The results presented here indicate that connectivity, subsystem and seasonality, as well as non-native species occurrence (NNOccur), non-native species richness (NNRich), and native species richness (NATRich) have a significant influence on taxonomic and functional beta-diversity patterns of small-bodied fish between lakes. Environmental factors influenced the replacement component for both taxonomic and functional diversity, while biotic factors (NNOccur and NNRich) influenced the richness component of each. For sites that contributed the most to beta-diversity (LCBD), both native and non-native species richness were related to the three diversity components (total, richness and replacement BD) for taxonomic and functional approaches. In contrast, NNOccur was related to the LCBD of BDrepl and LCBD of BDrich of both approaches. Seasonality influenced both taxonomic and functional LCBD, while connectivity did not influence any component of LCBD.
Taxonomic and functional beta-diversity
Even though some species vary in resource use, abundance and distribution according to some functional traits (Violle et al. 2007), the present study found very similar patterns for the taxonomic and functional beta-diversity, being influenced by the same variables. This result is potentially worrying since any change in taxonomic diversity would likely have a direct impact on functional diversity. In general, taxonomic beta-diversity was greater than functional beta-diversity, which may indicate functional redundancy since changes in functional characteristics do not occur in proportion to changes in taxonomic diversity. In fact, tropical regions are known to have sizeable functional redundancy due to their extraordinary biodiversity (Toussaint et al. 2016), but this does not guarantee that a loss of taxonomic diversity would not impact the ecosystem services provided (Cadotte et al. 2011; Vitule et al. 2017).
High non-native species richness
Almost half of the fish species collected in the present study are not native to the region, and are present in most sampled lakes. At least 66 non-native fish species have been recorded for the Upper Paraná River floodplain (Ota et al. 2018), which may be potential competitors of native species for resources. These introductions are mainly associated with the construction of the Itaipu reservoir in 1982, which flooded the geographic barrier Salto de Sete Quedas that effectively separated two ichthyofaunistic regions, the Upper and lower Paraná River basins (Langeani et al. 2007; Júlio Júnior et al. 2009). The subsequent construction of a quasi-natural canal for fish transposition in 2002 became a big problem (Vitule et al. 2012). Introducing non-native species into a new environment can directly or indirectly change ecological multifunctionality (Constán-Nava et al. 2015; Moi et al. 2021; Petsch et al. 2022a). The large number of non-native species in the study area can become a critical problem since environments dominated by non-native species tend to be more susceptible to stochastic extinction of native species (Erős et al. 2020).
The influence of non-native species on beta-diversity
The present results show that both the richness of native species and the occurrence and richness of non-native species influenced beta-diversity similarly, mainly the rich component. A biotic acceptance mechanism, common in Neotropical environments, can explain this influence (Ortega et al. 2018; Muniz et al. 2021), since, in these environments, native and non-native fish species respond similarly to increases in available resources (Muniz et al. 2021). According to the biotic acceptance hypothesis, natural ecosystems favorable for the survival of native species, such as those with high resource availability, tend to accommodate the establishment and coexistence of non-native species (Stohlgren et al. 2006; Fridley et al. 2007; Santos et al. 2018).
The influence of environmental factors on beta-diversity
Some environmental factors were found to be related to both taxonomic and functional beta-diversity, such as subsystem and lake connectivity. In contrast, seasonality was only related to the taxonomic beta-diversity. Environmental factors were found to be important mainly for the replacement component, showing that these factors are essential in exchanging species between environments. Indeed, environmental factors directly influence fish community diversity (e.g., Agostinho et al. 2004; Petsch 2016; López-Delgado et al. 2020; Lopes et al. 2022), sometimes even more than spatial factors (López-Delgado et al. 2020).
The hydrological regime is the main component of floodplain seasonality (Junk et al. 1989; Agostinho et al. 2004). The water flow of the Upper Paraná River floodplain is currently regulated by upstream dams, however, the hydrological regime is still the main force influencing the structure and functioning of communities (Agostinho et al. 2004). The alternation between dry and flood periods leads to both limnological and resource availability oscillations (Gomes et al. 2012; Quirino et al. 2017; Petsch et al. 2022b). While there is a greater homogenization of physical and biological characteristics in the flood period (Agostinho et al. 2004; Thomaz et al. 2007; Granzotti et al. 2018), the floodplain sites have less connection with the main channels in the dry period, becoming more susceptible to wind disturbances and tributary inputs, leading to greater dissimilarity (Agostinho et al. 2004; Thomaz et al. 2007; Granzotti et al. 2018). These limnological distinctions can act as environmental filters by selecting fish species that tolerate the conditions of each environment (Leibold et al. 2004; Fernandes et al. 2014), which may contribute to increasing beta-diversity.
