Latitudinal gradients of parasite richness: a review and new insights from helminths of cricetid rodents

Abstract

The latitudinal diversity gradient (LDG), or the trend of higher species richness at lower latitudes, has been well documented in multiple groups of free-living organisms. Investigations of the LDG in parasitic organisms are comparatively scarce. Here, I investigated latitudinal patterns of parasite diversity by reviewing published studies and by conducting a novel investigation of the LDG of helminths (parasitic nematodes, trematodes and cestodes) of cricetid rodents (Rodentia: Cricetidae). Using host-parasite records from 175 parasite communities and 60 host species, I tested for the presence and direction of a latitudinal pattern of helminth richness. Additionally, I examined four abiotic factors (mean annual temperature, annual precipitation, annual temperature range and annual precipitation range) and two biotic variables (host body mass and host diet) as potential correlates of parasite richness. The analyses were performed with and without phylogenetic comparative methods, as necessary. In this system, helminths followed the traditional LDG, with increasing species richness with decreasing latitude. Nematode richness appeared to drive this pattern, as cestodes and trematodes exhibited a reverse LDG and no latitudinal pattern, respectively. Overall helminth richness and nematode richness were higher in areas with higher mean annual temperatures, annual precipitation and annual precipitation ranges and lower annual temperature ranges, characteristics that often typify lower latitudes. Cestode richness was higher in areas of lower mean annual temperatures, annual precipitation and annual precipitation ranges and higher annual temperature ranges, while trematode richness showed no relationship with climate variables when phylogenetic comparative methods were used. Host diet was significantly correlated with cestode and trematode species richness, while host body mass was significantly correlated with nematode species richness. Results of this study support a complex association between parasite richness and latitude, and indicate that researchers should carefully consider other factors when trying to understand diversity gradients in parasitic organisms.

Influence of elevation on the species–area relationship

“…Qian, Fridley, and Palmer (2007) also found that z consistently decreased with increasing latitude for the North American flora, whereas Lyons and Willig (2002), using bats and marsupials of North and South America, found that z increased with latitude, while c exhibited a reverse pattern. Thus, despite the great interest of ecologists and biogeographers in latitudinal gradients (Fattorini & Ulrich, 2012; Lomolino, Riddle, Whittaker, & Brown, 2010; Preisser, 2019), it is not clear if and how z and c vary with latitude.…”

What would it take to describe the global diversity of parasites?

“…In total, we estimated 103,078 species of helminth parasites of vertebrates, most strongly represented by trematodes(44,262), followed by nematodes(28,844), cestodes(23,749), and acanthocephalans(6,223). Using an updated estimate of bony fish richness significantly increased these estimates from previous ones, with over 37,000 helminth species in this clade alone.Birds and fish were estimated to harbor the most helminth richness, but reptiles and amphibians had the highest proportion of undescribed diversity.…”

Integrating climate and host richness as drivers of global parasite diversity

“…In addition, climate is a strong correlate of species diversity for free-living organisms, especially at large spatial grains and extents (Field et al, 2009). Previous studies on parasites have also emphasized the prominence of climate as an important global predictor of parasite diversity (Dunn et al, 2010;Guernier et al, 2004;Preisser, 2019). Some proposed explanations for how climate affects species diversity are related to the speed of evolutionary processes, the amount of available energy and to species tolerances (see Currie et al, 2004).…”

“…There is usually a strong correlation between species richness and climatic components leading to various hypotheses of how climate affects species diversity (Currie et al, 2004;Field et al, 2009). However, macroecological studies on parasite diversity are primarily focused on host-related drivers and latitudinal gradients (Poulin, 2014;Morand, 2015; but see Guernier et al, 2004;Preisser, 2019). Most studies found a positive association between host and parasite diversity regardless of host and parasite taxa (Kamiya et al, 2014a).…”

“…Temperature and precipitation belong to the core of some prominent climate-based explanations, but how they affect parasite diversity remains largely unexplored (but see Guernier et al, 2004;Preisser, 2019). For instance, according to the climate stability hypothesis, the absence of marked climatic extremes has a positive effect on net speciation rates by lowering extinction rates and promoting specialization (Fine, 2015;McKenna & Farrell, 2006).…”