Three species of tiny wasps were particularly useful

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pests.Three species of tiny wasps were particularly useful in attacking Hawaiian fruit flies. Over a four-year period, populations of wasps successively replaced each other until by 1951 only one species persisted. Similar studies of introduced wasps that attack scale insects in southern California had similar results. Two species of barnacle are found on the rocky coasts of Scotland. One species grows in the upper reaches of the intertidal zone, where it is exposed to air daily at low tides.The other species is found farther down on the rocks, where it is less frequently and intensely exposed.In the 1950s,Joseph Connell discovered that the shallow-water species could survive deeper water if the other species was removed.The species’ absence there under natural conditions was interpreted as a result of competition. In the 1950s, Robert MacArthur found what he thought at first was an example of species that could coexist and compete. He was studying warblers in the coniferous forests in the northeastern United States and southeastern Canada.He found five species
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510 C HAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations? apparently feeding in the same trees.Detailed observations revealed,however,that com- petition between species was minimal. Although there was some overlap, each species had its own preferred region within the trees in which it fed. Intense competition had not resulted in the exclusion of species in this case. Instead, resources were partitioned among the species (Figure 15-18). The yellow-rumped warbler is the most cosmopolitan of the five species studied by MacArthur. Its breeding range spans the continent. Notice that, in the trees MacArthur studied, these birds fed in the lower branches. In other regions, where the other war- blers are not present, yellow-rumped warblers feed mainly in the tops of trees.This abil- ity to go where there is less competition goes a long way toward explaining why this species is the most numerous and widespread warbler in North America. Numerous studies of a range of organisms verify that competition is often mini- mized by resource partitioning . So,what are the results of competition between species? First of all,notice that com- petition is a density-dependent limiting factor.In an ideal environment in which resources are plentiful and population numbers are low, competition is not a factor. Potential competitors can find all they need without interfering with each other. This situation doesn’t last for long. As populations grow, interactions between competitors intensify. There are two possible results: One species will exclude the other, or the two species will partition resources so that competition is minimized and tolerable. In Predator–Prey Interactions, One Population Benefits at the Expense of the Other One of the most obvious interactions between populations is a matter of which eats which. Populations regularly exploit other populations as sources of nutrients and ener- gy. As a result, the fates of predator and prey populations are intimately intertwined.
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