2.1.1 Historical framework: Population richness

Extended to the population level, the greater time and area for species to live in the tropics may have also provided ample time for populations to differentiate across a heterogeneous tropical habitat (Mittelbach et al., 2007; Rosenzweig, 1995; Terborgh, 1973). Therefore, across the very large number of species at low latitudes, we might expect high numbers of populations overall (TotPopR; Table 1). This high TotPopR might still apply to tropical environments in the Western Hemisphere even though contemporarily they have a smaller geographic area relative to North America, because the low latitude environments have been open and inhabited longer by species than habitable area at higher latitudes. On the other hand, the smaller contemporary area of tropical environments also means that there is now less opportunity for new populations to diverge within species (PopPerSpp) compared to within temperate environments (see also related concepts on species range sizes and diversification rates in the ecological and evolutionary frameworks below, respectively). Moreover, the lower number of temperate species in North America have had less time residing in open (connected) habitats, resulting in incomplete speciation but greater population structure across a species’ range. Such incomplete speciation in turn would likely lead to higher PopPerSpp than found at low latitudes and reduce or perhaps even eliminate tropical versus temperate differences in TotPopR. Aside from the ample time that species have had occupying low latitudes in the past, we next consider how historical adaptations can structure future evolution, and the implications for population richness.

The ancestral niche of a species clade determines future regions and habitat that the clade can disperse to and persist in, a phenomenon known as phylogenetic niche conservatism (Ackerly, 2003; Peterson, Soberon, & Sanchez-Cordero, 1999; Ricklefs & Latham, 1992; Wiens, 2004; Wiens & Donoghue, 2004). A species can then only broaden its niche through niche evolution and dispersal (Sandel et al., 2011; Wiens & Donoghue, 2004), which would influence population differentiation across a species’ range. For example, niche evolution may result in an increase in genetic differentiation among populations at range edges due to strong directional selection, similar to expectations from the central-marginal hypothesis (Eckert, Samis, & Lougheed, 2008; Guo, 2012; Hargreaves & Eckert, 2019; Willi, Fracassetti, Zoller, & Buskirk, 2018). With more differentiated populations at range edges, we might expect high latitudes to have higher PopPerSpp, as species that have expanded their ranges outwards from the tropics would likely have more populations in these high latitude areas. Conversely, low latitude clades would again be expected to have lower PopPerSpp. These clades are typically older, having had more time for speciation of the edge populations to occur, thus resulting in fewer populations in the remaining range.

Overall, when considering historical influences, we would expect higher PopPerSpp at high latitudes relative to low latitudes. Predictions about TotPopR are less clear in part because they are dependent on the magnitude of the difference in species richness and PopPerSpp between tropical and temperate regions. Nonetheless, more similar levels of TotPopR across latitudes seem plausible. For example, while the sheer number of species in the tropics may result in a high TotPopR at low latitudes, multiple factors discussed here may also generate high or higher TotPopR at high latitudes; or at least TotPopR might not be proportional to species richness or PopPerSpp. In short, these points illustrate how species richness and population richness may be uncoupled in many instances.

2.1.2 Historical framework: Genetic diversity

The historical extent of habitat and its availability can also influence current patterns of genetic diversity, but we would expect an opposite pattern from population richness—higher genetic diversity at low to intermediate latitudes. For example, since more time has passed for evolution to occur at lower latitudes, genetic diversity would accumulate across species (TotGenDiv) (Adams & Hadly, 2013). Average genetic diversity across populations within a tropical species (GenPerSpp) could also follow the same pattern as TotGenDiv, since older clades tend to have more genetic diversity (Willi et al., 2018). Conversely, habitat has been available for less time at high latitudes so organisms may not have had sufficient time to accumulate as much genetic diversity.

In addition to purely just having had more time or not, clade age and history would play a large role in structuring patterns of genetic diversity. For example, many North American clades have experienced glacial fragmentation across their ranges, followed by founder effects after glacial retreat. Founder effects like these can lead to an overall decrease of genetic diversity within a species (GenPerSpp) (Galbreath & Cook, 2004; Green, Sharbel, Kearsley, & Kaiser, 1996; Hewitt, 2000; Provan & Bennett, 2008; Stewart, Lister, Barnes, & Dalen, 2010; Willi et al., 2018). Additionally, adaptation to colder or new environments (i.e. niche evolution, Wiens & Donoghue, 2004) tends to elicit strong directional selection, often leading to losses in GenPerSpp (Eckert et al., 2008; Hargreaves & Eckert, 2019; Pierce, Gutierrez, Rice, & Pfennig, 2017).

