More selective than most chemical

   pesticides in that they ordinarily destroy

only unwanted species, biocontrol.

Line agents (such as insects, fungi, and

(5) viruses) eat, infect, or parasitize tar-

   geted plant or animal pests. However,

   biocontrol agents can negatively affect

   nontarget species by, for example,

   competing with them for resources: a

(10) biocontrol agent might reduce the bene-

fits conferred by a desirable animal

species by consuming a plant on which

the animal prefers to lay its eggs.

Another example of indirect negative.

(15) consequcnces occurred in England

when a virus introduced to control

rabbits reduced the amount of open

ground (because large rabbit popu-

lations reduce the ground cover), in

(20) turn reducing underground ant nests

and triggering the extinction of a blue

butterfly that had depended on the

nests to shelter its offspring. The

paucity of known extinctions or disrup-

(25) tions resulting from indirect interactions

may reflect not the infrequency of such

mishaps but rather the failure to look

for or to detect them: most organisms

likely to be adversely affected by

(30) indirect interactions are of little or

no known commercial value and the

events linking a biocontrol agent with

an adverse effect are often unclear.

Moreover, determining the potential

(35) risks of biocontrol agents before they

are used is difficult, especially when a

nonnative agent is introduced, because,

unlike a chemical pesticide, a biocontrol

agent may adapt in unpredictable ways.

(40) so that it can feed on or otherwise harm

new hosts.

有色部分这句话没看懂逻辑关系，既然large rabbit popu-

前提：兔子破坏草地，蚂蚁在荒地建窝，蝴蝶在蚂蚁窝下崽

逻辑：兔子减少，草地增加，荒地减少,蚂蚁窝减少，蝴蝶减少

the ground cover 草地

open ground荒地

解释的非常好，终于明白了，我也犯糊涂。

以为是兔子吃草，引进病毒后兔子少了，那蚂蚁不是应该更多了吗？

呵呵。

原来是：

the ground cover 草地

open ground荒地

这两个词没理解清楚！