Abstract
Through physical state changes in biotic or abiotic materials, ecosystem engineers modulate resource availability to other organisms and are major drivers of evolutionary and ecological dynamics. Understanding whether and how ecosystem engineers are interchangeable for resource users in different habitats is a largely neglected topic in ecosystem engineering research that can improve our understanding of the structure of communities. We addressed this issue in a cavity-nest web (1999–2011). In aspen groves, the presence of mountain bluebird (Sialia currucoides) and tree swallow (Tachycineta bicolour) nests was positively related to the density of cavities supplied by northern flickers (Colaptes auratus), which provided the most abundant cavities (1.61 cavities/ha). Flickers in aspen groves provided numerous nesting cavities to bluebirds (66%) and swallows (46%), despite previous research showing that flicker cavities are avoided by swallows. In continuous mixed forests, however, the presence of nesting swallows was mainly related to cavity density of red-naped sapsuckers (Sphyrapicus nuchalis), which provided the most abundant cavities (0.52 cavities/ha), and to cavity density of hairy woodpeckers (Picoides villosus), which provided few (0.14 cavities/ha) but high-quality cavities. Overall, sapsuckers and hairy woodpeckers provided 86% of nesting cavities to swallows in continuous forests. In contrast, the presence of nesting bluebirds in continuous forests was associated with the density of cavities supplied by all the ecosystem engineers. These results suggest that (i) habitat type may mediate the associations between ecosystem engineers and resource users, and (ii) different ecosystem engineers may be interchangeable for resource users depending on the quantity and quality of resources that each engineer supplies in each habitat type. We, therefore, urge the incorporation of the variation in the quantity and quality of resources provided by ecosystem engineers across habitats into models that assess community dynamics to improve our understanding of the importance of ecosystem engineers in shaping ecological communities.
Highlights
Ecosystem engineers are organisms that modulate resource availability to other organisms by maintaining or creating new habitat through physical state changes in biotic or abiotic components of the ecosystems [1]
To assess the variation in the importance of the ecosystem engineers in relation to habitat, we investigated the relationships between the presence of nests of two secondary cavity nesters in two different habitats, and the density of cavities supplied by four types of ecosystem engineers: three cavity excavating birds and rot fungi/insects
Sapsucker cavities were used in lower proportions than flicker and hairy woodpecker cavities in continuous forests, whereas only a few cavities supplied by rotfungi/insects were used by bluebirds for nesting (Fig. 2A)
Summary
Ecosystem engineers are organisms that modulate resource availability to other organisms by maintaining or creating new habitat through physical state changes in biotic or abiotic components of the ecosystems [1]. Through this environmental modification, ecosystem engineers change the selective pressures to which other organisms are exposed (i.e. the ‘‘niche construction’’ process, see [2,3]). Some studies on burrowing mammals have found that the effects of ecosystem engineering vary spatially in extensive geographic ranges [12,17], but whether habitat types mediate the specific associations between ecosystem engineers and resource users after controlling for the confounding effect of the large spatial variation in ecosystem engineering remains unclear. Assessment of the specific associations between ecosystem engineers and resource users in different habitat types at local spatial scales is, important to shed light on whether such associations vary with habitat
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