Habitat modification can create conditions favorable to the establishment of non-indigenous species. Soil disturbed in construction and agriculture is open for colonization by non-indigenous weeds, which in turn may provide habitats for the non-indigenous insects that evolved with them. Human-generated changes in fire frequency, grazing intensity, as well as soil stability and nutrient levels similarly facilitate the spread and establishment of non-indigenous plants. When human changes to natural environments span large geographical areas, they effectively create passages for species movement between previously isolated locales. The rapid spread of the Russian wheat aphid to fifteen states in just two years following its 1986 arrival has been attributed in part to the prevalence of alternative host plants that are available when wheat is not. Many of these are non-indigenous grasses recommended for planting on the forty million or more acres enrolled in the U.S. Department of Agriculture Conservation Reserve Program.

    A number of factors perplex 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 agriculture pests, provides data only when such species enter via closely-examined routes. Finally, some comparisons between pathways defy quantitative analysis—for example, which is more “important”： the entry path of one very harmful species or one by which many but less harmful species enter the country?

36.Which of the following statements about species movement is best supported by the text?
［A］ Human factors affect species movement rates more than its long-term amount.
［B］ Natural expansions of species account for their slow natural contractions.
［C］ Natural environments created by human activities contribute much to species movement.
［D］ Long-range movement of species depends on the geographic extent of human mobility.

37-According to the text, the U.S. Department of Agriculture
［A］ contributed to the fast distribution of the Russian wheat aphid to broad areas.
［B］ failed to isolate the Russian wheat aphid in limited locales.
［C］ provides data about foreign species entering the country by regulated routes.
［D］ is responsible for introducing harmful plants onto federal lands.

38.It can be inferred that all of the following affect the movement of species EXCEPT
［A］ earth fertility.
［B］ import restrictions.
［C］ natural obstacles.
［D］ fire disasters.

39.To determine the entry pathway for a non-native species is LEAST likely to depend on