequent population collapse.

Another important effect of exploitation is that it can change size and age structure of a population. Most exploitation targets older or larger individuals, for example, in timber harvesting, trophy hunting, and fishing. Large individuals are often quickly removed and continued exploitation in combination with increased reproduction may shift the populations toward smaller and younger individuals (Figure 1). This can be problematic, for example, in fish populations where old individuals often contribute disproportionately to the total reproductive output, and therefore to the reproductive surplus that can be sustainably harvested. A single female red snapper (Lutjanus campechanus) of 61 cm length and 12.5 kg weight, for example, produces the same number of eggs (9 300 000) as 212 females of 42 cm and 1.1 kg each.

How vulnerable a population is to overexploitation is determined partly by the intensity of exploitation pressure and partly by its life-history traits. Slow population growth rate, high age at maturity, low fecundity, and the tendency to aggregate in large groups are traits that make populations particularly vulnerable to overexploitation. Elephants and sea turtles, for example, have all of these traits and are acutely threatened by overexploitation.

Continued exploitation can be a strong selection pressure that has been shown to cause rapid evolutionary change, for example, in fish and mammal populations. Exploited populations often manifest changes in life-history traits such as age and size at maturity, and growth rate. Typically, the age and size at maturity are depressed and somatic growth is slowed. This latter trend is of course highly undesirable from the exploiters perspective, as it slows the rate of sustainable exploitation. In the North Atlantic and Pacific groundfish communities, for example, mean reductions in age (21%) and size (13%) at maturity were significant, as were changes in mean age (5%) and size (18%) of spawners. The consequences of these changes for total population growth rate remain unclear, however.

If a population is overexploited, the initial decline in population density will continue, in some cases toward extinction. Examples include the extinct Steller sea cow (Hydrodamalis gigas), the largest and the only cold-water member of the Sirenia to which manatees and dugongs also belong, and the Great Auk (Pinguinis impennis) a flightless seabird, and the largest of the alcid family. Declining populations that are threatened by overexploitation often show a range of symptoms such as range contractions, loss of subpopulations, and loss of genetic diversity. Loss of genetic variability in subpopulations is of particular concern due to the long timescale on which such variability has evolved, and the correspondingly large timescale necessary for it to be recovered. Loss of genetic diversity can cause secondary problems, such as inbreeding depression, and decreased adaptability or resilience in the face of environmental change.

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