- Top of page
- Materials and methods
The mechanisms driving the distribution of species have been of interest in the field of biology for over 150 years. Darwin (1859) placed emphasis on the importance of behavioural interactions, competition, and its potential influence in limiting the distributions of species (McDonald 2003). Disentangling the factors that determine species distributions has been and continues to be a central goal of ecology and evolutionary biology (Brown 1984; Krebs 1985; Brown & Lomolino 1998; Gaston 2003; Case et al. 2005; Holt & Keitt 2005; Parmesan et al. 2005) especially as modern climate change and increasing anthropogenic pressure are currently reshaping the geographical distributions of many plants and animals (Parmesan & Yohe 2003; Hampe & Petit 2005). In fact, ecologists are now being called upon to predict changes in species distributions in response to exotic invasions, habitat alteration and loss, land-use change, and pollution (Parmesan et al. 2005); therefore, it is of utmost importance that we further our understanding of the influence of abiotic and biotic factors and their interplay on species distributions.
If competition between parapatric species at range margins affects the geographical distribution of one or both species, then ecologically suitable areas for both species in a contact zone should be especially important for investigating the dynamics of species interactions. However, contact zone studies still suffer from the inherent difficulty of determining the relative influence of abiotic conditions on biotic interactions (Anderson, Peterson & Gómez-Laverde 2002). That is, abiotic conditions may be biased towards the ecological requirements of one species over the other. Therefore, we suggest that with the ever-increasing use of ecological niche modelling techniques that combine spatially explicit environmental data and locality information of specimens (e.g. Elith et al. 2006), researchers will be able to accurately disentangle the effects of the environment from biotic interactions. Many niche modelling techniques implicitly include the effects of species interactions because a species’ current location is a result of both abiotic and biotic interactions (Araújo & Guisan 2006). Therefore, niche modelling cannot be used in isolation; additional experimental and manipulative studies should be used to assess the relative roles of biotic and abiotic factors on populations. In this study, we combine the use of broad-scale ecological niche models with fine-scaled studies of biotic interactions in order to examine how abiotic and biotic factors interact at contact zones.
Ecological niche modelling uses climate data associated with museum occurrence localities to generate a model of habitat suitability (high or low probability of species occurrence) which provides an objective way to help control for climate as a random variable in experimental design. Field studies are often limited to ‘ideal’ reciprocal transplant situations, such as obvious steep climatic clines found in montane regions (e.g. Berven 1981; Waser & Price 1985; Wang et al. 1997; Link et al. 2003; Angert & Schemske 2005; Iraeta et al. 2006). Unfortunately for ecologists, the majority of species do not interact at such narrow or easily defined contact zones. Therefore, in order to study how competitive interactions shape present-day distributions of wide-ranging species, ecological niche modelling can provide an a priori framework, allowing one to define areas that contain a habitable suite of climatic variables for both interacting species or to differentiate between climatic zones found across a species’ range.
In this study, we examine the influence of environmental conditions on intra- and interspecific competition at the southern range margin of the terrestrial salamander species Plethodon glutinosus Green 1818 (northern slimy salamander). For amphibians, the environment is known to drive broad-scale patterns of diversity (e.g. Buckley & Jetz 2007), and in salamanders, intraspecific and interspecific interactions have been found to shape the distributions of species in nature (Hairston 1987; Walls 1990; Jaeger & Forester 1993; Marshall, Camp & Jaeger 2004). Within the family Plethodontidae (over 70% of known salamanders), the social behaviour of members of the genus Plethodon has been extensively studied, and territoriality is believed to be common and mediate inter- and intraspecific competition (Hairston 1987; Jaeger & Forester 1993; Mathis et al. 1995; Marvin 1998a,b; Maerz & Madison 2000). The occurrence of competitive interactions between salamander species that share parapatric or narrowly overlapping ranges has been extensively documented in salamanders of this genus (e.g. Jaeger 1970; Hairston 1980a,b). Within plethodontid salamander assemblages, interactions can range from interference competition to outright competitive exclusion, all of which can influence spatial distributions (Hairston 1987; Jaeger & Forester 1993; Mathis et al. 1995; Marvin 1998a,b; Maerz & Madison 2000). At its southern range limit, P. glutinosus shares a contact boundary in western Kentucky, Tennessee, and west-central Alabama with the closely related Plethodon mississippi Highton 1989 (Mississippi slimy salamander) (Fig. 1). Specifically, we ask (i) Are climate conditions at the contact zone significantly different than conditions in the interior or core area of the range of P. glutinosus? (ii) Do abiotic conditions at the range boundary influence competitive interactions among these salamanders? and (iii) Do individuals from core and edge populations show regional adaptation in regards to competition?
Figure 1. The distribution of Plethodon glutinosus and Plethodon mississippi, and the area of range overlap based on Lannoo (2005). The range of P. glutinosus is shown with hatching; the range of P. mississippi is shown shaded in grey, and the area of range overlap is shown crosshatched.
Download figure to PowerPoint
- Top of page
- Materials and methods
It is frequently the case that the outcome of competitive interactions will be variable and environmentally specific (Dunson & Travis 1991; Gómez-Mestre & Tejedo 2002). In fact, differences in the competitive ability of plants and insects from range-edge and interior populations have been observed in multiple species (Hoffman & Blows 1994). Yet, defining the mechanisms behind this remains a challenge due to the number of causative factors that could be responsible for differences in competitive ability. It has frequently been suggested that range-edge populations are more prone to extinction, exist at lower density, and are genetically less diverse than core populations because they persist under less favourable abiotic conditions (Lawton 1993; Vucetich & Waite 2003; Hampe & Petit 2005); therefore, individuals from range-edge populations may be locally adapted but perform poorly under different abiotic conditions (Hoffman & Blows 1994). However, this hypothesis has received little attention, largely due to the difficulty in quantifying the abiotic differences between core and edge habitats.
