Ecosystem engineers show variable impacts on habitat availability for cavity nesters in South American temperate forests

Ecosystem engineers are organisms that impact their environment and
co-existing species by creating or modifying habitats, and thus they play
important roles as drivers of community assembly. We examined whether
cavity characteristics and/or habitat attributes associated with cavities
provided by four ecosystem engineers, influence the presence of nests of
three secondary cavity-nesting birds [Aphrastura spinicauda (Thorn-tailed
Rayadito), Tachycineta leucopyga (Chilean Swallow), and Troglodytes aedon
(Southern House Wren)], and whether these variations influence their
reproductive success. We tested this by: i) assessing nest presence in
cavities supplied by ecosystem engineers and ii) quantifying the
reproductive success of secondary cavity nesters as a function of cavity
characteristics and habitat attributes supplied by ecosystem engineers.
Between 2009 and 2022, we recorded 757 cavities in 546 trees in old-growth
and second-growth forests in a Global Biodiversity Hotspot in the Andes of
southern Chile. Insect/fungi and Pygarrhichas albogularis (White-throated
Treerunner) play a key role as the primary producers of cavities.
Insect/fungi generated the cavities for 82% of A. spinicauda nests and 95%
of T. aedon nests; in contrast, 57% of T. leucopyga nests were cavities
excavated by P. albogularis. Characteristics of cavities (size of cavity
entrance, volume, and height above ground) were associated with nest
presence of secondary cavity nesters and with reproductive success of A.
spinicauda and T. aedon. Habitat attributes such as tree density and
canopy cover influenced nest presence for A. spinicauda and T. leucopyga,
but did not correlate with the reproductive success of any secondary
cavity nester. Bamboo density and forest type were related to reproductive
success of A. spinicauda and T. aedon. Diameter at Breast Height of trees
was related to the reproductive success of T. leucopyga. This study
contributes to understanding the importance of various ecosystem engineers
for the conservation of secondary cavity-nesting birds in temperate
forests and beyond.