Why Do Species Co-Occur? A Test of Alternative Hypotheses Describing Abiotic Differences in Sympatry versus Allopatry Using Spadefoot Toads

Areas of co-occurrence between two species (sympatry) are often thought to arise in regions where abiotic conditions are conducive to both species and are therefore intermediate between regions where either species occurs alone (allopatry). Depending on historical factors or interactions between species, however, sympatry might not differ from allopatry, or, alternatively, sympatry might actually be more extreme in abiotic conditions relative to allopatry. Here, we evaluate these three hypothesized patterns for how sympatry compares to allopatry in abiotic conditions. We use two species of congeneric spadefoot toads, Spea multiplicata and S. bombifrons, as our study system. To test these hypotheses, we created ecological niche models (specifically using Maxent) for both species to create a map of the joint probability of occurrence of both species. Using the results of these models, we identified three types of locations: two where either species was predicted to occur alone (i.e., allopatry for S. multiplicata and allopatry for S. bombifrons) and one where both species were predicted to co-occur (i.e., sympatry). We then compared the abiotic environment between these three location types and found that sympatry was significantly hotter and drier than the allopatric regions. Thus, sympatry was not intermediate between the alternative allopatric sites. Instead, sympatry occurred at one extreme of the conditions occupied by both species. We hypothesize that biotic interactions in these extreme environments facilitate co-occurrence. Specifically, hybridization between S. bombifrons females and S. multiplicata males may facilitate co-occurrence by decreasing development time of tadpoles. Additionally, the presence of alternative food resources in more extreme conditions may preclude competitive exclusion of one species by the other. This work has implications for predicting how interacting species will respond to climate change, because species interactions may facilitate survival in extreme habitats

Predicting environmental suitability for a rare and threatened species (Lao newt, Laotriton laoensis) using validated species distribution models.

The Lao newt (Laotriton laoensis) is a recently described species currently known only from northern Laos. Little is known about the species, but it is threatened as a result of overharvesting. We integrated field survey results with climate and altitude data to predict the geographic distribution of this species using the niche modeling program Maxent, and we validated these predictions by using interviews with local residents to confirm model predictions of presence and absence. The results of the validated Maxent models were then used to characterize the environmental conditions of areas predicted suitable for L. laoensis. Finally, we overlaid the resulting model with a map of current national protected areas in Laos to determine whether or not any land predicted to be suitable for this species is coincident with a national protected area. We found that both area under the curve (AUC) values and interview data provided strong support for the predictive power of these models, and we suggest that interview data could be used more widely in species distribution niche modeling. Our results further indicated that this species is mostly likely geographically restricted to high altitude regions (i.e., over 1,000 m elevation) in northern Laos and that only a minute fraction of suitable habitat is currently protected. This work thus emphasizes that increased protection efforts, including listing this species as endangered and the establishment of protected areas in the region predicted to be suitable for L. laoensis, are urgently needed

Principal components of the abiotic environment.

<p>Means (+/− s.e.) for the first two principal components describing variation in the eight environmental variables used to build ecological niche models. Different letters indicate significantly different means; each group (<i>S. multiplicata</i> in allopatry, <i>S. bombifrons</i> in allopatry, and sympatry), is significantly different from the other two.</p

Loadings on the first two principal components for the eight environmental variables used in the Maxent model.

<p>Loadings near 0 (i.e., −0.1 to 0.1) are denoted with an “*”.</p

Range maps of predicted sympatry.

<p>Range maps of predicted sympatry between <i>Spea multiplicata</i> and <i>S. bombifrons</i>. The value for each 1 km sq pixel was calculated by multiplying the logistic value of both species, and values range from 0 (white) to 1 (dark green). Sites used in the environmental analysis are indicated by points. Specifically, blue squares represented collection locations for <i>S. bombifrons</i> that occurred in areas predicted to be allopatric for that species; orange circles represent collection locations for <i>S. multiplicata</i> records that were predicted to be allopatric for that species, whereas gray triangles represent collection locations for either species in areas predicted to be sympatric.</p

Range maps for <i>Spea multiplicata</i>

<p><b>and </b><b><i>S. bombifrons</i></b><b>.</b> Range map showing the total range of both <i>S. multiplicata</i> and <i>S. bombifrons</i>.</p

Maxent response curves for the Biotic Model.

<p>The response curves of the Biotic Model for (a) <i>S. bombifrons</i> and (b) <i>S. multiplicata</i>. These curves show how the logistic output changes along an ‘environmental gradient’. Here, the environmental gradient is the predicted output of the other species used to create the Biotic Model. The red line shows the average of the 10 replicate runs, while the blue bands shows +/− one standard deviation. At low logistic values for one species, the other species has a low logistic value as well. Both species thus show a similar response to the environment (i.e. environments good for one species tend to be good for the other).</p

Alternative hypotheses for abiotic conditions underlying species distributions.

<p>A representation of three alternative patterns of environmental variation underlying sympatric and allopatric populations of two species across an environmental gradient. (a) Under hypothesis 1, species co-occur at intermediate environmental conditions where niches of the two species overlap. (b) Under hypothesis 2, biotic factors mediate co-occurrence such that species co-occur most commonly under extreme conditions. (c) Under hypothesis 3, sympatry and allopatry are governed primarily by dispersal ability, resulting in no environmental differences between sympatric and allopatric populations.</p

Environmental variation between sympatry and allopatry.

<p>Box-and-whisker plots showing environmental space occupied by predicted allopatric populations of <i>S. bombifrons</i> (abbreviated “<i>Sb</i> allopatry”), predicted allopatric populations of <i>S. multiplicata</i> (abbreviated “<i>Sm</i> allopatry”), and predicted sympatric populations of both species for each environmental variable used in the Maxent models. Non-overlapping notches are roughly equivalent to 95% confidence intervals, and therefore provide strong evidence that the medians differ <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032748#pone.0032748-R1" target="_blank">[48]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0032748#pone.0032748-Chambers1" target="_blank">[58]</a>. An “*” above the sympatry box indicates variables for which sympatric sites are significantly hotter or drier than both allopatric sites.</p

Is the future already here? The impact of climate change on the distribution of the eastern coral snake (Micrurus fulvius)

Anthropogenic climate change is a significant global driver of species distribution change. Although many species have undergone range expansion at their poleward limits, data on several taxonomic groups are still lacking. A common method for studying range shifts is using species distribution models to evaluate current, and predict future, distributions. Notably, many sources of ‘current’ climate data used in species distribution modeling use the years 1950–2000 to calculate climatic averages. However, this does not account for recent (post 2000) climate change. This study examines the influence of climate change on the eastern coral snake (Micrurus fulvius). Specifically, we: (1) identified the current range and suitable environment of M. fulvius in the Southeastern United States, (2) investigated the potential impacts of climate change on the distribution of M. fulvius, and (3) evaluated the utility of future models in predicting recent (2001–2015) records. We used the species distribution modeling program Maxent and compared both current (1950–2000) and future (2050) climate conditions. Future climate models showed a shift in the distribution of suitable habitat across a significant portion of the range; however, results also suggest that much of the Southeastern United States will be outside the range of current conditions, suggesting that there may be no-analog environments in the future. Most strikingly, future models were more effective than the current models at predicting recent records, suggesting that range shifts may already be occurring. These results have implications for both M. fulvius and its Batesian mimics. More broadly, we recommend future Maxent studies consider using future climate data along with current data to better estimate the current distribution