请教：gwd-25-13 to 15中一句话的理解

ysl246

GWD25-Q 13 to 15:

Grassland songbirds often nest in

the same grassland-wetland complexes

as waterfowl, particularly in a certain

Line part of those complexes, namely,

 (5) upland habitats surrounding wetlands.

Although some wildlife management

procedures directed at waterfowl, such

as habitat enhancement or restoration,

may also benefit songbirds , the impact

(10) of others, especially the control of

waterfowl predators, remains difficult to

predict. For example, most predators

of waterfowl nests prey opportunistic-

ally on songbird nests, and removing

(15) these predators could directly increase

songbird nesting success. Alterna-

tively, small mammals such as mice

and ground squirrels are important

in the diet of many waterfowl-nest

(20) predators and can themselves be

important predators of songbird

nets. Thus. Removing waterfowl-nest

predators could affect songbird nesting

success through subsequent increases

(25) in small-mammal populations.

In 1995 and 1996, researchers

trapped and removed certain waterfowl-

nest predators. primary raccoons and

striped skunks, then observed subse-

(30) quent survival rates for songbird nests.

Surprisingly. They observed no sig-

nificant effect on songbird nesting

success. This may be due to several

factors. Neither raccoons nor striped

(35) skunks consume ground squirrels,

which are important predators of song-

bird nests. Thus, their removal may

not have led to significant increases

in populations of smaller predators.

(40) Additionally, both raccoons and striped

skunks prefer wetlands and spend little

time in upland habitats; removing these

species may not have increased the

nesting success of songbirds in the

uplands enough to allow detection.

高亮这部分是什么意思？opportunistically和Alternatively怎么理解？

ysl246

upup

smilecrystal

例如：大部分捕食WN的捕食者偶尔捕SN，移除这部分捕食着可以直接增加S筑巢成功率。
同时小哺乳动物（如M和GS）是对很多捕食WN的捕食者很重要的食物，他们自己也是吃SN的捕食者。
因此，移除这部分捕食者（捕食WN的捕食者偶尔捕SN的捕食者）能够通过影响后续的小哺乳动物数量的增加来影响S筑巢成功率。