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. 入侵外来物种可通过捕食、资源种间竞争、疾病介导的竞争等多种过程，导致本土物种走向灭绝。脊椎动物体内应激激素水平的升高，可能与上述过程存在关联，并进一步促成本土物种的存活与繁殖能力衰退。
2. 北美入侵性灰松鼠（Sciurus carolinensis）被引入后，欧亚红松鼠（Sciurus vulgaris）已在不列颠群岛大部分区域以及意大利北部部分地区绝迹。本研究从粪便样本中提取糖皮质激素代谢物（glucocorticoid metabolites），以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上升。
3. 本研究发现，在7个与入侵性灰松鼠共存的本土红松鼠栖息地中，其糖皮质激素浓度是5个无入侵物种栖息地的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上升。当我们通过实验手段降低入侵性灰松鼠的种群数量后，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时期间出现了显著下降。
4. 因此，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水平。
5. 鉴于糖皮质激素的持续升高会降低个体生长速率与繁殖率，本研究结果与既往研究结论一致：入侵性灰松鼠的共存会导致红松鼠体型更小、繁殖输出更低。