How Much Nature Is Enough?

By Andrew C.Revkin

Even some ardent conservationists acknowledge that the diversity of life on Earth cannot be fully sustained as human populations expand use more resources nudge the climate and move weedlike pests and predators from place to place.

Given that some losses are inevitable the debate among many experts has shifted to an uncomfortable subject what level of loss is acceptable. The discussion is taking place at both the local and global levels How small can a fragment of an ecosystem be and still function in all its richness and thus be considered preserved﹖ And as global biodiversity diminishes is it a valid fallback strategy to bank organisms and genes in zoos DNA banks or the like or does this simply justify more habitat destruction﹖ Is nature on ice a sufficient substitute for the real thing﹖ Some conservation groups have strenuously avoided or even attacked such calculations and strategies. They say there is no safe diminution of habitat as long as human understanding of ecology is as sketchy as it is a fallback strategy is unthinkable. Furthermore banking nature in a deep freeze or database of gene sequences cannot capture context. For instance even if a vanished bird was someday reconstituted from its genes would it warble with the same fluency as its ancestors﹖ On the other side of the debate those considering what the smallest viable habitats are or how to expand archives as an insurance policy say that recent trends have proved that old conservation strategies are no longer sufficient. A few decades ago the issue seemed fairly uncomplicated identify biological “hot spots” or species of concern and establish as many reserves as possible. But the picture has grown murky.

Twentyfour years ago Dr. Thomas E. Lovejoy and other biologists began a remarkable experiment on the fasteroding fringe of rain forest near the Brazilian city of Manaus. They established 11 forest tracts ranging from 2.5 to 250 acres each surrounded by an isolating sea of pasture similar to what is advancing around most other tropical forests. Among the many findings an analysis published last week on birds in the lower layers of greenery found that it would take a fragment measuring at least 2 500 acres—10 times as large as the biggest one in the experiment—to prevent a decline of 50 percent in those bird varieties in just 15 years or so.

In the understated language of science the new study in The Proceedings of the National Academy of Sciences concludes “This is unfortunate when one considers that for some speciesrich areas of the planet a large proportion of remaining forest is in fragments smaller than 2 500 acres.”

In the face of this and other evidence a growing group of conservation biologists say try everything at the same time. “Clearly the most effective way to protect biodiversity is to protect natural areas ” said Dr. Peter H. Raven the director of the Missouri Botanical Garden “and to find those organisms most endangered in nature and somehow protect them in typeculture collections botanical gardens zoos seed banks or whatever.” But most important he said is to find ways to limit human pressures on the world’s last wild places by slowing population growth and using resources more efficiently. One pioneer of genetic deconstruction Dr. J. Craig Venter agrees with Dr. Raven. Dr. Venter has moved from sequencing the DNA of humans and other species to assaying genes in entire ecosystems most recently the waters of the Sargasso Sea. In five 50gallon samples gathered in February he said his team had found 1 million distinct genes quite a haul compared with the 26 000 or so of a human being. And that is the tiniest scratch in the surface he added. His is one effort among many. Britain has a Millennium Seed Bank a growing archive of all the country’s plants. The San Diego Zoo has its parallel Frozen Zoo an archive of thousands of DNA samples and cell lines from a host of species. Nonetheless given the overwhelming complexity of nature Dr. Venter added “we’re better off trying to preserve the diversity of what we have rather than trying to regenerate it in the future.”

連一些積極的自然資源保護論者都承認，隨著人口的膨脹、消耗更多的自然資源、引起氣候的變化，以及造成大量害蟲和捕食動物的遷移等，地球上的生物多樣性肯定不會完全地持續下去。

許多專家的爭論焦點已經轉到了一個令人不安的話題，假使一些損失是不可避免的，多大程度的損失是可以容忍的呢?關于這個問題的辯論在局部和全球范圍兩個層面上同時展開：生態系統的一小部分可以小到何種程度仍能維持其完整、豐富的功能，從而可以認為是受到了保護呢?在全球生物多樣性減少的過程中，把生物有機體和基因保存在諸如動物園、基因庫之類的地方是一種有效的保全策略嗎?或者這樣做僅僅為更多的(動、植物)棲息地的破壞提供了借口?冷藏的自然能夠充分地代替真正的自然嗎?一些自然資源保護組織一直極力避開甚至反對這樣的推論和策略。他們說，只要人類對生態系統的認識還是一知半解的，那就不存在對棲息地的安全縮小;因此也就談不上什么保險策略了。更何況，把自然生態深凍起來或者將其存入基因序列數據庫并不能保存與其相關的背景。比如，就算一只滅絕的鳥兒某一天被人們從它的基因中重新組合出來，它的啁啾聲能像其先輩們一樣婉轉動聽嗎?另一方面，尋求最小可行棲地的人們或試圖擴大現有檔案庫作為一種保全策略的人們則說，最近的趨勢已經表明，舊的保護策略不再夠用了。幾十年之前，問題似乎還不是那么復雜：只要確認出那些受到威脅的生態地區或者令人擔憂的物種，然后建立盡可能多的保護區就是了。然而，目前這個狀況已經變得模糊起來。

24年以前，托馬斯?E?洛夫喬伊博士和其他一些生物學家在巴西馬瑙斯市附近遭受快速侵蝕的熱帶雨林地區邊緣開始了一項備受矚目的實驗。他們建立了 11塊森林試驗區，面積大小從2.5英畝到250英畝不等，每一塊都被一片分割開來的廣袤草場包圍起來，這些草場與正在向大多數其他熱帶雨林周邊推進的草場相類似。在大量的調查結果中，上周出版的一篇關于生活在綠地較低層的鳥類的分析報告發現，至少需要一塊2500英畝的森林區域——相當于實驗中劃出來的最大的一塊試驗區的10倍——才能防止那些鳥類的品種在僅僅15年左右的時間里減少50%。

《美國國家科學院學報》上新刊登的一篇專題研究論文用毫不夸張的科學語言總結道：“當你考慮到在地球上一些物種資源豐富的地區，留存下來的森林中一大部分是小于2500英畝的分散小塊時，這是多么不幸。”

面對這些情況和其他證據，愈來愈多的自然資源保護生物學家說：應該同時嘗試所有可行的辦法。美國密蘇里州植物園園長彼得?H?雷文博士說：“顯然，最有效的保護生物多樣性的方法是保護自然棲息地，同時，還要找出那些自然界中最為瀕危的物種，用某種方法把它們保護起來，比如，把它們放入物種培育采集庫、植物園、動物園、種子銀行等諸如此類的地方。”他說，但最為重要的是通過減緩人口增長和更有效地利用資源找到減少人類對世界最后原始生態地區的壓力的方法。遺傳解構學的先驅之一，J?克雷格?文特爾博士同意這一看法。文特爾博士從對人類和其他物種的DNA基因排序的研究轉到了對整個生態系統的基因分析，最近開始了對馬尾藻海海水的研究。他說，他的小組在2月份(指2003年2月份——譯者)收集的5份50加侖的樣本中發現了上百萬種不同的基因類型，這與人類個體具有的約26000種基因相比實在是太多了。他補充說，這不過才觸及到皮毛而已。他所做的只是許多努力中的一部分。英國有一個“千禧年種子銀行”。它不斷擴大，收藏了該國所有植物。(美國)圣地亞哥動物園有一座與其相應的“冷藏動物園”，其中保存了許多物種的數千個DNA樣本和細胞株的資料。即使如此，考慮到自然界極為復雜，文特爾博士補充說：“如果我們盡力保護好現存生物的多樣性，而不是試圖在將來去重新創造它，我們才更明智。”