Seed consumption by small fish follows peak seed availability in a tropical dry forest river

Seed consumption and dispersal by fish has been more extensively described in natural Neotropical large river systems of Amazonia, where ichthyochory follows a seasonal gradient associated with a floodpulse that creates long-lasting seasonally flood zones. It has been shown that it is relevant in maintaining the plant community structure of wetlands, but its effects on plant communities in seasonal dry forests are largely unknown. The Amacuzac hydrological system, which runs through one of the most extensive seasonal tropical dry forests of Central Mexico, shows a marked climatic seasonality, which in turn determines a transient increase in river flow and its potential to overflow, resulting in a drag effect on banks, and in the river carrying three times more seeds in the rainy than in the dry season. Another characteristic of the system is the coexistence of native and non-native fish species, due to the fact that the middle part of the river receives discharges from four tributaries that cross important urban, industrial, and agricultural districts as well as an extensive network of ornamental fish farms. Therefore, we hypothesize that rates of seed consumption by fish would correspondingly increase during rainy seasons, and that both native and non-native fish would consume seeds in similar proportions. We evaluated seed consumption by fish by (a) identifying and estimating the frequency and number of fish species whose stomachs contained seeds, and determining the percentage of each fishes' diet corresponding to seeds with respect to total food items, and (b) by evaluating if there were seasonal differences in the consumption of intact seeds by fish and between native vs. non-native species. We found two native species - Astyanax aeneus and Notropis moralesi, and two non-native species - Aequidens rivulatus and Amatitlania nigrofasciata containing seeds in their stomachs. The number of individuals with seeds was significantly higher in the rainy season than in the dry season, with a higher proportion of non-native fishes carrying seeds, but only in the rainy season. We found significant differences between the fish species in the proportion of seeds consumed. Fishes follow the peak of seed availability after a flood pulse released them from the soil seed banks. Thus, fishes consume fruits/seeds on a seasonal basis, however in this case it is explained by a different mechanism other than the timing of seed production by trees. As with hydrochory in tropical dry forests, seed consumption/dispersal by fish inhabiting rivers within this ecosystem has been overlooked in the ecology and conservation literature. This evidence is relevant for tropical dry forest ecosystem dynamics and management strategies in riparian corridors.鱼类对种子的取食与传播，在亚马孙地区的新热带大型自然河流水系中已有较为充分的研究——在此类区域，鱼类传播（ichthyochory）遵循与洪水脉冲（floodpulse）相关的季节梯度，后者会形成长期存在的季节性泛滥区。已有研究表明，鱼类传播对维持湿地植物群落结构具有关键作用，但其对季节性干旱森林植物群落的影响在很大程度上仍未明确。阿马库萨克（Amacuzac）水文系统贯穿墨西哥中部最广袤的季节性热带干旱森林之一，具有显著的气候季节性，这进而决定了河流流量的短暂上升与漫溢潜力，对河岸产生拖曳侵蚀效应，同时使得雨季河流携带的种子数量较旱季高出三倍。该水系的另一特征是本土与外来鱼类物种共存，原因在于河流中段接收了四条支流的汇入，这些支流流经重要的城市、工业与农业区域，同时覆盖了庞大的观赏鱼养殖场网络。因此我们提出假说：鱼类取食种子的速率会在雨季相应提升，且本土与外来鱼类取食种子的比例相近。我们通过两种途径评估鱼类的种子取食情况：(a) 鉴定并统计胃内含种子的鱼类物种的出现频率与个体数量，并计算每种鱼类的食物组成中，种子占总食物类别的百分比；(b) 评估鱼类对完整种子的取食是否存在季节差异，以及本土与外来物种间是否存在此类差异。我们共发现四种胃内含种子的鱼类：两种本土物种为Astyanax aeneus与Notropis moralesi，两种外来物种为Aequidens rivulatus与Amatitlania nigrofasciata。胃内含种子的个体数量在雨季显著高于旱季；外来鱼类携带种子的比例更高，但这一现象仅出现在雨季。我们还发现，不同鱼类物种的种子取食比例存在显著差异。鱼类会在洪水脉冲将土壤种子库（seed banks）中的种子释放后，跟随种子可获得性的峰值进行取食。因此鱼类的果实/种子取食具有季节性，但本研究中的这一现象并非由树木的种子生产时间所驱动，而是由另一套不同的机制所介导。与热带干旱森林中的水媒传播（hydrochory）类似，栖息于该生态系统河流中的鱼类对种子的取食与传播，在生态学与保护相关文献中长期被忽视。该研究结果对于热带干旱森林的生态系统动态及河岸带（riparian corridors）的管理策略具有重要参考价值。