Data from: Stress in biological invasions: introduced invasive grey squirrels increase physiological stress in native Eurasian red squirrels

1. Invasive alien species can cause extinction of native species through processes including predation, interspecific competition for resources, or disease-mediated competition. Increases in stress hormones in vertebrates may be associated with these processes and contribute to the decline in survival or reproduction of the native species.
2. Eurasian red squirrels (Sciurus vulgaris) have gone extinct across much of the British Isles and parts of Northern Italy following the introduction of North American invasive grey squirrels (Sciurus carolinensis). We extracted glucocorticoid metabolites from faecal samples to measure whether the presence of the invasive species causes an increase in physiological stress in individuals of the native species.
3. We show that native red squirrels in seven sites where they co-occurred with invasive grey squirrels had glucocorticoid concentrations that were three times higher than those in five sites without the invasive species. Moreover, in a longitudinal study, stress hormones in native red squirrels increased after colonisation by grey squirrels. When we experimentally reduced the abundance of the invasive grey squirrels, the concentration of faecal glucocorticoid metabolites in co-occurring red squirrels decreased significantly between pre- and post-removal periods.
4. Hence, we found that the invasive species acts as a stressor which significantly increases the concentrations of glucocorticoids in the native species.
5. Given that sustained elevations in glucocorticoids could reduce body growth and reproductive rate, our results are consistent with previous studies where the co-occurrence of the invasive grey squirrel was associated with smaller size and lower reproductive output in red squirrels.

1. 外来入侵物种可通过捕食、资源种间竞争或疾病介导的竞争等过程，造成本土物种的灭绝。脊椎动物体内应激激素（stress hormones）水平升高可能与上述过程相关，并会加剧本土物种的存活能力与繁殖成功率下降。
2. 欧亚红松鼠（Sciurus vulgaris）在北美入侵性灰松鼠（Sciurus carolinensis）被引入后，已在不列颠群岛大部分区域以及意大利北部部分地区区域性灭绝。本研究通过提取粪便样本（faecal samples）中的糖皮质激素代谢物（glucocorticoid metabolites），检测入侵物种的存在是否会引发本土红松鼠个体的生理应激水平升高。
3. 研究结果表明，在7个与入侵性灰松鼠共存的采样位点中，本土红松鼠的糖皮质激素浓度是5个无入侵物种共存位点的3倍。此外，在一项纵向研究（longitudinal study）中，本土红松鼠的应激激素水平在灰松鼠定居后出现显著上升。当我们通过实验降低入侵性灰松鼠的种群丰度后，共存红松鼠的粪便糖皮质激素代谢物浓度在移除前后两个阶段间出现了明显下降。
4. 综上，本研究证实入侵物种可作为一种应激源，显著提升本土物种体内的糖皮质激素（glucocorticoids）浓度。
5. 鉴于糖皮质激素（glucocorticoids）的持续升高会降低个体的身体生长速率与繁殖率，本研究结果与既往研究结论一致——即入侵灰松鼠的共存会导致红松鼠体型更小、繁殖输出更低。