Spatial Congruence Analysis (SCAN): A method for detecting biogeographical patterns based on species range congruences

Abstract

Species with congruent geographical distributions, potentially caused by common historical and ecological spatial processes, constitute biogeographical units called chorotypes. Nevertheless, the degree of spatial range congruence characterizing these groups of species is rarely used as an explicit parameter. Methods conceived for the identification of patterns of shared ranges often suffer from scale bias associated with the use of grids, or the incapacity to describe the full complexity of patterns, from core areas of high spatial congruence, to long gradients of range distributions expanding from these core areas. Here, we propose a simple analytical method, Spatial Congruence Analysis (SCAN), which identifies chorotypes by mapping direct and indirect spatial relationships among species. Assessments are made under a referential value of congruence as an explicit numerical parameter. A one-layered network connects species (vertices) using pairwise spatial congruence estimates (edges). This network is then analyzed for each species, separately, by an algorithm which searches for spatial relationships to the reference species. The method was applied to two datasets: a simulated gradient of ranges and real distributions of birds. The simulated dataset showed that SCAN can describe gradients of distribution with a high level of detail. The bird dataset showed that only a small portion of range overlaps is biogeographically meaningful, and that there is a large variation in types of patterns that can be found with real distributions. Species analyzed separately may converge on similar or identical groups, may be nested in larger chorotypes, or may even generate overlapped patterns with no species in common. Chorotypes can vary from simple ones, composed by few highly congruent species, to complex, with numerous alternative component species and spatial configurations, which offer insights about possible processes driving these patterns in distinct degrees of spatial congruence. Metrics such as congruence, depth, richness, and ratio between common and total areas can be used to describe chorotypes in detail, allowing comparisons between patterns across regions and taxa.

Ground spiders (Chelicerata, Araneae) of an urban green space: intensive sampling in a protected area of Rome (Italy) reveals a high diversity and new records to the Italian territory

“…To describe the biogeographical composition of the Italian spider fauna, species were assigned to chorotypes, i.e. groups of species with similar distributions ( Fattorini 2015 , Fattorini 2016 , Fattorini 2017a , Gatto and Cohn-Haft 2021 ). Chorotypes are established by an inductive and recursive process in which species distributions are mapped, their contours are compared and species with similar ranges are classified with the same group, i.e.…”

“…Chorotypes are established by an inductive and recursive process in which species distributions are mapped, their contours are compared and species with similar ranges are classified with the same group, i.e. they form a given chorotype ( Fattorini 2015 , Fattorini 2016 , Gatto and Cohn-Haft 2021 ). After a chorotype is defined by the overlap of multiple species distributions, any other species showing a similar distribution can be assigned to that chorotype.…”

The Role of Vegetation in Elevational Diversity Patterns of Tenebrionid Beetles in Central Italy

“…Tenebrionids were classified biogeographically by using chorotypes. Chorotypes are groups of generalized distributional patterns based on inductive methods: individual species distributions are mapped and species with similar distributions are grouped together to define chorotypes as recurrent patterns [75][76][77]. Once a chorotype is defined, any additional species exhibiting a similar distribution can be ascribed to that chorotype.…”

Variations in Plant Richness, Biogeographical Composition, and Life Forms along an Elevational Gradient in a Mediterranean Mountain

“…Chorotypes refer to groups into which species with similar distributions can be classified [43][44][45]. Chorotypes are established by an inductive and recursive process in which species distributions are mapped, their contours are compared, and species with similar ranges are classified with the same group, i.e., they form a chorotype [43,44,46]. After a chorotype is defined by the overlap of multiple species distributions, any other species showing a similar distribution can be assigned to that chorotype.…”

“…As "abstractions" used to express recurrent species distributions, chorotypes are also roughly similar to the "generalized tracks" of Croizat [48], although, in the case of chorotypes, no shared history is implied. Chorotypes are widely used both to shortly indicate species distributions and to make hypotheses about the origin of plant and animal assemblages [43,44,46]. As species with similar distributions should also have similar macroecological needs [31], the analysis of the chorotype composition of local species assemblages can be used to draw inferences about which ecological and historical factors have shaped such assemblages.…”