Biogeographical evaluation and conservation assessment of arboreal leafhoppers in the Mexican Transition Zone biodiversity hotspot

Mexico harbours a diverse fauna comprising more than 1,400 leafhopper species, 60% of which appear to be strictly endemic, with many apparently restricted to particular habitats and host plants. The aims of this study were to identify areas of high species richness and endemism, and determine the biogeographic affinities of species in the diverse arboreal tribe Athysanini to provide data useful for conservation prioritization.


| INTRODUC TI ON
Studies of biodiversity and biogeography are key to understanding the historical processes that led to present distributions of species and natural communities, and also to delineating areas of high endemism in need of conservation prioritization (Morrone, 2019;Sanmartín et al., 2001Sanmartín et al., , 2012Slatyer et al., 2007). The Americas harbour a diverse biota including many species that appear to require unique or specific environmental conditions (Ceballos & Ehrlich, 2006;Marques et al., 2006;Pinkus-Rendón et al., 2006;Sanchez & Parmenter, 2002;Sánchez-González & López-Mata, 2005; Tovar- Sanchez, 2009). Zones with high biodiversity and endemism occur throughout North America and South America (Van Jaarsveld et al., 1998). In North America, there are eight recognized biodiversity hotspots between the Nearctic and Neotropical regions and two of these occur in Mexico (Brooks et al., 2006;Myers et al., 2000;Norman, 2003).
Cicadellidae, commonly known as leafhoppers, is one of the ten largest insect families worldwide. Within the order Hemiptera, it is the most species-rich family with more than 23,500 described species (Dmitriev, 2003;S. H. McKamey, pers. comm.) and is widespread and abundant in all major geographical regions (Bartlett et al., 2018).
Of the nineteen cicadellid subfamilies, Deltocephalinae is the largest, comprising 39 tribes, of which Athysanini is the largest and most widespread with 228 genera and 1,123 species (Zahniser & Dietrich, 2013). These insects feed on a wide variety of angiosperms and gymnosperms and many species appear to be highly specialized on particular plant genera or species (Dietrich, 2009;Hamilton & Whitcomb, 2010). They are abundant insects that occur in all places inhabited by vascular plants and are easily sampled using conventional methods (e.g. sweeping, light traps). Thus, they are useful as indicators of overall biodiversity and habitat quality (Biedermann et al., 2005;Wallner et al., 2013).
Compared with many other insect groups, the leafhopper fauna of Mexico is relatively well studied. In Mexico, ~1,426 species of Cicadellidae are recorded, most belonging to the subfamily Deltocephalinae and nearly 11% are in the tribe Athysanini (Zanol, 2008). Prof. Dwight DeLong and colleagues from Ohio State University collected leafhoppers intensively during the 1930s and 40s and described numerous genera and species, a large proportion of which appear to be endemic and highly threatened by modern anthropogenic activities (DeLong, 1943(DeLong, , 1945(DeLong, , 1980DeLong & Hershberger, 1948;Pinedo-Escatel et al., 2016). More recent field work by the authors has yielded many additional occurrence records.
All these data combined provide an opportunity to examine patterns of biogeography and endemism in the Mexican leafhopper fauna for conservation proposes.
Endemic Mexican forest-dwelling arboreal leafhopper genera and species are mostly placed in the tribe Athysanini, which is polyphyletic according to recent phylogenetic analyses (Zahniser & Dietrich, 2013;Lu et al., unpublished data). However, these analyses also suggest that the New World genera of this tribe belong to a single lineage that also includes the smaller endemic New World tribes Bahitini, Pendarini, and Scaphytopiini. Thus, this lineage could be used as an indicator of biogeographic relationships among New World forest ecosystems. Although host plant data for individual species of this group remain incomplete, available data suggest that many species and genera are highly host specific and some alternate between woody and herbaceous hosts during different life stages, tying them closely to particular habitat types and plant communities. Available occurrence data also indicate that many genera and  (Krishnankutty et al., 2016;Wang et al., 2017;Xue et al., 2020;Zahniser & Dietrich, 2015), but none examined New World biogeographic patterns in detail or considered the unique fauna of the Mexican Transition Zone (MTZ) as either a potential distributional bridge or a centre of origin for these sap-sucking insects. Understanding the biogeographical affinities of this leafhopper tribe in combination with a broader phylogenetic and ecological framework might help to comprehend the evolutionary processes that promoted their diversity, current distributions and conservation basis.

| Geographical setting
The project focussed on the Mexican landscapes between 14°-32°N latitude and 86°-117°W longitude, spanning the Nearctic and Neotropical biogeographical zones of the country. The varied topography and climate support a diverse terrestrial biota which includes both northern and southern elements (Halffter & Morrone, 2017).
The north-south trending mountain ranges provide a natural corridor between regions (Valdez et al., 2006).

