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

教授：上一次，我们谈到了消极的栖息地选择，比方说。

Pro: OK. Well, last time we talked about passive habitat selection, like plants for example.

植物都不会对于它们生长地点做积极地选择。

They don't make active choices about where to grow.

它们被一些比如说是风之类的原因分散了。

They are dispersed by some other agent, like the wind.

然后如果种子在一片合适的栖息地着陆，它们会生长的不错，并且繁殖。

And if the seeds land in a suitable habitat, they do well and reproduce.

但是如果是积极的栖息地选择，生物就会在生理意义上选择它们生长繁殖的地方。

With active habitat selection, an organism is able to physically select where to live and breed.

并且因为动物的栖息地是如此的重要，以至于我们认为动物物种倾向于选择使它们的后代有最好的生存几率的地方。

And because an animal’s breeding habitat is so important, we’d expect animal species to have developed preferences for particular types of habitats, places where their offspring have the best chance of survival.

所以，让我们通过几个例子来看一下这些偏好的影响，

So let's look at the effect this preference can have by looking at some examples.

学生：它是生物通常生长生活的地方或者环境

Stu: Well, it’s basically the place or environment where an organism normally lives and grows.

教授：对了，正像我们讨论的，对于一个栖息地，必须包含有一些关键因素。显然，食物、水，然后还有合适的气候，以及自我保护的空间。

Pro: Right, and as we've discussed, there are some key elements that a habitat must contain: food obviously, water and it’s got to have the right climate and spaces for physical protection.

我们可以看到当这些因素被移除的时候，这些栖息地的选择是多么的重要。

And we saw how important habitat selection is when we look at habitats where some of these factors are removed, perhaps through habitat destruction.

我刚刚读了一些关于一种叫珩科鸟的资料。

Um, I just read about a shorebird, the plover.

珩科鸟生存在海边，以小型的贝壳、昆虫还有植物为食。

The plover lives by the ocean and feeds on small shellfish, insects and plants.

它们会躲在沙子里，与沙子混为一体，所以可以很好的躲在捕食鸟类之下。

It blends in with the sand, so it’s well-camouflaged from predator birds above.

但是，它们总是在几乎没有遮护的低洼地里面产蛋。

But it lays its eggs in shallow depressions in the sand with very little protection around them.

所以如果有人或者狗在沙滩上，在巢中的蛋和幼鸟几乎是很脆弱，很容易受伤害的。

So if there are people or dogs on the beach, the eggs and fledglings in the nests are really vulnerable.

在加州的海滩有很多的人工建筑。现在珩科鸟已经是一个濒危物种。

Out in California where there has been a lot of human development by the ocean, the plovers are now a threatened species.

所以动物保护主义者正在尝试着为它们建立一个新的栖息地。

So conservationists tried to recreate a new habitat for them.

他们做了一个人工的海滩以及防止人类和狗进入的沙洲。

They made artificial beaches and sand bars in areas inaccessible to people and dogs.

在这些地方，珩科鸟的数量增加了不少。

And the plover population is up quite a bit in those places.

好了，这就是一个关于栖息地变糟糕的一个例子。

Ok. That is an instance where a habitat is made less suitable.

但是现在，如果两个栖息地都很合适，那么动物会表现出怎样的选择呢？

But now, what about cases where an animal exhibits a clear choice between two suitable habitats?

In cases like that, does the preference matter?

蓝莺是一种生活在北美的鸣鸟。

The blue warbler is a songbird that lives in North America.

它们很显然的偏好于有着稠密的灌木丛阔叶树森林。

They clearly prefer hard wood forests with dense shrubs, um... bushes underneath the trees.

它们通常在灌木丛中筑巢，而不是在树上。

They actually nest in the shrubs, not the trees.

所以，它们倾向于接近地面，但是这些蓝莺也会在有着稀疏灌木丛的森林里面筑巢。

So they're pretty close to the ground, but these warblers also nest in forests that have low shrub density.

通常，年轻的蓝莺将会在这些地方，因为更好的地方已经被主流的成年蓝莺所占领。

It is usually the younger warblers that nest in these areas because the preferred spots where there are a lot of shrubs are taken by the older, more dominant birds.

并且这些栖息地的选择貌似将会影响繁殖的成功与否。

And the choice of habitat seems to affect reproductive success.

因为，更加成熟，更加有经验的鸟将会在高度稠密的灌木丛中筑巢，相较至于在低密度灌木丛筑巢的鸟，它们明显有着更多的后代，这也就表明了在那里筑巢对于下一代的数量有着很大的影响。

Because the older, more experienced birds who nest in the high density shrub areas have significantly more offspring than those in low density areas, which suggests that the choice of where to nest does have an impact on the number of chicks they have.

但是，一个动物偏好的环境并不总是与高的繁殖率相关。

But a preferred environment doesn't always seem to correlate with greater reproductive success.

比方说，在欧洲，关于黑顶莺做过了一些研究，

For example, in Europe, studies have been done of blackcap warblers.

这些黑顶被发现在两种不同的环境里面，

The Blackcap can be found in two different environments.

它们偏好的环境是在溪流附近的森林里面。

Their preferred habitat is forests near the edge of streams.

