Stable isotope analysis (SIA) of modern and historical samples to investigate the potential influence of urbanization on carbon and nitrogen isotope rations in murid rodents

The study used stable isotope analysis (SIA) of modern and historical samples to: 1) investigate the potential influence of natural history traits and environmental factors that also included metrics of urbanization on carbon (δ13C) and nitrogen (δ15N) isotope ratios in murid rodents from Gauteng Province, South Africa; and 2) evaluate their potential trophic ecological shifts due to urbanization over a historical time scale. SIA was conducted on biologically inert fur samples of modern and historical samples (i.e., museum-preserved samples collected over a time scale) of three invasive (Rattus rattus, R. norvegicus and R. tanezumi) and three indigenous (Aethomys ineptus, Mastomys coucha and Otomys angoniensis) murid rodents. Generalized linear models (GLM), analysis of variance (ANOVA) and Tukey’s post-hoc analyses showed that sex, age, and sampling year had minimal impact on δ13C and δ15N isotope ratios, but not sampling locality and season. These results were supported by the analysis of percentage trophic overlaps based on sample size-corrected standard ellipse areas (SEAc). Collectively, these results suggest that in the last century, short-term seasonal fluctuations accounted for a larger proportion of isotopic variation than long-term environmental changes. The results also showed that qualitative categorical land use data as a proxy for urbanization did not adequately account for changes in rodent isotope ratios. Consequently, an attempt was made to identify quantifiable potential sources of short-term variation in δ13C and δ15N stable isotope ratios in these rodents. The identified quantitative variables included monthly median temperature and monthly total rainfall as short-term metrics of environmental change and Normalized Difference Vegetation Index (NDVI) as a metric for vegetation cover and impervious surface area (ISA), both as proxies for urbanization over a historical time scale. The analyses found that both metrics of short-term environmental factors and proxies of urbanization had no impact on δ13C ratios in any of the three invasive and three indigenous murid rodent species investigated, while δ15N ratios of the insectivorous indigenous A. ineptus and M. coucha were found to be influenced by monthly median temperature and impervious surface area. Overall, the largest %SSQs were due to error (i.e., residual), suggesting that apart from the metrics analysed and the interaction between them, there may be other unexplained components that are also responsible for the variation. These results suggest that the murid rodent populations investigated may be shielded from the negative impacts of short-term environmental factors and urbanization through the availability of excess food resources in one of the most urbanized landscapes in South Africa. These results also highlight the potential impact of urban landscapes on the trophic ecology of invasive and indigenous murid rodents and their negative impacts on the conservation of other threatened species within urban settings.

本研究运用稳定同位素分析（SIA）对现代与历史样本进行了研究，旨在：1）探讨自然历史特征及环境因素（包括城市化指标）对南非豪登省鼠类（啮齿类动物）碳（δ13C）和氮（δ15N）同位素比率的潜在影响；2）评估城市化在历史时间尺度上对其潜在的营养生态位变迁的影响。SIA对现代和历史样本（即，在时间尺度上收集的博物馆保存样本）中三种入侵性（Rattus rattus、R. norvegicus 和 R. tanezumi）和三种土著（Aethomys ineptus、Mastomys coucha 和 Otomys angoniensis）鼠类的生物惰性毛样进行了分析。广义线性模型（GLM）、方差分析（ANOVA）和Tukey事后检验显示，性别、年龄和采样年份对δ13C和δ15N同位素比率的影响最小，但采样地点和季节则不然。这些结果得到了基于样本量校正的标准椭圆面积（SEAc）的百分营养重叠分析的支持。总体而言，这些结果暗示，在上个世纪，短期季节性波动在同位素变化中占比更大，超过了长期环境变化。结果还显示，以定性土地使用数据作为城市化的代理指标并不能充分解释啮齿类动物同位素比率的改变。因此，研究试图识别δ13C和δ15N稳定同位素比率短期变化的量化潜在来源。识别的量化变量包括月均温度和月总降水量作为环境变化的短期指标，以及归一化植被指数（NDVI）作为植被覆盖和不可渗透地表面积（ISA）的指标，均作为城市化历史时间尺度的代理指标。分析发现，短期环境因素指标和城市化代理指标对所研究的三个入侵性和三个土著鼠类的δ13C比率均无影响，而食虫的土著A. ineptus和M. coucha的δ15N比率被发现受到月均温度和不可渗透地表面积的影响。总的来说，最大的百分平方和（%SSQs）归因于误差（即，残差），这表明除了所分析的指标及其相互作用之外，可能还存在其他未解释的成分，也是导致变异的原因。这些结果提示，所研究的鼠类种群可能通过在南非城市化程度最高的景观中获取丰富的食物资源而免受短期环境因素和城市化的负面影响。这些结果还凸显了城市景观对入侵性和土著鼠类的营养生态学的影响，及其对城市环境中其他受威胁物种保护的消极影响。