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Why do species co-occur? A test of alternative hypotheses describing abiotic differences in sympatry versus allopatry using spadefoot toads.

Chunco AJ, Jobe T, Pfennig KS - PLoS ONE (2012)

Bottom Line: 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.This work has implications for predicting how interacting species will respond to climate change, because species interactions may facilitate survival in extreme habitats.

View Article: PubMed Central - PubMed

Affiliation: Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America. chunco@email.unc.edu

ABSTRACT
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.

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Maxent response curves for the Biotic Model.The response curves of the Biotic Model for (a) S. bombifrons and (b) S. multiplicata. 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).
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pone-0032748-g004: Maxent response curves for the Biotic Model.The response curves of the Biotic Model for (a) S. bombifrons and (b) S. multiplicata. 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).

Mentions: The Biotic Model evaluated whether the predicted presence of one species could predict the presence of the other species. Examining the response curves for the Biotic Model shows how the logistic output changes along the environmental gradient (where the “environmental gradient” is the predicted output of the other species). We found that the Biotic Model performed reasonably well, and that the relationship was roughly positively linear (Figure 4). At low logistic values for one species, the other has a low logistic value as well, so both species show a roughly similar response to the environment (Figure 4). These results therefore indicate that the habitat requirements between the two species are similar. Nevertheless, this Biotic Model is the poorest performing model of the four that we considered. Moreover, the predicted areas of species presence were greater than in any of the other models, suggesting some over-prediction. This indicates that, although the requirements of S. multiplicata and S. bombifrons are similar, there are important differences between them in how they respond to the environment. Thus, models that include climate variables are still a better approach than using only the presence of one species to predict the distribution of the other.


Why do species co-occur? A test of alternative hypotheses describing abiotic differences in sympatry versus allopatry using spadefoot toads.

Chunco AJ, Jobe T, Pfennig KS - PLoS ONE (2012)

Maxent response curves for the Biotic Model.The response curves of the Biotic Model for (a) S. bombifrons and (b) S. multiplicata. 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).
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3316550&req=5

pone-0032748-g004: Maxent response curves for the Biotic Model.The response curves of the Biotic Model for (a) S. bombifrons and (b) S. multiplicata. 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).
Mentions: The Biotic Model evaluated whether the predicted presence of one species could predict the presence of the other species. Examining the response curves for the Biotic Model shows how the logistic output changes along the environmental gradient (where the “environmental gradient” is the predicted output of the other species). We found that the Biotic Model performed reasonably well, and that the relationship was roughly positively linear (Figure 4). At low logistic values for one species, the other has a low logistic value as well, so both species show a roughly similar response to the environment (Figure 4). These results therefore indicate that the habitat requirements between the two species are similar. Nevertheless, this Biotic Model is the poorest performing model of the four that we considered. Moreover, the predicted areas of species presence were greater than in any of the other models, suggesting some over-prediction. This indicates that, although the requirements of S. multiplicata and S. bombifrons are similar, there are important differences between them in how they respond to the environment. Thus, models that include climate variables are still a better approach than using only the presence of one species to predict the distribution of the other.

Bottom Line: 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.This work has implications for predicting how interacting species will respond to climate change, because species interactions may facilitate survival in extreme habitats.

View Article: PubMed Central - PubMed

Affiliation: Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America. chunco@email.unc.edu

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus