A symbiotic relationship is an interaction between two or more species in which one species lives in or on another species. There are three main types of symbiotic relationships: parasitism, commensalism, and mutualism. The first and the third can be key factors in the structure of a biological community; that is, all the populations of organisms living together and potentially interacting in a particular area.
Parasitism is a kind of predator-prey relationship in which one organism, the parasite, derives its food at the expense of its symbiotic associate, the host. Parasites are usually smaller than their hosts. An example of a parasite is a tapeworm that lives inside the intestines of a larger animal and absorbs nutrients from its host. Natural selection favors the parasites that are best able to find and feed on hosts. At the same time, defensive abilities of hosts are also selected for. As an example, plants make chemicals toxic to fungal and bacterial parasites, along with ones toxic to predatory animals (sometimes they are the same chemicals). In vertebrates, the immune system provides a multiple defense against internal parasites.
At times, it is actually possible to watch the effects of natural selection in host-parasite relationships. For example, Australia during the 1940 s was overrun by hundreds of millions of European rabbits. The rabbits destroyed huge expanses of Australia and threatened the sheep and cattle industries. In 1950, myxoma virus, a parasite that affects rabbits, was deliberately introduced into Australia to control the rabbit population. Spread rapidly by mosquitoes, the virus devastated the rabbit population. The virus was less deadly to the offspring of surviving rabbits, however, and it caused less and less harm over the years. Apparently, genotypes (the genetic make-up of an organism) in the rabbit population were selected that were better able to resist the parasite. Meanwhile, the deadliest strains of the virus perished with their hosts as natural selection favored strains that could infect hosts but not kill them. Thus, natural selection stabilized this host-parasite relationship.
In contrast to parasitism, in commensalism, one partner benefits without significantly affecting the other. Few cases of absolute commensalism probably exist, because it is unlikely that one of the partners will be completely unaffected. Commensal associations sometimes involve one species' obtaining food that is inadvertently exposed by another. For instance, several kinds of birds feed on insects flushed out of the grass by grazing cattle. It is difficult to imagine how this could affect the cattle, but the relationship may help or hinder them in some way not yet recognized.
The third type of symbiosis, mutualism, benefits both partners in the relationship Legume plants and their nitrogen-fixing bacteria, and the interactions between flowering plants and their pollinators, are examples of mutualistic association. In the first case, the plants provide the bacteria with carbohydrates and other organic compounds, and the bacteria have enzymes that act as catalysts that eventually add nitrogen to the soil, enriching it. In the second case, pollinators (insects, birds) obtain food from the flowering plant, and the plant has its pollen distributed and seeds dispersed much more efficiently than they would be if they were carried by the wind only. Another example of mutualism would be the bull's horn acacia tree, which grows in Central and South America. The tree provides a place to live for ants of the genus Pseudomyrmex. The ants live in large, hollow thorns and eat sugar secreted by the tree. The ants also eat yellow structures at the tip of leaflets: these are protein rich and seem to have no function for the tree except to attract ants. The ants benefit the host tree by attacking virtually anything that touches it. They sting other insects and large herbivores (animals that eat only plants) and even clip surrounding vegetation that grows near the tree. When the ants are removed, the trees usually die, probably because herbivores damage them so much that they are unable to compete with surrounding vegetation for light and growing space.
The complex interplay of species in symbiotic relationships highlights an important point about communities: Their structure depends on a web of diverse connections among organisms.
共生关系是两种或更多物种之间的一种交互作用，其中一个物种要么在另一个物种中生存要么依赖另外一个物种生存。 共生关系共有三种类型：寄生、共栖和互利共生。 其中第一种和第三种是一个生物群落结构的关键要素。所谓生物群落，指的是在某个特定区域内，所有生物体共同生存并且潜在地相互影响。
寄生现象是一种捕食式的关系，其中，寄生物通过削弱其寄主而获得自身所需食物。 寄生物的形体往往小于寄主。 绦虫是寄生的例子之一，它生存在较大型动物的肠道中，并吸收寄主体内的营养。 自然选择是导致能适应环境的个体或群体存活的过程，自然选择青睐那些最能够寻找寄主并能依靠寄主存活的寄生虫。 同时，防御能力强的寄主也被选择出来。比如说，有些植物会产生对真菌和细菌寄生物有毒的化学物质，也会产生那些对捕食动物有毒的化学物质（有时这些化学物质是一样的）。 而对于脊椎动物来说，其身体的免疫系统可以对体内的寄生物进行多层防御。
有时候，寄生关系的自然选择效应也可能在现实中被观察到。 比如说，二十世纪四十年代，亿万只欧洲兔在澳大利亚泛滥成灾。 兔子肆虐了广袤的土地，并给牛羊业带来了极大的威胁。 1950年，为了控制兔灾，澳大利亚特意引进了一种名为粘液瘤病毒的可影响兔子的寄生虫。 通过蚊子的快速传播，兔子数量急剧减少。 然而，这种寄生虫对于生存下来的兔群的后代就没有那么致命了，而且这种伤害逐年减小。 显然，该兔群的遗传性状（生物体的基因结构）经过了自然选择，已经具备了更好地抵抗粘液瘤病毒的能力。 同时，由于自然选择更倾向于那些能够感染寄主但不致其死的品系，这种病毒最致命的品系也逐渐地衰败了。 这样，自然选择便使得寄主-寄生虫的关系趋于稳定。
与寄生关系相反，在共生关系之中，一方受益，也不会给另一方带来严重影响。 然而在现实中，纯粹的共生关系几乎不存在，因为很难有一方会完全不受影响。 共生关系有时候表现为这样一种方式，一个物种寻觅的食物经由另外一个物种不经意地暴露出来。 比如说，有一些以昆虫为食的鸟类会被放牧中的牛群赶出草地。 很难说这个对牛群会带来什么影响，但这样的关系也许正以一种我们尚未认知到的方式在帮助或阻碍着它们。
第三种关系，互利共生，是指共生双方能够互利互惠。 其中典型的例子有豆科植物和固氮细菌，以及开花植物和授粉生物。在第一个例子中，植物可以为细菌提供碳水化合物以及其他一些化合物，而这些细菌则能产生一种起催化作用的酶，它最终增加土壤中的氮元素来丰富土壤。 在第二个例子中，授粉生物（昆虫、鸟类等）从开花植物中获取食物，而植物则可以通过它们来传递花粉和种子，这比仅仅依靠风来传递要高效得多。 还有一个互利共生的例子是生长在美国中南部的牛角金合欢树。 这种树为一种伪蚁属的蚂蚁提供了栖居地。 这些蚂蚁住在大的中空的荆棘丛中，汲取金合欢树分泌出来的糖分。 它们还吃树叶末端的黄色组织：这个部分富含蛋白质。但是除了吸引蚂蚁，这些蛋白质似乎对树本身没有任何功能。 而这些蚂蚁们则可以帮助它们的寄主攻击外界几乎所有的威胁。 它们会叮咬昆虫和食草动物（只以植物为食的动物），甚至可以削减生长在树周围的其他植物。 一旦蚂蚁被清除掉，这种树就难以存活，很可能是因为它们被食草动物损害而无力与周围的其他植物争夺阳光和生长空间。 共生关系中物种间错综复杂的相互影响揭示了群落中很重要的一点，即结构的建立依赖于生物间千变万化的联系网络。
Which of the following statements about commensalism can be inferred from paragraph 1?