While it is true that living organisms are profoundly affected by their environment, it is equally important to remember that many organisms are also

(5) capable of altering their habitat significantly, sometimes limiting their own growth.The influence of the biological component of an cosystem is often greater in fresh waters that in

(10) marine or terrestrial systems, because of the small size of many reshwater bodies.Many of the important effects of organisms are related to their physiology, especially growth and

(15) respiration.By their growth many species can deplete essential utrients within the system.Thus limiting their own growth or that of other species.Lund has demonstrated that in Lake

(20) Windermere the alga Asterionella is unable to grow in conditions that it itself has created.Once a year, in the spring, this plant starts to grow rapidly in the lake, using up so much

(25) silica from the water that by late spring there is no longer enough to maintain its own growth.The population decreases dramatically as a result.

Which of the following is an example of the type of organism described in lines 3-7?

A kind of ant that feeds on the sweet juice exuded by the twigs of a species of thorn tree that grows in dry areas.

A kind of fish that, after growing to maturity in the ocean, returns to fresh water.

A kind of flower that has markings distinctly perceptible in ultraviolet light to the species of bee that pollinates the flower.

A kind of tree with seeds that germinate readily only in a sunny spot and then develop into mature frees that shade the area below them.

A kind of butterfly, itself nonpoisonous, with the same markings as a kind of butterfly that birds refuse to eat because it is poisonous.

答案：(D)

尽管生物会深受其环境的影响是确凿无疑的，但同样重要的是，我们应记住，

许多生物也能极大地改变它们的栖息地，从而在某些情况下限制着它们自身的生

长。某一生态系统中的生物组成部分，其影响在淡水环境中要比在海洋或陆地系统

中经常要来得大，因为淡水中许多生物体的身体尺寸都很小。生物体所受到的许多

重要影响均与它们的生理形态有关，尤其是生物体的生长和呼吸。通过它们自身的

生长，许多生物种类能够减少该生态系统范围内具有重要意义的营养物，从而限制

它们自身的生长或其它生物种类的生长。伦德(Lund) 证明，在温德尔湖(Lake

Windermere)中，硅藻属(Asterionella)这类水藻无法在它自身所创造的条件中

生长。每年一次，在春季，这类植物在湖中开始急剧生长，耗费掉水中如此之多的

二氧化硅，以至于至春季后期，再也没有充分的二氧化硅来维持它自身的生长。这

样一来，整个种群数量便急剧减少。