GenPerSpp and TotGenDiv therefore may be lowest at high latitudes due to the strong influence of historical events and the recent establishment of populations [e.g. after the Last Glacial Maximum 23,000–18,000 yr bp (Hewitt, 2004)], leading to less time for alleles to accumulate in a given population from mutations. Miraldo et al. (2016) may support this expectation in finding higher mitochondrial genetic diversity (TotGenDiv) at low latitudes. However, re-analyses of their data (Gratton et al., 2017; Schluter & Pennell, 2017) showed a systematic northward bias in spatial autocorrelation and that the pattern was not consistent across species (i.e. GenPerSpp). Despite this, Schluter and Pennell (2017) demonstrated that mammalian and amphibian mitochondrial genetic diversity, equivalent to GenPerSpp here, has a slightly negative slope with latitude. Another study found some evidence for scale- and taxa-dependent latitudinal gradients in genetic diversity (Millette et al., 2019). These results are mostly consistent with the expectations from a historical perspective, wherein species at low latitudes have experienced more time for genetic diversity to accumulate, but some nuances appear to blur gradient patterns.

2.1.3 Historical framework: Conclusion

In general, historical hypotheses tend to predict higher PopPerSpp at high latitudes, similar levels of TotPopR across latitudes and more genetic diversity at low latitudes. While the historical time and area available for diversification may form the foundation from which species evolve, current patterns of intraspecific diversity may be the product of both historical and contemporary processes. Changes in population differentiation and genetic diversity can occur relatively quickly throughout time (e.g. tens to hundreds of years instead of thousands or millions) due to stochastic processes as well as increasing anthropogenic impacts (Goossens et al., 2006; Riley et al., 2006; Weider, Lampert, Wessels, Colbourne, & Limburg, 1997). Human impacts can cause species range barriers more quickly than otherwise expected, greatly reducing connectivity and thus increasing the likelihood of population differentiation (Ascensão et al., 2016; Cheptou, Hargreaves, Bonte, & Jacquemyn, 2017; Meyer, Kalko, & Kerth, 2009; Riley et al., 2006). Alternatively, human influences can homogenize populations by the movement of individuals through introductions, translocations, stocking and supplementation (Johnson et al., 2010; Tringali & Bert, 1998). The roles and effects of more contemporary ecological hypotheses are discussed in more depth below.

2.2.1 Ecological framework: Population richness

A prominent hypothesis of population dynamics is Rapoport’s rule (Ruggiero & Werenkraut, 2007; Stevens, 1989), the positive correlation of geographic range size with latitude, focusing on the climatic variation that organisms are exposed to and adapted for. Temperate species tend to populate large geographic ranges, so they experience a wide range of climatic variation; tropical species, conversely, are limited to small geographic ranges due to specialization, with some exceptions (Ruggiero & Werenkraut, 2007; Stevens, 1989). If a species has a small range, there is less area for populations to become isolated and differentiated, and the species will likely have fewer populations. Additionally, each population may have smaller population sizes as Currie et al. (2004) noted that both population size and density of individuals decrease towards low latitudes. This could interfere with the ability of populations to become established in a new area if they have small suitable ranges within which to disperse. If consistent across taxa, species with large range sizes may, in general, harbour more PopPerSpp, leading to a reverse latitudinal trend for population richness relative to species richness.

The size of a species’ range would also influence gene flow and subsequent population differentiation. Greater distances between populations in large-ranged species would make gene flow more difficult, all else being equal. Thus, according to Rapoport’s rule, we would expect populations at or near the edge of a species’ range to experience lower gene flow. Range-edge populations are more likely to be geographically isolated and more differentiated from neighbouring populations, particularly for large-ranged species (Eckert et al., 2008; Hargreaves & Eckert, 2019; Pelletier & Carstens, 2018; Stevens, 1989; Willi et al., 2018). A likely consequence across a species’ range would be more distinct populations at or near range edges, and fewer within the ‘core’ due to increased gene flow (Pelletier & Carstens, 2018). This could result in higher TotPopR in areas where many species range edges overlap extensively, that is, at low latitudes. Overall, larger range sizes tend to be associated with increased distances between populations, leading to the expectation of increasing PopPerSpp with range size.

