问题 问答题

影响群落结构的因素有哪些

答案

参考答案:影响群落结构的因素有以下几个。
(1)竞争:如果竞争的结果引起种间的生态位的分化,将使群落中物种多样性增加。
(2)捕食:泛化种的作用:捕食提高多样性、过捕多样性降低:特化种的作用:捕食对象为优势种,多样性增加;捕食对象为劣势种,降低多样性。
(3)干扰:在陆地生物群落中,干扰往往会使群落形成断层(gap),断层对于群落物种多样性的维持和持续发挥着很重要的作用。群落在中等程度的干扰水平下能维持高多样性。
(4)空间异质性:环境的空间异质性:环境的空间异质性愈高,群落多样性也愈高;植物群落的空间异质性:植物群落的层次和结构越复杂,群落多样性也就越高。

名词解释
单项选择题

Largely for "spiritual reasons", Nancy Manos started home-schooling her children five years ago and has studiously avoided public schools ever since. Yet last week, she was enthusiastically enrolling her 8-year-old daughter, Olivia, in sign language and modern dance classes at Eagleridge Enrichment—a program run by the Mesa, Ariz. , public schools and taught by district teachers. Manos still wants to handle the basics, but likes that Eagleridge offers the extras, "things I couldn’t teach. " One doubt, though, lingers in her mind. why would the public school system want to offer home-school families anything

A big part of the answer is economics. The number of home-schooled kids nationwide has risen to as many as 1.9 million from an estimated 345,000 in 1994, and school districts that get state and local dollars per child are beginning to suffer. In Maricopa County, which includes Mesa, the number of home-schooled kids has more than doubled during that period to 7,526, at about $ 4,500 a child, that’s nearly $ 34 million a year in lost revenue.

Not everyone’s happy with these innovations. Some states have taken the opposite tack. Like about half the states, West Virginia refuses to allow home-schooled kids to play public-school sports. And in Arizona, some complain that their tax dollars are being used to create programs for families who, essentially, eschew participation in public life. "That makes my teeth grit," says Daphne Atkeson, whose 10-year-old son attends public school in Paradise Valley. Even some committed home-schoolers question the new programs, given their central irony., they turn home-schoolers into public-school students, says Bob Parsons, president of the Alaska Private and Home Educators Association. "We’ve lost about one third of our members to those programs. They’re so enticing. "

Mesa started Eagleridge four years ago, when it saw how much money it was losing from home schoolers—and how unprepared some students were when they re-entered the schools. Since it began, the program’s enrollment has nearly doubled to 397, and last year the district moved Eagleridge to a strip mall (between a pizza joint and a laser-tag arcade). Parents typically drop off their kids once a week; because most of the children qualify as quarter-time students, the district collects $ 911 per child. "It’s like getting a taste of what real school is like," says 10-year-old Chad Lucas, who’s learning computer animation and creative writing.

Other school districts are also experimenting with novel ways to court home schoolers. The town of Galena, Alaska, (pop. 600) has just 178 students. But in 1997, its school administrators figured they could reach beyond their borders. Under the program, the district gives home-schooling families free computers and Internet service for correspondence classes. In return, the district gets $ 3,100 per student enrolled in the program—$ 9.6 million a year, which it has used partly for a new vocational school. Such alternatives just might appeal to other districts. Ernest Felty, head of Hardin County schools in southern Illinois, has 10 home-schooled pupils. That may not sound like much— except that he has a staff of 68, and at $ 4,500 a child, "that’s probably a teacher’s salary," Fehy says. With the right robotics or art class, though, he could take the home out of home schooling.

What can we infer from the last paragraph()

A. The tuition the home-schoolers have to pay for the public school is very high

B. Public school system gains much profit from the home-schoolers

C. Home-schoolers do not want to receive education at home any more

D. Public school system tries to attract the home-schoolers back to school