Abstract
In severe and variable conditions, specialized resource selection strategies should be less fre- quent because extinction risks increase for species that depend on a single and unstable resource. Psithyrus (Bombus subgenus Psithyrus) are bumblebee parasites that usurp Bombus nests and display inter-specific variation in the number of hosts they parasitize. Using a phylogenetic comparative frame- work, we show that Psithyrus species at higher elevations display a higher number of hosts species com- pared with species restricted to lower elevations. Species inhabiting high elevations also cover a larger temperature range, suggesting that species able to occur in colder conditions may benefit from recruit- ment from populations occurring in warmer conditions. Our results provide evidence for an 'altitudinal niche breadth hypothesis' in parasitic species, showing a decrease in the parasites' specialization along the elevational gradient, and also suggesting that Rapoport's rule might apply to Psithyrus.
Highlights
Optimal resource selection is crucial for all organ‐ isms and directly impacts fitness (Parker and Stu‐ art 1976)
The bum‐ blebee species richness is higher at intermediate values of degree‐days, corresponding to mid‐ elevations, and reaches its lowest value in the coldest conditions at high elevation
Host specialization is known to vary between Psithyrus species (Rasmont 1988), but it was un‐ known whether, as for latitudinal gradients, a spe‐ cialist‐to‐generalist progression could be found along an elevation cline
Summary
Optimal resource selection is crucial for all organ‐ isms and directly impacts fitness (Parker and Stu‐ art 1976). Different resource selection strategies may not generate identical costs and benefits in mild com‐ pared to severe environments. Resource specialization is expected to be more advanta‐ geous in stable environments, where fluctuations of resources’ availability are low, and intense competition favours niche partitioning (MacArthur 1955). More generalist species should be advantaged in severe environments where re‐ sources are fluctuating, since they do not depend on a restricted range of resources that may be‐ come temporarily unavailable (Currie 1991). Parasites obtain resources from a host in the form of nutrients (e.g., body fluids), and habitats (e.g., nest). The latter is particu‐ larly widespread in social insects, because nest parasites benefit from the workforce of non‐
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