2.2.2 Ecological framework: Genetic diversity

A species’ range size could also reflect levels of genetic diversity, where small ranges may have lower genetic diversity per species (GenPerSpp) (Fine, 2015). Small population size or density, typical of tropical species, can lead to increased levels of ecological or genetic drift (Fine, 2015; Mittelbach et al., 2007; Siqueira et al., 2019). While genetic drift has not received much empirical support as an explanation for the species richness gradient, it could be more relevant for intraspecific diversity. As patch size or a species’ range size is correlated with population size (Bernos & Fraser, 2016; Currie et al., 2004), it is a reasonable assumption that species with small ranges have, on average, smaller population sizes. This would lead to increased levels of genetic drift and an increased possibility of inbreeding, and hence lower GenPerSpp. Alternatively, for species that are population rich, different populations could drift in different directions, and show an inflated GenPerSpp collectively across the species. Herein the combined analysis of genetic diversity metrics such as observed heterozygosity and mean number of alleles (MNA) within species (see GenPerSpp in Glossary) could provide a better indication of genetic diversity within a species or taxonomic group. MNA responds to inbreeding, population bottlenecks and genetic drift more quickly than heterozygosity (Allendorf, 1986; Allendorf & Luikart, 2007; Nei, Maruyama, & Chakraborty, 1975) and could indicate which populations are at risk of low genetic diversity. Additionally, heterozygosity can relate to long-term effective population size (see Glossary; Bazin et al., 2006; Hansson & Westerberg, 2002) and in some instances, adaptation to environmental change (Fraser et al., 2019; Saccheri et al., 1995). Assessing either MNA or heterozygosity alone might mask some of these potential patterns and thus it is important to distinguish between the two metrics. However, larger geographic ranges may not necessarily correspond to higher levels of GenPerSpp. As previously discussed, many high latitude organisms have also experienced glacial fragmentation that has resulted in a history of small population size and thus lower overall GenPerSpp and TotGenDiv (Galbreath & Cook, 2004; Green et al., 1996; Hewitt, 2000; Provan & Bennett, 2008; Stewart et al., 2010).

A species’ range size and dispersal abilities also influence the extent of gene flow between core and edge populations, affecting the maintenance of genetic diversity (Bohonak, 1999; Martinez et al., 2018; Pelletier & Carstens, 2018; Willoughby et al., 2017). For example, having a large geographic range but being mobility-limited, such as in small rodents, reduces the likelihood of gene flow between northern and southern populations strictly because these would be so far apart. Conversely, species with small geographic ranges may have comparatively more gene flow across their range and have fewer genetically distinct populations. Fishes show interesting patterns: large ranges typical of marine or anadromous fishes tend to have higher genetic diversity and lower population differentiation, whereas freshwater fishes with typically limited dispersal capabilities tend to have lower genetic diversity and higher population differentiation (DeWoody & Avise, 2000; Martinez et al., 2018; Medina et al., 2018). Thus, species that are not as capable of dispersing large distances may show stronger latitudinal patterns for both population and genetic diversity, regardless of range size, due to differences in population differentiation across their range (Bohonak, 1999). Populations may become more easily differentiated in large ranges, and as a result, local adaptation may inflate total genetic diversity across all populations (TotGenDiv) within a species’ range. Interestingly, however, non-migratory vertebrate species tend to have more genetic diversity than their migratory counterparts, except for birds, which show the opposite pattern (Willoughby et al., 2017). This could be an indication that migratory species more frequently encounter fragmentation that causes reductions in genetic diversity, potentially blurring the otherwise expected pattern of increasing genetic diversity with range size.

2.2.3 Ecological framework: Conclusion

Overall, biological differences between taxonomic groups can play a large role in determining population richness and genetic diversity gradients. In general, we expect large-ranged, limited dispersers to show more intraspecific diversity, and small-ranged, capable dispersers to show lower intraspecific diversity. If intraspecific diversity generally increases with range size, we would expect both higher population richness and genetic diversity at higher latitudes. However, dynamics within particular taxonomic groups could cause population richness and genetic diversity gradients to be much more idiosyncratic than species gradients.

