Gut-passage time and the role of the Greater Rhea (<i>Rhea americana</i>) influencing seed dispersal and predation in Neotropical grasslands and savannas

The Greater Rhea (<i>Rhea americana</i>) is the largest (32–35 kg) flighless bird inhabiting grasslands and savannas of South America. Its diet comprises several fruit species, but the outcome of fruit ingestion (seed dispersal or predation) is largely unknown. We investigated the role of Greater Rheas in seed dispersal using fruit feeding trials with captive animals and seed germination experiments for 11 plants from the Brazilian savannas (Cerrado). We also performed a meta-analysis with a total 29 species included in seed germination experiments following fruit ingestion by Rheas. Rheas have a long gut passage time (GPT) (mean ± SD: 61.6 ± 13.5 hours), allowing them to carry seeds to long distances. In our experiment, seeds within heavier fruits and those with lipid-rich fruit pulp had shorter and longer GPT, respectively, which influences the potential distances of seed dispersal achieved. The impact of seed ingestion varied from seed destruction (for three species) to successful (three species) or faster germination (two species) compared to controls. Results were inconclusive for three species. Dispersal is done at the cost of considerable seed losses, probably due to prolonged seed abrasion inside the gut. Similar results were found in the meta-analysis. Given the consumption of many fruits combined with a long GPT and large home range, Rheas may influence the fate of a great amount of seeds for several plants. Current pervasive population declines of Rheas can disrupt these processes, affecting plant regeneration, particularly for species that depend on them for long-distance seed dispersal.大美洲鸵（Greater Rhea，*Rhea americana*）是体重可达32–35千克的最大型无翼鸟类，栖息于南美洲的草原与稀树草原地带。其食谱涵盖多种果实类群，但取食果实后的生态结局——即种子传播或是种子捕食——目前仍尚不明确。本研究通过对圈养大美洲鸵开展果实投喂试验，并针对来自巴西稀树草原（塞拉多，Cerrado）的11种植物开展种子萌发实验，以此探究大美洲鸵在种子传播过程中发挥的作用。此外，本研究还针对共29种经大美洲鸵取食果实后的植物的种子萌发实验数据开展了荟萃分析。大美洲鸵具有较长的肠道滞留时间（gut passage time，GPT），其平均值±标准差为61.6±13.5小时，这使得它们能够将种子携带至较远的距离。本实验结果显示，重量更大的果实内部的种子，以及果肉富含脂质的种子，其肠道滞留时间分别更短与更长，这会对种子传播的潜在距离产生影响。大美洲鸵取食对种子的影响存在显著差异：3个物种的种子会被完全破坏，3个物种的种子可成功萌发，另有2个物种的种子萌发速度较对照组更快；另有3个物种的实验结果尚无定论。种子传播过程伴随着可观的种子损失，这大概率是由于种子在消化道内受到了长时间的摩擦磨损所致。荟萃分析得到了与之相似的结果。鉴于大美洲鸵会取食大量果实，加之其较长的肠道滞留时间与广阔的活动范围，它们可能会对多种植物的大量种子的命运产生显著影响。当前大美洲鸵种群的普遍衰退可能会破坏这一生态过程，进而影响植物的自然更新，尤其是那些依赖其进行长距离种子传播的植物物种。