Unraveling the cavity-nesting network at large spatial scales: The biogeographic role of woodpeckers as ecosystem engineers

Aim: Cavities are usually a limiting resource for several forest-dwelling
vertebrates, with effects that propagate through ecological networks.
Although diverse assemblages of primary excavators (e.g., woodpeckers) are
assumed to increase cavities, other forest resources can also limit
populations of primary excavators and cavity users, thus undermining the
ecological role of excavators over different scales. We aim to test the
biogeographical-scale relationships between primary excavators and cavity
users by distinguishing the contribution of forest characteristics.
Location: Southern South America Methods: We used species distribution
models, which combine bioclimatic and remote sensing derived variables, to
map the richness of vertebrates composing the cavity network of temperate
and Mediterranean forests of South America. Based on a resampling
procedure for ensuring spatial independence, we fitted structural equation
models to estimate causal relationships between forest characteristics and
cavity-user vertebrates. Results: Secondary cavity users (obligated,
habitat generalists and forest specialists) were positively and strongly
influenced by the richness of primary excavators, while mammal richness
was more influenced by tree richness. The richness of trees and Nothofagus
tree species influenced positively the richness of primary excavators and
secondary cavity users. Canopy height and net primary productivity
affected positively secondary cavity users. Main conclusions: Our results
confirm the role of primary excavators as ecosystem engineers but
highlight the importance of considering large spatial scales when
analyzing cavity-nesting networks. Biogeographical patterns of tree
diversity and forest structure can be important drivers of cavity-nesting
networks that remain hidden when studies are conducted over fine spatial
scales.

研究目的：树洞通常是多种森林栖息脊椎动物的限制性资源，其影响会沿生态网络逐级传递。尽管学界普遍假设初级掘洞者（primary excavators，如啄木鸟）的多样类群可增加树洞资源总量，但其他森林资源同样可能限制初级掘洞者与树洞利用者的种群规模，进而在不同空间尺度上削弱掘洞者的生态功能。本研究旨在通过区分森林特征的贡献，探究生物地理尺度下初级掘洞者与树洞利用者之间的关联。
研究区域：南美洲南部
研究方法：本研究采用结合生物气候变量与遥感衍生变量的物种分布模型（species distribution models），绘制南美洲温带与地中海森林中构成树洞网络的脊椎动物物种丰富度空间分布。为保障空间独立性，我们通过重采样流程构建结构方程模型（structural equation models），以量化森林特征与树洞利用脊椎动物间的因果关系。
研究结果：次级树洞利用者（secondary cavity users，包括专性树洞利用者、栖息地广适种与森林特化种）的丰富度显著且正向地受初级掘洞者丰富度的影响；而哺乳动物丰富度则更多受树木物种丰富度的调控。树木及南山毛榉（Nothofagus）树种的丰富度，对初级掘洞者与次级树洞利用者的丰富度均存在正向影响。林冠高度与净初级生产力（net primary productivity）同样对次级树洞利用者产生正向作用。
主要结论：本研究结果证实了初级掘洞者作为生态系统工程师（ecosystem engineers）的核心作用，但同时强调，在分析树洞筑巢网络时需重视大空间尺度的影响。当研究仅在精细空间尺度开展时，树木多样性与森林结构的生物地理模式作为树洞筑巢网络的重要驱动因子，往往会被忽视。