Presence of an invasive species reverses latitudinal clines of multiple traits in a native species

1 INTRODUCTION

Latitudinal gradients and clines are key features of biogeography found across taxa and ecosystems (De Frenne et al., 2013; Fuhrman et al., 2008; Hawkins, Porter, & Felizola Diniz-Filho, 2003; Roy, Jablonski, Valentine, & Rosenberg, 1998). These patterns describe how factors such as diversity, abundance, and species’ traits vary across space and in association with biotic and abiotic environmental factors (Willig, Kaufman, & Stevens, 2003). The study of latitudinal clines has provided insight into how key organismal traits, such as body size and fecundity, impact fitness (Cardillo, 2002; Jonsson & L'abée-Lund, 1993; Sibly, 1991). Latitudinal clines are incorporated into theory and development of models explaining life-history variation and the role of plasticity and adaptation in shaping traits (Niewiarowski, 1994; Reznick & Ghalambor, 2001). Some latitudinal clines are well known in certain taxa, such as those described by Bergmann's, Allen's, and Gloger's Rules, and are considered to be driven by abiotic factors such as temperature and season length (Meiri & Dayan, 2003; Millien et al., 2006). Much research has focused on the role that abiotic factors, especially thermal regimes (Chen, Hill, Ohlemüller, Roy, & Thomas, 2011; De Frenne et al., 2013; Huey, Gilchrist, & Hendry, 2005), play in structuring latitudinal clines. However, biotic factors, such as mutualisms, symbioses, parasite–host interactions, or predator–prey relationships, may also be important drivers of clines and have received increasing study (Merino et al., 2008; Moles, Bonser, Poore, Wallis, & Foley, 2011; Schemske, Mittelbach, Cornell, Sobel, & Roy, 2009). Crucially, both abiotic and biotic factors theorized to drive formation of clines are shifting as a result of global environmental change.

Research into the biogeographical impacts of abiotic and biotic factors has taken on a new importance in the current era of rapid anthropogenic change (Araújo & Luoto, 2007). Changes including warming, increasing climatic variability, and invasion of non-native species (Butchart et al., 2010) produce novel selective regimes (Parmesan, 2006; Sax et al., 2007). Some species may not be able to successfully adapt to these changes, especially those from polar (Kerby & Post, 2013; Mills et al., 2013) or alpine regions (Dullinger et al., 2012). However, many species do adapt to novel selective regimes (Strauss, Lau, & Carroll, 2006), and biogeographical patterns, including latitudinal clines, can be altered in the process. Clines in genetic variation, body size, and phenology have shifted to track changes in abiotic conditions (Gardner, Heinsohn, & Joseph, 2009; Millien et al., 2006; Yang & Rudolf, 2010). Similarly, species introduced to novel environments recreate clines found in their native range and develop new clines, within as few as 20 years (Huey et al., 2005; Huey, Gilchrist, Carlson, Berrigan, & Serra, 2000; Johnston & Selander, 1964; Lee, 2002; Liu, Li, & McGarrity, 2010). Studying effects of rapid change on clines can help us understand environmental factors underlying the formation of clines and provide insight into how novel ecological pressures alter species’ traits (De Frenne et al., 2013; Teplitsky & Millien, 2014).

Much research on how global change alters latitudinal clines has focused on how changes in abiotic conditions drive shifts in genotypes and phenotypes (Edeline, Lacroix, Delire, Poulet, & Legendre, 2013; Gardner et al., 2009; Millien et al., 2006; Rombouts et al., 2009). Some types of global environmental change, including biological invasions, provide valuable insights into the role of biotic factors in driving the formation and alteration of latitudinal gradients and clines (Bruno, Fridley, Bromberg, & Bertness, 2005). Invasive species occur over large geographical extents and can apply strong selective pressures to native species (Salo, Korpimäki, Banks, Nordström, & Dickman, 2007; Sax & Gaines, 2008). This provides an opportunity to assess how rapid changes in biotic factors affect existing clines. Additionally, while most studies of changes in biogeographical clines address one focal trait, under some conditions, including biological invasions, multiple traits within a species can shift simultaneously (Langkilde, 2009; Shine, Brown, & Phillips, 2011). Here, we examine the role of novel predation pressure, a biotic factor, in altering latitudinal clines in multiple traits in a single species using a system of invasive predatory ants and native lizards.

