Species Co-Occurrence and Population Dynamics in Annual Fish Assemblages in the Lower Río Uruguay Basin
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Environ Biol Fish https://doi.org/10.1007/s10641-019-00854-x Species co-occurrence and population dynamics in annual fish assemblages in the lower Río Uruguay basin Daniel García & Marcelo Loureiro & Emanuel Machín & Martin Reichard Received: 30 July 2018 /Accepted: 11 February 2019 # Springer Nature B.V. 2019 Abstract Austrolebias is a genus of annual killifishes indications that adult A. bellottii can survive a short from subtropical and temperate grasslands of South period of habitat desiccation in wet mud, leading to America. They coexist in assemblages of up to five coexistence of an older A. bellottii cohort with a younger species in pools with variable connection to permanent A. nigripinnis cohort. Males disappeared from popula- water bodies and complete their entire life cycle within tions at a higher rate, especially towards the end of the 1 year, terminated by summer desiccation of their hab- season. The overall lifespan of both common species itat. To quantify community and population characteris- was up to 7 months in pools that did not experience mid- tics of Austrolebias assemblages, we sampled a set of 18 season desiccation. This study provides fundamental pools in the lowlands of the confluence of Río Negro demographic parameters for the annual fish in this and Río Uruguay in western Uruguay, with 16 sampling region. trips conducted over the entire seasonal cycle in 2015. Austrolebias bellottii was a ubiquitous and generalist Keywords Annual fish . Community assembly. species, A. nigripinnis was more common in pools Cyprinodontiformes . Seasonal dynamics . Sex ratio influenced by active floodplain, A. elongatus (a large predatory species) was rare overall, and four A. alexandri was found only in two pools. Unexpected- Introduction ly, many pools desiccated in winter (mid season) and some of them supported a second cohort after their re- Ephemeral pools are exploited by fishes of diverse life- inundation. Fish abundance and fish density declined history strategies. While most fishes exploit ephemeral steadily prior to pool desiccations. There were habitats temporarily, others possess special adaptations to the fluctuating and challenging environmental condi- tions, such as air breathing and the ability to escape from Electronic supplementary material The online version of this č article (https://doi.org/10.1007/s10641-019-00854-x) contains desiccating pools via crawling or jumping (Pola ik and supplementary material, which is available to authorized users. Podrabsky 2015). Very few fish taxa, however, are : : adapted to complete their entire life cycle in ephemeral D. García M. Loureiro E. Machín pools. Adaptations include dry-season aestivation of Departamento de Ecología y Evolución, Instituto de Biología, Facultad de Ciencias, Universidad de la República, Iguá, adult fish in wet mud (Delaney et al. 1974;Chewetal. 4225 Montevideo, Uruguay 2004) and survival of embryos protected in diapausing stages in some species of cyprinodontiform fishes, M. Reichard (*) The Czech Academy of Sciences, Institute of Vertebrate Biology, named annual fishes. Květná 8, Brno, Czech Republic Annual fish populations survive the dry period as e-mail: [email protected] eggs encased in the desiccated substrate. The eggs Environ Biol Fish possess an ornamented external layer (Loureiro and de Austrolebias, coexistence of two or three species is com- Sá 1996;Thompsonetal.2017) that protects the em- mon and some assemblages comprise five species bryo from desiccation and enables gas exchange coexisting in a single pool (Loureiro et al. 2016). (Wourms and Sheldon 1976). The embryonic develop- Coexisting species typically belong to different phyloge- ment of annual fishes comprises up to three diapausing netic clades and morphotypes, including a large predatory stages (Wourms 1972a, b, c) that shield developing morphotype. The predatory Austrolebias morphotype embryos from harsh environmental conditions and syn- (elongated and robust body, with a total of five species) chronize hatching with habitat inundation (García et al. mayfeedonotherannualfishes(Costa2009;Loureiro 2018). After hatching, annual fish develop rapidly, et al. 2016) and are a rare component of the assemblage reaching sexual maturity within a few weeks (Blažek (Costa 2009). Other annual fish morphotypes correspond et al. 2013; Vrtílek et al. 2018a), and reproducing daily to medium or small-sized species with compressed and to fully exploit temporary nature of their habitat. Annual ellipsoidal to elongate bodies and are considered gener- life history appears to have evolved at least six times in alist predators with broadly overlapping diets (Laufer Cyprinodotiformes (Furness et al. 