Spiders in a Desert City: What the Behavior and Microclimate of Western Black Widows Can Teach Us About the Impacts of Urbanization

With the planet rapidly urbanizing, understanding the ecological effects of urbanization is a grand challenge for modern biology. For example, increased city temperatures known as the urban heat island effect, disproportionately impact nocturnal taxa and this consideration is widely overlooked. Slight shifts in the thermal microclimate have a cascade of ramifications that directly impact species density and distribution. Animal behavior is a trait that may explain why some species thrive after urbanization when others go locally extinct. In this study we followed 22 adult females of the western black widow, Latrodectus hesperus, from both urban and undisturbed Sonoran Desert habitats. We began looking for differences between urban and desert spiders under field conditions: boldness, voracity, web size and body condition. Both urban and desert spiders were then brought to the laboratory to see how their behavior changed. We found no behavioral differences between urban and desert spiders in the field or the laboratory. We did find that spider behavior differed between the field and the laboratory. Specifically, boldness in the laboratory was significantly lower compared to the field. Voracity was more repeatable in the laboratory versus the field, and boldness was strongly positively correlated with voracity in the laboratory, but not in the field. These behavioral shifts from the field to the laboratory favor the conclusion that black widow behavior is highly plastic and context dependent. Lastly, we monitored web temperature of black widow microhabitat continuously for an entire year using thermochron data loggers. We found microhabitat temperatures differences between urban and desert sites were greatest at night and absent during the daytime. We uncovered a seasonal effect with the highest magnitude temperature difference occurring during the springtime. Additionally, behavior was significantly correlated with field temperatures; the boldest spiders come from the warmest webs. However, we found little evidence that temperature predicts spider body condition or voracity, and body condition does not predict its behavioral expression. Our results highlight the importance of studying animal behavior to increase understanding of the factors that shape distribution and density in a lethal pest species.

随着全球城市化进程加速，解析城市化的生态效应已成为现代生物学面临的重大挑战。例如，被称为城市热岛效应（urban heat island effect）的城市气温升高现象，对夜行性类群造成了不成比例的影响，但这一点却被广泛忽视。热微环境的微小变化会引发一系列连锁反应，直接影响物种的密度与分布格局。动物行为作为一种性状，或许可以解释为何部分物种在城市化进程中得以存续繁衍，而另一些则在当地走向灭绝。在本研究中，我们对来自城市与未受干扰的索诺兰沙漠栖息地的22只成年雌性西部黑寡妇蜘蛛（Latrodectus hesperus）进行了追踪。我们首先在野外条件下观测两类蜘蛛的差异指标，包括大胆性、贪食性、网幕尺寸与身体状况。随后将城市与沙漠生境的蜘蛛均带回实验室，以观测其行为是否发生改变。研究结果显示，无论是野外还是实验室环境中，两类蜘蛛均未表现出行为差异。但我们确实发现，蜘蛛的行为在野外与实验室环境之间存在显著差异。具体而言，实验室环境下的大胆性评分显著低于野外环境；贪食性在实验室环境中的重复性显著高于野外；且实验室环境下大胆性与贪食性呈显著正相关，而野外环境中二者并无相关关系。上述从野外到实验室的行为转变，佐证了黑寡妇蜘蛛的行为具有高度可塑性且依赖于环境背景的结论。最后，我们使用热敏数据记录仪（thermochron data loggers）对黑寡妇蜘蛛微生境的网幕温度进行了一整年的连续监测。研究发现，城市与沙漠生境的微生境温度差异在夜间最为显著，日间则无明显差异。此外还存在季节效应，温度差异的峰值出现在春季。另外，蜘蛛行为与野外环境温度呈显著相关：最大胆的个体均来自温度最高的网幕。但我们几乎未发现温度可预测蜘蛛身体状况或贪食性的证据，且身体状况同样无法预测其行为表现。本研究结果凸显了开展动物行为研究的重要性，有助于加深我们对这种致命害虫物种的分布与密度影响因素的认知。