Abstract
While ecosystem engineering is a widespread structural force of ecological communities, the mechanisms underlying the inter-specific associations between ecosystem engineers and resource users are poorly understood. A proper knowledge of these mechanisms is, however, essential to understand how communities are structured. Previous studies suggest that increasing the quantity of resources provided by ecosystem engineers enhances populations of resource users. In a long-term study (1995-2011), we show that the quality of the resources (i.e. tree cavities) provided by ecosystem engineers is also a key feature that explains the inter-specific associations in a tree cavity-nest web. Red-naped sapsuckers ( Sphyrapicus nuchalis ) provided the most abundant cavities (52% of cavities, 0.49 cavities/ha). These cavities were less likely to be used than other cavity types by mountain bluebirds ( Sialia currucoides ), but provided numerous nest-sites (41% of nesting cavities) to tree swallows ( Tachycineta bicolour ). Swallows experienced low reproductive outputs in northern flicker ( Colaptes auratus ) cavities compared to those in sapsucker cavities (1.1 vs. 2.1 fledglings/nest), but the highly abundant flickers (33% of cavities, 0.25 cavities/ha) provided numerous suitable nest-sites for bluebirds (58%). The relative shortage of cavities supplied by hairy woodpeckers ( Picoides villosus ) and fungal/insect decay (<10% of cavities each, <0.09 cavities/ha) provided fewer breeding opportunities (<15% of nests), but represented high quality nest-sites for both bluebirds and swallows. Because both the quantity and quality of resources supplied by different ecosystem engineers may explain the amount of resources used by each resource user, conservation strategies may require different management actions to be implemented for the key ecosystem engineer of each resource user. We, therefore, urge the incorporation of both resource quantity and quality into models that assess community dynamics to improve conservation actions and our understanding of ecological communities based on ecosystem engineering.
Highlights
Key ecosystem engineers are organisms that facilitate and modulate resource availability to other organisms via physical modification or creation of habitat [1,2]
We investigated the mechanisms underlying the specific associations between ecosystem engineers and resource users by examining the responses of two secondary cavity nesters to the variation in the quantity and quality of tree-cavities supplied by four types of ecosystem engineers: three avian excavators and rot fungi/insects [16]
We investigated the mechanisms underlying the specific associations between ecosystem engineers and secondary cavity users in an avian nest web community
Summary
Key ecosystem engineers are organisms that facilitate and modulate resource availability to other organisms via physical modification or creation of habitat [1,2]. Despite the recognized importance of ecosystem engineering as a structural force that enhances the biodiversity of ecological communities, the underlying mechanisms of the inter-specific associations between ecosystem engineers and the users of the resources provided via ecosystem engineering are poorly known in most ecosystems. A better knowledge of these mechanisms is relevant to elucidate if resources supplied by different ecosystem engineers provide different benefits to a given resource user [6]. If this proved true, niche differentiation associated with the differential quality of resources provided by ecosystem engineers might promote coexistence of ecologically similar species by overcoming potential negative effects of inter-specific competition [13,14]. Management actions oriented to preserve populations of resource users require proper identification of the respective roles of potential key ecosystem engineers for each resource user
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