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One important natural check is a limit on the amount of suitable habitat for each species. Insect problems arose with the intensification of agriculture-the devotion of immense acreage to a single crop. Such a system set the stage for explosive increases in specific insect populations. Obviously then, an insect that lives on wheat can build up its population to much higher levels on a farm devoted to wheat than on one in which wheat is intermingled with other crops to which the insect is not adapted.
一個重要的自然控制就是對于適合于每個物種的棲息地的限制。農(nóng)業(yè)生產(chǎn)高度集中，大量的土地被用來種某一種莊稼，昆蟲問題也就接踵而至。這樣的系統(tǒng)使某種特定的昆蟲數(shù)量激增。很明顯，以小麥為生的昆蟲，如果是在僅種植小麥的地里，它的數(shù)量會很多，而如果是在那些小麥和不適合這種昆蟲生長的其他農(nóng)作物混種的地里，它的數(shù)量就會少很多。
Another factor in the modern insect problem is the spreading of thousands of different kinds of organisms from their native homes. Some hundred million years ago, flooding seas cut many land bridges between continents and living things found themselves confined in what an ecologist calls "colossal separate nature reserves." There, isolated from others of their kind, they developed many new species. When some of the land masses were joined again, about 15 million years ago, these species began to move out into new territories—a movement that is not only still in progress but is now receiving considerable assistance from man.
導致現(xiàn)代昆蟲問題的另外一個因素是成千上萬種不同種類的生物從它們原來的棲息地向新的區(qū)域蔓延入侵。幾億年前，泛濫的大海切斷了許多大陸之間的聯(lián)系，生物只能被限制在如同一位生態(tài)學家所說的“巨大的獨立的一然保護區(qū)”內(nèi)。在那里，它們被與其他同類隔絕開，進化出了許多新的物種。大約1500萬年前，當一些大陸重新連通之時，這些物種開始遷移到新的地區(qū)——這個運動現(xiàn)在仍在進行中，而且正在得到人類的大力幫助。
The importation of plants is the primary agent in the modern spread of species, for animals have almost invariably gone along with the plants.
植物的進口是當代昆蟲種類傳播的主要原因，因為動物幾乎總是跟著植物遷移的。
