“狸”是一个萌萌哒汉字，观之则有花斑状、毛绒绒之感。古人戏将家猫称之为“狸奴”。陆务观在《十一月四日风雨大作》中曾有温暖的两句：溪柴火暖面粘软，我与狸奴不出门。陆游一生为情所困，与表妹唐琬的爱情颇多曲折、不得相守，报国又有“胡未灭，鬓先秋，泪空流”的苍凉，所幸晚年得此狸奴相伴。狸奴又向花间戏晚晴的画面，也许千年前就给大文豪带去了些许的慰藉。脑补陆铲屎官拿着小鱼口呼“阿狸”，顿使爱国诗人的形象多了几分温暖和亲切，也使得“狸”字观之可亲。
“狸”在中文中泛指长脊类哺乳动物，善伏之兽。毛皮珍贵，因此“狸裘”价值斐然。大概古人把毛绒绒的萌物统称为“狸”。今天要说的“河狸”，就是极具代表性的小可爱一枚。
河狸是美洲土著的啮齿类动物，以树根、树皮为食，擅长游泳和潜水。不知道为什么儿童文学和影视偏爱啮齿类的老鼠，米老鼠、猫和老鼠、舒克和贝塔都以老鼠为原型，而老鼠的远房表哥河狸却无人问津。事实上河狸表哥不容小觑。首先呢，他是顾家的暖男一枚。河狸爱情忠贞，一生只爱一个人。成家以后，就过着你负责带娃我负责看家的岁月静好的日子。其次，河狸是阳光向上的事业型少年。河狸擅长筑坝。他们用无坚不摧的牙齿咬断树木，然后将树枝一根一根立于水中，再用柳条、树叶、泥巴穿插于树枝之间，建成堤坝，提高局部水位，使得他们的洞穴处于“半水下”的状态。每个上门提亲的河狸小哥都有两层洋楼：下层是豪华水下河景房，冬暖夏凉；上层高出水面，从此夏天的晚上露天看星星看月亮从诗词歌赋谈到人生哲学不是梦。小哥可不是随随便便的泥瓦工人，那是真正的资深土木工程师，在加拿大建成的河狸水坝是美国最大水坝的两倍之大，在太空上依稀可见。这样的河狸小哥，难怪是麻省理工和加州理工的吉祥物，实是可遇不可求的人才。
本来有趣的灵魂已经万里挑一了，河狸小哥却连好看的皮囊也不放过。河狸的裘毛暖和又防水，是冬天保暖炫富的必备品之一。不过也是因为这一身光鲜亮丽的皮毛，使得美洲河狸遭受了灭顶之灾。实是匹夫无罪，怀璧其罪。1929年，北加州克拉马斯山谷中最后一只河狸也被捕杀了。
随着河狸的消失，加州和与之比邻的俄勒冈州的水流和湖泊也发生了显著的变化。因着河狸水坝的减少，水流速度增加，河床被水流侵蚀越馅越低。水势难以漫过河堤，河畔的土地便逐渐的枯竭，不久便呈干旱燥裂的状态。曾经被河狸水坝围起来的平静的河水是鱼儿栖息繁衍的圣地，也是水鸟恬然忘机之所。失去了河狸，鱼鸟都无家可归。
生态系统的破坏和重建是一个漫长的过程。人们终于认识到河狸筑坝的重要性。然而河狸已经没有了，人类只能自己动手。可是人工的堤坝与河狸的坝相比，笨拙又不易拆除，导致大量淤泥和有害物质堆积，应变性很低，成本却很高。生态学者们开始另想办法，把河狸“请”回来。他们自己动手在河堤上建筑河狸水坝的类似物（beaver dam analogs "BDA"), 像河狸一样使用树枝和柳条，然后放三两只河狸在附近。
善良的河狸不计前嫌，看到建了一半的堤坝，欣然接过善后工作，再次勤劳地日落而作，日出而息，为生态系统的改善鞠躬尽瘁。在BDA和河狸的共同合作下，水流速度减缓，泥沙下降，河床上升。绕过堤坝的水冲击河岸、拓展河宽，形成了支流和池塘。青树翠蔓，蒙络摇缀，参差披拂。
河狸的积极参与使得生态环境迅速改善。BDA建成后1-3年，水流覆盖面积增加了3倍以上，支流增加1200%，鱼类增加52%。温暖的河狸用勤劳的双手筑起自己的生活和家园，与此同时，也造福了周边的植被和鱼虾。
衣沾不足惜，但使愿无违。
不过虽然益处多多，河狸的回归也并不完全是一帆风顺的。人们早已在曾经被支流覆盖的土地上架桥铺路。如今河狸归来，时而被淹没的公路令人头痛。人类曾经长驱直入，捕杀河狸以做裘帽，如今它们再次登台，人们也还是只从自己的利益角度出发和思考。世间安得双全法呵。生态和经济是一场有舍有得的博弈。河狸只是管中窥豹，可见一斑。第六大生物灭绝正在发生，亡羊而补牢，但求未为晚也。
悟以往之不谏，知来者之可追。实迷途其未远，觉今是而昨非。
Compared to their distant rodent cousins mice, beavers seem to be extremely under-represented in literature and on the screen. Sure, they sometimes appear as supporting actors, like (the "thoughtfully"-named) beaver Mr. Busy in Lady and the Tramp, who helped to bring about the famous spaghetti smooching scene by biting off Lady’s muzzle. Redwall, a fantasy land dedicated to valiant warrior rodents, features only one unnamed beaver. Disney's Zootopia merely pans the camera past a row of beaver construction workers. This scarcity of attention seems extremely unfair in light of the fact that beavers are such amazing animals made for tales and for the screen: extremely fluffy, highly industrious, heart-warmingly family-oriented. Surely they deserve dedicated screen time in their own right.
