Abstract
Functional biogeography, or the study of trait-based distributional patterns, not only complements our understanding of spatial patterns in biodiversity, but also sheds light on the underlying processes generating them. In parallel with the well-studied latitudinal diversity gradient, decades-old ecogeographical rules also postulate latitudinal variation in species traits. Notably, species in the tropics are predicted to have smaller body sizes (Bergmann's rule), narrower niches (MacArthur's rule) and smaller geographical ranges (Rapoport's rule) than their counterparts at higher latitudes. Although originally proposed for free-living organisms, these rules have been extended to parasitic organisms as well. In this review, I discuss the mechanistic hypotheses most likely to explain latitudinal gradients in parasite traits, and assess the empirical evidence obtained from comparative studies testing the above three rules as well as latitudinal gradients in other parasite traits. Overall, there is only weak empirical support for latitudinal gradients in any parasite trait, with little consistency among comparative analyses. The most parsimonious explanation for the existence of geographical patterns in parasite traits is that they are primarily host-driven, i.e. ecological traits of parasites track those of their hosts, with a direct influence of bioclimatic factors playing a secondary role. Thus, geographical patterns in parasite traits probably emerge as epiphenomena of parallel patterns in their hosts.This article is part of the theme issue ‘Infectious disease macroecology: parasite diversity and dynamics across the globe’.
Highlights
Biogeography has revealed large-scale patterns in the distribution of species across space, and the underlying mechanisms and processes generating those patterns [1]
Latitude serves as a convenient proxy for a range of bioclimatic factors, such as solar radiation and environmental stability, which together may act to favour and maintain high species diversity at low latitudes
The latitudinal diversity gradient applies to at least some parasite taxa when species richness is measured per host species [8–10]
Summary
Biogeography has revealed large-scale patterns in the distribution of species across space, and the underlying mechanisms and processes generating those patterns [1]. These three ecogeographical rules (Bergmann’s, MacArthur’s and Rapoport’s) pertain to three of the most important traits for parasite fitness and/ or disease epidemiology: body size, host specificity (the number of host species that can possibly be used at a given life stage, a measure of parasite niche breadth), and parasite geographical range size, respectively The first of these traits, parasite body size, is positively correlated with parasite fecundity across diverse parasitic taxa [30,31], whereas the other two properties play major and complementary roles in determining transmission success and population growth, as hedges against local extinction risk, and as determinants of zoonotic potential [32–34]. I synthesize the findings into an integrated latitudinal gradient of parasite traits, to reveal what potential selective forces have shaped their differential evolutionary paths across the globe
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