Keystone Species in an Ecosystem - Essay Example

appears here] appears here] appears here] appears here] Keystone Species Keystone species are species central to an ecosystem, species upon which nearly all other species depend. Several keystone species have been identified in the wild, but it is not easy to predict which species will be keystone because the connections between species in food webs are often complex and obscure. For instance, large cats act as keystone predators in Neotropical forests. They limit the number of medium-sized terrestrial mammals, which in turn control forest regeneration. On Barro Colorado Island, Panama, jaguars (Felis onca), pumas (Felis concolor), and ocelots (Felis pardalis) have been removed. The populations of big-cat prey including the red coati (Nasua nasua), the agouti (Dasyprocta variegata), and the paca (Agouti paca) are about ten times higher than on Cocha Cashu, Peru, where big cats still live. However, this increase may result from natural population variability rather than the lack of jaguars and pumas. The extreme removal of herbivores and frugivorous mammals would drastically affect forest regeneration, altering tree species composition, but the effects of modest changes in densities are less clear. The sea otter (Enhydra lutris) is a keystone predator par excellence (Duggins 1980). A population of around 200,000 once thrived on the kelp beds lying close to shore from northern Japan, through Alaska, to southern California and Mexico. In 1741, Vitus Bering, the Danish explorer, reported seeing great numbers of sea otters on his voyage among the islands of the Bering Sea and the North Pacific Ocean. Some furs were taken back to Russia and soon this new commodity was highly prized for coats. Hunting began. In 1857, Russia sold Alaska to the United States for $7,200,000. This cost was recouped in forty years by selling sea otter pelts. In 1885 alone, 118,000 sea otter pelts were sold. By 1910, the sea otter was close to extinction, with a world-wide population of fewer than 2,000. It was hardly ever seen along the Californian coast from 1911 until 1938. The inshore marine ecosystem changed where the sea otter disappeared. Sea urchins, which were eaten by the otters, underwent a population explosion. They consumed large portions of the kelp and other seaweeds. While the otters were present, the kelp formed a luxuriant underwater forest, reaching from the sea bed, where it was anchored, to the sea surface. With no otters to keep sea-urchins in check, the kelp vanished. Stretches of the shallow ocean floor were turned into sea-urchin barrens, which were a sort of submarine desert. Happily, a few pairs of sea otters had managed to survive in the outer Aleutian Islands and at a few localities along the southern Californian coast. Some of these were taken to intermediate sites in the United States and Canada where they were protected by strict measures. With a little help, the sea otters staged a comeback and the sea urchins declined. The lush kelp forest grew back and many lesser algae moved in, along with crustaceans, squids, fishes, and other organisms. Grey whales migrated closer to shore to park their young in breaks along the kelp edge while feeding on the dense concentrations of animal plankton. Keystone predators sometimes are more effective within certain parts of their range. The sea star (Pisaster ochraceus) is a keystone predator of rocky intertidal communities in western North America (Paine 1995). This starfish preys primarily on two mussels Mytilus californianus and Mytilus trossulus. A study along the central Oregon coast showed that three distinct predation regimes exist (Menge, B. A., and T. L. Freidenburg. 2001). Strong keystone predation occurs along wave-exposed headlands. Less strong predation by sea stars, whelks, and possibly other predators occurs in a wave-protected cove. Weak predation occurs at a wave-protected site regularly buried by sand. If keystone species do exist, how do the links within a system dominated by such a species actually function Paine proposed that the trophic link between a top consumer and its resource would be a 'strong' one, while those between other consumers and their food might be 'weak'. Only the 'strong' links would be important in determining community structure, but on these strong links might depend whole 'modules' of weakly interacting species that flourish or disappear depending upon the presence or absence of the strong link. In the example of starfish and mussels, starfish would be the top consumer, the mussel its major resource, and the whole network of grazers and suspension feeders that appear when the starfish is present would be the 'module'. (Paine, R. T, 1966) Paine used another example to demonstrate the keystone species hypothesis, and this has been much discussed. In the beds of giant kelp, Macrocystis, off the Pacific coast of North America, there used to be large numbers of sea otters (Enhydra lutris). This species was decimated in the nineteenth century by hunting, and has only recently been reestablished now that hunting has been banned. One of the major food