When several individuals of the same species or of several different speciesdepend on the same limited resource, a situation may arise that is referred to as competition. The existence of competition has been long known to naturalists; its effects were described by Darwin in considerabledetail. Competition among individuals of the same species (intraspecies competition), one of the major mechanisms of natural selection, is the concern of evolutionarybiology. Competition among the individuals of different species (interspecies competition) is a majorconcern of ecology. It is one of the factors controlling the size of competing populations, and extreme cases it may lead to the extinction of one of the competingspecies. This was described by Darwin for indigenous New Zealand species of animals and plants, which died out when competingspecies from Europe were introduced.
No serious competition exists when the major needed resource is in superabundant supply, as in most cases of the coexistence of herbivores (plant eaters). Furthermore, most species do not dependentirely on a single resource, if the majorresource for a species becomes scarce, the species can usually shift to alternative resources. If more than one species is competing for a scarceresource, the competingspecies usually switch to different alternative resources. Competition is usually most severe among close relatives with similar demands on the environment. But it may also occur among totally unrelated forms that compete for the same resource, such as seed-eating rodents and ants. The effects of such competition are graphically demonstrated when all the animals or all the plants in an ecosystem come into competition, as happened 2 million years ago at the end of Pliocene, when North and South America became joined by the Isthmus of Panama. North and South American species migrating across the Isthmus now came into competition with each other. The result was the extermination of a large fraction of the South American mammals, which were apparently unable to withstand the competition from invading North American species----although added predation was also an important factor.
To what extent competition determines the composition of a community and the density of particularspecies has been the source of considerablecontroversy. The problem is that competition ordinarily cannot be observed directly but must be inferred from the spread or increase of one species and the concurrent reduction or disappearance of another species. The Russian biologist G. F. Gause performed numerous two-species experiments in the laboratory, in which one of the species became extinct when only a single kind of resource was available. On the basis of these experiments and of field observations, the so-called law of competitive exclusion was formulated, according to which no two species can occupy the same niche. Numerous seeming exceptions to this law have since been found, but they can usually be explained as cases in which the two species, even though competing for a majorjointresource, did not really occupy exactly the same niche.
Competition among species is of considerableevolutionary importance. The physical structure of speciescompeting for resources in the same ecologicalniche tends to graduallyevolve in ways that allow them to occupy different niches. Competingspecies also tend to change their ranges so that their territories no longer overlap. The evolutionary effect of competition on species has been referred to as “speciesselection”; however, this description is potentially misleading. Only the individuals of a species are subject to the pressures of natural selection. The effect on the well-being and existence of a species is just the result of the effects of selection on all the individuals of the species. Thus speciesselection is actually a result of individualselection.
Competition may occur for any needed resource. [■]In the case of animals it is usually food; in the case of forest plants it may be light; in the case of substrate inhabitants it may be space,
as in many shallow-water bottom-dwelling marine organisms. [■]Indeed, it may be for any of the factors, physical as well as biotic, that are essential for organisms. [■]Competition is usually the more severe the denser the population. [■]Together with predation, it is the most important density-dependent factor in regulating population growth.
13.Look at the four squares [■] that indicate where the following sentence could be added to the passage.Where would the sentence best fit?
That is, as the density of a population increases, competition has a greater impact and leads to greater mortality..
D 要插入的句子开头的 that 一定是指代前文的一句话。并且跟 density 有关。而原文第一次出现 density 有关的词是在 D 选项前的一句话“denser”，插入后发现，插入的就是在进一步说明 D 前的那一句。而后文又补充了 predation 和 competition 对 density 的共同作用，逻辑关系吻合，意思通顺。