Recent climate changes in the study area have been responsible for a decrease in rainfall through a decline in extreme rainfall events in the region (Zandonadi et al. 2016), making the seasons more similar. These changes can directly impact fish of the Upper Paraná River floodplain since the data presented here showed that seasonality influences their beta-diversity. Langer et al. (2018) found variation in annual rainfall to have a significant influence on fish beta-diversity and suggested that climate change and anthropogenic water level stabilization can impact this diversity. In addition, the insertion of dams upstream of the study site has dramatically modified variation in hydrometric level over the years, reducing the frequency and intensity of floods, which has directly influenced fish diversity (Gubiani et al. 2007; Alves et al. 2021).
For both taxonomic and functional beta-diversity, the Paraná subsystem showed an opposite influence on beta-diversity compared to the other two subsystems. This difference may be associated with two intrinsic environmental characteristics of this subsystem. The first is the presence of a cascade of reservoirs upstream of the Upper Paraná River floodplain, which directly influences the habitats of the Paraná subsystem, while having a substantially smaller influence for the Baía and Ivinhema subsystems (Agostinho et al. 2004; Roberto et al. 2009). The second concerns the fluctuation of the hydrometric level, which is independent in each of the three subsystems (Souza-Filho et al. 2004), leading to environmental changes and high fish diversity in these locations (Agostinho et al. 2004; Lopes et al. 2022).
Habitat connectivity was another factor explaining the beta-diversity of both facets. This was expected as the disconnection of lakes from a main channel generally reduces fish diversity (Liu and Wang 2010). Although these subsystems are connected, the presence of different types of habitats and tributaries within them increases environmental heterogeneity and, consequently, regional biodiversity (Agostinho et al. 2007), especially during dry periods when several environments are disconnected.
Local contribution to beta-diversity
In general, this study found that environmental and biological characteristics influence LCBD. High LCBD values are generally negatively related to species richness (Heino and Grönroos 2017; Landeiro et al. 2018). Here, both BDtotal LCBD and BDrich LCDB were, in general, negatively related to native and non-native species richness, at least to some extent. Sites with low species richness can contribute to beta-diversity as they harbor unique compositions (Legendre and De Cáceres 2013). Therefore, they may be ecologically unique and more susceptible to impacts. However, despite the negative response, after a certain number of species, the increase in species richness (native or non-native) generated an increase in LCBD values for taxonomic BDtotal and BDrich and functional BDtotal.
This positive relationship between LCBD and richness was also observed by Kong et al. (2017), who suggested that it reflects the introduction of novel species (e.g., migratory species) in communities. Thus, sites of both low and high-richness require special attention in terms of environmental protection (Legendre and De Cáceres 2013), as both may represent unique species and traits. On the other hand, the results of BDrepl LCBD showed a greater species substitution in an intermediate species richness, indicating that in a high richness there is only an increment of species, while in a low richness there is only the presence of common species. The results of the present study did not show a relationship between the occurrence of non-native species and total taxonomic and functional LCBD. Nonetheless, they did show a relationship between the LCBD of BDrepl and LCBD of BDrich, both functional and taxonomic with opposite trends. These results show the importance of decomposing beta-diversity into two components to fully understand the predominant processes occurring in a community (Legendre 2014). Thus, sites with the occurrence of non-native species proved important for the replacement of species. In contrast, places without non-native species are more important for the richness component. Since non-native species comprised about half of the species and were present in 76.5% of the sampled points studied, sampled points lacking them generally had lower species richness, leading to a peak of LCBD for the richness component.
The results presented here also showed an influence of environmental variables (seasonality and connectivity) on LCBD. There is less connectivity between environments and a decrease in food availability during the dry period, compared to the flood period (Junk et al. 1989; Quirino et al. 2017). In addition, there is an intensification of some biological interactions during the dry period, such as predation and competition for resources (Thomaz et al. 2008; Fernandes et al. 2014), making each environment more unique, a crucial factor for LCBD.