items of the sea otter is a sea urchin, Strongylocentrotus, and it has been hypothesized that the decline in sea otters allowed the sea urchins to survive in greater numbers than before. The sea urchins are grazers, and Paine attributed to their increased grazing pressure the decline of algal abundance on the California coast. More than this, some authors have suggested that, since urchins are capable of deforesting some areas, providing an extreme contrast to areas of dense kelp where urchin densities are low, the urchins trigger a switch from one 'stable state' to another. Within their geographical range, the otters may mediate these switches through their predation on the urchins. Here, then, is a possible hierarchy of strong interactions, sometimes called a 'cascade' effect. Discussion and testing of the hypothesis that sea otters are keystone species in kelp beds has been intense: are they keystone species or are they 'just another brick in the wall' One line of evidence comes from following changes in the kelp canopy after sea otters reoccupied areas in which they had long been extinct. At Monterey, for instance, the sea bed had high densities of sea urchins in the late 1950s. Ten years after sea otters reached the area, the urchins were confined to crevices, and giant kelps were abundant. It certainly seems that otters can have a striking effect on the ecosystem. It is, however, important to know how widespread this effect actually is. Surveys of shallow subtidal reefs outside the present range of the otters, but within their ancestral range, have found only a few sites with 'urchin deforestation'. Only in this small fraction of sites could sea otters therefore act as keystone species if they returned. At most sites (89%), the urchins are present in patches, or are restricted to crevices, while only 2% are completely dominated by algae. Many sites changed their characteristics over the survey period, suggesting that rather than switching between 'stable states', the sites show intrinsic variability in composition. Kelp communities may not, therefore, generally be structured around interactions between sea otters, sea urchins and macroalgae. Instead, variation in the communities may depend upon a whole suite of factors, the 'bricks in the wall': the availability of algal propagules; the nature of the substrate, nutrient and light levels; incidence of storm action; as well as the effects of otters and urchins. The 'hierarchical' view that sea otters ameliorate the grazing effects of sea urchins is appealing because of its simplicity, but it does not take into account these other factors. (Hixon, Mark A. and William N. Bronstoff, 1983) There have been many other comments on the system. Tests to mimic the action of sea otters by manually removing numbers of urchins have failed to produce large changes in kelp density. In general, the otterurchin interaction appears to be important in some areas, but not in others, and it has been suggested that it is not an effective factor over very wide areas. But this leads to the question of how large an area does a species have to dominate in order to be called a keystone species The scale of experiments has, by definition, been quite small, yet the concept is applied to large but undefined areas. In the long run then, the question of whether species can be called 'keystone species' or not may be a question of scale. On the mid shore in New South Wales, the dominant organisms in regions of moderate exposure are barnacles, limpets and whelks. Croll, D. A., J. L. Maron, J. A. Estes, E. M. Danner, and G. V. Byrd. (2005) investigated the roles of the four most common species in this community by means of experimental manipulations. These species are the barnacle, Tesseropora rosea; the large patellid limpet, Cellana tramoserica, which feeds on open rock surfaces; the small acmaeid limpet Patelloida latistrigata, which mostly grazes in the interstices between barnacles; and the predatory whelk, Morula marginalba. The experiments of Underwood et al. differ somewhat from many that have been discussed so far, in that they examined interactions by varying the densities of species, instead of simply removing them. For example, to study the effect of the limpet Cellana on the survival of barnacles, they excluded the whelk Morula and confined Cellana at varying densities per enclosure. Over 10 months, barnacles showed high survival where there were two or four Cellana per enclosure, but low survival where there were no Cellana or where Cellana were at high densities. At low Cellana densities, macroalgae grow and choke the barnacles, while at high Cellana densities the limpets crush the newly settled cyprids and young barnacles. At intermediate densities, the limpets graze down the macroalgae sufficiently to encourage barnacle settlement and growth. In parallel experiments, where whelks were allowed access, barnacle survival was low at all Cellana densities predation here takes over as a dominant factor. (Estes, J.A., M.T. Tinker, T.M. Williams, & D.F. Doak, 1998) To study the inverse interaction how barnacles affect Cellana Underwood et