2.3.1 Evolutionary framework: Population richness

Temperate clades are seeing an increase in speciation rates, and this is at least in part due to the opening of available habitat (Schluter, 2016; Schluter & Pennell, 2017). Schluter (2016) describes this as the ecological opportunity hypothesis, wherein areas having more open niches tend to correspond with faster diversification rates. Low latitudes have historically had a wider range of niches and higher rates of speciation than higher latitudes, giving the tropics a ‘head start’ to accumulate species. If this is the case, the tropics could have ‘maxed out’ on speciation rates and population richness; species are now restricted to small ranges due to specialization of niches and have on average fewer populations (PopPerSpp). Note again it is possible that if every species has at least one population, the tropics could have higher TotPopR than temperate regions simply due to having much higher species richness, although this effect could be mediated if high latitude species have much higher PopPerSpp. This is where comparing PopPerSpp and TotPopR is useful. The tropics might have a higher or similar absolute number of populations (TotPopR), but higher latitudes—which tend to have larger species ranges (Stevens, 1989) and now increasing speciation rates—would have a higher PopPerSpp (Figure 1).

Increasing diversification rates at high latitudes has consequences for PopPerSpp. As climate shifts open historically inhospitable regions in temperate areas, more diversification is facilitated (Schluter, 2016; Schluter & Pennell, 2017; Weir & Schluter, 2007). This diversification process leads to higher PopPerSpp as species begin moving into novel habitats, without completing speciation due to insufficient time. Faster diversification rates should lead to more populations diverging, thus leading to an increase in PopPerSpp, and TotPopR for a given area over time.

2.3.2 Evolutionary framework: Genetic diversity

Diversification rates can also play a role in a species’ ability to adapt and maintain genetic diversity. For example, if a species experiences faster diversification rates at the edge of its range due to strong directional selection pressures, a given population could become locally adapted and may see a drop in genetic diversity relative to other populations (Eckert et al., 2008; Ellegren & Galtier, 2016; Guo, 2012; Hargreaves & Eckert, 2019; Willi et al., 2018). Higher diversification rates could lead to more rapid population differentiation, leading to decreases in fitness should ongoing gene flow occur (Schluter, 2016; Seidel, Rockman, & Kruglyak, 2008) and encouraging further speciation. Because this process of diversification has likely already occurred at low latitudes, they may remain a hotspot for genetic diversity across species (TotGenDiv). However, we expect patterns of within-species genetic diversity (GenPerSpp) to be sensitive to taxonomy, particularly for older clades, which may have accumulated more genetic diversity, whether such clades originated at high or low latitudes.

2.3.3 Evolutionary framework: Conclusion

Evolutionary processes such as diversification rates influence intraspecific diversity expectations by affecting the trajectory of populations within species. While PopPerSpp is expected to be highest at high latitudes due to incomplete diversification within species, TotPopR may have similar levels across latitudes regardless due to many populations across a much larger number of species having already diverged or partially diverged over time. Genetic diversity is predicted to be typically higher at low latitudes, especially for TotGenDiv where diversification across species has led to the accumulation of genetic diversity. On the other hand, trends for GenPerSpp are much more variable across taxonomic groups due to different diversification rates and decreases in genetic diversity from niche specialization and/or novel adaptations.

4.1.1 H1: Geographic distribution hypothesis

We term the first hypothesis the geographic distribution hypothesis, which posits that a positive relationship exists between a species’ geographic range size and its population richness. PopPerSpp should therefore increase with increasing latitude because temperate species ranges are typically larger than in tropical species. Broadly speaking, we also expect different vertebrate groups to show different strengths for this pattern because of inherent differences between dispersal capabilities and environments inhabited (Sandel et al., 2011). For example, relative to other vertebrates, freshwater and anadromous fish species may show greater TotPopR and/or greater PopPerSpp across their ranges due to the easily fragmented nature of aquatic freshwater habitats through natural barriers (Tatarenkov, Healey, & Avise, 2010; Wofford, Gresswell, & Banks, 2005; Underwood, Mandeville, & Walters, 2016), dams (Roberts, Angermeier, & Hallerman, 2013; Wofford et al., 2005; Underwood et al., 2016), and the connectivity between fluvial environments and lakes (Hébert, Danzman, Jones, & Bernatchez, 2000; Underwood et al., 2016). Amphibians and reptiles (collectively, herptiles) may also show strong patterns between range size and PopPerSpp due to their generally limited ability for dispersal (Araújo, Pearson, & Rahbek, 2005; Green et al., 1996; Medina et al., 2018; Sandel et al., 2011) that leads to high subpopulation differentiation across a given species’ range. Birds and some mammals, conversely, tend to have greater dispersal capabilities than herptiles and some freshwater fishes (Araújo et al., 2005; Medina et al., 2018; Munguía, Townsend Peterson, & Sánchez-Cordero, 2008; Servín, Sánchez-cordero, & Gallina, 2003; Sutherland, Harestad, Price, & Lertzman, 2000). Thus, we expect these groups will have a lower TotPopR than fishes and herptiles due to homogenization of population structure, but more variable PopPerSpp depending on the specific species’ dispersal ability.

