翻譯原文：
　　One basic weakness in a conservation system based wholly one economic motives is that most members of the land.community have no economic value. Yet these ereatures are members of the biotic community and, if its stability depends on its inteyrity, they are entitled to continuance。

　　When one of these noneconomic categories is threatened and, if we happen to love it, we invert excuses to give it economic importance.at the beginning of century songbirds were supposed to be disappearing. 46. Scientists jumped to the resure with some distimctly shaky evidence to the effect that insects would eat us up of birds failed to control them, the evideuce had to be comic in order to be valid。

　　It is painful to read these round about accounts today. we have no land ethic yet. 47. But we have at least drawn near the point of admitting that birds should continue as a matter of intrinsic right, regardless of the presence or absence of economic advantage to us。

　　A panallel situation exists in respect of predatory mamals and fish-eating birds. 48. Time was when biologists somewhat overworked the evidence that these creatures preserve the health of game by killing the physically weak,or that they prey only on “worthless species”。

　　Some species of tree have been read out of the party by economics-minded foresters because they grow too slowly .or have too low a sale vale to pay as imeber crops. 49. In Europe ,where forestry is ecologically more advanced, the non-commercial tree species are recognized as members of native forest community, to be preserved as such , within reason。

　　To sum up: a system of conservation based solely on economic self-interest is hopelessly lopsided. 50. It tends to ignore, and thus eventually to eliminate, many elements in the land community that lack commercial value, but that are essential to its healthy functioning. Without the uneconomic pats。

　　46.Scientists jumped to the rescue with some distinctly shaky evidence to the effect that insects would eat us up if birds failed to control them。

　　【詞匯理解】

　　jump to the rescue(趕來拯救)，shaky(站不腳的)，to the effect that(其大意是);eat up(吃光)，fail to(沒有)。

　　【結(jié)構(gòu)理解】

　　that引導(dǎo)的是個同位語從句，if引導(dǎo)的是個條件狀語從句。

　　【參考譯文】

　　科學(xué)家們提出一些明顯站不住腳的證據(jù)迅速來拯救，其大意是：如果鳥類無法控制害蟲，那么這些害蟲就會吃光我們?nèi)祟悺?/p>

　　47. But we have at least drawn nearer the point of admitting that birds should continue as a matter of intrinsic right, regardless of the presence or absence of ecnomic advantage to us。

　　【詞匯理解】

　　at least(至少)，draw near(靠近，接近于) regardless of(盡管)， presence or absence of (有還是沒有)

　　【結(jié)構(gòu)分析】

　　that引導(dǎo)的是個賓語從句，as常接在動詞后面，翻譯成：作為。Regardless of引導(dǎo)的是個讓步狀語從句。

　　【參考譯文】

　　但我們至少幾乎也承認(rèn)這樣一點：不管鳥類是否給我們帶來經(jīng)濟上的好處，但鳥類作為生物其固有的權(quán)利應(yīng)該繼續(xù)存在。

　　48. Time was when biologists somewhat over-worked the evidence that these creatures preserve the health of game by killing the physically weak, or that they prey only on "worthless" species。

　　【詞匯理解】

　　Time was when(曾幾何時)，overworked(過度使用，濫用)，game(獵物)，prey on(捕殺)。

　　【結(jié)構(gòu)分析】

　　that引導(dǎo)的是個同位語從句，or這個并列連詞，連接的是個并列的同位語從句。

　　【參考譯文】

　　曾幾何時，生物學(xué)家們有點過度使用這個證據(jù)，即這些物種通過殺死體質(zhì)弱者來保持獵物的正常繁衍或者這些生物捕殺的僅僅是毫無價值的物種。

　　49. In Europe, where forestry is ecologically more advanced, the non-commercial tree species are recognized as members of the native forest community, to be preserved as such, within reason。

　　【詞匯理解】

　　foestry林業(yè);ecologically(從生態(tài)上來說)，advanced(先進的，發(fā)達的)，be recognized as(看作是)，native forest community(原始森林群落)，as such(類似地，這樣地)within reason(合理范圍內(nèi))。

　　【結(jié)構(gòu)分析】

　　where引導(dǎo)的是個定語從句。

　　【參考譯文】

　　在歐洲，林業(yè)在生態(tài)方面更加發(fā)達，無商業(yè)價值的樹種被看作是原生森林群落的一部分，而得到合理的保護。

　　50. It tends to ignore, and thus eventually to eliminate, many elements in the land community that lack commercial value, but that are essential to its healthy functioning。

　　【詞匯理解】

　　it指代前面出現(xiàn)the system of conservation，tend to(傾向于，易于)， land community(土地共同體)，lack(缺乏)，commercial value(商業(yè)價值)，第二個that是個代詞，指代前面的出現(xiàn)的many elements。

　　【結(jié)構(gòu)分析】

　　and是個并列連詞，連接的是to do sth這樣的動詞不定式，但其后省略了與前面相同的動詞tends，第一個that是個定語從句，修飾前面的名詞community，but引導(dǎo)的是個表示轉(zhuǎn)折的連詞。

　　【參考譯文】

　　這一系統(tǒng)易于忽視，因而比較終會消除掉這個土地共同體里的許多要素(成員)，雖然這些要素(成員)缺乏商業(yè)價值，但這些要素(成員)對這個共同體的健康運行來說是必要的。