Data from: Urbanized landscapes favored by fig-eating birds increase invasive but not native juvenile strangler fig abundance

Propagule pressure can determine the success or failure of invasive plant range expansion. Range expansion takes place at large spatial scales, often encompassing many types of land cover, yet the effect of landscape context on propagule pressure remains largely unknown. Many studies have reported a positive correlation between invasive plant abundance and human land use; increased propagule pressure in these landscapes may be responsible for this correlation. We tested the hypothesis that increased rates of seed dispersal by fig-eating birds, which are more common in urban habitats, result in an increase in invasive strangler fig abundance in landscapes dominated by human land use. We quantified abundance of an invasive species (Ficus microcarpa) and a native species (F. aurea) of strangler fig in plots spanning the entire range of human land use in South Florida, USA, from urban parking lots to native forest. We then compared models that predicted juvenile fig abundance based on distance to adult fig seed sources and fig-eating bird habitat quality with models that lacked one or both of these terms. The best model for juvenile invasive fig abundance included both distance to adult and fig-eating bird habitat terms, suggesting that landscape effects on invasive fig abundance are mediated by seed-dispersing birds. In contrast, the best model for juvenile native fig abundance included only presence/absence of adults, suggesting that distance from individual adult trees may have less effect on seed limitation for a native species compared to an invasive species undergoing range expansion. However, models for both species included significant effects of adult seed sources, implying that juvenile abundance is limited by seed arrival. This result was corroborated by a seed addition experiment that indicated that both native and invasive strangler figs were strongly seed limited. Understanding how landscape context affects the mechanisms of plant invasion may lead to better management techniques. Our results suggest that prioritizing removal of adult trees in sites with high fig-eating bird habitat may be the most effective method to control F. microcarpa abundance.

繁殖体压力（propagule pressure）可决定入侵植物范围扩张的成败。范围扩张通常发生在大空间尺度上，往往涵盖多种土地覆盖类型，但景观背景对繁殖体压力的影响仍鲜为人知。诸多研究已报道入侵植物丰度与人类土地利用呈正相关；这类景观中升高的繁殖体压力或许正是该相关性的成因。我们验证了这一假说：在城市生境中更为常见的食无花果鸟类所介导的种子扩散速率提升，会导致人类土地利用主导的景观中入侵性绞杀榕（strangler fig）的丰度增加。我们在美国南佛罗里达州的样地中，对入侵性绞杀榕物种（小叶榕，Ficus microcarpa）与本土绞杀榕物种（F. aurea）的丰度进行了量化，样地覆盖了从城市停车场到原生林的全部人类土地利用类型梯度。随后，我们将基于成年榕种子源距离与食无花果鸟类生境质量预测幼榕丰度的模型，与缺失其中一项或两项指标的模型进行了对比。入侵性绞杀榕幼株丰度的最优模型同时纳入了成年个体距离与食无花果鸟类生境两项指标，这表明景观对入侵性绞杀榕丰度的影响是由种子扩散鸟类所介导的。与之相反，本土绞杀榕幼株丰度的最优模型仅纳入了成年个体的存在/缺失指标，这表明与正在经历范围扩张的入侵物种相比，单个成年树木的距离对本土物种的种子限制效应较弱。不过，两个物种的模型均显示成年种子源存在显著效应，这意味着幼株丰度受到种子到达量的限制。该结果得到了种子添加实验的佐证：实验表明本土与入侵性绞杀榕均受到强烈的种子限制。厘清景观背景如何影响植物入侵的机制，或可助力开发更高效的管理技术。我们的研究结果表明，在食无花果鸟类生境丰富的区域优先移除成年个体，或是控制小叶榕（Ficus microcarpa）丰度的最有效手段。