2015). Most annual et al. 2009;Keppeleretal.2015). Given that Austrolebias fishes occur in tropical regions, with the rainy season species live in small replicated water bodies, they repre- during the warmest phase of the year. However, some sent an ideal system to understand aspects of community Neotropical killifishes have colonized ephemeral pools assembly (Arim et al. 2010) and community ecology in subtropical and temperate regions, where the wet (Laufer et al. 2009; Canavero et al. 2014; Keppeler phase of pools occurs during the coldest period of the et al. 2015; Reichard et al. 2018). However, there is year and pools desiccate in summer (Berois et al. 2015). limited information on seasonal dynamics in temperate In the present study, we investigated population and and subtropical annual fish assemblages, most of which community characteristics of Austrolebias annual fishes derives from the Atlantic coastal lagoon region (Lanés from subtropical region of Uruguay. The genus et al. 2014, 2016). Specifically, there is a lack of infor- Austrolebias (Rivulidae) is the most diverse genus of mation on the seasonal dynamics (that represent their annual fishes in subtropical South America, with more entire life cycle) of the widely distributed species of Río than 40 species described (Loureiro et al. 2016;Alonso Uruguay basin. et al. 2018a).Thecentreofitsrangeisinthelowlandsof The pools where Austrolebias occur can be entirely the La Plata and Los Patos basins. While many isolated from permanent water bodies, or may be tem- Austrolebias species are endemic to small areas and porarily connected to lakes, streams or rivers during categorized as vulnerable, endangered or critically endan- occasional floods, enabling the invasion of non-annual gered(RosaandLima2008;Volcánetal.2009, 2010; fishes such as characids and poecilids (Vaz Ferreira et al. Louriero and Bessonart 2017;Alonsoetal.2018a), other 1966; Lanés et al. 2016). Austrolebias populations ap- species are common and geographically widespread pear adapted to these incursions (Lanés et al. 2016), (Costa 2009). The size of habitat where Austrolebias though non-annual fishes are reported to eliminate an- occur varies from small pools to extensive flooded nual fish populations in the tropics (Nico and meadows. Austrolebias are specialised to ephemeral Thomerson 1989). In the Atlantic coastal lagoons and pools where they complete their entire life cycle and their the Río Paraná basin of subtropical South America, populations are not found in permanent habitats. Across other genera of annual fishes occur with Austrolebias most of their range (including our study area), the pools but in the Río Uruguay basin, where the current study inundate in early autumn (March–April) once tempera- was conducted, Austrolebias is the only annual fish ture decreases to the levels when precipitation exceeds genus that occurs in this region. evaporation and persist until late spring (November–De- In this study we aimed to describe the seasonal pat- cember) when evaporation increases (García et al. 2018). terns of species co-occurrence and sex ratio of annual The hatching of Austrolebias species is highly fishes that coexist in the wetlands of the Río Negro basin synchronised with pool inundation (March–April) in Uruguay (Río Uruguay/La Plata basin). We predicted (García et al. 2018), with juveniles attaining sexual existence of one core species with a common occurrence maturatity in approximately 8 weeks (Berois et al. 2012). in most pools co-occurring with other, more specialised While some pools in subtropical and temperate grass- annual fish species. We further predicted a steady sea- land are inhabited by only a single species of sonal decrease in annual fish abundance (as a response Environ Biol Fish to deterioration habitat conditions and the negative ef- The study area is composed of five sections that form fects of ageing) rather than a sudden steep decline in the clusters of pools with similar characteristics and spatial terminal phase of the pools (Vrtílek et al. 2018b). Lastly, proximity (Fig. 1, Table 1). The BRincón^ cluster (sec- we predicted a seasonal decrease in the proportion of tion A, six pools) is located in the Río Negro floodplain males in the populations as a response of male-male and was occasionally flooded from the river. Depending competition for females (Passos et al. 2014). on the flood magnitude, the river flooded different sets Three Austrolebias species were expected to be found of pools. Small floods affected only 3A, 4A and in our study area. Austrolebias bellottii (Steindachner, 6A. medium floods also affected 1A and 5A. Pool 2A 1881) (maximum size 70 mm) and Austrolebias was not affected by flooding at all during the sampling nigripinnis (Regan, 1912) (maximum size 40 mm) are season (García pers. obs.). The BCanaleta^ cluster of