LTREB: On the Consequences of Removing Ground-dwelling Mammals for Tropical Forest Diversity: Towards a New Conceptual Framework

[0212054]我们正在进行一项长期、大规模的实验，以解决解释热带森林树种多样性维持的主要假设之一，即哺乳动物作为食草动物、种子捕食者和传播者，是热带树木多样性和丰度的关键决定因素。这个实验本质上是对Janzen-ConnellHypothesis的一个社区层面的测试。Janzen-ConnellHypothesis预测，陆地哺乳动物的移除将最终导致热带树木多样性的下降。在这种情况下，哺乳动物优先捕食数量丰富的树种的幼体，并最终控制这些树种。研究系统是巴拿马中部Barro ColoradoIsland （BCI）和邻近大陆（Gigante半岛）的成熟阶段热带潮湿森林。自1994年以来，我们在8个大型（1355平方米）围栏封闭区和相邻的围栏控制区监测了将地面生活哺乳动物排除在外的后果。在BCI上建立了4对处理和对照样地，在相似的成熟林内的Gigante上建立了4对处理和对照样地。在开始任何处理之前，我们对所有样地50 cm高的木本植物进行了使用、测量、鉴定和标记。此外，我们还对每个大样地的28个小样地的新招募幼苗进行了量化。我们现在总共跟踪了6万多棵幼苗和树苗的命运。总的来说，哺乳动物的排除导致了幼苗密度的持续增长。迄今为止的实验结果表明，至少在短期内（~10年），排除哺乳动物可能会显著增加幼苗层的木本物种多样性。因此，我们的结果与Janzen-Connell假说的社区水平预测形成鲜明对比。我们提出了一个新的概念模型，该模型表明，从热带森林中短期移除哺乳动物（2-25年）的后果，无论是通过实验排除还是更广泛地通过捕猎，都将首先导致木本物种多样性的增加，然后可能会减少（见图1）。这一结果将首先出现在幼苗层（1-50厘米高），然后以波的形式在较大的种子层中传播。然而，即使在7年后，迄今为止的数据仍然模棱两可。在封闭样地内明确记录的多样性短期增加可能是1998年厄尔尼诺-南方涛动（ENSO）事件的短暂影响。具体而言，ENSO事件造成了严重和延长的旱季，增加了幼苗死亡率。由于封闭区域内的幼苗密度较高，这些地点可能缓冲了干旱的影响。我们认为有必要继续这个实验，并在幼苗层进入小树苗级时监测下一阶段的情况。这项工作的保护意义是明确的；在许多热带森林保护区、公园和碎片中，对哺乳动物的过度开发可能会对森林多样性产生长期的负面影响，这种影响可能在几十年内不会显现出来。只有通过长期实验，我们才能明确区分短期动态和长期森林轨迹。

0212054CarsonLTREBWe have underway a long-term, large-scale experiment that is addressing one of the primaryhypotheses proposed to explain the maintenance of tree species diversity in tropical forests, namely,that mammals acting as herbivores, seed predators, and dispersers, are critical determinants of tropicaltree diversity and abundance. This experiment is essentially a community-level test of the Janzen-ConnellHypothesis that predicts that the removal of ground-dwelling mammals will ultimately cause adecline in tropical tree diversity. Under this scenario, mammals preferentially prey on the juveniles oftree species that are abundant and ultimately keep these tree species in check.The study system is mature-phase tropical moist forest in central Panama on Barro ColoradoIsland (BCI) and the adjacent mainland (Gigante Peninsula). Since1994, we have monitored theconsequences of excluding ground-dwelling mammals in 8 large (1355m2) fenced exclosures andadjacent unfenced control plots. Four pairs of treatment and control plots were established on BCI and4 pairs were located on Gigante in similar mature forest. Prior to initiating any treatments, wecensused, measured, identified, and tagged all woody plants 50 cm in height in all plots. Inaddition, we quantified newly recruited seedlings in 28 smaller subplots in each large plot. In total,we have now followed the fate of more than 60,000 individual seedlings and saplings. Overall, theexclusion of mammals led to a substantial and sustained increase in seedling density.The results of this experiment to date suggest that excluding mammals may significantlyincrease woody species diversity of the seedling layer at least in the short-term (~10 yrs). Thus, ourresults contrast sharply with the community-level predictions of the Janzen-Connell hypothesis. Wepropose a new conceptual model that suggests that the consequences of the short term removal ofmammals (2-25 yrs) from tropical forests, whether by experimental exclusion or more broadly viaover hunting, will cause first an increase in woody species diversity and thereafter potentially adecrease (see Fig. 1). This result will first appear in the seedling layer (1-50 cm in height) and thenmove as a wave through larger size classes. The data to date, however, even after 7-years, remainequivocal. The short term increase in diversity clearly documented inside exclosure plots may be atransient effect of the 1998 El Nino Southern Oscillation (ENSO) event. Specifically, the ENSO eventcaused a severe and extended dry season that increased seedling mortality. Because there was a higherseedling density inside the exclosures, these sites may have been buffered from the drought. Webelieve that it is imperative that we continue this experiment and monitor the next phase as the seedlinglayer moves into the small sapling size class. The conservation implications of this work are clear; theoverexploitation of the mammalian fauna in many tropical forest reserves, parks, and fragments mayhave long-term negative consequences for forest diversity that may not become apparent for decades.It is only through long-term experiments that we can unequivocally separate out short-term dynamicsfrom long-term forest trajectories.