Red imported fire ants (Solenopsis invicta) were introduced through the port of Mobile, AL, USA, in the 1930s and have spread throughout the southeastern Unites States (Figure 1). They consume and can exert novel selective pressures upon native species including eastern fence lizards (Sceloporus undulatus) (Allen, Epperson, & Garmestani, 2004; Langkilde, 2009). Lizards, and especially fence lizards, have been valuable organisms for studying ecogeographical rules (Angilletta, Niewiarowski, Dunham, Leaché, & Porter, 2004; Du, Warner, Langkilde, Robbins, & Shine, 2010; Niewiarowski, 1994). Fence lizard populations are known to differ in a suite of traits, including antipredator behavior, stress (glucocorticoid) responsiveness, and morphology, according to whether or not they have been invaded by fire ants (Graham, Freidenfelds, Mccormick, & Langkilde, 2012; Langkilde, 2009). Lower latitude populations of fence lizards, which have been invaded by fire ants for longer periods of time (Figure 1), are more behaviorally responsive to fire ants and have correspondingly longer hindlimbs and higher stress responsiveness, adaptations which support increased survival of fire ant attacks (Graham et al., 2012; Langkilde, 2009), as compared to higher latitude, fire ant-naïve populations. These previous studies assume a causal effect of fire ant presence but have not addressed whether climatic factors or pre-existing latitudinal clines in antipredator behavior, stress responsiveness, or hindlimb length might explain the observed patterns in these traits as well as, or better than, the presence of fire ants. For these three traits, we examined whether observed patterns represent pre-existing clines or instead are due to changes in pre-existing clines that correspond to novel pressure by invasive fire ants.

2 MATERIALS AND METHODS

3 RESULTS

In areas uninvaded by fire ants, fence lizard populations show a decreasing incidence of crypsis behavior with increasing latitude (Figure 2a; β = −0.312, F_1,8 = 20.080, p < 0.001). However, in areas invaded by fire ants, this latitudinal cline is reversed (difference in slopes: t = 5.824, p < 0.001), and lizards are most likely to break crypsis and flee in response to fire ants in lower latitude populations with longer history of fire ant invasion (Figure 2a; β = 0.393, F_1,8 = 13.78, p < 0.001, n = 487).

In areas uninvaded by fire ants, stress responsiveness of female fence lizards shows a positive, but nonsignificant trend with latitude (β = 0.121, F_1,8.23 = 2.528, p = 0.149). However, this relationship is significantly different in populations from fire ant-invaded areas (Figure 2b; difference in slopes: t = 2.423, p = 0.017), where populations at the lowest latitudes have the greatest increases in CORT following exposure to a standardized stressor (β = −0.269, F_1,12.78 = 3.640, p = 0.079, n = 95).

Fence lizard specimens collected prior to fire ant invasion across the full latitudinal range of this species show a positive relationship between relative hindlimb length and latitude (Figure 3; β = 0.0018, F_1,80.6 = 5.681, p < 0.020, n = 165), with potential drivers of this pattern including temperature and precipitation (Supporting Information Table S2). This trend is not different from the relationship between hindlimb length and latitude in present-day fence lizard populations uninvaded by fire ants (Figure 2c; β = 0.0031, F_1,7.4 = 9.692, p < 0.016, n = 706; difference in slopes of historical vs. present-day: t = −1.101, p > 0.271). The direction of this cline is reversed, however, in populations invaded by fire ants (difference in slopes of uninvaded vs. invaded: t = 4.444, p < 0.001), which have proportionally longer limbs at lower latitudes (Figure 2c; β = −0.0044, F_1,8.5 = 9.949, p < 0.013, n = 744).

Models constructed using climate data had higher AICc values than models including fire ant presence for all three traits (Table 1).

4 DISCUSSION

For all three traits we examined in fence lizards—antipredator behavior, physiological stress responsiveness, and relative hindlimb length—we found evidence that clines are significantly different where lizards coexist with fire ants than in areas which remain unoccupied by fire ants. The putative trait changes we observed in areas invaded by fire ants are all in directions that confer adaptive advantages to lizards responding to fire ant attacks.

