死水、活水疑似原文(摘自forevercoco帖子里面的资料）

briarbaby

看了forevercoco的帖子里面附送的材料，觉得其中一部分和大家的回忆很像的感觉，就截取下来，供大家参考~附上原帖的地址，大家可以下载原来的文章~http://forum.chasedream.com/GMAT_Math/thread-558855-1-1.html?SearchText=

Temporary watershave often been considered distinct from permanent ponds. Much of the earlyliterature on temporary pond fauna was concerned with the question of survivalin an ephemeral aquatic environment. This focus on the adaptations to life intemporary ponds is understandable. The existence of a dry phase presents anobvious challenge to any “aquatic” animal. The persistence of these organismsrequires that they survive the dry period in situ, migrate to other aquatichabitats, undergo a transition to a terrestrial phase, or recolonize followingextinction. All of these strategies are found among temporary pond fauna.

While there is agreat variety of particular adaptations to ephemeral ponds, most residentspecies persist by reaching some critical developmental stage prior to ponddrying. Examples include holometabolous and hemimetabolous insects andamphibians that undergo metamorphosis into a terrestrial form. In addition,many invertebrates (e.g. cladocera, clam shrimp, copepods, fairy shrimp,mayflies, seed shrimp, tadpole shrimp) must reach adulthood and then produceencysted eggs that can endure the dry phase within the pond basin. For thesegroups, hydroperiod represents a maximum limit on development time. That thislimit is frequently an active constraint on the performance and distribution offreshwater organisms is suggested by numerous reports of catastrophic mortalityand even local extinction associated with pond drying events. Experiments directedat the role of hydroperiod have been of two primary types. A number ofartificial pond experiments have been conducted in which hydroperiod is imposedas a treatment . In addition, several experiments have transplanted speciesamong natural ponds that vary in permanence. Together, this study providesevidence that drying associated mortality is often high and is correlated withdevelopmental rate of the species.

In order to succeedin the most temporary aquatic environments, developmental rates must bephenomenal. A seed shrimp (Conchostroca) of the Saharan Desert is able toachieve maturity within just five days after pond filling. Spade foot toads(Scaphiopus couchii) of the southwestern deserts of the United States can reachmetamorphosis in just 10 days after hatching. In more permanent environments,the restriction on developmental rates is relaxed. North American frogs, forinstance, have larval periods that range up to three years in length. Studiesof tadpoledistributions show that species with longer larval periods tend to befound in more permanent ponds. As hydroperiod increases, so does the potentialspecies pool. Correspondingly, a number of studies have noted correlationsbetween permanence and species richness. While the total number of species mightincrease with permanence, there is also significant turnover in species composition.Most species that live in temporary ponds are absent from permanent ponds. Thispattern suggests that some factor, or suite of factors, can often preventspecies from occupying habitats that are apparently suitable givendevelopmental constraints.

While there aremany possible reasons why a species may be restricted from living in morepermanent habitats than those it occupies, predation may be the mostwidespread. Predatory species are often entirely absent within the mosttemporary habitats and are typically numerous and diverse within permanentfishless ponds. Along the intervening portion of the gradient, several studieshave shown that the identity, density, and size distribution of predatorschanges with even small differences in permanence. For example, Woodward reportedan approximately fourfold increase in aquatic predator density in permanentversus temporary desert ponds in New Mexico. Similar trends have been foundamong Michigan ponds where predator density triples between annual temporaryponds and permanent ponds. In these permanent ponds, average body lengths ofpredatory salamander, beetle larvae, and dragonfly larvae were longer by 100%,36%, and 46%, respectively, than those of their counterparts in temporary pond.