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-
lations reduce the ground cover，那引入来控制它的virus怎么会导致 reduced the amount of open ground？然后又导致reducing underground ant nests？
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