Functional biogeography of parasite traits: hypotheses and evidence

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’.

Infectious disease macroecology: parasite diversity and dynamics across the globe

“…[49,[119][120][121]) could be used to accelerate macroecological exploration of mutualistic networks and other understudied organismal systems. More importantly, findings presented in our issue highlight the need to look beyond the availability of their symbiotic partners and suggest that a deep understanding of the climatic niche of symbionts themselves is crucial for predicting their future distributions under global change [45,56,80].…”

“…Although parasites must rely on their hosts to complete their reproductive cycles [40,41], the diversity and distribution of parasites at large scale are constrained by both their hosts and the external environmental condition (e.g. Poulin [45]; Martins et al [56]). Therefore, parasitic organisms and free-living organisms are alike in the sense that their macroecological patterns are governed by complex processes, with both abiotic and biotic forces acting in concert to drive their diversity and distribution dynamics.…”

“…A review by Poulin [45] highlights trait-based biogeography as a new, promising research area in disease biogeography. In particular, the discussion is focused on three classic geographic patterns of species trait variation, laying out potential mechanisms in the context of parasite fitness and/or disease epidemiology.…”

Phanerozoic parasitism and marine metazoan diversity: dilution versus amplification

“…While we cannot directly test the amplification and dilution hypotheses in deep time in the same way as in living systems, we can test the relationships between the occurrence of parasitic traces (and the prevalence of parasitic traces) with the diversity of their hosts through the history of animal life. There is at least some support that diversity patterns in parasites might be closely linked to that of their hosts [70]. How have the number of occurrences of parasitism within geologic periods and the prevalence of parasitism changed since the first known occurrence in the early Cambrian?…”

Convergent patterns of body size variation in distinct parasite taxa with convergent life cycles

“…Although meant to explain patterns seen in free-living animals, Bergmann's rule can also apply to parasitic metazoans. For example, tapeworm species parasitic in pelagic sharks are larger if their hosts live at higher latitudes (Randhawa & Poulin, 2009; for other examples, see Poulin, 2021). However, the body sizes of parasitic species within any higher taxon are often correlated positively with the body sizes of their hosts (e.g., Harvey & Keymer, 1991;Maestri et al, 2020;Morand et al, 1996;Poulin, 1996), a phenomenon known as Harrison's rule (Marshall, 1981).…”