Narrator: Listen to part of a lecture in a biology class.

旁白：听一段生物学的讲座。

Student: Professor, since we are going to talk about changes in animal populations in the wild, I’d like to ask about something I read in an article online, about how the population size of some animal species can affect other animal species, and how other environmental factors come into play too.

学生：教授，既然我们打算要说一下野生动物数量的变化，我想针对我在网上读到的一篇文章问一些问题，关于某些动物数量变化对于其他动物的影响，以及其他一些环境因素又是怎么产生作用的。

Professor: Right. Relationships between animal species in a given ecosystem can get pretty complex.

教授：是的。在特定的生态系统中，动物之间的关系可以变得非常的复杂。

Because in addition to predator-prey relationships, there are other variables that affect population size.

因为除了捕食者-食饵的关系之外，还有很多其他变量会影响种群数量大小。

Student: The article mentioned that populations of predators and their prey might go up rapidly and then decline all of a sudden.

学生：这篇文章提到了捕食者和他们的食饵的数量可能会急剧上升，然后突然下降。

Student: Oh. Yeah! I read about that in my ecology class.

学生2：在生态学的课上我读到了那个。

It happens in cycles.

它周期性地发生。

I think that’s called a boom-and-bust cycle. Right?

我认为那个叫做繁荣与萧条的交替循环。对不对？

Professor: OK. Well, hold on a second.

教授：好的，稍等一下。

First I want to go over some key concepts.

首先我想复习一下一些关键的概念。

Let’s say there was a species that had access to plenty of food and ideal conditions.

假如有一个物种可以获得大量的食物和理想的生存条件。

Under those circumstances, its population would increase exponentially, meaning it would increase at an ever-accelerating pace.

在这种情况下，它的数量就会以指数方式增长，意思是它以持续加速的节奏增长。

Student: Wow! That sounds a little scary.

学生：哇欧！这听上去有点吓人。

Professor: Well, it doesn’t usually happen.

教授；这个并不会常常发生。

Like you said, a rapid population growth is often followed by a sudden decline.

就像你说的，在一个快速的增长之后常常是突然的下降。

But we do occasionally see exponential growth in nonnative species when they are transplanted into a new environment.

但是我们偶尔会看见，当一些非本土的物种被转移到新的环境中后，出现指数式增长。

Um…because they face little competition and have favorable growing conditions.

额……那是因为他们面临非常少的竞争，并且它们有着最佳的生长环境。

But for most species, most of the time, resources are finite.

但是，对于大部分的物种来说，大部分的时间来说，资源都是有限的。

There’s only so much available…which leads me to my point.

那里只有那么一些是可以获得的，这个便引到了我想说的观点。

Every ecosystem has what we call a carrying capacity.

每一个生态系统都有我们所说的承载能力。

The carrying capacity is the maximum population size of a species that can be sustained by the resources of a particular ecosystem.

承载能力指的是在某一生态系统中的资源所能维持的这种物种的最大的数量。

Resources are, of course, food, water, and just as important, space.

当然，资源指就是食物、水和同样重要的空间。

Although every species has a maximum rate at which the population of that species could increase, assuming ideal conditions for the species in its environment.

尽管每一个物种都有它可以增长的最大的速度，假定在它的环境中所有的条件都非常理想，

There are always going to be environmental factors that limit population growth.

总会有一些环境因素会限制这种物种数量的增长。

This is called environmental resistance.

这个就被称作为环境阻力。

Environmental resistance is important because it stops populations from growing out of control.

环境阻力是非常重要的，因为它阻止这个种群的数量增长失控。

Factors such as food supply, predation and disease affect population size, and can change from year to year or season to season.

像食物供给、捕食者及疾病这些因素影响物种数量，在每一年或每个季节都会有所不同。

Student: OK. I think I get it.

学生：好的，我明白了。

Professor: Well, let’s look at a case study.

教授：好的，让我们来看一个案例。

That should make things clear.

那个会把事情说明白的。

Some years ago, some of my colleagues conducted an experiment in an oak forest involving three different species: white-footed mice, gypsy moths and oak trees.

很多年前，我的一些同事们在栎树橡木林做了实验，牵涉到3个不同物种：白足鼠、舞毒蛾和橡树。

OK. Now let me explain what the situation is in this forest.

好的，现在让我来解释一下在这个树林中的情况是怎样的。

Oak trees produce acorns, and acorns are a primary food source for white-footed mice.

橡树会生产橡子，而橡子是白足鼠的主要食物来源。

Another food source for the white-footed mice is the gypsy moth.

对于白足鼠来说另一个食物来源就是舞毒蛾。

So the size of the gypsy moth population is controlled by the white-footed mice, which is a good thing because gypsy moth caterpillars are considered pests.

因此舞毒蛾的数量受到白足鼠的控制。这是一件好事，因为舞毒蛾毛毛虫被人作为是一种害虫。

They strip away the leaves from the oak trees every ten years or so.

每十年左右他们就会把橡树的叶子都剥离。

Student: So the mice eat both acorns from the oak trees and gypsy moths.

学生：那么白足鼠既吃橡树的橡子又吃舞毒蛾。

And the gypsy moth caterpillars eat oak tree leaves.

然后舞毒蛾毛毛虫吃橡树的叶子。

Professor: Right. Now, what makes this set of relationships particularly interesting is that oak trees only produce a large number of acorns every few years.

教授：是的。使这样的一系列关系非常有趣的是，橡树每几年才生产出大量的橡子。

Student: So during the years with fewer acorns, the white-footed mice have to deal with a smaller food supply.

学生：在橡子很少的几年，白足鼠不得不解决食物资源少的问题。

Professor: Yes. But in the years with large amounts of acorns, the mice have more food, which leads to…?

教授：是的，但是在橡子多的那几年，白足鼠有更多的食物，就导致……？

Student: The white-footed mice population growing.

学生：白足鼠的数量增长。

Professor: And the gypsy moth population decreasing.

教授：但是舞毒蛾的数量下降了。

Student: How can we know that for sure?

学生：你怎么就那么肯定？

It seems like a big jump from more acorns to fewer gypsy moths.

这好像跳得太快，从更多的橡子到减少的舞毒蛾。

Professor: Well, we can know for sure because in this oak forest, the researchers decided to test the links between acorns and the two animal species.

教授：我们可以那么肯定是因为在这个橡树林里，研究员们决定去测试这两个动物种类和橡子之间的关联。

In some parts of the forest, they had volunteers drop a large number of extra acorns on the forest floor.

在森林的一些地方，他们让志愿者们在地上放大量的额外的橡子。

And in another section of the forest, they removed a number of white-footed mice.

而在森林的另一个地方，他们移除了一些白足鼠。

In the forest areas where extra acorns had been dropped, the gypsy moth population soon went into a significant decline.

放了额外的橡子的森林区域，舞毒蛾的数量很快就大量减少。

But in the section of the forest where the white-footed mice had been removed, the gypsy moth population exploded.

但是在白足鼠被移除的地方，舞毒蛾的数量大量增加。