Data_Sheet_1_Long-Term Habitat Degradation Drives Neotropical Macrophyte Species Loss While Assisting the Spread of Invasive Plant Species.docx

The spread of invasive macrophyte species is a pressing threat to neotropical shallow lakes. Yet there are few studies addressing the full extent of biotic and abiotic changes that may occur in response to invasive species. Less is known of how other human-induced stressors such as eutrophication and lake draining may interact over time with invasive macrophytes to influence biodiversity. We combined limnological observations with paleoecological data from Fúquene Lake, Colombia, a eutrophic neotropical shallow lake, to provide information on the current and long-term (decades-centuries) dynamics of the spread of two well-established invasive plants Eichhornia crassipes and Egeria densa. We found a unique in macrophyte species composition in areas currently dominated by Egeria and Eichhornia. Eichhornia-dominated areas had 14 macrophyte species, turbid (secchi=19 ± 6 cm) and poorly oxygenated (3.94 ± 2.61 ppm) waters whereas Egeria-rich areas supported 5 species and had clearer (secchi=51 ± 12 cm) and better-oxygenated (6.06 ± 2.4 ppm) waters. Historical macrophyte community shifts were linked to eutrophication and lake level variation and characterized by the loss of charophytes and bryophytes before 1500 CE and subsequent reductions in Nymphaea sp., Potamogeton illinoensis and Najas guadalupensis in the early 1900s (lake draining). Eichhornia crassipes (since 1500 CE) and E. densa (early 1900s) occurred well before proposed dates of introduction (1950s and 1990 respectively). Both species have rapidly expanded since the 1990s along with Azolla filiculoides in response to an inflow water diversion scheme and heavy nutrient loads. Our results suggest that the spread of Eichhornia and Egeria was not responsible for native macrophyte species loss, but that their current dominance is exerting synergistic and antagonistic secondary effects on plant assemblages through habitat modification, competitive exclusion and promotion of habitat heterogeneity across the lake. It could therefore be misleading to suggest that invasive plants causes macrophyte species loss in degraded lakes. We suggest that aggressive species like Eichhornia, Azolla, and Egeria require hydrologically stable and eutrophic environments to spread; thus, management actions should focus on controlling these two factors. Our study demonstrates the need to use a long-term approach to fully-understand the effects of invasive macrophytes.

入侵性大型水生植物（macrophyte）的扩散，对新热带区浅水湖泊构成了严峻威胁。然而，目前鲜有研究探讨入侵物种可能引发的生物与非生物变化的全维度范围。此外，对于富营养化、湖泊疏干等其他人为胁迫因子，如何随时间推移与入侵大型水生植物相互作用并影响生物多样性，学界认知仍较为匮乏。我们结合湖沼学观测数据与古生态学数据，以研究富营养化新热带区浅水湖泊——哥伦比亚富奎内湖（Fúquene Lake）——的现状与长期（数十年至数百年）动态，聚焦两种已广泛定植的入侵植物：凤眼蓝（Eichhornia crassipes）和黑藻（Egeria densa）的扩散过程。我们发现，当前以黑藻（Egeria densa）和凤眼蓝（Eichhornia crassipes）为优势种的区域，其大型水生植物群落组成具有独特性。以凤眼蓝为优势的区域共分布有14种大型水生植物，水体浑浊（赛氏盘透明度=19±6厘米）且溶解氧含量较低（3.94±2.61 ppm）；而以黑藻为主的区域仅分布5种大型水生植物，水体更清澈（赛氏盘透明度=51±12厘米）且溶解氧含量更高（6.06±2.4 ppm）。历史上的大型水生植物群落演替与富营养化及湖泊水位变化密切相关，其特征为：公元1500年前轮藻（charophytes）与苔藓植物（bryophytes）大量消失；20世纪早期（湖泊疏干阶段），睡莲属（Nymphaea sp.）、伊利诺伊眼子菜（Potamogeton illinoensis）及瓜达卢佩茨藻（Najas guadalupensis）的种群数量持续下降。凤眼蓝（Eichhornia crassipes）自公元1500年左右便已存在，黑藻（Egeria densa）则于20世纪早期出现，二者的出现时间均早于学界此前提出的引入时间（分别为20世纪50年代与20世纪90年代）。自20世纪90年代起，受引水工程与高营养负荷的影响，这两种入侵植物与细叶满江红（Azolla filiculoides）一同快速扩散。本研究结果显示，凤眼蓝与黑藻的扩散并非导致本土大型水生植物物种丧失的原因，但二者当前的优势地位通过生境改造、竞争排斥以及促进全湖生境异质性，对植物群落产生了协同与拮抗的次生效应。因此，若认为入侵植物会导致退化湖泊中的大型水生植物物种丧失，这一观点可能存在误导性。我们认为，凤眼蓝、细叶满江红与黑藻等入侵性强的物种，需要水文稳定且富营养化的环境才能实现扩散；因此，相关管理措施应聚焦于管控这两类影响因子。本研究表明，若要全面厘清入侵大型水生植物的生态效应，需采用长期研究视角。