Official 08 Set 2

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置顶

Active Habitat Selection

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What is the main purpose of the lecture?
  • A. To compare active habitat selection with passive habitat selection

  • B. To show that most habitat preferences in animals are learned

  • C. To compare the habitat requirements of several bird species

  • D. To examine the consequences of habitat selection by animals

显示答案 正确答案: D

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

    FEMALE PROFESSOR: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’re 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 these preferences can have by looking at some examples, but first let's recap.What do we mean by habitat? Frank?

    MALE STUDENT:Well, it's basically the place or environment where an organism normally lives and grows.

    FEMALE PROFESSOR:Right. And as we’ve discussed, there’re 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.And we saw how important habitat selection is when we looked at habitats where some of these factors are removed, perhaps through habitat destruction.Um, 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 create 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—ah, 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 the 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 the reproductive success.Because the older and 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.Ah, their preferred habitat is forests near the edges 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.

  • 旁白:听一篇动物行为的演讲。

    教授:上一次,我们谈到了消极的栖息地选择,比方说,植物无法主动选择在哪里生长,它们被一些比如说风之类的中介传播。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。

    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。并且因为动物的繁殖栖息地非常重要,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。所以,让我们通过几个例子来看一下这些偏好的影响,但是首先我们复习一下。栖息地是什么意思,frank?

    学生:它是生物通常生长生活的地方或者环境。

    教授:对了,正像我们讨论的,对于一个栖息地,必须包含一些关键因素:很显然有食物、水,然后还有合适的气候,以及自我保护的空间。我们看到了栖息地的选择是多么的重要,当我们看着栖息地的这些因素被移除,也许被破坏的时候。我刚刚读到一篇关于一种叫珩科鸟(plover)的文章。

    珩科鸟生存在海边,以小型的贝壳、昆虫还有植物为食。它们会躲在沙子里,与沙子混为一体,对于处在上方的捕食者来说,这是很好的伪装。但是,它们总是在几乎没有遮护的低洼地里面产蛋。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。在加州的海滩附近有很多的住宅小区,现在珩科鸟已经是一个濒危物种。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。他们建造了人们和狗无法进入的人工沙滩和沙洲。在这些地方,珩科鸟的数量增加了不少。

    好了,这就是一个关于栖息地变糟糕的一个例子。但是现在,如果一个动物在两个栖息地都很合适的情况下表现出明显的选择,在这种情况下,它们的偏好重要吗?让我们来看看蓝莺(bluewarbler)。

    蓝莺是一种生活在北美的鸣鸟。它们很显然的偏好于有着稠密的灌木丛阔叶树森林。它们通常在灌木丛中筑巢,而不是在树上,所以,它们离地面很近,但是这些蓝莺也会在有着稀疏灌木丛的森林里面筑巢。通常,年轻的蓝莺将会在这些地方筑巢,因为有浓密灌木的首选的筑巢点已经被占主导地位的成年蓝莺所占领。

    并且这些栖息地的选择貌似将会影响繁殖的成功与否。因为,那些在高度稠密的灌木丛中筑巢的更加成熟,更加有经验的鸟,相较于在低密度灌木丛筑巢的鸟,它们明显有着更多的后代。这也就表明了在哪里筑巢对于下一代的数量有着很大的影响。

    但是,一个动物偏好的环境并不总是与高的繁殖率相关。比方说,在欧洲,关于黑顶莺做过了一些研究,我们把它们叫做黑顶。

    这些黑顶被发现在两种不同的环境里面。它们偏好的环境是在溪流附近的森林里面。但是,黑顶也住在远离水的松林中。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。为什么呢?

    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。所以溪流附近的地区有着更加稠密的物种密度,也就意味着对于资源的竞争很激烈,以同样的食物进食,在同样的地点筑巢,这就降低了主要栖息地的适宜性,尽管这是它们所偏好的地区。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)和主栖息地一样成功,仅仅是因为有相对较少的相同物种在那里生存。所以对资源的竞争是决定一个栖息地是否合适的另一个重要影响因素。

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    题型分类:主旨题
    原文定位:So let's look at the effect the preference can have by looking at some examples, but first let’s recap.
    选项分析:
    文章开头教授先回顾了上次课的内容,然后说明了这次课程的主要内容,“look at the effect the preference can have”,即 the consequences,D选项正确。

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