那天我架著錄音機在茂林山路等了四個小時。
我去過那裡兩次，都是四月。第一次，蝶群從山谷掠過，翅膀拍動的頻率疊在一起變成一種低沉的絲鳴，像一條河流從空中通過。我試著把那個聲音收進麥克風，收不完——聲景太大，我的設備太小。第二次，我帶了更好的設備。山路很安靜。蝴蝶沒來。
2026年清明前後，雲林縣林內鄉統計的紫斑蝶北遷數量約4.1萬隻。往年同期，那個數字是30到40萬。高公局有一套「國道蝶道」機制：蝶量每分鐘達250隻，外側車道封閉，讓蝴蝶先過。今年，那個門檻沒有被觸發。
那條河流長什麼樣子
台灣紫斑蝶（Euploea spp.）與墨西哥帝王斑蝶是全球僅有的兩種大型越冬型蝶種。帝王斑蝶從美國東岸飛去墨西哥，每年遷徙吸引全球生態攝影師跟拍，是北美人熟悉的秋季奇觀。台灣的版本發生在清明到穀雨之間：蝴蝶從高雄茂林、六龜越冬谷出發，沿著西部山麓一路北返，走的是幾千年演化出來的路線。
高峰期，那條路線上的密度大到公路設備必須介入。想像一條雙線道山路，每六十秒有兩百五十隻蝴蝶橫穿而過，持續數小時——那不是「很多蝴蝶」，那是一條活的紫色河流，從山這邊流向山那邊。現在這條河流在4.1萬這個數字面前，細得幾乎消失。
廊道是怎麼斷的
廊道不是某一年突然斷掉的，是一段段消失的。
紫斑蝶幼蟲需要特定的食草才能完成羽化，爬森藤是其中關鍵的一種。這類植物在農業區被視為雜草——它們長在田埂邊、山腳溝渠旁、果園邊緣。農民打除草劑，順帶清掉；施殺蟲劑，讓幼蟲沒有生存空間。農民不是惡意的，農藥是工業農業的標準配備，台灣農藥使用密度長期居全球前列，這不是哪個農民的個人問題，是整個農業系統幾十年來的慣性。
廊道的邏輯是這樣的：蝴蝶遷徙需要沿途補充能量，食草和蜜源植物必須串連成連續的帶狀棲地，任何一段農業區把植被清乾淨，廊道就在那裡斷開。斷開一段，蝴蝶繞路；斷開三段，種群數量開始衰減；斷開到某個臨界點，整個北返規模就垮下來了。生態學家指出農業區域農藥使用是廊道破壞的主要原因，但這條因果鏈要花幾十年才會被看見。
農民自己決定的那一刻
高雄農業改良場場長羅正宗說過一句話：「跟土地當朋友。」這句話很容易被當成政府宣傳口號略過，但六龜和茂林發生的事情，比口號具體。
農改場輔導六龜、茂林地區農民投入有機友善耕作，策略包括農田草生帶維護、淺山保育軸帶緩衝區、減少化學藥劑、輪作管理。關鍵在於保留田埂邊的食草植物，特別是爬森藤這類蝶幼蟲的宿主植物，不再視之為需要清除的雜草。
這個轉型沒有靠大規模補貼撐起來。農民自己做了選擇。有農民說：蝴蝶回來，他就知道土地沒有生病。這不是詩意，這是一個在土地上工作幾十年的人，學會用蝴蝶當監測指標。廊道趨於完整之後，旗山、美濃的農田也加入了這個體系——同一個計畫還連帶讓水雉繁殖成功率提升、高屏溪沿岸草鴞族群開始恢復。
保育的時間尺度從來不配合新聞週期。黃石公園灰熊復育從150頭到700頭花了將近40年。紫斑蝶廊道的重建沒有40年可以等，但窗口還沒有關死。
聲音還在那裡
如果廊道重建成功，我會再回去那條山路。帶同樣的設備，同樣的時間，等那個低沉的絲鳴再次從山谷掠過。
4.1萬隻蝴蝶不是零。它是一個很小的數字，但還不是終點。茂林山谷還有越冬的族群，六龜的農田邊已經開始留著食草，農改場的廊道計畫還在推進。那條河流沒有消失，它只是細到幾乎聽不見。
問題是細到這個程度，還能等多少個春天。
— 林郁翔 (Jason)
延伸閱讀
41,000 Butterflies and the Road Gone Silent
I set up my recorder on a mountain road in Maolin and waited four hours. Nothing came.
