As expected, our results indicate that a highly diverse clade approach can be used to predict the environmental integrity of the streams in this study site. There is a growing need to identify rapid and effective biological indicators for measuring environmental health and quality (Kutcher and Bried, 2014; Voß and Schäfer, 2017). However, the use of indicator species has been criticized for several reasons, most notably the lack of justification given for selecting a particular indicator (Siddig et al., 2016). However, Mendoza-Penagos et al. (2021) recently demonstrated that a focus on odonate families for environmental monitoring could be advantageous in terms of cost- and time effectiveness and the quality of the information obtained. Moreover, these authors concluded that rapid assessments can be conducted effectively by selecting an adequate taxonomic level.
In this sense, adult damselflies and dragonflies (Odonata) have several attributes that make them appropriate for environmental evaluations (Dolný et al., 2012; Kutcher and Bried, 2014; Berquier et al., 2016; Miguel et al., 2017; Šigutová et al., 2019). Odonata species are increasingly used as indicators of water quality (Chovanec and Waringer, 2001; Simaika and Samways, 2009, 2011, 2012; Harabiš and Dolný, 2010; Silva et al., 2010; Kutcher and Bried, 2014; De Oliviera-Jr. et al., 2015; Berquier et al., 2016; Golfieri et al., 2016; Valente-Neto et al., 2016), multiple modalities of climatic change (Hassall, 2015), habitat integrity (Mendes et al., 2017), forest fragmentation (Renner et al., 2016), logging activities (Calvão et al., 2016), land use intensification (Rocha-Ortega et al., 2019) and the effects of urbanization (Bried and Samways, 2015). Pinto et al. (2012) demonstrated a higher wing fluctuating asymmetry in adults of Argia tinctipennis when riparian vegetation was removed, while Rocha-Ortega et al. (2019) showed that adult Odonata body size seems to be a more suitable variable for measuring the integrity of original vegetation than the more commonly used species richness indices.
In tropical regions, nine Odonata species were classified as useful indicators of ecosystem health (De Oliviera-Jr. et al., 2015). Four of these have been associated with more degraded streams (three species from Anisoptera and one from Zygoptera), while five Zygoptera species were indicative of optimal environmental conditions (De Oliviera-Jr. et al., 2015). These findings suggest the use of sets of species that are sensitive to environmental conditions.
The SUs downstream were lower in habitat integrity (measured with the VBHAS) since they presented a low score for velocity (depth regimes), with categories of poor quality in El Palmar (SU7) and El Arenal (SU11). This is in contrast to the midstream and upstream SUs, which present optimum and suboptimum habitat integrity, respectively. Some species, such as Argia pipila and A. pocomana, are associated with riffles, while other species are associated with pools (Erythemis plebeja, Neoerythromma cultellatum, Tramea calverti). Accordingly, further studies of rapid assessments have found that Argia modesta (Silva et al., 2010) and A. indocilis (Dalzochio et al., 2011) are abundant in sites with higher habitat integrity scores. In contrast, Montero-Junior et al. (2015) found that the population size of Argia species decreased when areas of urbanization and deforestation increased. Consequently, the mixture of specialist and generalist species acted to conceal the relationship with VBHAS, which was evaluated at a higher taxonomic level (order).
Classical species diversity measures use species richness, heterogeneity or evenness; however, as Magurran (2004) stated, no index is universally applicable across all ecological assemblages. Consequently, there is no single index that can summarize species diversity concepts (Whittaker, 1972; Purvis and Hector, 2000; Morris et al., 2014). Traditional diversity indices represent various phenomena because some indices prioritize species richness over uniformity (or vice versa). As a result, communities could be ranked differently depending on the index applied (Hurlbert, 1971). According to Stirling and Wilsey (2001), species diversity should be measured using a compound statistical metric. Since traditional diversity indices cannot summarize species diversity, a composite statistical measure is required (Guisande et al., 2017). In this context, Di Battista et al. (2016) provide a scalar measure that leads to a community classification, using the area under the curvature function (Supplementary Material Appendix 5) and avoiding bias towards either dominant or rare species.
