Why are there so many species? Spatio-temporal heterogeneity and northern bumblebee communities

It is proposed that (1) the different body sizes and proboscis lengths of bumblebee species lead to different optimal foraging patterns in different species. The morphological differences affect niche relations, depending on the pattern of the resource environment. Tongue morphology allows only a basic set of three to four species per community, differing by proboscis length and frequency in indulging in robbery. To explain considerably higher community species numbers (frequently between six and eleven species), (2) the effect of superabundant resources was examined. Data suggest that species-rich bumblebee communities exist locally by the Schlaraffenland effect. As few of the bumblebee communities are located at fields of superabundant resources, or in non-competitive environments, other coexistence mechanisms were sought for. (3) Intensified intraspecific competition relative to interspecific competition may theoretically allow coexistence, but supporting evidence is still lacking. (4) Spatio-temporal heterogeneity was advocated as the main coexistence mechanism: the energy available for colony growth changes seasonally and depends to a large extent on the location of the nest relative to dynamically changing resources. This may lead to temporal reversions in the competitive relations between the colonies and species. All three tests available suggested that spatio-temporal heterogeneity suffices in explaining the high species numbers in northern bumblebee communities. However, specific features of bumblebee foraging behaviour call for measurements of withinand between-colony competition.

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