但是，黑顶也住在远离水的松林中。

However, blackcaps also live in pine woods away from water.

在这两种地区都做了有关于繁殖成功率的研究。结果很令人惊异的表明在这两种地方的繁殖率基本一致。

Studies have been done on the reproductive success rates for the birds in both areas, and the result showed surprisingly that the reproductive success was essentially the same in both areas: the preferred and the second choice habitat.

结果是，事实上，在溪流边缘的森林里面住着比远离溪流地区四倍多的鸟。

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 from the stream.

所以溪流附近的地区有着更加稠密的人口密度，也就意味着对于资源的竞争很激烈，而竞争就降低了动物在这些主要栖息地的适应能力，尽管这是它们所偏好的地区。

So this stream edge area had a much denser population, which meant more members of the same species competing for resources, wanting to feed on the same things or build their nests in the same places, which lower the suitability of the prime habitat even though it's their preferred habitat.

所以研究结果显示当在主栖息地的竞争者数量达到一定的量时，第二选择的栖息地将会在（繁殖上）和主栖息地一样能够的成功，仅仅是因为有相对较少的物种在那里生存。

So the results of the study suggest that when the number of the competitors in the prime habitat reaches a certain point, the second rank habitat becomes just as successful as the prime habitat, just because there are fewer members of the same species living there.

所以相互争夺资源也是判断一个栖息地是否合适的重要影响因素。

So it looks like competition for resources is another important factor in determining if a particular habitat is suitable.

What element of the plover's habitat in California was threatened?

What does the professor illustrate with the example of the blue warbler?

Why does the professor mention the population density of blackcaps in two different habitats?

According to the professor, why did some blackcaps choose a secondary habitat?

What can be inferred about the professor when she says this:

Pro: OK. Well, last time we talked about passive habitat selection, like plants for example, they don't make active choices about where to grow. They are dispersed by some other agent, like the wind. And if the seeds land in a suitable habitat, they do well and reproduce. 
With active habitat selection, an organism is able to physically select where to live and breed. And because an animal’s breeding habitat is so important, we’d expect animal species to have developed preferences for particular types of habitats, places where their offspring have the best chance of survival. So let's look at the effect this preference can have by looking at some examples. 
But first let’s recap. What do we mean by habitat? Frank? 
Stu: Well, it’s basically the place or environment where an organism normally lives and grows. 
Pro: Right, and as we’ve discussed, there are some key elements that a habitat must contain, food obviously, water, it’s got to have the right climate and spaces for physical protection. And we saw how important habitat selection is when we look at habitats where some of these factors are removed, perhaps through habitat destruction. I just read about a shorebird, the plover. 
The plover lives by the ocean and feeds on small shellfish, insects and plants. It blends in with the sand, so it’s well-camouflaged from predator birds above. But it lays its eggs in shallow depressions in the sand with very little protection around them. So if there are people or dogs on the beach, the eggs and fledglings in the nests are really vulnerable. Out in California where there has been a lot of human development by the ocean. The plovers are now a threatened species. So conservationists tried to recreate a new habitat for them. They made artificial beaches and sand bars in areas inaccessible to people and dogs. And the plover population is up quite a bit in those places. 
Ok. That is an instance where a habitat is made less suitable. But now, what about cases where an animal exhibits a clear choice between two suitable habitats? In cases like that, does the preference matter? Well, let's look at the blue warbler. 
The Blue warbler is a songbird that lives in North America. They clearly prefer hard wood forests with dense shrubs, bushes underneath the trees. They actually nest in the shrubs, not the trees. So they’re pretty close to the ground, but these warblers also nest in forests that have low shrub density. It is usually the younger warblers that nest in these areas because the preferred spots where there are a lot of shrubs are taken by the older, more dominant birds. 
And the choice of habitat seems to affect reproductive success. Because the older, more experienced birds who nest in the high density shrub areas have significantly more offspring than those in low density areas, which suggests that the choice of where to nest does have an impact on the number of chicks they have. 
But a preferred environment doesn't always seem to correlate with greater reproductive success. 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 edge of streams. 
However, blackcaps also live in pine woods away from water. Studies have been done on the reproductive success rates for the birds in both areas, and the result showed surprisingly that the reproductive success was essentially the same in both areas--- the preferred and the second choice habitat. 
But a preferred environment doesn't always seem to correlate with greater reproductive success. 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 edge of streams. 
However, blackcaps also live in pine woods away from water. Studies have been done on the reproductive success rates for the birds in both areas, and the result showed surprisingly that the reproductive success was essentially the same 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 from the stream. So this stream edge area had a much denser population, which meant more members of the same species competing for resources, wanting to feed on the same things or build their nests in the same places, which lower the suitability of the prime habitat even though it’s their preferred habitat. So the results of the study suggest that when the number of the competitors in the prime habitat reaches a certain point, the second rank habitat becomes just as successful as the prime habitat, just because there are fewer members of the same species living there. So it looks like competition for resources is another important factor in determining if a particular habitat is suitable. 
Ok. That is an instance where a habitat is made less suitable. But now, what about cases where an animal exhibits a clear choice between two suitable habitats? In cases like that, does the preference matter? Well, let's look at the blue warbler.