[Pollinator] bee nests sites as a limiting factor

[David Inouye]

For solitary bees in Germany, it appears that nest site availability 
limits population growth rates.

Steffan-Dewenter, I. and S. Schiele. 2008. Do resources or natural 
enemies drive bee population dynamics in fragmented habitats. Ecology 
89:1375-1387.

The relative importance of bottom-up or top-down forces has been 
mainly studied for herbivores but rarely for pollinators. Habitat 
fragmentation might change driving forces of population dynamics by 
reducing the area of resource-providing habitats, disrupting habitat 
connectivity, and affecting natural enemies more than their host 
species. We studied spatial and temporal population dynamics of the 
solitary bee Osmia rufa (Hymenoptera: Megachilidae) in 30 fragmented 
orchard meadows ranging in size from 0.08 to 5.8 ha in an 
agricultural landscape in central Germany. From 1998 to 2003, we 
monitored local bee population size, rate of parasitism, and rate of 
larval and pupal mortality in reed trap nests as an accessible and 
standardized nesting resource. Experimentally enhanced nest site 
availability resulted in a steady increase of mean local population 
size from 80 to 2740 brood cells between 1998 and 2002. Population 
size and species richness of natural enemies increased with habitat 
area, whereas rate of parasitism and mortality only varied among 
years. Inverse density-dependent parasitism in three study years with 
highest population size suggests rather destabilizing instead of 
regulating effects of top-down forces. Accordingly, an analysis of 
independent time series showed on average a negative impact of 
population size on population growth rates but provides no support 
for top-down regulation by natural enemies. We conclude that 
population dynamics of O. rufa are mainly driven by bottom-up forces, 
primarily nest site availability.