To test the geographic distribution hypothesis, we used a generalized linear model fitted with a gamma distribution where the number of populations for a given species (i.e. PopPerSpp, Supporting Information Table S1) was our dependent variable (n = 625 species, 5,172 populations; see Supporting Information Appendix S1: H1), while the natural logarithm of range size (km²), latitudinal extent (decimal degrees), and taxonomic class (amphibian, bird, anadromous or freshwater fish, mammal, reptile) were fixed effects. PopPerSpp and range size for each species can be found in Supporting Information Table S1. We also tested the linear relationships between range size and PopPerSpp for each taxonomic group (Figure 3a,b). These linear relationships were significant for all taxonomic groups combined (p  .001, R² = .03; Figure 3a), and fish separately (p = .003, R² = .07; Figure 3b), although they did not explain much variation in the data. There were no significant relationships between range size and PopPerSpp within amphibians, birds, mammals or reptiles (Figure 3b). For the generalized linear model, both the natural logarithm of range size and the latitudinal extent were significant (p = .022, < .001 respectively, Figure 3a, Supporting Information Table S2). The discrepancy across taxonomic groups could be due to a lack of thorough sampling across species ranges. When assessing taxonomic groups separately, amphibians, reptiles and birds tended to have data that were sparsely sampled across species ranges compared to other species, especially fishes. To account for this, we recommend future studies estimate the area represented by each population so that the percent of the species’ range that has been sampled can be included.

4.1.2 H2: Overlapping range hypothesis

Areas that have extensive species range overlap may have lower PopPerSpp due to higher competition, smaller range sizes, etc. (Kennedy et al., 2018; Pelletier & Carstens, 2018). For instance, lower latitudes are more likely to have high species richness, moderate TotPopR and lower PopPerSpp. Species richness is to the point of oversaturation at low latitudes (Schluter, 2016), and tropical species are generally restricted to smaller ranges (Currie et al., 2004; Stevens, 1989). The combination of small range sizes and fewer open niches would lower the number of intraspecific populations able to differentiate (or speciate with time), because fewer opportunities for local adaptation or population differentiation are available to occur across a species’ range (Schluter & Pennell, 2017; Weir & Schluter, 2007). Collectively, one might expect TotPopR to increase as species richness increases, but PopPerSpp to decrease with increasing species richness (overlapping range hypothesis).

To test the overlapping range hypothesis, we calculated the species richness in 500-km² equal area grid cells generated in the Behrmann projection across the American continents and correlated it using a linear model with both the absolute population richness (TotPopR) and the number of sampled populations of each species (PopPerSpp) (Figure 3c, Supporting Information Table S3; see Supporting Information Appendix S1: H2). The number of species in an area was positively correlated with TotPopR (p < .001, R² = .75; Figure 3c, Supporting Information Table S3), and PopPerSpp (p < .001, R² = .22, Figure 3d). While our analysis does not show the expected trend for PopPerSpp, this may again be due to incomplete population sampling for each species in the dataset. We note that the slopes of the two relationships (4.74 and 0.08 for TotPopR and PopPerSpp, respectively; Supporting Information Table S3) do provide some indication that the trends between the two population richness metrics are different and that different mechanisms may underpin them. Perhaps as species richness increases, PopPerSpp does not increase at a corresponding rate, indicating that species richness has some impact on the capacity for evolution of population richness within a species. If the actual number of populations within a species’ range is known, we expect this positive relationship between PopPerSpp and species richness to break down further, showing the negative relationship as predicted, or a very weak relationship.

