Fish on fire: Shifts in Amazonian fish communities after floodplain forest fires

Severe droughts can lead to fires that cause massive tree mortality in even the wettest and most isolated Amazonian forests. After repeated fires, blackwater floodplain forests can remain in an open vegetation state of arrested succession that facilitates a transition towards a white-sand “savanna-like” vegetation. These vegetation shifts from closed-canopy floodplain forests to open fire scars, and eventually white-sand savannas, may have profound implications for fish communities that depend on floodplain forests for food and recruitment. In turn, changes in fish communities may contribute to the arrested forest recovery after fires as fish are important seed dispersal agents for many floodplain tree species. To explore the impacts of floodplain forest fires on fish communities, we sampled fish in unburnt forests, fire scars, and white-sand savannas in the middle Rio Negro basin (Brazil) during two consecutive flooding seasons. We compared the abundance, species richness, and the taxonomic and trophic composition of fish assemblages across the three habitat types. We found significant shifts in fish assemblages in fire scars compared to unburnt floodplain forests. Also, as fire scars increased in size, total fish biomass decreased strongly. Fish communities in unburnt floodplain forests seem to be characterized by a higher proportion of smaller and omnivorous fish species than fish communities in burned floodplain forests and white sand savannas. Fish assemblages in fire scars and white-sand savanna were not significantly different. Synthesis and Applications. Amazonian fish communities change after floodplain forest fires. Unburnt forests have diverse fish communities, with a large proportion of unique and small omnivorous species. In contrast, carnivores and detritivores become more common in fire scars and white-sand savannas formed after forest fires. Less omnivore fish after forest fires can reduce tree seed dispersal and forest regeneration, affecting ecosystem functioning and the services provided by Amazonian forests. Fish are also a primary source of food and income for people living in Amazonian floodplains. Preventing forest fires is therefore of fundamental importance for the conservation of Amazonian aquatic and terrestrial ecological communities as well as for the livelihood of people.严重干旱可引发火灾，即便在最湿润、最偏远的亚马孙森林中，也会造成大量树木死亡。经历反复火灾后，黑水河漫滩森林（blackwater floodplain forests）可能会停留在植被开放的演替停滞（arrested succession）状态，进而向白沙类稀树草原（white-sand savanna-like）植被类型转变。这类植被从郁闭冠层漫滩森林（closed-canopy floodplain forests）向开阔火烧迹地、最终演变为白沙稀树草原的转变，可能对依赖漫滩森林获取食物并完成种群补充的鱼类群落产生深远影响。反之，鱼类群落的变化也可能延缓火灾后森林的恢复进程——因为鱼类是诸多漫滩树木物种的重要种子传播媒介（seed dispersal agents）。 为探究漫滩森林火灾对鱼类群落的影响，我们在连续两个洪水季中，于巴西内格罗河盆地（Rio Negro basin）中游流域的未过火森林、火烧迹地与白沙稀树草原中开展鱼类采样。我们对比了三类生境中鱼类组合群（fish assemblages）的丰度、物种丰富度，以及分类学与营养级组成。 研究发现，与未过火的漫滩森林相比，火烧迹地内的鱼类组合群发生了显著变化。同时，随着火烧迹地面积扩大，鱼类总生物量大幅下降。未过火漫滩森林的鱼类群落，相较于过火漫滩森林与白沙稀树草原的鱼类群落，以小型杂食性（omnivorous）鱼类占比更高为特征。火烧迹地与白沙稀树草原的鱼类组合群则无显著差异。 研究总结与应用启示。亚马孙鱼类群落在漫滩森林火灾后会发生改变。未过火森林拥有多样的鱼类群落，其中独特小型杂食性鱼类占比颇高。与之相反，肉食性（carnivores）与食碎屑鱼类（detritivores）在森林火灾后形成的火烧迹地与白沙稀树草原中更为常见。森林火灾后杂食鱼类减少，会削弱树木种子传播与森林更新能力，进而影响亚马孙森林的生态系统功能（ecosystem functioning）与服务价值。此外，鱼类是亚马孙漫滩地区居民的主要食物与收入来源。因此，预防森林火灾对于保护亚马孙水生与陆地生态群落，以及当地居民的生计（livelihood）均具有根本性意义。