Beavers are natives of North America. In 1836, when Stephen Meek wandered through the Klamath Mountains, he named what was later known as Scott Valley after one of its most abundant inhabitants - beavers. Beaver Valley was an especially apt name, as it not only described the valley’s inhabitants, it also saluted the valley’s architects. Beavers are nature's very own civil engineers. They use their powerful front teeth to gnaw through trees, dragging logs into river beds, which they then set in upright positions. Next they weave willow branches, leaves and mud in between the logs to create dams. These ingenious dams help to slow water flow, creating a pond haven for beavers to then build their homes - lodges with underwater entry ways. The peaceful ponds the beavers create are also utilized by fish and birds. No other extant animal is known to have more a paw in shaping its landscape than these amazing architects. And beaver dams are no mean feat. The largest known beaver dam is twice the size of the Hoover dam and can be seen from space. It is no wonder then that both MIT and Caltech are proud homes of the Beavers.
However, Stephen didn’t know (or think much about) this when he first encountered beavers in Beaver Valley. What he registered was an abundant source of fine fur and beaver-pelt. As such, beavers were hunted almost to extinction in Northern California and Oregon, setting into action a chain of disastrous events.
Loss of beaver dams resulted in speedier waters, which carved deep into the river beds through erosion. Plains that were usually under water became exposed, dried and barren. Vegetation died. Fish lost their breeding ponds, birds lost quiet resting areas and hunting territories.
The land and its inhabitants pleaded for the return of its furry architect.
Humans tried to fix the problem, to no avail. It turned out that building lodges and dams with manpower is back-breaking (duh) and expensive (huh...). In addition, man-made dams are permanent structures that trap water for far too long, creating excessive sedimentation that spirals down into a muddy mess. So, being the shrewd creatures that we are, humans cast around for cheap labor. Hmmm beavers anyone?
Now this is why beavers deserve to be heroes in tales. Not only are they extremely handsome and talented, they also have shining hearts of gold - the defining quality of heroes through the ages. When they saw humans building what they called “beaver dam analogs” (BDA) - vertical poles with woven branches, the survivors of the fur trade, along with new beavers reintroduced by tentative humans, didn’t hesitate to lend a paw. Their instincts kicked in and they labored to add more dams while fortifying existing ones.
The results are mind-blowing. The dams diverted streams to cut into banks, widening the channel and decelerating water flow. Sedimentation raised the stream bed, allowing the long awaited-for water to again spread into flood plains and replenish ground water. Irrigation brought back plant life, and fish and birds again thrived in the diverse maze of ponds and streams (see figures above). In some places, within 1-3 years of the introduction of BDAs and beavers, water coverage more than tripled, side streams increased by 1200%, and fish were 52% more likely to survive.
Of course this does not come without skepticism and concerns. What used to be flood plains that welcomed water diverted by beaver dams are now a conglomerate of human civilization: infrastructure that did not like to be underwater, not even occasionally. Hunting beavers had been disastrous for the ecosystem, but reintroducing them into the currently precarious equilibrium could bring just as much harm.
The beavers homeward bound journey is fraught with doubt and uncertainties from the human perspective. We seem to forget that it is in the beavers’ ancestral home that we have set up fort, and refused to share. Even now, we still think of “useful” and “harmful” only in terms to our own species: the maintenance of “our” lands, protection of “our” trees, what we stand to gain and lose economically. And with the threat of the sixth mass extinction hanging over our heads, perhaps even happening as we sit back and contemplate the situation, maybe it is time we gave thought to the wishes and well-beings of others.
Hail the homeward bound beavers.
Reference
Goldfarb, Ben. (2018). Beaver dams without beavers? Artificial logjams are a popular but controversial restoration tool. Science. 10.1126/science.aau3887.
