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Multidimensional trait morphology predicts ecology across ant lineages

Citation

Sosiak, Christine; Barden, Phillip (2020), Multidimensional trait morphology predicts ecology across ant lineages, Dryad, Dataset, https://doi.org/10.5061/dryad.kh1893243

Abstract

1. Understanding the link between ecology and morphology is a fundamental goal in biology. Ants are diverse terrestrial organisms, known to exhibit ecologically-driven morphological variation. While relationships between individual traits and ecologies have been identified, multidimensional interactions among traits and their cumulative predictive power remain unknown. Because selective pressures may generate convergent syndromes spanning multiple traits, we applied multivariate analyses across a wide sampling of taxa to assess ecomorphological variation in an integrative context.

2. How well does morphology predict ecology? Moreover, are there quantitatively-supported ant ecomorphs? We investigated the links between trait morphology and ecology by assembling a morphometric dataset spanning over 160 species within 110 genera. Because ants occupy a wide range of ecologies, we compiled natural history data on nesting microhabitat, foraging stratum, and functional role into 35 defined niche combinations. This tripartite ecological classification and our morphological dataset were optimized under dimension reduction techniques including Principal Component Analysis, Principal Coordinate Analysis, Linear Discriminant Analysis, and Random Forest supervised machine learning.

3. Our results describe ant ecomorphospace as comprising regions of shared, generalized morphology as well as unique phenotypic space associated with specialized ecologies. Dimension reduction and model-based approaches predict ecology with 77-85% accuracy and Random Forest analysis consistently outperforms LDA. While accounting for shared ancestry, we found eye, antennal scape, and leg morphology to be most informative in differentiating among ecologies. We also note some heterogeneity between trait significance in each ecological aspect (nesting niche, foraging niche, functional role). To increase the utility of ecomorphological classification we simplified our 35 observed niche combinations into 10 ecomorph syndromes, which were also predicted by morphology. The predictive power of these machine learning methods underscores the strong role that ecology has in convergently shaping overall body plan morphology across ant lineages. We include a pipeline for predictive ecomorphological modeling using morphometric data, which may be expanded with additional specimen-based and natural history data.

Methods

Our dataset includes ant trait measurements from 15 subfamilies, 113 genera, and 167 species, with associated ecological niche information. Morphometric sampling included linear measurements of 12 cephalic traits and 5 post-cephalic traits, with 5 additional traits that were not subsequently included in our analyses but are presented here, after preliminary analyses indicated overlap with other metrics, variation due to measurement artifacts, or lack of significance across all niche aspects. Fifteen selected traits have been previously correlated with ecology. All measurements were conducted on point-mounted specimens under stereo microscopy. Including raw measurements in dimension reduction techniques such as principal component analysis can result in body size driving the overwhelming majority of variation in a dataset, masking other potentially important contributors. To explore the impact of body size, we created two datasets for analyses: a dataset comprising raw measurements and a size-corrected dataset using only ratios. Tab 1 (all data (raw measurements)) is the raw measurements dataset; tab 2 (all data (ratio measurements)) is the size-corrected ratios dataset; tab 3 (tree-matched data (raw meas.)) holds a dataset of raw measurements pruned to taxa that are included in Blanchard and Moreau's 2017 comprehensive ant phylogeny to facilitate phylogenetic corrections; tab 4 (tree-matched data (ratio meas.)) includes the same taxa as in tab 3 but with the size-corrected ratio measurements. 

Usage Notes

All measurements included here are in millimetres.

