Plant-mediated community structure of spring-fed, coastal rivers

Quantifying ecosystem-level processes that drive community structure and function is key to the development of effective environmental restoration and management programs. To assess the effects of large-scale aquatic vegetation loss on fish and invertebrate communities in Florida estuaries, we quantified and compared the food webs of two adjacent spring-fed rivers that flow into the Gulf of Mexico. We constructed a food web model using field-based estimates of community absolute biomass and trophic interactions of a highly productive vegetated river, and modeled long-term simulations of vascular plant decline coupled with seasonal production of filamentous macroalgae. We then compared ecosystem model predictions to observed community structure of the second river that has undergone extensive vegetative habitat loss, including extirpation of several vascular plant species. Alternative models incorporating bottom-up regulation (decreased primary production resulting from plant loss) versus coupled top-down effects (compensatory predator search efficiency) were ranked by total absolute error of model predictions compared to the empirical community observations. Our best model for predicting community responses to vascular plant loss incorporated coupled effects of decreased primary production (bottom-up), increased prey search efficiency of large-bodied fishes at low vascular plant density (top-down), and decreased prey search efficiency of small-bodied fishes with increased biomass of filamentous macroalgae (bottom-up). The results of this study indicate that the loss of vascular plants from the coastal river ecosystem may alter the food web structure and result in a net decline in the biomass of fishes. These results are highly relevant to ongoing landscape-level restoration programs intended to improve aesthetics and ecosystem function of coastal spring-fed rivers by highlighting how the structure of these communities can be regulated both by resource availability and consumption. Restoration programs will need to acknowledge and incorporate both to be successful.量化驱动群落结构与功能的生态系统级过程，是制定高效环境修复与管理方案的核心基础。为评估大规模水生植被丧失对佛罗里达河口鱼类与无脊椎动物（invertebrate）群落的影响，我们对两条毗邻且汇入墨西哥湾的泉水补给型河流的食物网（food web）进行了量化对比分析。我们以高生产力植被覆盖河流的野外实测群落绝对生物量（absolute biomass）与营养相互作用（trophic interactions）数据为基础，构建了食物网模型，并模拟了维管植物（vascular plant）衰退结合丝状大型藻类（filamentous macroalgae）季节性生产力变化的长期动态。随后，我们将生态系统模型的预测结果与第二条河流的实测群落结构进行比对：该河流已发生大范围植被生境丧失，包括多个维管植物物种的局部灭绝。我们依据模型预测结果与实测群落观测数据的总绝对误差（total absolute error），对两类替代模型进行了排序：一类为上行调控（bottom-up regulation，即植物丧失导致初级生产力（primary production）下降），另一类为耦合下行效应（top-down effects，即捕食者搜索效率的补偿性变化）。本研究用于预测维管植物丧失后群落响应的最优模型，整合了三类调控效应：初级生产力下降的上行调控作用、低维管植物密度下大型鱼类猎物搜索效率提升的下行调控作用，以及丝状大型藻类生物量增加时小型鱼类猎物搜索效率下降的上行调控作用。研究结果表明，滨海泉水补给型河流生态系统中维管植物的丧失，可能改变食物网结构，并最终导致鱼类生物量出现净下降。上述结论高度契合当前正在推进的景观级修复计划：该计划旨在通过揭示这类群落如何同时受资源可获得性与捕食作用调控，提升滨海泉水补给型河流的景观美学价值与生态系统功能。若要取得成功，此类修复计划必须同时兼顾这两类调控机制并将其纳入方案设计。