Frequently, environmental conditions at a species range margin are less favourable than conditions in the interior portion of the range (Whittaker 1956; Parsons 1991; Lawton 1993; Brown, Stevens & Kaufman 1996; Sagarin & Gaines 2002; Vucetich & Waite 2003). In our study, climate conditions at the CZ in Alabama were significantly different from those in the interior portion of the range of P. glutinosus (Fig. 4). These different selective environments are likely to drive patterns of resource use and the outcome of competitive interactions. As the CZ is suitable to varying degrees for P. glutinosus and P. mississippi, it is likely that the southern range limit of P. glutinosus is determined by the combined affect of environment and species interactions. Although it is hard to argue that two field sites can be broadly representative of conditions in the core and range-edge, our data suggest that abiotic tolerance may shape competitive interactions in this species.
In this study, individuals of P. glutinosus from both core and range-edge populations faired significantly worse under all competition treatments at the AL field site. However, salamanders from range-edge populations did better in their local conditions in Alabama than those from core populations (Fig. 5). Yet individuals of P. glutinosus from both populations did equally well in the VA field site, suggesting that those individuals from the CZ may be better adapted for the relatively more stressful conditions in that environment. It is possible that this could be a factor that contributes to the maintenance of this range limit. However, these findings are not surprising because populations of widespread species often vary in their degree of specialization due to the different abiotic and biotic conditions found throughout their range (Gómez-Mestre & Tejedo 2002). By creating a more tangible abiotic niche (ENMs), we were able to disentangle the effect of regional variation in competitive ability with the effect that abiotic conditions have on species interactions.
The results from the factorial anova indicated that salamanders from core and range-edge populations did not differ in their response to competition itself (Table 2). The percentages of mass lost by salamanders in inter- and intraspecific competition treatments were quite similar suggesting that the response to competition itself may not vary geographically. However, the differences in response to competition became apparent when abiotic factors (field site) were added (Fig. 5; Table 2). This suggests that abiotic factors can influence competitive interactions.
If salamanders show regional adaptation in terms of competition, then individuals of P. glutinosus from core and range-edge populations would be expected to perform best under native conditions. In this study, individuals of P. glutinosus did have significantly different responses (in terms of mass loss) to competition at the two field sites (Table 2; Fig. 5). Individuals of P. glutinosus from range-edge populations performed better than individuals from core populations at the AL field site (Fig. 5). Yet, at the VA field site, there was little difference in mass loss by individuals of P. glutinosus from both populations (Fig. 5). These results suggest that abiotic tolerance may shape competitive interactions (Fig. 5; Table 2). That said, because we only had two field sites in this study, we must be cautious in our inferences; the factors responsible for the observed differences are unclear at this time and could be the result of several factors. It is possible that individuals of P. mississippi discriminated between adult P. glutinosus from AL and VA, a type of dear enemy recognition, albeit at a broad geographical scale (Jaeger 1981). Dear enemy recognition (Fisher 1954; Wilson 1975) is the widespread phenomenon of reduced aggression between adjacent territorial neighbours in comparison to aggression directed towards intruding unfamiliar individuals. This has been observed in salamanders of the genus Plethodon (Jaeger 1981; Walls & Roudebush 1991). Also, previous studies have found that P. glutinosus individuals will respond aggressively towards heterospecifics (Nishikawa 1985; Marvin 1998a; Rissler et al. 2000; Price & Secki Shields 2002; Marshall et al. 2004) with the intensity of interspecific competition varying geographically with the highest intensity being in areas of narrow sympatry (Hairston 1980a,b; Nishikawa 1985). It is possible that what we observed was the result of a learned response by individuals of P. glutinosus from range-edge populations to frequent competitive interactions with P. mississippi in a narrow geographical area of favourable conditions in the CZ (Fig. 3). An alternative explanation is that individuals from range-edge and core populations have inherent behavioural differences that result in differences in competitive ability. It is possible that individuals of P. glutinosus from range-edge populations have behavioural adaptations to local conditions that decrease the cost of interspecific interactions; perhaps they are more aggressive and win interspecific encounters more often than individuals from core populations.
The results of our study suggest that differences in abiotic tolerances between individuals from core and range-edge populations may shape competition; therefore, abiotic conditions may strongly influence competitive interactions. In order to determine how competitive abilities vary among individuals from core and range-edge populations, we encourage researchers to use ENMs to determine reciprocal transplant sites in core and edge environments. Future studies should incorporate additional field sites within the CZ and in the interior of the range to understand variation within core and edge areas. In addition, incorporating laboratory behavioural studies would also address if individuals from different populations have inherent behavioural differences. Much insight could also be gained by the incorporation of physiological studies to address the differences in abiotic tolerances of individuals from core and range-edge populations.
In summary, the geographical range of a species is influenced by a complex interplay of abiotic conditions and biotic interactions that operate at varying spatial scales across geographical space (Dunson & Travis 1991). A complication of disentangling these factors is that species interactions themselves can be affected by abiotic conditions (Coulson et al. 2001; Gaston 2003). The incorporation of ENMs and the analysis of spatially explicit climate data provide a novel method that can be used to help unravel the effects of abiotic factors on these interspecific interactions. It is likely that understanding the ecological dynamics of species boundaries will greatly enhance our understanding of a wide range of biological phenomena as range limits are entry points to understanding the ecological niche (Holt & Keitt 2005).