Raw trait measurement definitions are as follows: head_width is the width of the head at the broadest point in frontal view; head_length is the length of the head at the longest point in frontal view; webers_length is the length of the mesosoma from the anterodorsal margin of the pronotum to the posteroventral margin of the mesosoma; procoxal_length is the length of the procoxa in lateral view; eye_length is the maximum length of the eye; tarsal_claw_length is the maximum length of the tarsal claw; tarsal_claw_width is the maximum width of the tarsal claw; mesosoma_height is the height of the mesosoma taken at a right angle to Weber's length; mandible_length_profile is the length of the mandible from insertion point to apical tooth in lateral view; mandible_angle is the angle between the plane of the mandible and plane of the head capsule; eye_height_total is the maximum total height of the head capsule; eye_height_dorsal is the height of the head from the midpoint of the eye to the dorsal margin of the head capsule; eye_height_ventral is the height of the head from the midpoint of the eye to the ventral margin of the head capsule; eye_length_total is the total length of the head in lateral view; eye_length_anterior is the length of the head from the midpoint of the eye to the anterior clypeal margin; eye_length_posterior is the length of the head from the midpoint of the eye to the posterior margin of the head capsule; mandible_length_face is the length of the mandible from insertion point to apical tooth in frontal view; metafemur_length is the maximum length of the metafemur; scape_length is the maximum length of the scape; ventral_head_length is the maximum length of the head along the ventral margin; ventral_pronotal_length is the maximum length of the prothorax along the ventral margin; pronotal_width is the width of the pronotum at the broadest point in dorsal view. 

Ratio trait measurement definitions are as follows: head_shape is the head width divided by head length; procoxal_ratio is the procoxal length divided by Weber's length; eye_head_size is the eye length divided by the head length; thorax_shape is the pronotal width divided by Weber's length; pronotal_shape is the pronotal width divided by the mesosoma height; mandible_curvature is the mandible length in lateral view divided by the mandible length in frontal view; mandible_head_length is the mandible length in lateral view divided by the head length; eye_dorsoventral is the fraction of the head capsule from the dorsal margin to the midpoint of the eye divided by the total height of the head; eye_anteroposterior is the fraction of the head capsule from the anterior clypeal margin to the midpoint of the eye divided by the head length in lateral view; scape_proportion is the length of the scape divided by the head length; metafemur_proportion is the length of the metafemur divided by Weber's length; mesosoma_shape is the height of the mesosoma divided by Weber's length.

Functional role binning definitions in the "func_role" column are as follows: GP represents generalist predators with broad taxonomic diets; SP represents specialist predators with obligate feeding on a specific taxon; Om represents omnivores with broadly ranging diets; Py represents phytophagous feeding such as extrafloral nectaries and herbivory; Fg represents fungus-growing/farming; Tr represents trophobiotic relationships with other insects; Gn represents granivory; Mh represents mushroom-foraging. 

Nesting niche binning definitions in the "nest_niche" column are as follows: Cn represents carton-nesting or forming structured nests from plant material in trees and shrubs; Gr represents ground-nesting in dirt mounds, under stones, rock cracks, etc.; Lg represents lignicolous nesting or nesting in twig and tree cavities; Ll represents leaf litter-nesting in leaf litter interstitial spaces; Sb represents subterranean nesting entirely underground. 

Foraging niche binning definitions in the "forag_niche" column are as follows: Ab represents arboreal foraging in and on trees and shrubs; CR represents column-raiding behaviour that may be cooperative, nomadic, or raiding predation; Eg represents epigaeic foraging on the ground surface; Ll represents leaf litter foraging within leaf litter interstitial spaces; Sb represents subterranean foraging underground. 

Ecomorph syndrome definitions in the "ecomorph" column are as follows: om_grll_eg represents ground- or leaf litter-nesting epigaeic omnivores; om_leaf represents leaf litter omnivores; om_lg_ab represents lignicolous arboreal omnivores; om_cn_ab represents carton-nesting arboreal omnivores; om_sub represents subterranean omnivores; pred_gr_egll represents ground-nesting epigaeic or leaf litter predators; pred_leaf represents leaf litter predators; pred_lg_ab represents lignicolous arboreal predators; pred_cr represents column-raiding predators; pred_ll_g represents leaf litter-nesting epigaeic predators; pred_sub represents subterranean predators. 

Other notes: the "PGLS_data" column in the "all data (raw measurements)" tab indicates which taxa match the Blanchard and Moreau 2017 phylogeny for further phylogenetic corrections; the "ecology_ref" column indicates references for the ecological niche information for that taxon. Taxa that are identified to genus and subsequent morphospecies are denoted by _sp1, _sp2, etc.; instances where taxonomic identification was not entirely certain are denoted with a _cf.