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Listen to part of a lecture in an animal behavior class.

OK, well, last time we talked about passive
habitat
selection. likeplants for example, they don't make active choices about
where to grow. They are dispersed by some otheragent, like the wind, and if the seeds land in the suitable habitat, they dowell and reproduce. With active
habitat
selection, and
organism
is able to physically select
where
to live and breed. And because
the
animals' breeding habitat is so important, we expect animal
species
to develop
preferences
for particulartypes of habitats, places where their offspring have the best chance
to survival. So, let's look at the
effect these
preferences canhave by looking at some examples. But first let's
recap, what
do
we mean byhabitat? Frank?

Well, it's basically the place or environment
where
an organismnormally
lives and grows.

Right. And as we've discussed, there are
some
key elements forthe habitat must contain, food, obviously, water, and it's got to have the rightclimate, and its basis for physical protection. And we saw some habitat selection is when we look at habitats where someof these factors are removed, perhaps through habitats destruction. I just read about a short bird, the Plover.

The Plover lives by the ocean and feeds on small shellfish, insects, andplants. It blends in with the sands, so it's well camouflaged from predator birdsabove. But it lays its eggs in shallow depressions in the sand, with verylittle protection around them. So if there are people or dogs on the beach, theeggs and fledglings in the nest are really vulnerable. Outing California weather has been a lot of human development by the ocean, the Plovers arenow a threaten species. So conservationists tried to create a new habitat forthem. They made artificial beaches and sand bars in areas inaccessible topeople and dogs, and the plover population is up quiet a bit in those places.

OK, that's an instance where a habitat is made less suitable. But now, whatabout cases when an animal exhibits clear choice between two suitable habitats. In cases like that, does the preference matter? well, let's look at the bluewarbler.

The blue warbler is a songbird that lives in north America, they clearly preferhardwood forests with dense shrubs, bushes, underneath the trees. They actually nest inthe shrubs, not the trees. So they are pretty close to the ground, but these warblers also nest in forests that have low shrub density. It's usually theyounger warblers that nest in these areas because the preferred spots where there are a lot of shrubs are taken by the older, more dominant bird. And thechoice of habitat seems to affect reproductive success. Because the older,more experienced bird who nest in the high density shrub areas, have significantlymore offspring than those in low density areas, which suggests that the choiceof where to nest does have impact on the number of chicks they have. But a preferred environment does not always seem to correlate with greater reproductivesuccess. For example, in Europe, studies have been done of blackcap warblers. We just call them blackcaps.

The blackcap can be found in two different environments. Their preferred habitat is forests near the edges of streams. However, blackcapsalso live in pine woods away from water. Studies have been done on thereproductive success, for the birds in both areas, and the result showed, surprisingly, that the reproductive success was essentially thesame in both areas. The preferred and the second choice habitat. Well, why?

It turned out that there were actually four times as many bird pairs or couples living in the stream edge habitat, compared to the area away form the steam, so this stream edge area had a much denser population which meant more members of the samespecies competing for resources. Wanting to feed on the same thing or buildthe nests on the same places, which lower the suitability of the prime habitat even though it is their preferred habitat. So the results of the studysuggest that when the number of competitors in the prime habitat reaches a certain point, the second rank habitat becomes just as successful as theprime habitat, just because there are fewer members of the same species livingthere. So it looks like competition for resources is another important factorin determining if a particular habitat is suitable.

(二)生词难词

recap：扼要重述

correlate:vt.使相互关联 vi.(to，with)相关，关联

underneath:在...下面

preference：偏爱，优先权

camouflage：伪装