McNatty Alice, Abbott Kirsti L, Lester Philip J
School of Biological Science, Victoria University of Wellington, Wellington, New Zealand.
Oecologia. 2009 May;160(1):187-94. doi: 10.1007/s00442-009-1279-z. Epub 2009 Feb 12.
Invasive species can dramatically alter trophic interactions. Predation is the predominant trophic interaction generally considered to be responsible for ecological change after invasion. In contrast, how frequently competition from invasive species contributes to the decline of native species remains controversial. Here, we demonstrate how the trophic ecology of the remote atoll nation of Tokelau is changing due to competition between invasive ants (Anoplolepis gracilipes) and native terrestrial hermit crabs (Coenobita spp.) for carrion. A significant negative correlation was observed between A. gracilipes and hermit crab abundance. On islands with A. gracilipes, crabs were generally restricted to the periphery of invaded islands. Very few hermit crabs were found in central areas of these islands where A. gracilipes abundances were highest. Ant exclusion experiments demonstrated that changes in the abundance and distribution of hermit crabs on Tokelau are a result of competition. The ants did not kill the hermit crabs. Rather, when highly abundant, A. gracilipes attacked crabs by spraying acid and drove crabs away from carrion resources. Analysis of naturally occurring N and C isotopes suggests that the ants are effectively lowering the trophic level of crabs. According to delta(15) N values, hermit crabs have a relatively high trophic level on islands where A. gracilipes have not invaded. In contrast, where these ants have invaded we observed a significant decrease in delta(15) N for all crab species. This result concurs with our experiment in suggesting long-term exclusion from carrion resources, driving co-occurring crabs towards a more herbivorous diet. Changes in hermit crab abundance or distribution may have major ramifications for the stability of plant communities. Because A. gracilipes have invaded many tropical islands where the predominant scavengers are hermit crabs, we consider that their competitive effects are likely to be more prominent in structuring communities than predation.
入侵物种能够极大地改变营养级相互作用。捕食是通常被认为在入侵后导致生态变化的主要营养级相互作用。相比之下，入侵物种的竞争对本地物种数量下降的影响频率仍存在争议。在此，我们展示了托克劳这个偏远环礁国家的营养生态学是如何因入侵蚂蚁（长足捷蚁）与本地陆生寄居蟹（椰子蟹属物种）对腐肉的竞争而发生变化的。观察到长足捷蚁和寄居蟹数量之间存在显著的负相关。在有长足捷蚁的岛屿上，螃蟹通常被限制在入侵岛屿的周边。在这些岛屿上长足捷蚁数量最多的中心区域，很少能发现寄居蟹。蚂蚁排除实验表明，托克劳寄居蟹数量和分布的变化是竞争的结果。蚂蚁并没有杀死寄居蟹。相反，当长足捷蚁数量众多时，它们会通过喷射酸液攻击螃蟹，并将螃蟹驱赶至远离腐肉资源的地方。对自然存在的氮和碳同位素的分析表明，这些蚂蚁正在有效地降低螃蟹的营养级。根据δ¹⁵N值，在长足捷蚁未入侵的岛屿上，寄居蟹具有相对较高的营养级。相比之下，在这些蚂蚁入侵的地方，我们观察到所有蟹类的δ¹⁵N都显著下降。这一结果与我们的实验结果一致，表明长期被排除在腐肉资源之外，促使共存的螃蟹转向更食草的饮食。寄居蟹数量或分布的变化可能对植物群落的稳定性产生重大影响。由于长足捷蚁已入侵许多热带岛屿，而这些岛屿上主要的食腐动物是寄居蟹，我们认为它们的竞争效应在构建群落方面可能比捕食更为突出。
