Data and R code for: 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 and 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 upwards 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.