Populations

The population dynamics of otters present a somewhat bleak picture. Animal ecologists often categorize species, on the basis of their lifetime reproductive strategies, as 'r selected' or 'k selected', terms derived from population dynamics terminology. R-selected species produce many offspring within a short time, but invest little parental care in them; consequently many of their young die quickly, and the parents themselves tend to be relatively short lived (e.g. rabbits). K-selected species have few young, look after them for a long time and invest a great deal of energy in that; they tend to be longer lived (e.g. elephants). With few exceptions, otters clearly fall into the k-selected end of the spectrum, with large maternal investment resulting in only one or a few cubs. Their problem is that the adults themselves live for only a short time, compared with many other k-selected species.

One expects carnivores of the size of otters to have a long life, after surviving the vicissitudes of early independence, and indeed the potential longevity of most species is 15-25 years (in captivity). However, for North American river and Eurasian otters in the wild, the mean life expectancy at the age of 1 year is only some 3-4 years, in areas that have been studied (see Chapter 12). So far, there is little to suggest that other species fare any better, except the sea otter, with a mean life expectancy of 7-8 years.

This implies that an average Lutra orLontra female, even if she breeds every year, will produce hardly more than two litters, in some areas with a mean of fewer than two cubs per litter. Sea otters have only one cub at a time. Some individuals, of course, will live much longer than the average age, but that does not alter the general picture that otters have very little room for manoeuvre with such small numbers of offspring. A small increase in adult mortality, or a decrease in recruitment of cubs, will seriously affect the viability of a whole population.

Unusually for mammals, annual mortality in Eurasian otters in Britain increases with age (see Chapter 11), and this may also be the case in other otter species, except in sea otters. There is as yet no good explanation for this; a likely possibility is that it is a consequence of the high metabolic rate of these animals, the price of their high-expense foraging.

An alternative is that pollutants are accumulating with age, although there is no evidence for this.

The pattern of recruitment and mortality in otters can usefully be compared with that in another well studied mustelid of comparable size, the Eurasian badger Meles meles (Cheeseman et al. 1987). The adult mortality rate of these badgers is similarly high, but does not increase with age as in otters, and most deaths occur in young animals. Females produce more cubs per year and per lifetime than do otters, and this is then followed by high cub mortality. This implies that badger recruitment is more easily adjusted to food availability, or to adult population density, than the recruitment in otter populations.

In otters, both adult mortality and cub recruitment are affected by fish availability, at least in some species and populations, but probably everywhere (see Chapter 8). However, these two aspects of otter populations are not influenced by fish numbers in the same way. In Eurasian otters, adult mortality occurs especially during the annual periods of fish shortage (see Chapter 12), for instance in northern latitudes at times when waters are coldest (so foraging is most costly). Cub recruitment, at least along Shetland sea coasts, is correlated with prey abundance during the annual high season for fish, in midsummer. Whatever affects fish numbers and productivity in their environment is likely, along one of these pathways, also to have implications higher up the ladder in the aquatic community, in the populations of otters.

Other environmental effects on adult otter mortality, either direct, or indirect by reducing fish populations, may be caused by pollution, which affects aquatic habitats more than most others. Effects of pollution have to be considered in a population context, jointly with other mortality factors such as food shortage, and this has been attempted to some extent in the Shetland studies. One possible scenario in Shetland was that otter numbers were limited by food, and that older animals, with high levels of mercury in their bodies, would suffer the highest mortality rate. Diseases may also affect survival.

A typical characteristic of populations of otters is that the animals always occur in small numbers; this is true for all species, except for Enhydra, which may occur in 'rafts' of several hundred animals. Species such as the fox or the Eurasian badger can attain densities of up to some 30 per km2 in Britain (0.1-0.3 per hectare) (Cheeseman et al. 1987; Macdonald 1985), but for Lutra lutra no more than some 0.8 adults were found along 1 km of rich Shetland coast, or 0.3 per km of stream in a 'good' area in mainland Scotland. However, such figures for Eurasian otters translate into actual densities of up to 0.5 animals per hectare of fresh water, or 0.1 per hectare of suitable coastal water, that is, the actual densities per area of habitat are of the same order of magnitude as those of other, similarly sized carnivores (see Chapter 11). Crucially, in any one region there is less suitable habitat for otters than for those other terrestrial carnivores. Otter populations, despite densities per area of suitable habitat that compare well with other carnivores, cannot fall back on substantial numbers if something goes wrong.

Recent studies of the genetic variability of otter populations have concentrated on the Eurasian and the sea otter (see Chapter 11). There is relatively little genetic exchange between populations of Eurasian otters more than 100 km apart, and within populations the genetic variability is relatively low in that species. This is unrelated to recent population bottlenecks (e.g. the pollution disasters), so it may well be a general feature for other otter species. Some populations, such as the one in Shetland, are clearly genetically distinct, and even morphologically different. Sea otters have lost much of their genetic diversity because of severe population reduction in the recent past (see Chapter 12), and also in that species there is only restricted gene flow between neighbouring populations (see Chapter 11). Small genetic variability of otters may well render them more vulnerable to disease, as in the case of the African cheetah (Acinonyx jubatus) and other species (Wayne et al. 1986), and, at least for sea otters, disease, especially toxoplasmosis, has become a demonstrably important factor in populations (see Chapter 12).

Noticeably, in recent studies, more emphasis is being placed on factors other than food resources that may limit numbers of otters in populations. Disease is one such factor, but what has caught most recent attention is predation on otters. Sea otters, especially diseased ones, are being preyed upon by sharks (see Chapter 12), which leads to population declines, and the most dramatic increase in killer whale predation upon that species is part of a general 'megafauna collapse' (Springer et al. 2003). I have argued above that predators may also be an important selection pressure favouring group formation, in otters of many species. 'Top-down' factors may play a more important role than was recognized in the past, when we concentrated on the 'bottom-up' effects of resources.

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