A Critical Evaluation of Intrinsic and Extrinsic Factors Responsible for Niche Restriction in Parasites
A Critical Evaluation of Intrinsic and Extrinsic Factors Responsible for Niche Restriction in Parasites
复制标题
对寄生虫生态位限制的内在和外在因素的批判性评估
作者：
K. Rohde
讨论了寄生虫尤其是鱼类单殖吸虫生态位限制的内在和外在因素。种间竞争可能导致一种或几种物种的竞争排斥，或者可能导致一种或所有共存物种的微生境发生变化（相互作用位点分离），但没有证据表明这种影响会导致进化变化以及对竞争的避免，即选择性位点分离。以下观察结果表明，外在因素，尤其是竞争，在寄生虫中并不像通常所认为的那样具有重大的进化意义。竞争排斥的大多数影响也可能由种内拥挤引起。种间影响也可能是积极的，增加物种共存的机会。宿主介导的影响通常对引发这些影响的物种比对竞争物种更有害。具有重合或重叠微生境的寄生虫物种通常没有相互作用。相关物种通常具有广泛重叠的生态位。鱼类体外寄生虫可利用的生态位只有一小部分被占据。具有不同大小口器的物种可能在同一栖息地利用相同的食物，即这种差异不一定表明存在分离。物种内在因素影响的证据是，在许多情况下，微生境受到极大限制，即使不存在竞争物种，过去也不可能存在竞争物种，例如在高纬度地区只有一种体外寄生虫的鱼类物种。间接证据表明，导致寄生虫生态位限制的一个重要因素是选择增加种内接触从而促进交配。生态位多样化是自我增强的，在一个不断扩大的生态位空间中，如果没有对生态位限制的反向选择，种群将会被稀释到无法交配的程度。即使没有竞争，两个物种生态位完全重合的概率也极小，因此不允许将生态位差异用作竞争的证据。
Intrinsic and extrinsic factors responsible for niche restriction in parasites and particularly in Monogenea of fish are discussed. Interspecific competition may result in competitive exclusion of one or several species, or it may lead to a change in the microhabitat of one or all co-occurring species (interactive site segregation), but there is no evidence that such effects lead to evolutionary changes and avoidance of competition, i.e., to selective site segregation. That extrinsic factors, particularly competition, are not of such great evolutionary significance among parasites as is usually assumed, is indicated by the following observations. Most effects of competitive exclusion may also be caused by intraspecific crowding. Interspecific effects may also be positive, enhancing the chances of species co-occurring. Host-mediated effects are usually more harmful to the species evoking them than to competing species. Parasite species with coinciding or overlapping microhabitats often show no interactions. Related species commonly have widely overlapping niches. Only a small proportion of the niches available to ectoparasites of fish is filled. Species with mouthparts of different sizes may use the same food in the same habitat, i.e., such differences do not necessarily indicate segregation. Evidence for the effect of species intrinsic factors is that in many cases microhabitats are extremely restricted, even though competing species do not exist and cannot have existed in the past, for instance in fish species at high latitudes with only one species of ectoparasite. Circumstantial evidence shows that an important factor responsible for niche restriction in parasites is selection to increase intraspecific contact and thus mating. Niche diversification is self-augmenting, and in a continuously expanding niche space populations would be diluted to such a degree that mating would become impossible without the counteracting selection for niche restriction. The probability that two species show complete niche coincidence is infinitesimally small even without competition, and it is therefore not permissible to use niche differences as proof for competition.