While it is true that living organisms are profoundly affected by their environment, it is equally important to remember that many organisms are also capable of altering their habitat significantly, sometimes limiting their own growth. The influence of the biological component of an ecosystem is often greater in fresh waters than that in marine or terrestrial systems, because of the small size of many freshwater bodies. Many of the important effects of organisms are related to their physiology, especially growth and respiration. By their growth many species can deplete essential nutrients within the system, thus limiting their own growth or that of other species. Lund has demonstrated that in Lake 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 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.
24. Which of the following is an example of the type of organism described in the first sentence?
(A) 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.
(B) A kind of fish that, after growing to maturity in the ocean, returns to fresh water.
(C) A kind of flower that has markings distinctly perceptible in ultraviolet light to the species of bee that pollinates the flower.
(D) A kind of tree with seeds that germinate readily only in a sunny spot and then develop into mature trees that shade the area below them.（D）
(E) A kind of butterfly, itself nonpoisonous, with the same markings as a kind of butterfly that birds refuse to eat because it is poisonous.
25. It can be inferred from the passage that which of the followings is true about Asterionella plants in Lake Windermere?
(A) They are not present except in early spring.
(B) They contribute silica to the waters as they grow.
(C) They are food for other organisms.
(D) They form a silica-rich layer on the lake bottom.（E）
(E) Their growth peaks in the spring.
26. The passage indicates that organisms frequently have the strongest effects on their environment in
(A) oceans, since oceans contain the largest organisms living on Earth
(B) oceans, since oceans provide habitats for many different kinds of species
(C) freshwater bodies, since such effects become pronounced in relatively small spaces
(D) freshwater lakes, since nutrients in freshwater lakes are present only in small amounts（C）
(E) land areas, since there exist major influences of climate on the kinds of small organisms supported in land areas
27. The primary topic of the passage is the way in which
(A) organisms are affected by the amount of nutrients available
(B) organisms can change their own surroundings
(C) elements of freshwater habitats impede the growth of small organisms
(D) the reproduction of organisms is controlled by factors in the environment（B）
(E) plant matter in a given locale can increase up to a limit