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Aim: South America is considered a biologically hybrid continent. To the south, the Patagonian region harbours a unique biota strongly related to other southern continents. To the northern portion, tropical and subtropical areas from the Neotropical region show a more complex taxocoenosis related to the Nearctic and Afro‐Oriental regions. The South American Transition Zone (STZ) has been proposed to belong simultaneously to both regions. This work aimed to test the validity of STZ in the light of the distributions of an ancient freshwater taxon. Location: South America. Taxon: Ephemeroptera. Methods: We compiled a dataset including all mayfly species having at least one record in South America (8,268 records for 661 species). By using the Network Analysis Method (NAM), we analysed the validity and delimitation of the STZ. Results: The distributions of Ephemeroptera give rise to groups of cohesively sympatric species with a clear distinction between Patagonian and Neotropical regions. Although some degree of overlap occurs between them, the overlapping area does not match the STZ to a significant extent. The units of co‐occurring species recovered show that Neotropical groups mainly occupy the STZ. Main conclusions: Almost the entire provinces of Puna, Desert and Paramo are not supported as part of the STZ by mayfly distribution. The transition zone between Patagonian and Neotropical mayfly fauna involves Southern Puna and high Andes (south to 17° S latitude), Monte province, and a narrow portion of Patagonian steppe with Chubut River being the southern limit. [ABSTRACT FROM AUTHOR]

Abstract: Aim: Traditionally, South American aquatic insects have been divided into cold and warm adapted forms. Cold‐adapted forms inhabit freshwater systems from higher latitudes, or higher altitudes even around the Equator. Warm‐adapted groups are defined as those found in lower latitudes and altitudes. This work aims to answer the questions: Are mayfly assemblages geographically segregated according to geographical (latitude) and topographical (altitude) surrogates of temperature? If so, where is this transition located? Location: South America. Methods: We compiled a data set about the relative incidence of 52 mayfly genera in 326 sampled communities. They span from 0 to 4,320 m and from 47.77° S to 5.74° N latitude. By virtue of the compositional nature of the data set, we applied the statistical procedures behind the Aitchison compositional data analysis. We delimited groups of assemblages based on their Aitchison distances and projected the data points onto a biplot obtained through Principal Component Analysis adjusted to compositions (Aitchison PCA). Results: A strong correspondence among biological and geographical information was detected, with mayfly assemblages clearly segregated in space. Andesiops and Meridialaris are typical cold‐adapted forms; Baetodes, Leptohyphes and Thraulodes represent the warm group. Thermal groups can be separated by a curved line of altitude in function of latitude expressed in terms of a superellipse arc. Main conclusions: The classical ecological bipartition of mayflies into warm and cold freshwater groups is formalized quantitatively. The dividing line between warm and cold assemblages levels off at high altitudes (c. 3,300 m) around the Equator and falls to sea level at southern latitudes. The community bipartition line is useful for tracking global change through records of altitudinal displacement below and above of the warm/cold line of involved ecological groups. [ABSTRACT FROM AUTHOR]