資源品質和競爭對於四紋豆象資源利用策略的影響

Abstract

動物在環境中遭遇非連續分佈的資源時，如何利用以達到對個體存活或是後代數量的最大獲益是生態學上相當有趣的問題。本試驗以邊際價值理論 (marginal value theorem, MVT)為基礎，該理論描述個體會利用資源區直到收益降到低於環境平均收益時，就離開該資源區，使其適存值可達最佳值，並推測，當雌蟲遭遇品質較好的資源區會花費較長的停留時間。由於四紋豆象(Callosobruchus maculatus)能夠分辨寄主的品質 (大小、重量、數量、被寄生與否) 而調整產卵的行為，本試驗以四紋豆象為材料探討其資源利用的行為模式在遭遇不同資源品質時是否符合MVT的預測。試驗中提供豆象雌蟲不同品質的寄主資源，透過操作改變供試寄主的大小、數量及寄主上的卵數反應資源區的品質差異，測試及比較豆象在各資源區對資源利用的行為反應。以豆子大小及數量當成不同品質的資源區，結果顯示雌蟲產卵的行為符合預期，即，雌蟲在大豆區及多豆區皆停留較久。但是在以寄主豆上卵數為品質試驗的資源區，雌蟲停留的時間在多卵豆區 (品質差) 顯著較長，且產卵數顯著較乾淨豆區少，與預期結果不符。試驗中並觀察到部份雌蟲選擇較早放棄資源區，而部分雌蟲則會停留相當長的時間，推測豆象面臨已被高度利用的資源，個體間可能表現不同的資源利用策略。更進一步探討有競爭者存在時，對於雌蟲的資源利用策略有什麼樣的影響。結果顯示，競爭者的存在，顯著延長了雌蟲停留在資源區的時間，試驗中也觀察到部份雌蟲選擇較早放棄資源區，而部分雌蟲則會停留相當長的時間。由試驗結果推測，同種的卵可能代表著非直接的競爭者，在競爭者的壓力下，族群中的個體展現兩種不同時間分配策略，而不同的時間分配策略各有其適應上意義，較早離開的雌蟲，選擇放棄資源區尋找新的資源，而停留較久的雌蟲可能有其他的策略，如殺卵等，有待進一步釐清。

The marginal value theorem (MVT) predicts that a single forager should exploit resource until their instantaneous rate of fitness gain drops below the mean of expected rate in the whole environment. In other words, the higher the quality of resource patch, the longer an individual should remain on it. We tested this prediction in the seed beetle Callosobruchus maculatus, whose female can adjust its oviposition behaviors based on host qualities, such as size, number and egg density. We manipulated different host quality patch to investigate the behavior strategies of the seed beetles. Our results showed that the performance of female seed beetles matched the MVT’s prediction under treatments of host size and number. Surprisingly, when treated them with already-parasitized hosts, females performed binomonal behaviors. Some members of population tended to leave early, whereas the others tended to remain on the resource for longer time, further exploit the resource. The MVT assumes the condition without competitors, but animal do not always forage alone. Some theoretical model predicts that forager should stay in the patch longer than when competitors are present. Hence, we investigated the effect of direct competition on resource exploitation strategies. The effect of direct competition was similar to the treatment of already-parasitized hosts. It revealed that the egg density is not only serving as a host qualities indicator, but also represents the existence of competition, which causes the polymorphism of resource exploitation in the seed beetle. The adaptive value of different strategies will be discussed.