Can Niche Modeling and Geometric Morphometrics Document Competitive Exclusion in a Pair of Subterranean Rodents (Genus Ctenomys) with Tiny Parapatric Distributions?

Abstract

Species with similar ecological requirements coexisting in the same geographic region are prone to competitively exclude each other. Alternatively, they may coexist if character displacement acts to change the niche requirements of one or both species. We used two methodological approaches (ecological niche modeling [ENM] and geometric morphometrics) to test two hypotheses: given their behavioral, morphological, and ecological similarities, one species competitively excludes the other; and, character displacement enables their coexistence at two sites in which the species are known to occur in sympatry. The results from the ENM-based approach did not provide evidence for competitive exclusion; however, the morphometric analyses documented displacement in size of C. minutus. This result, suggests that C. minutus might exclude C. flamarioni from areas with softer soils and higher food availability. We stress the importance of using multiple methodological approaches when testing prediction of competitive exclusion. However, both methods had limited explanatory power given that the focal species possess truly peculiar distributions, being largely parapatric and restricted to narrow, small geographic areas with a strange distribution and there is a need to search for additional methods. We discuss the idiosyncrasy of the ENM-based approach when applied to organisms with subterranean habits.

Figure 1

Ecological niche models projected (in binary format) on the coastal plains of southern Brazil: (A) final Maxent model of abiotically suitable areas for Ctenomys flamarioni; (B) final Maxent model of abiotically suitable areas for C. minutus, and (C) areas of potential sympatry for both species. Parallel lines indicate location of known contact zone in the north (A) and the south (B). Abiotically suitable areas are indicated with shades of gray; increasingly stronger predictions are indicated with pregressively darker shades. Areas of potential sympatry are those where suitable environmental conditions exist for both species. The dashed line in “A” indicates the approximate limits of the sand dunes. Withe circles represent localities of C. flamarioni; black circles reprensent localities of C. minutus. Maps were obtained from “© OpenStreetMap contributors” (available at: www.openstreetmap.org; Open Street Map is made available under the Open Database License: http://opendatacommons.org/licenses/odbl/1.0/. Any rights in individual contents of the database are licensed under the Database Contents License: http://opendatacommons.org/licenses/dbcl/1.0/; and http://mapas.mma.gov.br/i3geo/datadownload.htm), and edited with QGis 2.18 software. The images were also edited using Corel Draw graphics Suite (X5).

Figure 2

Binary representation of the climatic suitability for Ctenomys flamarioni and C. minutus in areas of potential sympatry. Circles represent records of C. flamarioni; triangles represent records of C. minutus. Sites (image pixels) with higher suitability values for C. flamarioni are indicated in grey; sites (image pixels) with higher suitability values for C. minutus are indicated with black. Maps were obtained from “© OpenStreetMap contributors” (available at: www.openstreetmap.org; Open Street Map is made available under the Open Database License: http://opendatacommons.org/licenses/odbl/1.0/. Any rights in individual contents of the database are licensed under the Database Contents License: http://opendatacommons.org/licenses/dbcl/1.0/; and http://mapas.mma.gov.br/i3geo/datadownload.htm), and edited with QGis 2.18 software. The images were also edited using Corel Draw graphics Suite (X5).

Figure 3

Comparison strenghts of of predicted environmental suitability for C. flamarioni and C. minutus. These comparisons are based on final models projected at the known conctact zones of the focal species. (A) shows models projected at the northern area, whereas (B) does so for the southern area. Withe circles reprensent localities of C. flamarioni and black circles represent localities of C. minutus. The rectangle in the figure (A) represents the limits of the contact zone between the species. Maps were obtained from “© OpenStreetMap contributors” (available at: www.openstreetmap.org; Open Street Map is made available under the Open Database License: http://opendatacommons.org/licenses/odbl/1.0/. Any rights in individual contents of the database are licensed under the Database Contents License: http://opendatacommons.org/licenses/dbcl/1.0/; and http://mapas.mma.gov.br/i3geo/datadownload.htm), and edited with QGis 2.18 software. The images were also edited using Corel Draw graphics Suite (X5).

Figure 4

Boxplot showing skull centroid size variation in Ctenomys flamarion and C. minutus in sympatry and allopatry. Asterisk indicates a significant difference between groups. The central line show the median, and the square limits are showing the first and thrid quartiles, respectively.

Figure 5

Procrustes distances between groups. The dotted line represents the first group and the continuous line represents the second. For example, in the first image the dotted line represents Ctenomys flamarioni in allopatry, whereas the continuous line represents C. minutus in allopatry.

Figure 6

Scatter plot of the two first axes of a between-group principal component analysis for the ventral view of the skull for C. flamarioni and C. minutus. The predicted shape change along each axis is given. Solid circles represent C. flamarioni in allopatry; open circles represent C. flamarioni in sympatry; solid squares represent C. minutus in allopatry; open squares represent C. minutus in sympatry.

Figure 7

Localities of Ctenomys flamarioni (withe circles) and Ctenomys minutus (black circles) and study region used to calibrate models of the abiotically suitable areas. Areas in black and white show the realized distribution of C. flamarioni; areas in black represent the realized distribution of C. minutus; areas in gray indicate the location of ponds. Maps were obtained from “© OpenStreetMap contributors” (available at: www.openstreetmap.org; Open Street Map is made available under the Open Database License: http://opendatacommons.org/licenses/odbl/1.0/. Any rights in individual contents of the database are licensed under the Database Contents License: http://opendatacommons.org/licenses/dbcl/1.0/; and http://mapas.mma.gov.br/i3geo/datadownload.htm), and edited with QGis 2.18 software. The images were also edited using Corel Draw graphics Suite (X5).