[00:00.00]NARRATOR: Listen to part of a lecture in a biology class.[00:06.24]MALE 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.[00:23.42]FEMALE PROFESSOR: Right. Relationships between animal species in a given ecosystem can get pretty complex. [00:29.16]Because in addition to predator-prey relationships, there are other variables that affect population size.[00:35.24]MALE STUDENT: The article mentioned that populations of predators and their prey might go up rapidly and then decline all of a sudden.[00:41.58]FEMALE STUDENT: Oh. Yeah! I read about that in my ecology class. [00:44.88]It happens in cycles. [00:46.60]I think that's called a boom-and-bust cycle. Right?[00:49.91]FEMALE PROFESSOR: OK. Well, hold on a second. [00:52.31]First I want to go over some key concepts. [00:55.34]Let's say there was a species that had access to plenty of food and ideal conditions. [01:00.89]Under those circumstances, its population would increase exponentially, meaning it would increase at an ever-accelerating pace.[01:08.38]MALE STUDENT: Wow! That sounds a little scary.[01:11.30]FEMALE PROFESSOR: Well, it doesn't usually happen.[01:13.68] Like you said, a rapid population growth is often followed by a sudden decline. [01:18.68]But we do occasionally see exponential growth in nonnative species when they are transplanted into a new environment. [01:25.71]Um, because they face little competition and have favorable growing conditions.[01:30.79]But for most species, most of the time, resources are finite. [01:35.34]There's only so much available—which leads me to my point. [01:38.69]Every ecosystem has what we call a carrying capacity.[01:42.63]The carrying capacity is the maximum population size of a species that can be sustained by the resources of a particular ecosystem.... [01:51.90]Resources are, of course, food, water, and just as important, space.[01:56.84]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. [02:06.22]There are always going to be environmental factors that limit population growth. [02:10.99]This is called environmental resistance.[02:14.20]Environmental resistance is important because it stops populations from growing out of control. [02:20.22]Factors such food supply, predation and disease affect population size, and can change from year to year or season to season.[02:28.85]MALE STUDENT: OK. I think I get it.[02:30.60]FEMALE PROFESSOR: Well, let's look at a case study.[02:32.85] That should make things clear. [02:34.30]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.[02:43.97]OK. Now let me explain what the situation is in this forest.[02:48.76] Oak trees produce acorns, and acorns are a primary food source for white-footed mice. [02:54.88]Another food source for the white-footed mice is the gypsy moth. [02:58.70]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.[03:07.96]They strip away the leaves from the oak trees every ten years or so.[03:11.61]MALE STUDENT: So the mice eat both acorns from the oak trees and gypsy moths. [03:16.95]And the gypsy moth caterpillars eat oak tree leaves.[03:20.30]FEMALE PROFESSOR: Right. Now, what makes this set of relationships particularly interesting is that oak trees only produce a large number of acorns once every few years.[03:30.41]MALE STUDENT: So during the years with fewer acorns, the white-footed mice have to deal with a smaller food supply.[03:35.84]FEMALE PROFESSOR: Yes. But in the years with large amounts of acorns, the mice have more food, which leads to?[03:42.12]MALE STUDENT: The white-footed mice population growing.[03:44.56]FEMALE PROFESSOR: And the gypsy moth population decreasing.[03:47.55]MALE STUDENT: How can we know that for sure? [03:49.30]It seems like a big jump from more acorns to fewer gypsy moths.
[03:53.13]FEMALE 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. [04:02.21]In some parts of the forest, they had volunteers drop a large number of extra acorns on the forest floor.
[04:08.39]And in another section of the forest, they removed a number of white-footed mice. [04:12.82]In the forest areas where extra acorns had been dropped, the gypsy moth population soon went into a significant decline. [04:20.34]But in the section of the forest where the white-footed mice had been removed, the gypsy moth population exploded.
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