This preview shows page 1. Sign up to view the full content.
Unformatted text preview: se. But it one school of thought proposes dividing populations into now seems clear that all populations are regulated by a two groups. These ecologists posit that the relatively steady mixture of d ensity- dependent and density-independent populations have "density- dependent" growth parameters; effects in varying proportions. that is, rates of birth, death, and migration which depend
strongly on population density. The highly varying
populations have "density-independent" growth parameters,
with vital, rates buffeted by environmental events; these
rates fluctuate in a way that is wholly independent of
This dichotomy has its uses, but it can cause problems
if taken too literally. For one thing, no population can be
driven entirely by d ensity-independent factors all the time.
No matter how severely or unpredictably birth, death and
migration rates may be fluctuating around their long-term
averages, if there were no density-dependent effects, the
population would, in the long run, either increase or
decrease without bound (barring a miracle by which gains
and losses canceled exactly). Put another way, it may be
that on average 99 percent of all deaths in a population
arise from density-independent causes, and only one
percent from factors varying with density. The factors
making up the one percent may seem unimportant, and
their cause may be correspondingly hard to determine. Yet,
whether recognized or not, they will usually determine the
long-term average population density. 134 7.The author of the passage is primarily concerned with 9.Which of the following statements can be inferred (A) discussing two categories of factors that control from the last paragraph? population growth and assessing their relative (A) For irregularly fluctuating populations, doubling importance the number of observations made will (B) describing how growth rates in natural popula- probably result in the isolation of density- tions fluctuate over time and explaining why dependent effects. these changes occur (B) Density-dependent effects on population (C) proposing a hypothesis concerning population dynamics do not occur as frequently as do sizes and suggesting wa ys to test it density-independent effects. (D) posing a fundamental question about environ- (C) At present, ecologists do not understand any mental factors in population growth and pre- of the underlying causes of the density- senting some currently accepted answers dependent effects they observe in population (E) refuting a commonly accepted theory about dynamics.. population density and offering a n ew alter- (D) Density-dependent effects on growth parameters native are thought to be caused by some sort of
biochemical "signaling" that ecologists hope 8. Select the sentence in the passage that presents the eventually to understand. author‘s attitude toward the dichotomy discussed in the (E) It is sometimes possible to infer the existence of second paragraph. a dens...
View Full Document
This document was uploaded on 09/16/2013.
- Fall '13