OFFICIAL6 why does the professor mention the size of the Nightcap Oak population over the last few hundred years?

[00:00.00]Narrator: Listen to part of a lecture in a biology class.
[00:05.50]FEMALE PROFESSOR: OK, I have an interesting plant species to discuss with you today. [00:09.73]Uh, it's a species of a ve-e-ry rare tree that grows in Australia, Eidothea hardeniana, but it’s better known as the Nightcap Oak.
[00:18.82]Now, it was discovered only very recently, just a few years ago. Uh, it remained hidden for so long because it's so rare, [00:29.55]there're only about, oh, two hundred of ‘em in existence. [00:33.44]They grow in a rain forest, in a mountain rage range in the north part of New South Wales, which is, uh, a state in Australia. [00:42.29]So just two hundred individual trees in all.
[00:46.60]Now, another interesting thing about the Nightcap Oak is that it is… it represents… a—a very old…type, a kind of a tree that grew…a hundred million years ago. [01:00.06]Uh we found fossils that old that bear a remarkable resemblance to the tree. [01:05.12]So, it's a primitive tree, [01:07.66]a... a living fossil, you might say. [01:09.77]It's a relic from earlier times, and it has survived all these years without much change. [01:15.57]And... it—it’s probably a kind of tree from which other trees that grow in Australia today evolved.
[01:22.29]Just—just to give you an idea of what we're talking about, [01:24.66]here’s a picture of the leaves of the tree and its flowers. [01:27.69]II dunno how well you can see the flowers, [01:30.77]they're those little clusters sitting at the base of the leaves.
[01:34.74]OK, what have we tried to find out about the tree since we’ve discovered it? [01:40.05]Hmm well how... why is… is it so rare? [01:44.79]It is one of the first questions. [01:46.21]Uh, how is it, uh how does it reproduce? [01:50.50]It is another question. [01:51.48]Uh, maybe those two questions are actually related? Jim.
[01:56.69]MALE STUDENT: Hmm, I dunno, [01:58.88]but uh I can imagine that…for instance…uh seed dispersal might be a factor [02:04.24]I mean if the uh y'know if the seeds cannot really disperse in a wide area then you know the tree may not uh colonize new areas, [02:12.55]it can't spread from the area where it's growing.
[02:15.11]FEMALE PROFESSOR: Right, that’s that’s actually a very good answer. [02:18.24]Uh, of course, you might think there might not be many areas where the tree could spread into, uh because uh well it's it's very specialized in terms of the habitat. [02:29.43]But that’s not really the case here, [02:31.90]uh the the suitable habitat habitat that is the actual rain forest is much larger than than the few hectares where the Nightcap Oak grows.
[02:40.76]Now, this tree is a flowering tree as I showed you, [02:43.94]uh uh, it—it produces a fruit, much like a plum, [02:48.52]on the inset inside there's a seed with a hard shell. [02:51.55]Uh it it appears that the shell has to crack open or break down somewhat to allow the seed to soak up water. [02:58.34]If the Nightcap Oak remains if their seeds remain locked inside their shell, they will not germinate. [03:05.29]Now actually the seeds, uh they don't retain the power to germinate for very long, maybe two years.
[03:12.85]So there's actually quite a short window of opportunity for the seed to germinate. [03:17.25]So the shell somehow has to be broken down before this uh germination ability expires. [03:24.51]And... and then there’s a kind of rat that likes to feed on the seeds as well. [03:28.98]So, given all these limitations, not many seeds that the tree produces will actually germinate. [03:35.46]So this is a possible explanation for why the tree does not spread. [03:40.21]It doesn't necessarily explain how it became so rare but it explains why it doesn’t increase.
[03:46.64]OK, so it seems to be the case that this species, uh this Nightcap Oak, is not very good at spreading. [03:54.01]However, it seems, though we can't be sure, that it's very good at persisting as a population. [04:02.10]Uh, uh we, uh, there—there're some indications to suggest that the population of the Nightcap Oak has not declined over the last uh, y'know, many hundreds of years.
[04:14.38]So, it's stayed quite stable; [04:17.15]it—it's not a remnant of some huge population that has dwindled in the last few hundred years for some reason. [04:23.94]It’s not necessarily a species in retreat. [04:26.98]OK, so it cannot spread very well but it's good at maintaining itself. [04:32.71]It’s rare but it's not disappearing.
[04:35.85]OK the next thing we might wanna ask about a plant like that is what chances does it have to survive into the future. [04:44.00]Let's look at that.