Wetland plant community variation across replicate urban to rural gradients: non-native species as both drivers and passengers in systems impacted by anthropogenic land-use

Anthropogenic land-use change impacts ecological communities in urban and rural landscapes, and wetlands are particularly vulnerable despite the valuable ecosystem services they provide. Urbanized non-wetland systems are often enriched in non-native plant species, and similar patterns in wetlands would have implications for ecosystem function and biodiversity. We evaluated landscape-scale patterns of plant community diversity across gradients of rural to urban land-use, testing whether diversity was related to environmental conditions indicative of surrounding land-use. We surveyed vegetation and collected soil samples from 45 wetlands throughout Ohio, USA. Sites were categorized based on surrounding land-use as intense urban, moderate urban, or rural, representing 15 replicate urban to rural gradients. Non-native richness was 56% greater and non-native relative abundance 74% greater in intense urban sites compared to rural sites. Structural equation modeling indicated that high non-native relative abundance caused reductions in native plant richness but not native Shannon diversity, which was instead related to high concentrations of urban-associated soil contaminants such as cadmium and sodium. Our results support both versions of the driver-passenger model of invasion impacts, depending on the response: native richness is directly limited by competition with non-native species (the driver model), while native diversity is limited more by urban-associated stressors that also affect non-natives (the passenger model). The few wetlands remaining in highly urban areas thus experience a range of constraints affecting multiple dimensions of wetland health. We argue it is in these sites specifically where the benefits of restoring wetland ecosystems will be maximized.

人为土地利用变化会对城市与乡村景观中的生态群落造成影响，尽管湿地能够提供诸多宝贵的生态系统服务（ecosystem services），但其本身却尤为脆弱。城市化的非湿地生态系统通常会富集外来植物物种，若湿地呈现类似格局，将对生态系统功能（ecosystem function）与生物多样性（biodiversity）产生显著影响。本研究针对乡村到城市的土地利用梯度，分析了景观尺度下的植物群落多样性格局，旨在探究多样性是否与反映周边土地利用状况的环境条件相关。我们在美国俄亥俄州（Ohio）全境的45处湿地开展了植被调查并采集了土壤样本。研究位点依据周边土地利用类型被划分为高强度城市、中等强度城市以及乡村三类，涵盖了15组重复的城乡梯度样带。与乡村位点相比，高强度城市位点的外来物种丰富度提升了56%，外来物种相对多度则提升了74%。结构方程模型（Structural equation modeling）分析结果显示，较高的外来物种相对多度会降低本土植物丰富度，但对本土香农多样性（Shannon diversity）并无显著影响；而本土香农多样性反而与城市相关的土壤污染物（如镉（cadmium）与钠（sodium））的高浓度密切相关。本研究结果支持入侵影响的两种‘驱动者-乘客模型（driver-passenger model）’版本，具体取决于响应指标：本土植物丰富度直接受到外来物种竞争的限制（即驱动者模型），而本土多样性则更多受到同时会影响外来物种的城市相关胁迫因子的制约（即乘客模型）。因此，留存于高强度城市化区域的少量湿地，面临着一系列影响湿地健康多维度特征的限制因素。我们认为，恰恰是在这类位点中，湿地生态系统修复所能带来的效益能够实现最大化。