Non-indigenous (non-native) species of plants and animals arrive by way of two general types of pathways. First, species having origins outside the United States may enter the country and become established either as free-living populations or under human cultivation-for example, in agriculture, horticulture, aquaculture, or as pets. Some cultivated species subsequently escape or are released and also become established as free-living populations. Second, species of either U.S. or foreign origin and already within the United States may spread to new locales. Pathways of both types include intentional as well as unintentional species transfers. Rates of species movement driven by human transformations of natural environments as well as by human mobility-through commerce, tourism, and travel-greatly exceed natural rates by comparison. While geographic distributions of species naturally expand or 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. 　　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 To determine the entry pathway for a non-native species is LEAST likely to depend on

A:whether the species is considered to be a pest. B:whether the species enters by a closely-checked route. C:the rate at which the species extends geographically. D:the magnitude of the average number of the species.

(A number of) endangered (species) (increases) every year and natural (resources disappear).

A:A number of B:species C:representative D:resigned disappear

Cryptic (隐蔽的，隐藏的) species—animals that appear identical but are genetically quite distant—may be much more widespread than previously thought. The findings could have major implications in areas ranging from biodiversity (生物多样性) estimates and wildlife management, to our understanding of infectious diseases and evolution.
Reports of cryptic species have increased dramatically over the past two decades with the advent of relatively inexpensive DNA (脱氧核糖核酸) sequencing technology. Markus Pfenninger and Klaus Schwenk, of the Goethe-Universitat in Frankfurt, Germany, analyzed all known data on cryptic animal species and discovered that they are found in equal proportions throughout all major branches of the animal kingdom and occur in equal numbers in all biogeographical (生物地理学的) regions.
Scientists had previously speculated that cryptic species were predominantly found in insects and reptiles (爬行动物), and were more likely to occur in tropical rather than temperate (温带的) regions. "Species that are seemingly widespread and abundant could in reality be many different Cryptic species that have low populations and are highly endangered," says Pfenninger. Until the genetic information of all species in at least one taxon (分类单元) is thoroughly studied, no one will know just how many cryptic species exist. "It could be as high as 30%," Pfenninger says.
"I’m extremely surprised by their results, " says Alex Smith of the University of Guelph in Ontario, Canada. "It’s a call to arms to keep doing the broad kind of genetic studies that we are doing. "
Sampling as many individuals as possible, scientists hope to complete work on all fish and birds in another 5 to 10 years. Once either of these taxonomic groups is completed, Pfenninger says researchers will be able to decide how many cryptic species exist throughout the animal kingdom.
Examples of cryptic species include the African elephant. A 2001 study found the elephants were actually two genetically distinct, non-interbreeding (非杂交繁殖的) species, the African bush elephant and the African elephant. The species are currently listed as vulnerable and threatened, respectively, by the World Conservation Union (WCU).
The reclassifications are more than an academic exercise. They define populations that have evolved independently of each other and whose genetic differences can have significant consequences.
In the early 1900s misidentification of mosquito species based on morphology confused, attempts to control malaria (疟疾) in Europe. Ultimately, what was thought to be a single species was actually made up of six sibling (同胞，同属) species, only three of which transmitted the disease." The basic unit in biology is always the species, and you have to know what you are dealing with," Pfenninger says. Much previous research is now no longer used, he says, because it is not clear what species was being studied.

People were confused in their attempts to control malaria in Europe in the early 1900s, because scientists （　）

A:identified only one mosquito species instead of six species B:thought only three mosquito species transmitted disease C:thought there was only one mosquito species D:did not know what species was being studied

{{B}}第二篇{{/B}}

People were confused in their attempts to control malaria in Europe in the early 1900s, because scientists

A:identified only one mosquito species instead of six species. B:thought only three mosquito species transmitted disease. C:thought there was only one mosquito species. D:did not know what species was being studied.

{{B}}第二篇{{/B}}

People are confused in their attempts to control malaria in Europe in the early 1900s, because scientists ______.

A:identified only one mosquito species instead of six species B:thought only three mosquito species transmitted disease C:thought there was only one mosquito species D:did not know what species was being studied

Cryptic species animals that appear identical but are genetically quite distant--may be much more widespread than previously thought. The findings could have major implications in areas ranging from biodiversity estimates and wildlife management, to our understanding of infectious diseases and evolution.
Reports of cryptic species have increased dramatically over the past two decades with the advent of relatively inexpensive DNA sequencing technology. Markus Pfenninger and Klaus Schwenk, of the Goethe-Universitat, in Frankfurt, Germany, analyzed all known data on cryptic animal species and discovered that they are found in equal proportions throughout all major branches of the animal kingdom and occur in equal numbers in all biogeographieal regions.
Scientists had previously speculated that cryptic species were predominantly found in insects and reptiles, and were more likely to occur in tropical rather than temperate regions. "Species that are seemingly widespread and abundant could in reality be many different cryptic species that have low populations and are highly endangered," says Pfenninger. Until the genetic information of all species in at least one taxon is thoroughly studied, no one will know just how many cryptic species exist." It could be as high as 30%," Pfenninger says.
"I’m extremely surprised by their results," says Alex Smith of the University of Guelph in Ontario, Canada. "It’s a call to arms to keep doing the broad kind of genetic studies that we are doing."
Sampling as many individuals as possible, scientists hope to complete work on all fish and birds in another 5 to 10 years. Once either of these taxonomic groups is completed, Pfenninger says researchers will be able to decide how many cryptic species exist throughout the animal kingdom.
Examples of cryptic species include the African elephant. A 2001 study found the elephants were actually two genetically distinct, non-interbreeding species, the African bush elephant and the African elephant. The species are currently listed as vulnerable and threatened, respectively, by the World Conservation Union (WCU).
The reclassifications are more than an academic exercise. They define populations that have evolved independently of each other and whose genetic differences can have significant consequences.
In the early 1900s misidentification of mosquito species based on morphology confused attempts to control malaria in Europe. Ultimately, what was thought to be a single species was actually made up of six sibling species, only three of which transmitted the disease. "The basic unit in biology is always the species, and you have to know what you are dealing with," Pfenninger says. Much previous research is now no longer used, he says, because it is not clear what species was being studied.
People were confused in their attempts to control malaria in Europe in the earlyl900s, because scientists______.

A:identified only one mosquito species instead of six species B:thought only three mosquito species transmitted disease C:thought there was only one mosquito species D:did not know what species was being studied

This passage mentions all of the following causes for the extinction of many species EXCEPT

A:natural selection of species. B:various natural disasters. C:commercial trade and killing D:destruction of natural surroundings.