Can heat waves change the trophic role of the world’s most invasive crayfish? Diet shifts in Procambarus clarkii

In the Mediterranean basin, the globally increasing temperatures are expected to be accompanied by longer heat waves. Commonly assumed to benefit cold-limited invasive alien species, these climatic changes may also change their feeding preferences, especially in the case of omnivorous ectotherms. We investigated heat wave effects on diet choice, growth and energy reserves in the invasive red swamp crayfish, Procambarus clarkii. In laboratory experiments, we fed juvenile and adult crayfish on animal, plant or mixed diets and exposed them to a short or a long heat wave. We then measured crayfish survival, growth, body reserves and Fulton’s condition index. Diet choices of the crayfish maintained on the mixed diet were estimated using stable isotopes (13C and 15N). The results suggest a decreased efficiency of carnivorous diets at higher temperatures, as juveniles fed on the animal diet were unable to maintain high growth rates in the long heat wave; and a decreased efficiency of herbivorous diets at lower temperatures, as juveniles in the cold accumulated less body reserves when fed on the plant diet. Heat wave treatments increased the assimilation of plant material, especially in juveniles, allowing them to sustain high growth rates in the long heat wave. Contrary to our expectations, crayfish performance decreased in the long heat wave, suggesting that Mediterranean summer heat waves may have negative effects on P. clarkii and that they are unlikely to boost its populations in this region. Although uncertain, it is possible that the greater assimilation of the plant diet resulted from changes in crayfish feeding preferences, raising the hypotheses that i) heat waves may change the predominant impacts of this keystone species and ii) that by altering species’ trophic niches, climate change may alter the main impacts of invasive alien species.

全球变暖背景下，地中海盆地预计将迎来更长时长的热浪事件。这类气候变化通常被认为可惠及受低温限制的外来入侵物种，但同时也可能改变其取食偏好，对于杂食性外温动物而言尤为显著。本研究以外来入侵物种红色沼泽螯虾（Procambarus clarkii）为研究对象，探究热浪对其食谱选择、生长状况与能量储备的影响。在室内受控实验中，我们分别为幼虾与成虾投喂动物性饵料、植物性饵料或混合饵料，并将受试个体暴露于短时热浪或长时热浪环境中。随后我们测定了螯虾的存活率、生长情况、体内储备物质及富尔顿条件指数（Fulton’s condition index）。对于投喂混合饵料的个体，我们通过稳定同位素（13C与15N）分析来推断其食谱选择。研究结果表明：高温环境下动物性饵料的利用效率出现下降——投喂动物性饵料的幼虾在长时热浪中无法维持较高的生长速率；而低温环境下植物性饵料的利用效率同样降低，低温环境中投喂植物性饵料的幼虾体内储备物质积累量更少。热浪处理提升了螯虾对植物性饵料的同化效率，尤以幼虾群体更为突出，这使得它们在长时热浪中仍能维持较高的生长速率。与我们的初始预期相悖，长时热浪环境下螯虾的整体生存表现出现下降，这提示地中海夏季热浪可能对红色沼泽螯虾产生负面影响，且难以推动该区域的种群扩张。尽管目前尚无定论，但螯虾对植物性饵料同化效率的提升可能源于其取食偏好的改变，据此我们提出两项假说：其一，热浪可能改变该关键物种的主导生态影响；其二，气候变化通过重塑物种的营养生态位，进而改变外来入侵物种的主要生态效应。