Otters in ecosystems and competition with other species

As I am focusing on just some unmarried species of predator, there is a danger in seeing these animals out of context. Otters are an integral office of large, complicated ecosystems, and whatever affects them, affects an entire customs. Conversely, whatever is changing in the aquatic community is bound to accept some effect on otters, sometimes very dramatically, as in the marine coastal communities where sea otters live (see Chapter 12).

Many such ecosystem effects are on a very large calibration. For example, fish communities, affecting otters as discussed to a higher place, are beingness contradistinct by climate modify throughout regions, for instance in the North Sea (Genner etal. 2004), and eel species are declining on a continental scale (Wirth and Bernatchez 2003), as are Atlantic salmon. The causes of these changes may be thousands of kilometres from the actual effects. In full general, freshwater fish are amidst the categories of animals with the highest rates of extinction (Jenkins 2003), and the changes in the ecosystems in which sea otters occur have been related to whaling (Springer et al. 2003; Williams et al. 2004). Large-calibration aquatic ecosystem changes may likewise be due to aeriform pollution, or even to aquatic (mercury) pollution a long distance away (Gutleb etal. 1997).

On a smaller scale, fish predators such as otters are likely to be affected by the occurrence in their ecosystems of other fish-eaters, every bit competitors. Where prey availability sets the 'carrying capacity' of an area for otters, ane may expect that the presence of other animals with similar diets will bear on otter numbers. It is a scenario of interspecific competition, and one ready of competitors is other species of otter.

Contest betwixt otter species is not common, as huge regions in the world are inhabited past but one species, and ofttimes when there is more than i they alive in split habitats. River otters and ocean otters both use the Pacific coast of North America, but river otters stay closely inshore and feed on small fish (Bowyer et al. 1995); sea otters go out much farther and feed on big invertebrates, much less often on quite large fish in deeper waters (Riedman and Estes 1990). Consequently, there is no effective competition between these ii. Similarly, in that location are coasts in Republic of chile where the marine and the southern river otter both occur, the marine otter forth very exposed, crude and rocky shores, the southern river otter in much more sheltered parts (besides every bit in rivers and freshwater lakes), which keeps them well apart. These ii species have quite like diets. In this example it is non known whether this habitat separation is due to 1 otter actively excluding the other, or to an inherited habitat preference (Ebensperger and Botto-Mahan 1997).

In dissimilarity, the neotropical otter appears to occur wherever the giant otter is found, in the same rivers equally well as elsewhere, and in the Brazilian Pantanal I have seen them forage inside sight of each other. There is a deviation in micro-habitat between the two, with neotropicals spending almost all of their time inside short altitude of the bank, and giant otters foraging all over the river, taking larger fish (Muanis 2004; Muanis and Waldemarin 2004). The solitary, slower and smaller neotropical otters are more vulnerable to attack by piranhas, which live in deeper waters of rivers and oxbows, and almost half of the neotropicals in areas of the Pantanal show evidence of previous injury from piranha (especially tail mutilation; run across Chapter 12).

In south-eastern asia the Eurasian otter shares rivers and streams with the smoothen and small-scale-clawed otters, and the potential for competition is there. The diets of the three are rather unlike (Fig. 11.xi), every bit is their use of different parts of the rivers and of micro-habitats (run across Affiliate 4). Even so, there is much overlap betwixt them on all scores, and they even utilize and scent-mark each other's latrines (Kruuk et al. 1994a). I constitute some testify from tracks in the sand that Lutra actively avoids Lutrogale. The ii most like species in that general region are the Eurasian and the hairy-nosed otter, but equally far equally is known (at that place is much uncertainty; Sivasothi and Nor 1994) the only area where the 2 actually overlap is a small part of southern Sumatra: the geographical distributions of the two are nearly entirely exclusive.

in c 40

in o twenty

10 0

in 60

80 60 forty 20

Eurasian otter

Crab Frog Fish Mamm Snake Other

Smooth otter

Frog

Fish Mamm Serpent Other

Small-clawed otter

Crab Frog Fish Mamm Snake Other Dominating spraint content

Effigy 11.11 There is little competition for casualty between sympatric Eurasian, polish and modest-clawed otters in Huay Kha Khaeng,Thailand. (Data from Kruuk etal. 1994a.)

In southern Africa the Cape clawless and the Congo clawless otter are very similar, and there are relatively small regions where the geographical ranges of the ii overlap. However, nothing is known about interactions betwixt the 2, and where they occur in the same area they are potential competitors, despite dietary differences (see Chapter 7). More importantly, both species as well share much of their geographical ranges with the spotted-necked otter, and there are interesting observations on overlaps of interest, or lack thereof (Carugati and Perrin 2003a and b; Kruuk and Goudswaard 1990; Somers and Purves 1996). The diets of Cape clawless and spotted-necked are quite different, with fish for the spotted-necked, and crabs, fishes and others for the clawless. Although they live in the same waters, and their habitat selections take much in common, the spotted-necked never occurs in the ocean, whereas Cape clawless often do. Where they occur in neighbouring areas, such every bit in Nigeria, spotted-necked otters stay in freshwater rivers, Greatcoat clawless in the brackish mangroves (Angelici etal. 2005).

