Cumulative effects of natural and human disturbances on ecological networks of cavity nesters in managed boreal landscapes

在加拿大东部的北方针叶林中，具有中度火灾周期（100-200年）的野火下的自然干扰制度在历史上产生了森林林分的年龄结构，该结构由早期火灾后景观组成，嵌入在由高比例的成熟和原始森林覆盖类型主导的基质中。这种干扰制度被认为是森林生物多样性的重要决定因素，特别是对于那些在老森林和火灾后林分中衰老和死树的高可利用性下进化的生物。现在有充分的证据表明，平均年龄管理正在将这一土地基础转变为更年轻的森林覆盖类型的马赛克，引起大规模的变化，大大减少了老森林的自然比例，从而减少了腐烂木材的数量和空间分布，而腐烂木材是北方生态系统生物多样性的一个关键栖息地属性。此外，在北方森林中仍然发生自然干扰（野火、虫害暴发和大风）。因此，在受管理的北方生态系统中，采伐和自然干扰的累积影响正在迅速改变与大型腐烂树木有关的物种的森林条件。使用树洞的鸟类代表了在栖息地属性要求（腐烂木材）方面要求很高的一组物种，因此对森林管理高度敏感。在这些物种中，啄木鸟特别感兴趣，因为它们在加拿大北方森林的洞穴筑巢鸟类群落中扮演着重要的角色（主要的洞穴生产者）。在景观水平上，腐烂木材可得性的变化可能改变空腔产生者（啄木鸟）和次级使用者（猫头鹰、鸭子、雀形目动物和哺乳动物）的生态网络，从而改变管理的北方林地中旧森林斑块的功能多样性。因此，增加对洞巢鸟类的人口反应及其在新环境中产生的生态网络（巢网）的了解，对于全面评估北方森林老鸟类群落的可持续性至关重要。在本研究中，我们(1)测量了在管理景观中洞巢鸟群落的巢网结构和关键物种（啄木鸟）的繁殖活动；(2)评估了旧森林栖息地的功能作用，特别是线性带（河岸缓冲带和切割块分隔带），它们构成了洞巢鸟和其他旧森林鸟类的重要比例。(3)在景观水平上评价了经管理森林旧残余生境中洞巢鸟类群落的持续性。本研究将利用在永久地块（10-40公顷）的局部尺度上进行的对keystone洞巢鸟的巢网结构和适合度组成部分的深入研究（目标1），并将与更广泛的区域调查（目标2和3）联系起来，研究管理的北方森林景观中洞巢鸟的占用和持久性，以评估局部尺度上的细粒度模式和过程如何转化为区域尺度上更普遍的模式。通过测量洞穴筑巢鸟类的生态网络对森林覆盖变化的响应，本研究项目的主要创新在于从分类学的角度转向功能多样性的角度，而我们的工作将集中在洞穴筑巢群落之间的生态关系如何受到森林条件变化的影响。这将直接有助于我们的长期目标，即制定古老森林栖息地的保护目标，不仅可以保护管理森林中的物种，而且最重要的是鸟类群落的潜在功能，以洞穴筑巢群落为模型。

In the eastern Canadian boreal forest, natural disturbance regimes under wildfires with moderate fire cycles (100-200 years) historically generated an age structure of forest stands made up of early post-fire landscapes embedded in a matrix dominated by a high proportion of mature to old-growth forest cover types. Such disturbance regimes are recognized as important determinants of forest biodiversity, particularly for organisms that evolved under a high availability of senescent and dead trees in both old forest and post-fire stands. It is now well documented that even-aged management is shifting this land base into a much younger mosaic of forest cover types, inducing large-scale changes that considerably reduce the natural proportion of older forests and, hence, the amount and spatial distribution of decaying wood, a key habitat attribute for biodiversity in the boreal ecosystem. In addition, natural disturbances (wildfires, insect outbreaks and windthrows) still occur in the boreal forest. Cumulative effects of logging and natural disturbances in the managed boreal ecosystem are thus rapidly changing forest conditions for species associated with large decaying trees. Birds using tree cavities represent a demanding group of species in terms of habitat attributes requirements (decaying wood) and are thus highly sensitive to forest management. Among these species, woodpeckers are of particular interest considering their keystone role (main cavity producers) for cavity nesting bird communities in Canada's boreal forests. At the landscape level, the change in the availability of decaying wood may modify the ecological networks of cavity producers (woodpeckers) and secondary users (owls, ducks, passerines and mammals), and thus, the functional diversity in old forest patches across managed boreal lands. Increased knowledge of demographic responses of bird cavity-nesters and the ecological networks (nest webs) that they generate in this new environment is thus critical in the overall assessment of the sustainability of boreal old forest bird communities. In this proposal we (1) measure the nest web structure of cavity nesting bird communities and reproductive activity of keystone species (woodpeckers) within managed landscapes, (2) assess the functional role of old forest habitats, in particular linear strips (riparian buffers and cutblock separators) which make up an important proportion of remnant old forests for both cavity-nesting birds and other old forest bird associates, and (3) evaluate at the landscape level the persistence of the cavity-nesting bird community in old remnant habitats of managed forests. This research will draw on intensive work on nest web structure and fitness components of keystone cavity-producers conducted at a local scale in permanent plots (10-40 ha) (Objective 1) that will be linked with a more extensive regional investigation (Objectives 2 and 3) on occupancy and persistence of cavity nesters in managed boreal forest landscapes to assess how fine-grained patterns and processes at local scales translate into more generalized patterns at regional scales. By measuring how ecological networks of cavity-nesting birds respond to forest cover changes, the main innovation of this research program is to shift from a taxonomic to a functional diversity perspective whereas our work will focus on how ecological relationships among the cavity-nesting community are affected by changing forest conditions. This will directly contribute to our long term objective which is to develop conservation targets of old forest habitat that can be set for not only maintaining species in managed forests but foremost the underlying functioning of bird communities, using the cavity-nesting community as a model.