台灣二期稻作區澤蛙生活史策略之季節分化

Abstract

兩棲類幼體具有高度的表型可塑性，能因應水域環境的變化來調整變態策略（變態體型大小和變態時間），以提高上陸後的適存度。然而，水域環境會隨季節變化，但卻鮮有研究針對季節性環境變化，探討兩棲類族群所衍生之季節性變態策略；另一方面，不同地區的兩棲類族群，亦需面對各自水域環境的差異性。故研究兩棲類幼體因應時空（季節性與地域性）環境變化，所產生的變態策略差異或趨勢，能讓我們更深入瞭解兩棲類的表型可塑性能力。本論文以澤蛙蝌蚪為材料，以實驗操弄的方式，探討下列四個問題：(1)生活在具二期稻作(分別為春和夏季）稻田的澤蛙蝌蚪是否因應兩期稻作期的環境溫度差異，而有不同的變態策略？(2)在兩個環境因子（溫度和食物量）的交互作用下，春夏兩季澤蛙蝌蚪的變態策略有何差異？(3)在不同地點的二期稻作區之澤蛙蝌蚪族群，其春夏兩季變態策略是否具一致性？(4)不同作物區的灌溉模式（水稻田：不連續灌溉和筊白筍田：連續灌溉），是否會影響澤蛙蝌蚪此兩季變態策略的分化？針對上述問題，我設計了下列四個實驗來檢測。 實驗一在新北市新店安康農場，分別採集自兩期稻作5月（春季）和9月（夏季）的澤蛙蝌蚪，在食物無限量供應下進行溫度（攝氏22和29度）和繁殖群（春和夏季）的2 × 2複因子實驗。結果發現不論是春或夏季蝌蚪，當飼養在各自的季節溫度時（春季在22度；夏季在29度），生長速率較快，因此澤蛙蝌蚪變態策略具有季節性的差異。推測蝌蚪已適應各自所生長的季節溫度，因此呈現較佳的生長速率。在低溫（22度）下，春季蝌蚪變態體重明顯大於夏季蝌蚪；而在高溫（29度）下，兩季蝌蚪皆提高其生長速率，但是夏季蝌蚪速率顯著高於春季。此結果顯示，二期稻作區的澤蛙蝌蚪因應春夏兩季不同氣溫，採用不同的變態策略，春季蝌蚪於低溫環境採用「體型優勢」策略，而夏季蝌蚪則在高溫下採用「速率優勢」策略。 實驗二仍自安康農場採集春季和夏季的澤蛙蝌蚪，以溫度（22和29度）、食物量（限量和無限量）和繁殖群（春和夏季）進行2 × 2 × 2複因子實驗。結果顯示食物量改變了溫度對變態策略的影響。在食物供給無虞情況下，春夏蝌蚪變態策略與實驗一結果雷同；然而，在食物有限的情況下，兩季蝌蚪的變態體重皆明顯降低；低溫下的春季蝌蚪相對於夏季蝌蚪仍保有體重優勢，具有較大變態體重；但是高溫下的夏季蝌蚪卻失去其速率優勢，生長速率顯著低於春季蝌蚪。由此得知澤蛙蝌蚪因應溫度和食物量兩種環境因子的交互作用，會調整春夏兩群之變態策略。 實驗三採集了不同地點的二期稻作區澤蛙蝌蚪（安康、大溪和彰化），在食物無限量供應下進行溫度（22和29度）和繁殖群（春和夏季）2 × 2複因子實驗。結果發現三地澤蛙蝌蚪皆有春夏兩季變態策略之差異，但三地族群呈現程度不一的季節差異。緯度最北的安康族群，具有明顯的季節性變態策略，春季蝌蚪在低溫下具有體重優勢，而夏季蝌蚪在高溫下具有速率優勢。這可能是因安康的夏季溫度變異最大，加上此樣區稻作面積最小、族群狀態也較封閉。大溪和彰化兩地族群的季節性的變態策略都不若安康明顯，可能因為兩地的稻作面積都極大，週邊有連續族群可交流之故。最南的彰化族群，其春夏蝌蚪變態策略差異在三地中最不明顯，可能是因其兩季均溫最高且穩定。 實驗四比較生活於水稻田（大溪：不連續灌溉）與筊白筍田（三芝：連續灌溉）的澤蛙蝌蚪，在食物無限量供應下進行溫度（22和29度）和繁殖群（春和夏季）2 × 2複因子實驗。結果發現來自筊白筍田的澤蛙蝌蚪，並不具有春夏變態策略季節性差異，兩季蝌蚪的變態時間僅受溫度影響，相同溫度處理下，春夏蝌蚪具有相同的蝌蚪期天數。此外，春夏兩季蝌蚪在低溫下的變態體重及生長速率差異不大；而高溫下，春季蝌蚪生長速率反而比夏季較高。由此推測，來自不同作物區的澤蛙蝌蚪，灌溉模式的不同會影響其是否具有季節性變態策略。筊白筍田的澤蛙族群，因繁殖期間有持續的灌溉水，可視為永久性池塘，此與二期稻作調節性排乾灌溉水的暫時性池塘不同，故筊白筍田澤蛙蝌蚪不具季節性變態策略之差異。 綜合以上實驗，兩期稻作區澤蛙蝌蚪因應春夏氣溫條件差異，產生季節性最佳變態策略，在食物充沛時，春季蝌蚪於低溫環境採用「體型策略」，夏季蝌蚪在高溫下採用「速率策略」。在溫度和食物量的交互作用下，兩季蝌蚪會調整其變態策略；在食物受限情況下，春夏季(尤其是後者)蝌蚪均展現截然不同的變態策略。此外，在台灣北部與中部之二期稻作區蝌蚪皆具季節分化現象，但因各地氣候與環境仍有些微差異，北台灣的新店安康季節分群最明顯，其次為大溪，最後則為中部的彰化。而在台灣北部的筊白筍田，因持續有水以致春夏蝌蚪變態策略未有季節上的差異。本篇論文顯示，澤蛙蝌蚪在環境的時空變異下具有高度可塑性，除了提供澤蛙生活史中變態策略的相關研究，亦指出「時間分群研究」對繁殖季甚長的物種之重要性。

