艾氏樹蛙大蝌蚪數量效應對小蝌蚪的影響及其競爭模式

Abstract

艾氏樹蛙(Chirixalus eiffingeri)具有樹棲性蝌蚪(arboreal tadpole)、母蛙回巢餵卵、蝌蚪義務性食卵(obligatorily oophagous)、單季多產及生殖巢重覆利用(nest reused)等特殊習性，因而造成一生殖巢中前後兩批蝌蚪共存的現象。本實驗旨在探討艾氏樹蛙不同發育時期(stage)的蝌蚪共存在一個小樹棲水池中的種內競爭情形及其競爭模式。實驗在溪頭的竹林中進行，共分為兩部分。 第一部分主要探討兩批大小不同的蝌蚪共存在同一個棲所且資源（卵）頗具獨占性時所造成的競爭現象。實驗方法是將野外收集的蝌蚪依照體長分為大小兩群；再讓10隻小蝌蚪分別與0、10、20隻大蝌科共存，以觀察大蝌蚪數量改變對小蝌蚪的影響。另外，為確認其效應為“大”蝌蚪所造成，另設20隻小蝌科及30隻小蝌科二處理作為對照，以消去蝌蚪總數不同造成之影響。每個竹筒為一個實驗單位，共重覆11?12次。記錄大小蝌蚪每週的存活率及每個竹筒的蝌蚪變態率，並用相機拍攝蝌蚪以記錄體長變化。每個竹筒皆追蹤記錄至所有小蝌蚪變態或死亡為止。實驗設置的有效竹筒共52個，其中有蝌蚪變態的有35個，總竹筒成功率為67.3％。有大蝌蚪存在時，小蝌蚪的存活率、變態率均明顯下降，變態時間也明顯延後，顯示大蝌蚪的存在確實對小蝌蚪有明顯抑制作用。而當大蝌蚪數量增加時，小蝌科第5週以後的存活率差異逐漸變大且最後平均變態率明顯下降。結果顯示，當大蝌蚪存在時，小蝌蚪成長與發展受到抑制，可能是因為小蝌蚪很難取得食物，需待大部分大蝌蚪變態跳走後才可能吃到卵而開始長大，因此蝌蚪變態率會降低而蝌蚪期被拉長。當起始大蝌蚪數到達20隻時，小蝌科無法長大而不久後全部死亡。從此結果可以推論當兩批蝌蚪的重疊過大時，母蛙每次生殖的投資效益會大幅下降，但生殖巢的重覆利用與兩批子代適當的重疊可以增加每個生殖季中的總產子數。 第二部分探討大蝌科與小蝌蚪間的競爭方式。同樣先將蝌蚪分為大、小二類，實驗分為對照及實驗組，二組分別放入10隻小蝌蚪，在實驗組另放入內關有5隻大蝌蚪的有孔PP管，使管內外的水可以互相流通，但大蝌蚪無法與小蝌科競爭母蛙回來餵食的卵，對照組則只放入空的PP管。兩組共設置16個重覆。實驗設置完後每週記錄存活數和照相記錄體長至第4週。結果顯示兩組間的存活數和體重變化均無顯著不同。結果顯示化學幹擾性競爭(chemical interference competition)應不存在或是影響很小；換言之，野外艾氏樹珪共存大蝌蚪對小蝌蚪的抑制主要可能是以對食卵的利用性競爭(exploitative competition)為主，而造成利用性競爭的可能原因則是大小蝌蚪競爭能力的差異和單一食物（卵）的在空間和時間上的高度集中。

I studied the dynamic relationship between large and small Chirixalus eiffingeri tadpoles in the bamboo forests in Chitou from 1997-1998. The first experiment aimed to assess the numerical effects of large tadpoles on the small tadpoles. Tadpoles were categorized into small (ST) and large tadpoles (LT) according to their length and Gosner stages. Each treatment contained 10 ST and varied LT (none added, 10 added, and 20 added). Two additional treatments were set (20 ST and 30 ST, with no LT) to rectify the effects of the total number of tadpoles. Each treatment consisted of 11-12 replicates (or experimental units), and each bamboo stump is an experimental unit. The number and total length of tadpoles were monitored until all tadpoles had left the stump or died. The presence of LT significantly reduced the survivorship, growth, and metamorphic rate of ST. The increase of the number of LT significantly affected the survivorship and growth of ST. No small tadpoles in the stumps with 20 LT reached metamorphosis. These results suggest that LT were able to out-compete ST for eggs, and ST that obtained eggs only occasionally, or not at all, grew slowly. As their body size fell behind, ST took longer to reach metamorphosis. Prolonged starvation even resulted in higher mortality and a low metamorphic rate. The second experiment aimed to assess the presence of interference competition in Chirixalus eiffingeri tadpoles. A holed PP tube with 5 LT inside was inserted into a bamboo stump which contained 10 ST (designated as the experimental group), while the blank contained no LT (designated as the control group). The experimental and control groups each consisted of 16 replicates (i.e., bamboo stumps). I monitored the growth and survivorship of ST for 4 weeks. There was no significant difference in growth and survivorship of tadpoles between the two treatments throughout the study period, suggesting that interference competition was either lacking or negligible in the natural conditions. In other words, the exploitative competition is probably the more crucial element of the competition between two clutches of C.eiffingeri tadpoles.