Moreover, previous studies have analyzed the relationship between biotic (Simaka and Samways, 2011) and habitat integrity indices (Alves-Martins et al. 2018, De Oliveira-Junior and Juen, 2019), using estimates of diversity. Our study includes an analysis of the total diversity, abundance and species richness of odonates at different taxonomic levels (order, sub-order, family, and genera) with a habitat integrity index (VBHAS). We found that Argia species richness and diversity both increase with increasing habitat integrity indices. This relationship means that analysis at the genus level could be more accurate than at higher taxonomic levels for this particular region.
Unfortunately, little is known regarding the life cycles, geographic distribution and tolerance to environmental change of many odonate species in this region. It is therefore difficult to relate the environmental factors that affect each life cycle stage to presence or absence in a particular site. For this reason, we found insufficient information for this specific Neotropical region to compare our results with other indices; for example, the Odonata Habitat Index (Chovanec and Waringer, 2001) or the Dragonfly Biotic Index (Simaika and Samways, 2009).
Moreover, Raebel et al. (2010), Patten et al. (2015), Bried et al. (2016), Khelifa (2019) and Šigutová et al. (2019) discussed the advantages and disadvantages of using the adult stage and concluded that adult-based bioassessment methods are reliable and easily applied to various types of tropical forest habitats worldwide. For this study, we monitored a complete hydrological cycle using only resident adults and insects that showed any reproductive activity, such as tandem or oviposition behavior.
With regard to our hypothesis, we suggested that habitat assessment will correlate positively with a diverse clade, particularly the speciose Argia genus, and therefore expected a positive relationship since most of these species are typically running water specialists and sensitive to environmental integrity. Moreover, since these species belong to the same clade, they must share a common evolutionary and colonization history in this specific area. We found a robust positive relationship between VBHAS with Argia richness and diversity, regardless of geographic position (latitude, longitude, and elevation). Habitat integrity scores increase 12.06 times when Argia species richness increases. At the same time, habitat integrity scores increase 23.51 times when Argia diversity (surface area) increases, which is consistent with the theoretical fundamentals of rapid assessment evaluation (Azrina et al., 2006; Harabiš and Dolný, 2010; Harabiš and Dolný, 2012).
El Triunfo, the SU with the highest VBHAS, has eight of the 13 Argia species; and Zapote Mocho, the SU with the second highest VBHAS, has nine of the 13 Argia species, two of which are exclusive to this site, supporting the notion that Argia species are better indicators of habitat integrity. These results coincide with those of other studies that have related habitat quality to species of the Suborder Zygoptera (Dutra and De Marco, 2015; Monteiro-Júnior et al., 2015; Rodrigues et al., 2016) and the genus Argia (Novelo-Gutiérrez and Gómez-Anaya, 2009; Silva et al., 2010; Calvão et al., 2016).
In terms of establishing monitoring protocols for each of the three Soconusco basins in this region, environmental quality assessment is more efficient when using only the genus Argia (13 spp.) rather than all of the odonate species (40 spp.). Previous studies have shown a relationship between species assemblage and habitat integrity. For example, Miguel et al. (2017) and Schöder et al. (2020) found that Argia species were abundant under better habitat conditions (i.e., using an index of habitat integrity proposed by Nessimian et al., 2008).
The study of Mendoza-Penagos et al. (2021) found that abundance had the highest concordance with habitat integrity variations at the Coenagrionidae level, while that of Valente-Neto et al. (2016) focused on the genus composition. However, to our knowledge, no specific studies have used a diverse species clade belonging to Argia with habitat integrity indices. Argia occurs throughout the New World, with its highest diversity found in our study area (Caesar & Wenzel, 2009). Our approach also differs in that we tested different taxonomic levels (from order to genera) as predictors of habitat integrity. Nevertheless, this approach must be tested with other quality indices and other highly diverse clades in order to determine its accuracy.