When several individuals of the same species or of several different species depend 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 considerable detail. Competition among individuals of the same species (intraspecies competition), one of the major mechanisms of natural selection, is the concern of evolutionary biology. Competition among the individuals of different species (interspecies competition) is a major concern 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 competing species. This was described by Darwin for indigenous New Zealand species of animals and plants, which died out when competing species 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 depend entirely on a single resource, if the major resource for a species becomes scarce, the species can usually shift to alternative resources. If more than one species is competing for a scarce resource, the competing species 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 particular species has been the source of considerable controversy. 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 tow-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 major joint resource, did not really occupy exactly the same niche.

Competition among species is of considerable evolutionary importance. The physical structure of species competing for resources in the same ecological niche tends to gradually evolve in ways that allow them to occupy different niches. Competing species 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 “species selection”; 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 species selection is actually a result of individual selection.

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.

当同一物种的不同个体或不同的物种都依靠同一有限资源时，这种情况往往会引发成所谓的竞争。竞争的存在已为生物学家们所熟知，其结果也已被达尔文详尽叙述过。同一物种不同个体之间的竞争（种内竞争），作为自然选择的一个主要原理，现在属于进化生物学。不同物种个体之间的竞争（种间竞争）是生态学的主要部分。它是控制有竞争关系的种群数量的一个因素，而其极端情况可能导致竞争的一方灭绝。达尔文在描述新西兰本土动植物物种在与引进的欧洲物种的竞争中灭绝时讲到该情况。

当主要需求的资源能充分供给时，竞争就不会很激烈，正如很多情况下植食动物都能够共存。而且绝大部分物种并不只依靠一种资源，当某物种的主要资源匮乏时，它们常转向其它候选资源。如果多个物种竞争同一稀缺资源，他们常会转向不同的候选资源。近亲之间的竞争往往最激烈，因为他们对环境有相似需求。但激烈竞争也可能发生在毫不相关却需要同一资源的物种之间，例如吃种子的啮齿动物和蚂蚁。当一个生态系统中的所有动植物都参与到竞争中来时竞争的影响将表现得淋漓尽致，比如在两百万年前上新世末期当南、北美洲在巴拿马地峡处聚拢时所发生的一切。南、北美洲的物种可以穿越地峡而相互竞争。结果是大量南美洲哺乳动物因抵抗不了来自北美洲物种的竞争而灭绝----尽管过度捕杀也是一个很重要的因素。

竞争在多大程度上决定群落的组成和某特定物种的密度一直备受争议。问题是竞争通常无法直接观察得到而必须通过某一物种的扩张或增加而另外一种物种同时减少或消失的对比中推测出来。俄罗斯生物学家G. F. Gause进行了大量的两物种的室内实验，结果表明当只提供一种资源时其中一个物种将会灭绝。基于以上实验和实地观察，所谓的竞争灭绝法则是成立的，因为两个物种不可能同时完全占有同一有限资源。不过也发现了很多例外，但这些情况下两个物种，尽管会争夺某一主要资源，但它们争夺的资源不完全相同。

物种间的竞争对进化至关重要。争夺同一生态资源的物种趋向于朝着依赖不同资源的方向进化。相互竞争的物种会逐渐改变他们的活动范围使其领地不再重叠。竞争对进化的影响称为“物种选择”，但这一描述有很大误导性。只有某一物种中的个体才能面临自然选择的压力。某一物种的繁盛或生存正是其所有个体自然选择的结果。因此物种选择实际上是个体选择的结果。

任何必须的资源都可能引发竞争。这种资源对动物而言常是食物，对森林植物而言可能是阳光，对地面物种来说可能是空间，比如很多生活在浅海海床上生物。实际上，竞争对象可能是生物所必需的任何生物的或非生物的因素。通常生物密度越大，竞争越激烈。和捕杀一起，竞争是与生物密度相关的调控生物密度增长的重要因素。