Woodpeckers and other excavators maintain the diversity of cavity‐nesting vertebrates

Abstract

Woodpeckers and other excavators create most of the holes used by secondary cavity nesters (SCNs) in North American temperate mixedwood forests, but the degree to which excavators release SCNs from nest‐site limitation is debated. Our goal was to quantify how excavators maintain the diversity and abundance of secondary cavity nesters in a temperate forest through the creation of tree cavities. We examined the short‐ and long‐term (legacy) effects of excavators (principally woodpeckers, but also red‐breasted nuthatches and black‐capped chickadees) on forest biodiversity using longitudinal monitoring data (1,732 nest cavities, 25 sites, 16 years) in British Columbia, Canada. Sites with higher densities of excavator nests had more cavities available, higher species richness of SCNs and higher nest density of SCNs, indicating the importance of a standing stock of cavities. Years with higher nesting densities of excavators were followed by years with higher SCN diversity, indicating that the creation of nesting opportunities through fresh excavation releases SCNs from community‐wide nest‐site limitation. We also show that excavators leave a ‘legacy’ of biodiversity (species richness and abundance) at a site by accumulating cavities at rates faster than they become unusable by decay or destruction. By quantifying site‐level effects of cavity excavation on the SCN community, our study highlights the key role of excavators as ecosystem engineers that maintain forest wildlife biodiversity.

Climate variability and aridity modulate the role of leaf shelters for arthropods: A global experiment

“…The leaf shelters can act as refuges by mitigating climatic variation and harsh conditions, allowing predators to rapidly adjust their use of niche space according to their physiological tolerances. This result calls for further studies investigating direct measures of physiological benefits of shelter occupancy for a range of organisms, including the shelter builders (e.g., Trzcinski et al, 2021). The ability to forage from such shelters may still have a strong influence on their overall performance and on the overall imprint of climate on arthropod community change and behavioral adaptations.…”

Cavities excavated by woodpeckers limit populations of other cavity‐nesting birds

“…Trzcinski et al (2022) provide the best empirical evidence to date that SCNs are limited by the availability of cavities excavated by PCNs. The authors examine changes in the numbers of nests of cavity‐nesting species per hectare over a 16‐year period in 25 study sites varying from 7 to 32 ha in size.…”

“…The long‐term data analysed by Trzcinski et al (2022) come from the study of the North American, Douglas‐fir Pseudotsuga menziesii , coniferous forest ecosystem in which the nest web concept originated (Martin & Eadie, 1999). Their large‐scale study, combined with smaller scale experimental studies (Aitken & Martin, 2008, 2012), make a compelling case that SCN populations are limited by cavities created by PCNs in this system.…”

“…Their large‐scale study, combined with smaller scale experimental studies (Aitken & Martin, 2008, 2012), make a compelling case that SCN populations are limited by cavities created by PCNs in this system. Trzcinski et al (2022) use newly excavated cavities, which are readily located and identified, as a measure of cavity production, and monitor these cavities over time to assess cavity availability. Important challenges to determining whether the correlation between densities of woodpeckers and SCNs that exists in this system represents a causal relationship include environmental factors that affect densities of SCNs, annual variation in factors affecting SCN nesting, and temporal autocorrelation of numbers within sites.…”

“…The structure of cavity-nesting communities, known as nest webs, includes links between tree species that provide natural holes and bird species that nest in those holes (secondary cavity nesters, SCNs), tree species that provide substrates for cavity excavation and bird species such as woodpeckers that create cavities using those substrates (primary cavity nesters, PCNs), and between PCNs and SCNs. Trzcinski et al (2022) focus on the latter links and provide the most compelling empirical evidence to date that cavities created by PCNs specifically, as opposed to natural holes or holes generally, limit populations of SCNs. Using data from a long-term study, the authors employ three analyses to separate effects of availability of cavities from environmental factors such as food, habitat features and host tree abundance, while controlling for annual variation and autocorrelation within sites, to isolate the relationship between excavated cavities and SCN numbers.…”