OFFICIAL8 Why does the professor mention the population density of blackcaps in two different habitats?

[00:00.00]Narrator: Listen to part of a lecture in an animal behavior class.
[00:07.23]FEMALE PROFESSOR: OK, well, last time we talked about passive habitat selection. Like plants, for example—[00:15.55]they don't make active choices about where to grow— [00:17.97]they're dispersed by some other agent, like the wind. [00:21.42]And if the seeds land in a suitable habitat, they do well and reproduce.
[00:26.70]With active habitat selection, an organism is able to physically select where to live and breed, [00:33.37]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. [00:46.63]So let’s look at the effect these preferences can have by looking at some examples. [00:52.17]But first let's recap. [00:53.79]What do we mean by “habitat.” Frank?
[00:57.53]MALE STUDENT: Well, it’s basically the place or environment where an organism normally lives and grows.
[01:04.39]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. [01:17.77]And we saw how important habitat selection is when we looked at habitats where some of these factors are removed, perhaps through habitat destruction. [01:27.27]Um, I just read about a shorebird, the plover.
[01:30.51]The plover lives by the ocean and feeds on small shellfish, insects, and plants. [01:37.38]It blends in with the sand, so it’s well camouflaged from predator birds above. [01:42.50]But it lays its eggs in shallow depressions in the sand, with very little protection around them. [01:48.46]So if there’re people or dogs on the beach, the eggs and fledglings in the nests are really vulnerable.
[01:54.86]Out in California, where there's been a lot of human development by the ocean, the plovers are now a threatened species. [02:01.63]So conservationists tried to create a new habitat for them. [02:05.91]They made artificial beaches and sandbars in areas inaccessible to people and dogs. [02:11.23]And the plover population is up quite a bit in those places.
[02:15.89]OK, that’s an instance where a habitat is made less suitable. [02:20.95]But now what about cases where an animal exhibits a clear choice between two suitable habitats—[02:27.80]in cases like that, does the preference matter?  [02:30.83]Well, let’s look at the blue warbler.
[02:35.08]The blue warbler is a songbird that lives in North America. [02:38.36]They clearly prefer hardwood forests with dense shrubs—ah, bushes—underneath the trees. [02:46.24]They actually nest in the shrubs, not the trees. [02:49.64]so they're pretty close to the ground but these warblers also nest in forests that have low shrub density. [02:56.27]It’s 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.
[03:05.95]And the choice of habitat seems to affect reproductive success. [03:10.98]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.
[03:27.80]But a preferred environment doesn't always seem to correlate with greater reproductive success. [03:34.02]For example, in Europe, studies have been done of blackcap warblers—[03:38.51]we just call them blackcaps.
[03:41.73]The blackcap can be found in two different environments. [03:45.70]Ah, their preferred habitat is forests near the edges of streams. [03:50.78]However, blackcaps also live in pine woods away from water. [03:55.81]Studies've been done on the reproductive success rates for the birds in both areas and the results showed—surprisingly—that the reproductive success was essentially the same in both areas—the preferred and the second choice habitat. [04:10.07]Well, why?
[04:12.52]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. [04:24.39]So the 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 lowered the suitability of the prime habitat even though it’s their preferred habitat.
[04:42.84]So the results of the studies suggest that when the number of competitors in the prime habitat reaches a certain point, the second-ranked habitat becomes just as successful as the prime habitat, just because there are fewer members of the same species living there. [04:58.67]So it looks like competition for res urces is another important factor in determining if a particular habitat is suitable.