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Abstract
Much has been written about latitudinal trends in parasitoid diversity and biology, though it is widely recognised that they are a comparatively poorly known group. Here I show that for both braconid and ichneumonid wasps there are highly significant relationships between body size and the mean recorded latitude of species. Numbers of species per genus (surrogates of clades) peaks in the temperate zone for both families contrasting with data from the virtually complete inventories for mammals, birds and monocot plants, suggesting massive under-description of tropical parasitoid faunas. If the ichneumonoids may be expected to show similar trends to mammals, birds and other groups, the implication is that taxonomic work both in terms of active generic revisions, but also likely, the collecting and processing of museum specimens, and selection of taxa for revision, is woefully inadequate to allow latitudinal patterns in biology to be analysed.
Highlights
One of the best known and most studied patterns in ecology is the general increase in numbers of species towards the tropics [1]
Species richness of genera Currently accepted genera are taken here as surrogates for clades, and any latitudinal trends in the species richness of genera may plausibly be interpreted as indications of relative speciation and extinction rates and clade ages at different latitudes
Potential latitudinal gradients in body size between species of insects have received relatively little attention despite the probably misleading impression that most of the largest species exist in tropical locations - there are, afterall, many more species of most insect groups within the tropics
Summary
One of the best known and most studied patterns in ecology is the general increase in numbers of species towards the tropics [1]. Its generality has recently been assessed in a meta-analysis of more than 600 studies [2] and was found to be upheld in most cases, though with a small number of apparent exceptions [2,3] including shore birds, penguins, freshwater zooplankton and the enormously species rich parasitic wasp family Ichneumonidae (Insecta: Hymenoptera). That the enormous family Ichneumonidae was apparently less diverse at low latitudes dates back to the 1970’s when Owen & Owen [4] reported results of analysing Malaise trap samples from gardens in Leicester, UK, and Freetown, Sierra Leone This very unusual pattern soon became known as anomalous diversity [5], and has been replicated by a number of other studies, for example [6,7]. Unexpected was this apparent pattern that it spurred much further work, and many hypotheses were developed to explain it, some of which have been put to various levels of testing [8,9]
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