The population dynamics of small populations

Much of conservation biology is a crisis discipline. Managers are inevitably confronted with too many problems and too few resources. Should they focus attention on the various forces that bring species to extinction and attempt to persuade governments to act to reduce their prevalence; or should they restrict activities to identifying areas of high species richness where reserves can be set up and protected (see Section 22.4); or should they identify species at most risk of extinction and work out ways of keeping them in existence? Ideally, we should do all these things. However, the greatest pressure is often in the area of species preservation. For example, the remaining populations of pandas in China or of yellow-eyed penguins (Megadyptes antipodes) in New Zealand has become so small that if nothing is done the species may become extinct within a few years or decades. Responding to the crisis requires that we devote scarce resources to identifying some special solutions; more general approaches may have to be put on the back-burner.

The dynamics of small populations are governed by a high level of uncertainty, whereas large populations can be described as being governed by the law of averages (Caughley, 1994). Three kinds of uncertainty or variation can be identified that are of particular importance to the fate of small populations.

1 Demographic uncertainty: random variations in the number of individuals that are born male or female, or in the number that happen to die or reproduce in a given year or in the quality (genotypic/phenotypic) of the individuals in terms of survival/reproductive capacities can matter very much to the fate of small populations. Suppose a breeding pair produces a genetic effects and the persistence of a rare plant three kinds of uncertainty for small populations...

Figure 7.19 Relationships for 23 populations of Gentianella germanica between population size and (a) mean number of fruits per plant, (b) mean number of seeds per fruit and (c) mean number of seeds per plant. (d) The relationship between the population growth rate from 1993 to 1995 (ratio of population sizes) and population size (in 1994). (From Fischer & Matthies, 1998.)

Small Populations

Figure 7.19 Relationships for 23 populations of Gentianella germanica between population size and (a) mean number of fruits per plant, (b) mean number of seeds per fruit and (c) mean number of seeds per plant. (d) The relationship between the population growth rate from 1993 to 1995 (ratio of population sizes) and population size (in 1994). (From Fischer & Matthies, 1998.)

clutch consisting entirely of females - such an event would go unnoticed in a large population but it would be the last straw for a species down to its last pair.

2 Environmental uncertainty: unpredictable changes in environmental factors, whether 'disasters' (such as floods, storms or droughts of a magnitude that occur very rarely - see Chapter 2) or more minor (year to year variation in average temperature or rainfall), can also seal the fate of a small population. Even where the average rainfall of an area is known accurately, because of records going back centuries, we cannot predict whether next year will be average or extreme, nor whether we are in for a number of years of particularly dry conditions. A small population is more likely than a large one to be reduced by adverse conditions to zero (extinction), or to numbers so low that recovery is impossible (quasi-extinction).

3 Spatial uncertainty: many species consist of an assemblage of subpopulations that occur in more or less discrete patches of habitat (habitat fragments). Since the subpopulations are likely to differ in terms of demographic uncertainty, and the patches they occupy in terms of environmental uncertainty, the patch dynamics of extinction and local recolonization can be expected to have a large influence on the chance of extinction of the metapopulation (see Section 6.9).

To illustrate some of these ideas, take the demise in North America of the heath hen (Tympanychus cupido cupido)

(Simberloff, 1998). This bird was once extremely common from Maine to Virginia. Being tasty and easy to shoot (and also susceptible to introduced cats and affected by conversion of its grassland habitat to farmland), by 1830 it had disappeared from the mainland and was only found on the island of Martha's Vineyard. In 1908 a reserve was established for the remaining 50 birds and by 1915 the population had increased to several thousand. However, 1916 was a bad year. Fire (a disaster) eliminated much of the breeding ground, there was a particularly hard winter coupled with an influx of goshawks (Accipiter gentilis) (environmental uncertainty), and finally poultry disease arrived on the scene (another disaster). At this point, the remnant population was likely to have become subject to demographic uncertainty; for example, of the 13 birds remaining in 1928 only two were females. A single bird was left in 1930 and the species became extinct in 1932.

The heath hen provides one example of a relatively recent global extinction. At a different scale, local extinctions of small populations in insular habitat patches are common events for diverse taxa, often being in the range of 10-20% per year (Figure 7.20). Such extinctions are also observed on true islands. The detailed

... illustrated by the heath hen

Figure 7.20 Fractions of local populations in habitat patches becoming extinct each year. (After Fahrig & Merriam, 1994.)

Perennial herbs (rlverbank) Algae (rocky intertidal)

> Biennial herbs (coastal sand dunes)

Arthropods (patches of goldenrod)

(grassy sites) Amphibians (ponds)

Birds (forest fragments)

Small mammals (forest fragments)

Annual rate of local extinctions

records from 1954 to 1969 of birds breeding on Bardsey Island, a small island (1.8 km2) off the west coast of Great Britain, revealed that 16 species bred every year, two of the original species disappeared, 15 flickered in and out, whilst four were initially absent but became regular breeders (Diamond, 1984). We can build a picture of frequent local extinctions, which in some cases are countered by recolonization from the mainland or other islands. Examples such as these provide a rich source of information about the factors affecting the fate of small populations in general. The understanding gained is entirely applicable to species in danger of global extinction, since a global extinction is nothing more nor less than the final local extinction. Thus, of the high-risk factors associated with local extinctions, habitat or island area is probably the most pervasive (Figure 7.21). No doubt the main reason for the vulnerability of populations in small areas is the fact that the populations themselves are small. A local extinction of an endemic species on a remote island is precisely equivalent to a global extinction, since recolonization is impossible. This is a principal reason for the high rates of global extinction on islands (see Figure 7.16).

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