The first time I went, in April, the butterfly swarm moved through the valley like an audible river — thousands of wings beating at slightly different frequencies, layering into a low silk-thread hum. My microphone couldn’t hold it. The sound was too wide. The second time I brought better equipment. The road was quiet. The purple spotted butterflies didn’t show.
In the 2026 spring migration season, monitoring stations near Linnei Township in Yunlin County counted approximately 41,000 purple spotted butterflies crossing northward. In previous years, that same corridor carried 300,000 to 400,000. Taiwan’s Freeway Bureau operates a protocol called “National Highway Butterfly Lane”: when butterfly density reaches 250 per minute, the outer lane closes to let them pass. This year, that threshold was never triggered.
What a Butterfly River Sounds Like
Taiwan’s purple spotted butterflies (Euploea spp.) and Mexico’s Monarch butterfly are the only two large-scale overwintering migratory butterfly species on Earth. The Monarch migrates from the eastern United States to central Mexico — a spectacle familiar to North Americans as an autumn phenomenon. Taiwan’s version runs from early April to early May: butterflies leave the overwintering valleys in Maolin and Liugui in Kaohsiung and fly north along the western mountain foothills, following routes shaped across thousands of years of evolution.
At peak density, the traffic is dense enough that highway infrastructure has to intervene. The Freeway Bureau’s response mechanism — slowing cars, closing lanes — is one of the few places in the world where a government has built ecological migration into highway operations. That mechanism sat idle this spring.
How a Corridor Breaks
Corridors don’t collapse overnight. They disappear in segments.
Purple spotted butterfly larvae require specific host plants to complete metamorphosis — including creeping plants like climbing milkweed that grow along field edges, irrigation ditches, and orchard borders. To farmers, these are weeds. Herbicide clears them. Insecticide kills larvae that survive the first pass. No individual farmer is making a decision to destroy a migratory corridor — they’re following the standard operating logic of industrial agriculture, which Taiwan has practiced at high intensity for decades. Ecologists have identified agricultural pesticide use in these areas as a primary cause of corridor degradation.
The corridor’s logic is simple: butterflies need a continuous chain of food plants to refuel during migration. Break the chain in one place, they reroute. Break it in three places, population numbers begin dropping. Break enough of it, and the scale of the whole northward migration collapses.
The Decision Farmers Made Themselves
The Kaohsiung District Agricultural Research and Extension Station has been working with farmers in Liugui and Maolin to shift toward organic, pesticide-free cultivation. The approach involves maintaining vegetated buffer strips along field edges, preserving host plants for butterfly larvae, reducing chemical inputs, and managing crop rotation. The station’s director, 羅正宗, put it plainly: “Make friends with the land.”
Farmers didn’t receive large subsidies to make this shift. They decided. One farmer, after years without pesticide use, said he could tell whether his soil was healthy by whether the butterflies came back. That’s not metaphor — that’s a practical monitoring framework developed across decades of watching one piece of land. As the corridor has improved, farms in Qishan and Meinong have joined the same system, and the same farmland management changes have also raised jacana breeding success rates and begun to restore barn owl populations along the Gaoping River.
Ecological recovery operates on timescales that news cycles don’t accommodate. The recovery of grizzly bears in Yellowstone from around 150 individuals to over 700 took roughly 40 years. The butterfly corridor doesn’t have 40 years, but the window hasn’t closed.
The Sound Is Still There
I’ll go back to that mountain road if the corridor holds. Same equipment, same month, waiting for that low silk hum to come through the valley again.
41,000 is not zero. It’s a small number, but it isn’t an ending. The overwintering colonies still exist in Maolin. Host plants are being left standing along farm edges in Liugui. The question is how many more springs that corridor can absorb before it becomes too thin to recover.
That’s not a rhetorical question. It has a biological answer, and the answer is being counted right now.
— 林郁翔 (Jason)
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https://justfly.idv.tw/s/qNMgCVz