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contract over historical time intervals (tens to hundreds of years), species' ranges rarely expand
thousands of miles or across physical barriers such as oceans or mountains. A number of factors
confound quantitative evaluation of the relative importance of various entry pathways. Time
lags often occur between establishment of non-indigenous species and their detection, and
tracing the pathway for a long-established species is difficult. Experts estimate that non-
indigenous weeds are usually detected only after having been in the country for thirty years or
having spread to at least ten thousand acres. In addition, federal port inspection, although a
major source of information on non-indigenous species pathways, especially for agricultural
pests, provides data only when such species enter via scrutinized routes. Finally, some
comparisons between pathways defy quantitative analysis. For example, which is more
"important": the entry pathway of one very harmful species or one by which many but less
harmful species enter the country?
16. Which of the following statements about species movement is
best supported by the information in the passage?
A) Species movement is affected more by habitat modifications than by human mobility.
B) Human-driven factors affect the rate at which species move more than they affect the long-
term amount of such movements.
C) Natural expansions in the geographic distribution of species account for less Species
movement than do natural contractions.
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