A meta-analysis of the ecological significance of density in tree invasions

Species richness, resource availability, and disturbance are the primary factors considered in assessing the invasibil- ity of plant communities. Nonetheless, the density of individuals in a community is a common and easy trait to measure. The ecological significance of the density of both native and invasive tree species was assessed using a systematic review and formal meta-analysis. The densities of recipient communities and invasive exotic tree species in novel ranges were identified in the published literature. In addition, we compared by means of a meta-analysis: (i) densities of invasive versus native species in invaded communities; (ii) densities of native species in invaded versus uninvaded communities; and (iii) densities of invasive species along distance gradients from initial locus of invasion. Invasive trees were found at higher densities than native species in recipient communities. Invasions by woody species were also recorded in communities with relatively low densities of natives suggesting that (i) low density forests may be more susceptible to invasion and/or (ii) density of the recipient community may be reduced during the invasion process. In addition, comparison of native species densities between invaded and uninvaded stands from the same community suggests that invasive trees negatively affect density of native trees once established. There- fore, the widely reported low density and often richness of native plants in invaded communities cannot be directly linked to ecosystem susceptibility to invasion without considering concomitant impacts. These findings suggest that density is a key preliminary determinant or factor which should be considered when assessing tree invasion dynamics.

在评估植物群落的可入侵性时，物种丰富度、资源可获得性与干扰是核心考量因素。尽管如此，群落内的个体密度仍是一项常见且易于测定的特征。本研究通过系统综述与规范元分析，评估了本土与入侵乔木物种密度的生态学意义：从已发表文献中获取了新生境中接收群落（recipient community）与入侵外来乔木物种的密度数据；此外，本研究通过元分析完成了三项对比：① 入侵群落中入侵物种与本土物种的密度；② 同一群落内已入侵样地与未入侵样地中本土物种的密度；③ 沿入侵初始位点距离梯度分布的入侵物种密度。研究发现，在接收群落中，入侵乔木的密度显著高于本土乔木；同时，研究还在本土物种密度相对较低的群落中记录到木本植物入侵事件，这提示两点：其一，低密度森林可能更易遭受入侵；其二，入侵过程可能会降低接收群落的物种密度。此外，对同一群落内已入侵与未入侵样地的本土物种密度进行对比后发现，入侵乔木定殖后会对本土乔木的密度产生负面影响。因此，若未考虑伴随产生的生态影响，已发表研究中普遍报道的“入侵群落内本土植物密度偏低且往往物种丰富度更高”这一现象，无法直接与生态系统的入侵易感性建立因果关联。综上，本研究结果表明，密度是评估乔木入侵动态时需优先考量的关键前置因素。