Amphibian larvae have high plasticity in life history traits such as metamorphic age and size to cope with the varied aquatic environments. However, among the plentiful studies, few have investigated the effects of seasonal environmental changes. On the other hand, amphibian populations in diverse habitats or at different locations may face considerably distinct environmental variations. To better understand the life history responses of amphibian larvae to the spatial and temporal environmental variations, I used common garden experiments to investigate the metamorphic strategies of the rice paddy frog, <i>Fejervarya limnocharis</i>, tadpoles. I asked the following four questions: (1) Do <i>F. limnocharis</i> tadpoles living in two crops rice fields show cohort-dependent (spring vs. summer cohorts) metamorphic strategies in response to different temperature regimes in spring and summer? (2) How does food availability alter the responses of <i>F. limnocharis</i> spring and summer cohorts to seasonal temperature regimes? (3) Is cohort-dependent plasticity a common life-history strategy among different populations of <i>F. limnocharis</i> residing in two-crops rice fields? (4) Do <i>F. limnocharis</i> populations living in cultivated fields with different irrigation regimes exhibit differential cohort-dependent metamorphic strategies? I performed four experiments to tackle the four questions, respectively. In experiment 1, I collected spring (May) and summer (September) <i>F. limnocharis</i> tadpoles from Ankang rice fields. The tadpoles underwent a 2 x 2 temperature (22 vs. 29 ºC) by cohort (spring vs. summer cohort) factorial experiment. I found that both spring and summer tadpoles have higher growth rate when raised under their respective field temperatures. Moreover, spring and summer tadpoles used 'size advantage' and 'rate advantage' as cohort-dependent metamorphic strategies in response to seasonal temperatures, respectively. That is, spring tadpoles responded to low temperature with larger metamorphic weight, while the summer tadpoles did not. On the other hand, while both spring and summer tadpoles responded to high temperature with accelerating growth rates, summer tadpoles grew significantly faster than the spring ones. In the second experiment, I manipulated an additional factor: food availability (<i>ad libitum</i> vs. restricted) in a 2 x 2 x 2 temperature by food by cohort factorial experiment. The results were similar to those of experiment 1 when larvae were raised under <i>ad libitum</i> food. However, under restricted food, both spring and summer cohort reduced their metamorphic size. Spring tadpoles still had size advantage than summer ones at low temperature, whereas summer ones lost their 'rate advantage' at high temperature and had slower growth rates than spring ones. In the third experiment, I collected tadpoles from rice fields at three locations: northern (Ankang and Daxi) and central (Changhua) Taiwan to examine their similarity in cohort-dependent plasticity. The tadpoles underwent a 2 x 2 x 2 temperature by cohort by location factorial experiment. The results showed that all three populations had cohort-dependent plasticity. Inter-population variations exist: the most northern population, Ankang, showed the clearest cohort-dependent plasticity likely due to being at a strongly seasonal temperatures and largely isolated location. The other northern population, Daxi, displayed a moderate, while the central population, Changhua, showed the least cohort-dependent plasticity probably due to the reduced seasonality in temperature toward the south. In the fourth experiment, I collected tadpoles from rice paddies and water bamboo fields that differ in irrigation regimes (disrupted vs. constant irrigation). The tadpoles underwent a temperature by cohort by irrigation system factorial experiment. The results showed that tadpoles from disrupted irrigation populations had cohort-dependent metamorphic strategies, while tadpoles from constant irrigation populations did not. Overall, the four experiments reveal that <i>F. limnocharis</i> tadpoles have high plasticity, specifically strong cohort-dependence, in metamorphic traits. Spring and summer tadpoles used 'size advantage' and 'rate advantage' strategies, respectively in response to seasonal temperatures. Such strategies vary depending on food availability, geographic location, and irrigation system.