al. measured the growth rates of limpets placed in fenced areas containing various densities of barnacles. Growth rates were highest with no barnacles, and declined in proportion to barnacle density: Cellana does not in fact normally inhabit barnacle-covered areas, and measurements of weight showed that they eventually die of starvation if confined there. In contrast to this effect, the limpet Patelloida survives better with increasing numbers of barnacles, in the absence of whelks. Because Patelloida is smaller, it can feed between the barnacles, whereas the larger Cellana presumably has difficulty rasping with its radula on very rough barnacle-covered surfaces. As with Cellana, though, when Patelloida is enclosed together with the whelk Morula, the whelks eliminate any effect of barnacle density because of their high predation rate. From these experiments, it is apparent that there are major interactions amongst all the component species in the system. Density of barnacles affects both limpet species, and in the case of Cellana this is neither a linear nor an all-or-none effect, but one which is beneficial at intermediate densities. Cellana has important effects on the survival of barnacles. The whelks affect density of both the barnacles and Patelloida. The outcome of these interactions evidently varies with position on the shore, exposure, season and weather, but no interactions can be classified as 'weak' in the sense of Paine. Croll, D. A., J. L. Maron, J. A. Estes, E. M. Danner, and G. V. Byrd interpretation of this system is thus quite different from a keystone-species concept. Here the various members of the community are seen to interact to varying degrees, but no overriding consumer-resource interaction is seen to dominate. (Paine, R. T, 1969). There is, of course, no reason why the communities of rocky shores in eastern Australia studied by Underwood et al. should necessarily be controlled in the same way as those in America, New Zealand and Chile studied by Paine. It could well be that, in places where one species tends to form monocultures; there is a tendency for keystone predators to evolve. But once again, we come to a question of scale. Could keystone species evolve where patches of monoculture are, say, centimeters or meters in diameter Or must they be hundreds of meters or kilometers in size Until more studies have been made of the geographical and temporal scales over which particular interactions occur, discussion will continue. Another way of thinking about the organization of communities in more general terms is to assume that they involve many interactions, some trophic and others competitive; and then to ask whether the lower trophic levels depend more upon the actions of organisms in higher levels ('top-down' factors), or whether the higher trophic levels are constrained by variation in production at lower levels ('bottom-up' factors). There seems, in fact, little doubt that both these types of factor can influence communities. For instance, some 'bottom-up' factors such as the level of primary production may determine how many trophic levels can exist, and this will affect community structure directly-although there is little evidence to show how widespread this kind of control may be. Classical 'top-down' factors might be the effect of sea urchins on kelp beds, whether or not the urchins are considered keystone species and such factors are thought to be common. (Duggins, D. O, 1980) Menge, B. A., and T. L. Freidenburg (2001) considered how these factors might act, and interact, by studying two sites on the coast of Oregon in North America. Each site has both exposed and sheltered areas. At Boiler Bay, benthic plants dominate shores at all exposures. At Strawberry Hill, plants dominate in shelter while mussels and barnacles dominate at exposed sites, where they are accompanied by many predatory starfish. The exposed shores at instance, may allow a succession of algae to settle. Clearance of mussels may allow settlement of other suspension feeders, such as barnacles. Areas cleared should be small, so as not to interfere with the overall habitat and clearance of Ascophyllum nodosum should not be undertaken because of its long life-span. The effect of removing species depends upon the complexity of the food web. In a complex food web, removal of a plant at the bottom has little effect through the rest of the ecosystem. This is borne out by the limited impact of American chestnut (Castanea dentata) removal from the eastern forests owing to chestnut blight. Seven species of butterfly that feed exclusively on the chestnut are probably extinct but forty-nine other species of butterfly that also fed on the chestnuts found alternative food sources, as did the insect predators that fed on all fifty-six butterflies. On the other hand, removal of keystone predators or herbivores is not so innocuous an event a major shock cascades all the way down the food web and shakes to lowermost level. Kangaroo rats ( Dipodomys spp.) are a keystone herbivores in the desert-grassland ecotone in North America. They have a major impact on seed predation and soil disturbance. Twelve