Crab

The African otters share their habitat about everywhere with the water mongoose Atilax paludinosus, which too feeds in water and consumes crabs, insects and small mammals (no fish). Its interests potentially overlap especially with that of the clawless otters (Purves etal. 1994; Somers and Purves 1996). Yet, here again there are distinct differences in foraging methods and choice of micro-habitat, so contest, if present at all, is minimal. The mongoose takes larger venereal than the otter, probably because developed crabs frequently forage on land at night, but pocket-sized ones do not. A similar relationship exists between the diverse otter species in south-east asia and the crab-eating mongoose Herpestes urva (Kruuk etal. 1994).

The species that is most often thought to compete with the Eurasian otter, for food and space, is the introduced American mink Mustela vison (east.chiliad. Dunstone 1993; Lever 1978), despite the fact that otters are some seven times heavier than mink. There is some overlap in diet between them (Chanin 1981; Clode and Macdonald 1995), but there are large size differences in their prey, and there is no evidence that any overlap includes prey species that may be express by predation. The two often occur in the aforementioned habitats in fresh water or along sea coasts, even using the aforementioned dens (personal observation).

In North America ane finds a like relationship betwixt the mink and the river otter, the 2 even sleeping simultaneously in the same cover (Melquist et al. 1981). However, forth sea coasts in Alaska the habitat option of river otters and mink is slightly dissimilar, mink using more sheltered sites for foraging. All the same, there is much overlap, and the diet of the 2 species shows many similarities (Ben-David etal. 1995).

When mink encounters otter, mink avoids, whereas otters practise non appear to take much notice (personal ascertainment). It has been suggested more recently that, in Britain, mink alter their nutrient habits to a more terrestrial diet when Eurasian otters are present (Bonesi and Macdonald 2004a; Bonesi et al. 2004), and also that mink numbers decrease when otters move into an area (Bonesi and Macdonald 2004b). By and large, notwithstanding, the two are so different in size and feeding habits that contest is not probable to be fierce. A similar conclusion appears to exist valid in Republic of chile for the introduced American mink and the southern river otter (Medina 1997).

Not surprisingly, there are many animals whose contest with otters is far less subtle than mere interest in the same prey species. Straight kleptopar-asites are out there, such as bald eagles Haliaeetus leucocephalus, which dive on to sea otters that sally with a fish, and are often successful in stealing information technology (Watt et al. 1995). Yet, despite the fact that bald eagles are numerous along Aleutian and Alaskan coasts, overall, the amount of fish they take from sea otters is insignificant. From elsewhere there is the record of a bobcat Felis rufus stealing a coot from a river otter (Bergan 1990), and one may look foxes or coyotes to indulge similarly.

Caiman (Melanosuchus niger; Fig. 11.12) oftentimes steal fish from neotropical otters, even on land, and in the Pantanal, where caiman are very common, it is not rare to run into an eating otter surrounded by several of them (personal observation). Many other species are likely to compete with otters of all kinds, including predatory birds (including herons, cormorants), fish such as state highway, and other predators. However, 1 should not equate a dietary involvement in similar casualty species, with competition—the latter occurs only if one species reduces the numbers of the prey that can be taken by another predator. For instance, the various vertebrates that feed on the recently introduced crayfish Procambarus clarkii in Portugal (including Eurasian otters) do not necessarily compete with i another (Correia 2001).

Figure xi.12 Competition for neotropical and giant otters: caiman (Melanosuchus niger) are arable, consume large quantities of fish and steal from otters.

People may suffer when in competition with otters, as their line-fishing interests can be like. In this particular interaction, the aggressive competition from otters, of whichever species, causes fishing people to lose out when otters destroy nets to get at the fish (for case, personal ascertainment of Eurasian and smooth otters in Thailand), or when otters take large numbers of a species important to fisheries. Spotted-necked otters in Rwanda have nigh 15% of the catch from the nets of local fisher-folk (Lejeune (1989), and along Californian coasts fishing of red abalone collapsed after the sea otters moved into areas (Fanshawe et al. 2003; Wendell 1994). Otters frequently accept fish from fish-farms (run across Chapter 14). There is no evidence that otters have less prey available considering of people's fishing activities, simply, of class, otters do suffer from persecution because of their predation—a clear upshot of competition.

About contact betwixt otters and other fish-eating animals is far less dissentious, and should exist classed every bit commensalism rather than competition. These are cases such as the fascinating association between giant otters and freshwater dolphins Inia geoffren-sis in large rivers in Colombia (Defler 1983), where the dolphins appear to swim along offshore from groups of behemothic otters foraging along the edges. The dolphins accept fish that escape the otters. My own observations were somewhat less spectacular, of several kingfishers and swimming heron, catching fish that were disturbed and fled from smoothen otters in Thailand (Kruuk etal. 1993b).

Equally a generalization, competition between other otter species and different fish eaters has lilliputian upshot on numbers of any of the otter species where they occur at present. The just major exception is competition between humans and several of the otters, resulting in persecution of the animals. I has to accept, withal, that the status quo between otters and other piscivores has peradventure evolved after conflict in the distant past, conflict that is no longer evident. Information technology shows when different otter species accept largely exclusive geographical ranges, and when these geographical ranges do overlap we find variation in diet and/or habitat selection.

In general, when present-twenty-four hours, closely related carnivorous species are sympatric, competition between them has little result, because of clear differences in size (hence differences in prey selection), for example the mustelids in U.k. and elsewhere (Dayan and Simberloff 1994). Such regularities are hands upset by introduced species, such equally mink (Sidorovich etal. 1999). All the same, competition is of little import, in clear contrast to another interspecific human relationship: predation (come across Chapter 12).

CHAPTER 12

Continue reading here: Historic period structure life expectancy and rates of mortality

Was this article helpful?