翻译原文：
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。
【词汇理解】
jump to the rescue(赶来拯救)，shaky(站不脚的)，to the effect that(其大意是);eat up(吃光)，fail to(没有)。
【结构理解】
that引导的是个同位语从句，if引导的是个条件状语从句。
【参考译文】
科学家们提出一些明显站不住脚的证据迅速来拯救，其大意是：如果鸟类无法控制害虫，那么这些害虫就会吃光我们人类。
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。
【词汇理解】
at least(至少)，draw near(靠近，接近于) regardless of(尽管)， presence or absence of (有还是没有)
【结构分析】
that引导的是个宾语从句，as常接在动词后面，翻译成：作为。Regardless of引导的是个让步状语从句。
【参考译文】
但我们至少几乎也承认这样一点：不管鸟类是否给我们带来经济上的好处，但鸟类作为生物其固有的权利应该继续存在。
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。
【词汇理解】
Time was when(曾几何时)，overworked(过度使用，滥用)，game(猎物)，prey on(捕杀)。
【结构分析】
that引导的是个同位语从句，or这个并列连词，连接的是个并列的同位语从句。
【参考译文】
曾几何时，生物学家们有点过度使用这个证据，即这些物种通过杀死体质弱者来保持猎物的正常繁衍或者这些生物捕杀的仅仅是毫无价值的物种。
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。
【词汇理解】
foestry林业;ecologically(从生态上来说)，advanced(先进的，发达的)，be recognized as(看作是)，native forest community(原始森林群落)，as such(类似地，这样地)within reason(合理范围内)。
【结构分析】
where引导的是个定语从句。
【参考译文】
在欧洲，林业在生态方面更加发达，无商业价值的树种被看作是原生森林群落的一部分，而得到合理的保护。
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。
【词汇理解】
it指代前面出现the system of conservation，tend to(倾向于，易于)， land community(土地共同体)，lack(缺乏)，commercial value(商业价值)，第二个that是个代词，指代前面的出现的many elements。
【结构分析】
and是个并列连词，连接的是to do sth这样的动词不定式，但其后省略了与前面相同的动词tends，第一个that是个定语从句，修饰前面的名词community，but引导的是个表示转折的连词。
【参考译文】
这一系统易于忽视，因而最终会消除掉这个土地共同体里的许多要素(成员)，虽然这些要素(成员)缺乏商业价值，但这些要素(成员)对这个共同体的健康运行来说是必要的。