years after their removal from plots of Chihuahuan Desert scrub, tall perennial and annual grasses increased threefold and rodent species typical of arid grassland had colonized. Similarly, the cassowary is thought to be the sole disperser for over a hundred species of woody tropical rain-forest plants in Queensland, Australia. It usually inhabits large forests. Logging and habitat fragmentation have removed the bird from several areas, in which only small remnants of forest remain. A progressive and massive loss of trees is likely to follow, unless the cassowary adapts or adjusts its behaviour. For simple food chains, the situation is reversed. Highly specialized herbivores or carnivores are extremely vulnerable to the loss of their sole food source. The koala (Phascolarctos cinereus) is an arboreal folivore. It feeds almost exclusively on the foliage of gum trees (Eucalyptus). Although still widespread, the koala population is controlled in some areas where overpopulation would otherwise lead to defoliation, the death of scarce food trees, and an endangerment of the koala population. The sabre-toothed cats were one of the main branches of the cat family (Felidae) throughout much of the Tertiary. They had enormous upper canine teeth and probably specialized in preying on large, slow-moving, thick-hided herbivores, such as mastodons and giant sloths. They became extinct during the Pleistocene, most likely because their prey was at first thinned by extinctions and finally vanished. The examples suggest that introducing generalists should have a greater impact on an ecosystem than introducing specialists. Correspondingly, introductions into ecosystems containing generalist predators should have less of an impact than introductions of specialist predators. The black-tailed prairie dog is what biologists call a keystone species. Lose the keystone, and the whole ecosystem crumbles down with it. Researchers once believed that as many as 208 species depend on prairie dogs, but a recent review of published data by Natasha Kotliar and her colleagues at the U.S. Geological Survey in Colorado revealed that those estimates were probably too high. Kotliar found strong evidence for nine species relying on prairie dogs, including the endangered black-footed ferret, which feeds almost exclusively on the animals; the mountain plover, a bird that requires the disturbed grassland habitat prairie dogs provide; the burrowing owl, which uses prairie dog burrows for homes; and the ferruginous hawk, which preys on prairie dogs. Evidence for other species was not as strong, or data wasn't available, but Kotliar's study nonetheless confirmed the prairie dog's role as a keystone species. Prairie dogs do more than just serve as prey, they also perform a valuable service for the prairie-they disturb it. In addition to digging up the soil, prairie dogs clip the vegetation around their burrows, enhancing nitrogen uptake by these plants. "Natural disturbances are an important part of maintaining the prairie ecosystem," says Kotliar. "We're learning that if you change the natural disturbance regime, you alter the ecosystem and you may start losing species." Although they are short of stature, prairie dogs cast a long shadow over grasslands in the western United States. The rodents' burrows provide homes to a number of birds and other creatures, and the prairie dogs provide nutrition to raptors and other predators. As a result, the large towns where prairie dogs live have many more species than similar areas without the creatures. www.nwf.org/nationalwildlife/ article.cfmissueID=34&articleID Bibliography Croll, D. A., J. L. Maron, J. A. Estes, E. M. Danner, and G. V. Byrd. 2005. Introduced predatorstransform subarctic islands from grassland to tundra. Science 307:1959-1961. Duggins, D. O. 1980. Kelp beds and sea otters: an experimental approach. Ecology 61:447-453. Estes, J.A., M.T. Tinker, T.M. Williams, & D.F. Doak. 1998. Killer whale predation on sea otters linking oceanic and nearshore ecosystems. Science 282 Hixon, Mark A. and William N. Bronstoff. 1983. "Damselfish as keystone species in reverse: intermediate disturbance and diversity of reef algae." Science 220 Kotliar, N. B., Baker, B. W., Whicker, A. D. and Plumb, G. 1999. A critical review of assumptions about the prairie dog as a keystone species. Environmental Management 24: 177-192. Menge, B. A., and T. L. Freidenburg. 2001. Keystone species. Pages 613-631 in S. A. Levin, editor-in-chief. Encyclopedia of Biodiversity, Volume 3. Academic Press, San Diego, California, USA. Miller, Brian, Ceballos, Gerardo, and Richard P. Reading. 1994. "The Prairie Dog and Biotic Diversity." Conservation Biology 8(3):677-81. Paine, R. T. 1966. Food web complexity and species diversity. The American Naturalist 100:65-75. Paine, R. T. 1969. A note on trophic complexity and community stability. The American Naturalist 103:91-93. .Paine, R.T. (1995) A conversation on refining the concept of keystone species. Conserv. Biol. 9: 962-964. www.nwf.org/nationalwildlife/ article.cfmissueID=34&articleID Read More

 

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