Fence lizards often employ crypsis (i.e., remaining still and not fleeing) as an antipredator tactic. In areas uninvaded by fire ants, we found that use of crypsis by fence lizards decreases with latitude (Figure 2a). This pattern coincides with observations and predictions in other organisms that antipredator behavior should decrease with latitude, likely as a consequence of reduced predator diversity and abundance at higher latitudes (Díaz et al., 2013). However, in areas invaded by fire ants, using crypsis as an antipredatory behavior is ineffective, as lizards which do not flee from fire ants are envenomated by additional ants and may quickly be overwhelmed (Freidenfelds, Robbins, & Langkilde, 2012). In this situation, increases in fleeing behavior (i.e., decreases in crypsis) are adaptive and allow escape from and survival of fire ant attacks (Langkilde, 2009). In areas invaded by fire ants, the relationship between latitude and use of crypsis shifted to become positive, with the lowest levels of crypsis occurring in the southernmost populations. This pattern matches predictions that adaptive increases in lizard anti-ant behavior should be greater in populations exposed to fire ants for longer periods of time (Langkilde, 2009) and corresponds to other observations of predator-driven natural selection on behavior in lizards (Lapiedra, Schoener, Leal, Losos, & Kolbe, 2018).

Increases in circulating levels of glucocorticoid stress hormones, a form of physiological stress responsiveness, facilitate organismal responses to a variety of challenges (Romero, 2004) and may covary with environmental stressors across latitudes (Breuner et al., 2003). Generally, stress responsiveness in clutching females is predicted to decrease with latitude during the breeding season to maximize fitness (Wingfield, 1988). However, previous work in Sceloporus has shown no evidence for a latitudinal cline in stress responsiveness of breeding females (Hews & Abell Baniki, 2013). In areas uninvaded by fire ants, we found that stress responsiveness in breeding female fence lizards is weakly and nonsignificantly associated with latitude (Figure 2b), in agreement with previous research. In contrast, in populations of lizards co-occurring with fire ants, the relationship between latitude and responsiveness is significantly different. Lizards from populations at the lowest latitudes have the highest levels of stress responsiveness (Figure 2b), suggesting an incipient shift in stress physiology (McCormick, Robbins, Cavigelli, & Langkilde, 2017). This physiological response supports more effective escape from fire ants (Trompeter & Langkilde, 2011) and primes lizards to flee from future attacks (Langkilde, Thawley, & Robbins, 2017), thus supporting observed adaptive shifts in antipredator behavior.

In areas uninvaded by fire ants, we found a significantly positive cline between relative hindlimb length and latitude (Figure 2c). This relationship was the same as that observed in museum specimens collected from areas free of fire ants at the time of collection (Figure 3), suggesting the cline predates fire ant invasion. However, this cline is reversed in populations co-occurring with fire ants, with southern populations having longer hindlimbs. Increased hindlimb length in eastern fence lizards is adaptive in fire ant-invaded populations as this trait supports higher sprint speeds (Miles, 1994) during flee behaviors and improves removal of attacking fire ants by twitch behaviors (Langkilde, 2009). Additionally, this pattern of increasing hindlimb length at lower latitudes, where fence lizard populations have coexisted longer with fire ants, matches the prediction that this adaptive trait should increase with the evolutionary time that populations have been exposed to fire ants (Langkilde, 2009).

It is possible that environmental factors other than fire ant presence or contemporary processes co-occurring with fire ant invasion have driven the observed clines in these traits. However, models using our selected set of abiotic variables (temperature, precipitation, and elevation) performed worse than those including fire ant presence in predicting observed patterns in traits (Table 1). For CORT responsiveness, however, precipitation in the warmest quarter of the year performed similarly to the model including fire ant invasion (ΔAICc = 0.193), suggesting that this climatic factor could influence stress responsiveness. However, as there is no a priori prediction for this relationship in this system or related literature, and CORT responsiveness can play a role in facilitating adaptive behavior (Langkilde et al., 2017), we do not consider it strong evidence against the effect of fire ants on stress responsiveness. For hindlimb length, analyses using specimens preserved in museums show that abiotic factors, including precipitation in the warmest quarter of the year, may have contributed to the historic cline in this trait (Supporting Information Table S2). The current pattern in hindlimb length diverges strongly from this historical cline, suggesting that the pattern we observed is driven by recent, nonclimatic change.