| Insect model
We

| Distributional data compilation
A complete georeferenced occurrence data set of all known Athysanini leafhopper taxa inhabiting Mexico was included (Appendice S1 and S3). Data were obtained from openly available electronic repositories such as Global Biodiversity Information Facility (GBIF.org) and iNaturalist (iNaturalist.org), in addition to records from specialized taxonomic literature and specimen hold- were mapped and corrected when necessary, using ArcGIS 9.3 (ESRI, 2008) and Google Earth (Google, 2015). Ambiguous locations and doubtful records (i.e. cases where either the locality or species identification was uncertain) were omitted. Because most historic collections of insects were made haphazardly, collecting intensity is difficult to assess and has presumably varied among Mexican states. Thus, available historic data may be biased to some unknown extent by variation in sampling effort among regions of Mexico. Nevertheless, given our focus on a particular group of leafhoppers largely limited to tropical and montane forests, which have been intensively sampled in the past as well as through our own recent efforts, we expect the available occurrence data for Athysanini to provide a reasonably accurate reflection of the actual distributional patterns of this group in Mexico.
The WE formula (WE = Σ tl ∈ TL (r tl /R tl )) expresses scores of all species attributes (TL) in one grid cell; r refers to a single species value (tl) recorded per cell over R, which is the summation of WE values found in different grid cells over the studied area. Calculations follow taxa at level tl, with one taxon at the highest taxonomic level (=0, tribe Athysanini) and the lowest level being species (tl = 1) (for details see Aragón-Parada, 2018). CWE is calculated by dividing the WE value per species by total species number. Analyses were run using Biodiverse (Laffan et al., 2010). Species richness as a function of elevation was analysed by using categorical taxa occurrence points grouped into classes spanning 500 m a.s.l. Analyses and figures were plotted using R (Development Core Team, 2016).

| Biogeographical distribution
Biogeographical affinities of the tribe Athysanini were inferred under the concept of biogeographic regionalization proposed by Morrone et al. (2017) considering three major regions: Nearctic, Neotropical and the MTZ with their respective 14 provinces. Vegetation preference was also included following criteria of Rzedowski (1978Rzedowski ( , 2006. Elevation range data for each genus and species were also analysed. Map layouts were built by using QGIS (QGIS Development Team, 2015) with open-access territorial land schemes generated by the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO, 2019).

| Georeferenced data
A total of 643 geographical records for Athysanini ( Figure 2) were compiled representing 164 species and 50 genera (Table 1; Appendix S1). Eighty percent of taxa had valid names while the remaining 20% are new to science. Of the taxa recorded, 145 species (88%) and 31 genera (63%) are strictly endemic to Mexico (Appendix S2) whereas 19 species occur either to the north or south of Mexico. Guerrero state had the largest number, 199, of occurrence records followed by Michoacán, Estado de Mexico and Jalisco (72-59 records). Remaining states had fewer than 50 records.

| Species richness of biogeographical regions and provinces
The province has the highest genus diversity, 33, substantially more than the other provinces. The richest province for endemic species (ES) was the TVB with 69 species, followed by the BB with 67 species and the SMS with 24 species (Figure 5). The Californian (C) and SMOc provinces had no species recorded.  others with wider tolerance of higher elevations while non-endemic taxa were found along the entire altitudinal range (Figure 11).

| Conservation status
Applying IUCN criteria to 145 Mexican athysanine leafhopper species (Appendix S2) suggests that most of the evaluated species meet the criteria for listing under one of the protected categories.
Based on available data, 57 species met the criteria to be included in the Endangered (EN) category while 88 species were assessed as Critically Endangered (CR). By contrast, the categories Least Concern (LC) and Data Deficient (DD) included relatively few species ( Figure 12).

| D ISCUSS I ON
Identification of areas of high species richness and endemism (biodiversity hotspots) requires not only measurement of species richness but also reliable data on species distributions with regard to both geography and habitat (Myers et al., 2000;Norman, 2003). No similar studies have been attempted on this insect group but the present results strongly suggest that forest leafhoppers are useful indicators of threatened faunas and habitats (Arriaga et al., 2012;Bizuet-Flores et al., 2015;Blancas-Calva et al., 2010;Contreras-Medina et al., 2007;De-Nova et al., 2012;Escalante et al., 2004;Halffter & Morrone, 2017;Huidobro et al., 2006;Morrone, 2006Morrone, , 2017Morrone, , 2019Salinas et al., 2019;Salinas-Rodríguez et al., 2018). Similar studies should be conducted in other regions to provide a more comprehensive conservation assessment of the global leafhopper fauna.
Unfortunately, Mexico is one of the few tropical countries where the leafhopper fauna has been relatively well studied with extensive historical as well as recent occurrence data available. Our analysis of available data for the leafhopper tribe Athysanini indicates that, in Mexico, this group is not only highly endemic but also greatly threatened by habitat loss (Pinedo-Escatel et al., unpublished data).
These data may be useful for further conservation assessment and planning.
Based on 164 Athysanini species identified from Mexico, the diversity of Athysanini in this country appears to be similar to that of other megadiverse Neotropical countries such as Peru, Bolivia and Brazil (Zanol, 2008), although the latter countries may not have been Continuing a trend begun in the 1920s, recent field work in Mexico has led to discovery of new leafhopper species and genera (Appendix S1; Aguilar-Pérez et al., 2019;Pinedo-Escatel et al., 2016).
Ongoing surveys will continue to fill gaps in knowledge of this diverse insect fauna. Undoubtedly diversity is substantially higher than indicated by previous studies (Zanol, 2008;Pinedo-Escatel et al., unpublished data;Mariño-Pérez et al., 2012) and efforts are needed to sample additional regions and habitat types, particularly those threatened by human activities and not covered by previous surveys. Unfortunately, the region remains poorly evaluated with regard to conservation priorities although a growing body of evidence accentuates the need for further efforts to conserve endangered habitats in this area.
As often observed in other arthropods (Corona et al., 2009;Pinkus-Rendón et al., 2006), most of the species treated in this study are restricted geographically. Nevertheless, 20 of the athysanine species collected are more widespread as in certain grass-feeding leafhoppers, for example Chiasmini and Stenometopiini (Zahniser & Dietrich, 2013

ACK N OWLED G EM ENTS
We would like express our sincere thanks to botanists at the IBUG

CO N FLI C T O F I NTE R E S T
The authors declare that they have no known competing interests that could have appeared to influence the work reported in this paper.

Pe e r Rev iew
The peer review history for this article is available at https://publo ns.com/publo n/10.1111/ddi.13254.