4.1.3 H3: Range-restricted gene hypothesis

If species range size influences population size and gene flow between populations (Currie et al., 2004; Fine, 2015), and range size is also correlated with PopPerSpp (H2), then genetic diversity will be more strongly associated with range size than with latitude (range-restricted gene hypothesis)—although some latitudinal patterns may occur as a result of this association. Previous studies have found latitudinal trends for genetic diversity, where higher alpha and beta genetic diversity (equivalent to GenPerSpp and TotGenDiv, respectively) were observed at low latitudes (Adams & Hadly, 2013; Miraldo et al., 2016; Schluter & Pennell, 2017). Even when spatial autocorrelation (Gratton et al., 2017), number of DNA sequences and species identity (GenPerSpp) (Schluter & Pennell, 2017) were accounted for, authors found a latitudinal gradient in genetic diversity—although the slope of the relationship was very small (e.g. −0.002, Supplementary Methods in Schluter & Pennell, 2017). However, these data were based on mitochondrial genetic diversity (mitochondrial DNA, mtDNA) rather than nuclear DNA. mtDNA may not be selectively neutral (Bazin et al., 2006), which is important for standardized comparisons across species and populations. Moreover, mtDNA may not reflect genetic variation in the nuclear genome, which is integral for adaptation to environmental change (Ballard & Whitlock, 2004; Bazin et al., 2006; Ghalambor, McKay, Carrols, & Reznick, 2007; Hurst & Jiggins, 2005; Sgrò, Lowe, & Hoffmann, 2011). Conversely, microsatellite nuclear DNA variation can be a reasonable metric of genome-wide variation, and the polymorphic nature of microsatellite loci is able to better resolve population structure at fine scales (Angers & Bernatchez, 1998; Jarne & Lagoda, 1996; Väli, Einarsson, Waits, & Ellegren, 2008). Microsatellite-based estimates of GenPerSpp may show a weaker latitudinal pattern than the TotGenDiv metric adopted in past mtDNA studies (e.g. Miraldo et al., 2016). Although non-neutrality has been observed in some studies involving nuclear microsatellite loci (Ranathunge et al., 2018; Selkoe & Toonen, 2006; Wiehe, 1998), this does not appear to be widespread in MacroPopGen (Lawrence et al., 2019; see also Selkoe & Toonen, 2006).

We tested the range-restricted gene hypothesis using generalized linear mixed models and model selection where one model was constructed for each population-level genetic diversity metric as the dependent variable (observed heterozygosity, HO, and MNA); taxonomic identity was accounted for with random effects. Fixed effects for both models included range size, latitudinal extent, HO or MNA (i.e. HO for MNA, MNA for HO), and other study-specific metrics (for details see Supporting Information Appendix S1: H3). All population-specific data can be found in the MacroPopGen database (Lawrence et al., 2019). After model selection, the mixed model for heterozygosity included the interaction between MNA and the number of microsatellite loci as well as taxonomic class, and the random effects for study, species and family (Supporting Information Table S2):

The model for MNA only included interactions between HO and number of microsatellite loci, as well as taxonomic class, and the random effects for study, species and family:

The retention of HO, MNA and the number of microsatellite loci in the models is not entirely surprising and indicates that these factors are more associated with genetic diversity than range size or latitudinal extent, although this effect varies according to taxonomic grouping (Figure 3f). While these measures of genetic variation are sometimes (weakly) correlated (Comps, Gömöry, Letouzey, Thiébaut, & Petit, 2001), the two metrics still indicate differences in population processes, as we discussed in the ecological framework, where decreases in MNA do not always correspond with decreases in HO (Allendorf, 1986). Additionally, we used variance inflation factors to test for collinearity between variables and found no evidence for any statistically significant collinearity (see Supporting Information Appendix S1: H3). Thus, we wanted to include both metrics in model selection to test how the effects of range size would compare to the effect of each metric on each other. Indeed, when we tested a model that only included range size and latitudinal extent, only latitudinal extent (not range size) was significant. Figure 3e demonstrates this lack of significance for range size, while a positive relationship is found in Figure 3f (note only MNA is shown but results were similar for HO). Varying relationships among taxa were also supported by the different slopes of linear relationships shown in Figure 3f. The inclusion of HO, MNA and number of microsatellite loci could indicate that genetic diversity metrics are sensitive to the number of alleles present within a population, where more alleles and loci being present increases the likelihood of being heterozygous and vice versa (Figure 3f). Together, the results of these models suggest that genetic diversity is not particularly influenced by range size or the latitudinal breadth of a species’ range.