Crossland Michael R, Alford Ross A, Shine Richard
School of Biological Sciences A08, University of Sydney, Sydney, NSW, 2006, Australia.
Oecologia. 2009 Jan;158(4):625-32. doi: 10.1007/s00442-008-1167-y. Epub 2008 Oct 14.
Invasive species are widely viewed as unmitigated ecological catastrophes, but the reality is more complex. Theoretically, invasive species could have negligible or even positive effects if they sufficiently reduce the intensity of processes regulating native populations. Understanding such mechanisms is crucial to predicting ultimate ecological impacts. We used a mesocosm experiment to quantify the impact of eggs and larvae of the introduced cane toad (Bufo marinus) on fitness-related traits (number, size and time of emergence of metamorphs) of a native Australian frog species (Opisthodon ornatus). The results depended upon the timing of oviposition of the two taxa, and hence the life-history stages that came into contact. Growth and survival of O. ornatus tadpoles were enhanced when they preceded B. marinus tadpoles into ponds, and reduced when they followed B. marinus tadpoles into ponds, relative to when tadpoles of both species were added to ponds simultaneously. The dominant tadpole-tadpole interaction is competition, and the results are consistent with competitive priority effects. However, these priority effects were reduced or reversed when O. ornatus tadpoles encountered B. marinus eggs. Predation on toxic toad eggs reduced the survival of O. ornatus and B. marinus. The consequent reduction in tadpole densities allowed the remaining O. ornatus tadpoles to grow more rapidly and to metamorphose at larger body sizes (>60% disparity in mean mass). Thus, exposure to B. marinus eggs reduced the number of O. ornatus metamorphs, but increased their body sizes. If the increased size at metamorphosis more than compensates for the reduced survival, the effective reproductive output of native anurans may be increased rather than decreased by the invasive toad. Minor interspecific differences in the seasonal timing of oviposition thus have the potential to massively alter the impact of invasive cane toads on native anurans.
入侵物种通常被视为纯粹的生态灾难，但实际情况更为复杂。从理论上讲，如果入侵物种能够充分降低调节本地种群数量的过程强度，它们可能产生微不足道甚至积极的影响。了解这些机制对于预测最终的生态影响至关重要。我们进行了一项中宇宙实验，以量化引入的甘蔗蟾蜍（海蟾蜍，Bufo marinus）的卵和幼体对澳大利亚本土蛙类物种（饰纹姬蛙，Opisthodon ornatus）与适应度相关性状（变态幼体的数量、大小和出现时间）的影响。结果取决于两个物种产卵的时间，进而取决于相互接触的生活史阶段。相对于两种蝌蚪同时放入池塘的情况，当饰纹姬蛙蝌蚪先于海蟾蜍蝌蚪进入池塘时，其生长和存活得到增强；而当它们跟随海蟾蜍蝌蚪进入池塘时，则受到抑制。蝌蚪之间的主要相互作用是竞争，结果与竞争优先级效应一致。然而，当饰纹姬蛙蝌蚪遇到海蟾蜍卵时，这些优先级效应会减弱或逆转。对有毒蟾蜍卵的捕食降低了饰纹姬蛙和海蟾蜍的存活率。蝌蚪密度的相应降低使剩余的饰纹姬蛙蝌蚪生长得更快，并以更大的体型变态（平均质量差异超过60%）。因此，接触海蟾蜍卵减少了饰纹姬蛙变态幼体的数量，但增加了它们的体型。如果变态时增加的体型足以弥补存活率的降低，那么入侵蟾蜍可能会增加而非减少本土无尾两栖动物的有效繁殖输出。因此，产卵季节时间上的微小种间差异有可能极大地改变入侵甘蔗蟾蜍对本土无尾两栖动物的影响。
