Predators balance consequences of climate-change induced habitat shifts for range-shifting and resident species

While many species distributions are shifting poleward or up in elevation in response to a changing climate, others are shifting their habitats along localized gradients in environmental conditions as abiotic conditions become more stressful. Whether species are moving across regional or local environmental gradients in response to climate change, range-shifting species become embedded in established communities of competitors and predators. The consequences of these shifts for both resident and shifting species are often unknown, as it can be difficult to isolate the effects of multiple species interactions. Using a model system of insects in high-elevation ponds in the Rocky Mountains of Colorado, we sought to disentangle the effects of predation and intraguild interactions on the survival and development of a semi-permanent pond resident caddisfly Limnephilus externus and the habitat-shifting caddis Asynarchus nigriculus that is being forced into semi-permanent ponds as temporary ponds dry too quickly to complete development. We conducted a manipulative in-situ pond cage experiment in which L. externus and A. nigriculus caddisfly larvae in single-species treatments and together were exposed to the presence/absence of predatory Dytiscus diving beetle larvae. This approach allowed us to isolate the effects of intraguild interactions and predation on the survival and development of both the resident and habitat-shifting species. We found that intraguild interactions had strong negative effects on the resident and habitat-shifting species. Intraguild interactions reduced the survival of the resident L. externus and increased the variation in survival of the shifting A. nigriculus. However, Dytiscus predators reduced these negative effects, stabilizing the community by increasing L. externus survival and reducing variation in A. nigriculus survival. We also found that intraguild interactions reduced L. externus biomass but resulted in increased A. nigriculus development. A. nigriculus development was also increased by predation. Our results show that strong intraguild interactions between resident and shifting species are likely to have negative consequences for both species. However, the presence of predators reduces these negative consequences of the habitat shift on both the resident and the shifting.

受气候变化影响，诸多物种的分布范围正向极地或高海拔区域迁移；另有一些物种则会沿环境条件的局地梯度调整栖息地，当非生物环境胁迫加剧时，这类物种更易发生此类栖息地转移。无论物种是沿区域还是局地环境梯度迁移以响应气候变化，范围转移物种（range-shifting species）都会嵌入到已形成的竞争者与捕食者群落中。此类迁移对本土物种与转移物种的影响往往尚不明确，因为难以分离多种物种互作的效应。我们以科罗拉多州落基山脉高海拔池塘中的昆虫类群为模式系统，试图厘清捕食作用与集团内互作（intraguild interactions）对本土半永久池塘栖息石蛾Limnephilus externus，以及因临时池塘干涸过快无法完成发育而被迫迁入半永久池塘的栖息地转移石蛾Asynarchus nigriculus的存活与发育的影响。我们开展了操控性原位池塘笼实验，设置单物种处理组与混合物种处理组，将两种石蛾幼虫分别暴露于有/无捕食性龙虱（Dytiscus）幼虫的环境中。该实验设计使我们能够分离集团内互作与捕食作用对本土物种与栖息地转移物种存活与发育的影响。研究发现，集团内互作对本土物种与栖息地转移物种均产生强烈的负面效应。集团内互作会降低本土石蛾Limnephilus externus的存活率，并增加栖息地转移石蛾Asynarchus nigriculus的存活变异度。但龙虱捕食者可削弱这类负面效应：通过提升本土Limnephilus externus的存活率、降低Asynarchus nigriculus的存活变异度，稳定了群落结构。我们还发现，集团内互作会降低Limnephilus externus的生物量，但会提升Asynarchus nigriculus的发育速率；而捕食作用同样可促进Asynarchus nigriculus的发育。研究结果表明，本土物种与栖息地转移物种间强烈的集团内互作，大概率会对双方均产生负面影响；但捕食者的存在可缓解栖息地转移对本土物种与转移物种带来的这类负面效应。