Beyond the treeline: Are wood-nesting bees and wasps limited by nesting habitat at high elevations?

Although climate suitability is often thought to be the most important determinant of species ranges, non-climate variables such as biotic interactions, dispersal abilities, and habitat quality may play equally important roles as range-limiting factors. For some organisms, the constraints imposed by these non-climate variables may explain their failure to establish in novel habitats that appear suitable with respect to climate. Determining whether or not organisms can respond to rising temperatures via range shifts therefore requires understanding the extent to which non-climate variables limit species ranges. We examined the potential for range shifts in montane populations of wood-nesting bees and wasps in the Colorado Rocky Mountains and assessed the extent to which future range shifts to higher elevations may be limited by the lack of wood substrates beyond treeline. We tracked wood-nesting bee and wasp occupancy in artificial wooden nesting structures placed at increasing elevations from treeline. We also quantified the relative abundance of wood- versus ground-nesting bees and wasps at each site. Over the course of the study, wood-nesting bee or wasp nesting activity was observed at least once at all sites, regardless of distance from treeline. The total number of nests constructed at each site decreased with increasing elevation; however, the average number of offspring per nest remained constant for the three most commonly observed taxa. The abundance of wood-nesting bees and wasps declined with increasing elevation more rapidly than that of ground-nesting bees and wasps. These findings suggest that wood-nesting bees and wasps are capable of nesting beyond treeline, but that the extent to which their ranges can shift upward in elevation may be limited by the lack of nesting substrate. Our work highlights the importance of considering non-climate variables such as essential habitat requirements in evaluating the capacity of organisms to respond adaptively to climate change.