Key message

The high mortality and low recruitment of the myrmecophytic Acacia drepanolobium are not consistent with the demographic rates of monodominant species. The positive conspecific density dependence observed from the spatial analysis is consistent with the defensive benefits conferred by symbiotic ants to trees when they grow close to each other.

Abstract

Patches of savanna dominated by Acacia drepanolobium occur throughout East Africa on nutrient-rich vertisols, also known as black cotton soils. We assessed the survival and recruitment for all freestanding trees with diameter at knee height (dkh) ≥ 10 mm in one of such mono-dominated patches (47 ha) at the Mpala Research Centre, Kenya, with the aim of identifying demographic traits that might explain the dominance of this species. Over a mean 6-year interval, mortality and recruitment rates in the habitat were 4.55%/year and 1.42%/year respectively, resulting in a net loss of 17.8% of the initial individuals. Of the 30 species recorded from the first census, 11 decreased in abundance, nine increased, and the remainder 10 did not change in abundance. The monodominant A. drepanolobium had a high mortality (4.69%/year), a low recruitment (1.31%/year), and a 19% population decline. There was no evidence of conspecific negative density dependence for this species. Rather, we found a statistically significant positive correlation between the number of conspecific neighbors and individual-level probability of survival, consistent with the “shared defense” benefits that symbiotic ant colonies occupying multiple trees can confer to these latter in a small neighborhood. Thus, mortality of A. drepanolobium was higher in areas where it occurred in lower densities, which resulted in an increase in the spatial aggregation of conspecifics. Mortality increased with dkh size classes and was mostly caused by elephants and stem-boring beetles. The demographic rates during the study period in theory are inconsistent with those of monodominant species. The protection against herbivory conferred by mutualistic ants associated with this species remains the most probable explanation of its dominance in this habitat.

中文翻译：

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了解东非热带稀树草原相思树的独占性：来自人口统计学数据的见解

关键信息

支原体金合欢的高死亡率和低招募与单一优势种的人口统计率不一致。从空间分析中观察到的正种密度依赖性与共生蚂蚁在彼此靠近时对树木的防御作用相一致。

抽象的

在整个东非，营养丰富的杂草，也称为黑棉土，遍布以金合欢属植物为主的稀树大草原。我们在肯尼亚的Mpala研究中心评估了其中一个单一占主导地位的斑块（47公顷）中所有直径在膝盖高度（dkh）≥10 mm的直立树的生存和募集，目的是确定可能解释这个物种的优势。在平均6年的时间间隔内，栖息地的死亡率和招募率分别为4.55％/年和1.42％/年，导致最初个体的净损失为17.8％。在第一次人口普查中记录的30种物种中，丰度下降了11种，增加了9种，其余10种没有变化。单支A. drepanolobium死亡率高（4.69％/年），招聘率低（1.31％/年）和人口下降19％。没有证据表明该物种具有特定的负密度依赖性。相反，我们发现同种邻居的数量与个体生存水平之间存在统计学上的显着正相关，这与占据多棵树的共生蚂蚁群落可以在一个小邻域中赋予后者的“共享防御”优势相一致。因此，drepanolobium的死亡率在较低密度发生的地区较高，导致同种异体的空间聚集增加。死亡率随着dkh规模班级的增加而增加，主要是由大象和无聊的甲虫引起的。从理论上讲，研究期间的人口统计学特征与单一优势物种的不一致。与该物种相关的互食性蚂蚁赋予的对草食动物的保护仍然是其在该生境中优势的最可能的解释。