An additional consideration is that the invasion of non-native species may be related to other landscape-level factors, such as land use or habitat type, or co-occurs with other contemporary processes. For example, fire ants typically occupy open, disturbed habitats, so changes in land use could contribute to the pattern we observe here. However, fence lizards also favor open, disturbed habitat, and all of our study populations were found in similar disturbed edges of mixed forests, suggesting that land use differences are not driving observed clines. There may be other contemporary processes that we have not considered that could explain our results. These findings support further research into the importance of biotic environmental changes, such as the expansion of invasive species, vs. other contemporary processes in the formation of clines.

Our results show that adaptive changes to novel pressures can occur simultaneously across multiple pathways and encompass multiple traits (morphology, physiology, and behavior). The adaptive values of trait shifts are linked, as changes in stress responsiveness and hindlimb length support increased flight behaviors (Langkilde, 2009; Langkilde et al., 2017; Trompeter & Langkilde, 2011), allowing an integrated organismal response to a single novel biotic threat. Evidence suggests that heritable variation may underlie some trait shifts in fence lizards, as limb length in Sceloporus is heritable (Langkilde, 2009; Tsuji, Huey, Berkum, Garland, & Shaw, 1989) and changes in response to selection in lizards (Calsbeek & Irschick, 2007), while transgenerational factors affect both antipredator behavior (Robbins & Langkilde, 2012) and stress responsiveness (McCormick et al., 2017). However, the relative contributions of genetic variation, epigenetic effects, and phenotypic plasticity to the formation of these clines are not yet clear and bear further investigation.

These results reveal that pre-existing latitudinal clines and the continuing presence of selective regimes that drove their formation may not preclude adaptation to environmental change even if novel pressures run counter to pre-existing ones. In fact, latitudinal clines in a variety of traits are labile (Brommer, Hanski, Kekkonen, & Väisänen, 2015; Gardner et al., 2009; Huey et al., 2005; Umina, Weeks, Kearney, Mckechnie, & Hoffmann, 2005). Variability, whether heritable or plastic, is a precondition for adaptive responses (Ayrinhac et al., 2004; Hoffmann, Shirriffs, & Scott, 2005). The existence of a cline may indicate that enough variation exists within a species for a given trait to allow adaptation to a range of environmental conditions. Conversely, lack of variation in key traits, especially across gradients of important environmental factors, could indicate species that may be unlikely or unable to adapt to environmental change (Bell, 2013; Carlson, Cunningham, & PaH, 2014; Hoffmann & Sgrò, 2011).

Both biotic (e.g., invasive species, habitat alteration) and abiotic (e.g., climate change) factors are changing increasingly rapidly as a result of human activities (Butchart et al., 2010). Whether and how species can adapt quickly enough to novel pressures imposed by anthropogenic activities are currently two of the most pressing ecological questions (Carlson et al., 2014; Huey, Buckley, & Du, 2018). Our research demonstrates that reversal of clines in the presence of novel selective pressures can occur in multiple traits over a broad portion of a species’ range within a short time frame [<80 years, ≈40 generations (Langkilde, 2009)], a period similar to those observed for clinal shifts in response to climate change or species’ introductions to novel areas (Hoffmann & Sgrò, 2011; Huey et al., 2005; Johnston & Selander, 1964). Although these clinal shifts are adaptive, they deflect traits from their original evolutionary trajectories (Figure 2) and thus may mismatch animals to the conditions under which they historically evolved (e.g., Thawley & Langkilde, 2017). As fire ants invade more northern lizard populations, we expect trait values in these populations to respond by shifting from historical norms, providing a further powerful test of the ability of invasive species to reshape clines in this system. This work shows that biotic factors, such as the presence of a novel invader, may serve as selective pressures over large geographical extents, in this case, their presence not just shifting but actually reversing clines. Given that biotic factors can quickly alter latitudinal clines, future studies should address if biotic factors may mediate or be the primary cause of changes in patterns currently attributed to abiotic drivers such as climate change. As species introductions accelerate and novel ecological communities become more common (Moyle & Light, 1996; Strauss et al., 2006), biotic interactions may have an increasingly important role to play in structuring latitudinal gradients and clines.