June 13, 2011
The cichlid Mesonauta festivus is common and abundant among macrophyte stands along a large geographical range of the Amazonas and Paraná-Paraguay basins, in South America. This broad geographical range highlights the species’ dispersion ability, which can be attributed to specific biological and behavioral traits. However, the dispersion ability does not account for the broad geographical range alone, as the species must be able to establish populations in a range of environments, which include marginal areas of large rivers with different water types, floodplain lakes, and small terra-firme streams. In this work we investigated the specie’s ecology, biological traits and behavior in order to understand what and how its traits may have allowed it to attain such broad geographical range and aid in establishing local populations. Regarding its dispersion ability we stress the capability of swimming in the pelagic region, which is remarkable for this species and uncommon among Neotropical cichlids. Its vagility is high even when juveniles are under parental care. Regarding population establishment, the high environmental tolerance stands out, allowing the species to live under strikingly different abiotic conditions. In addition, the small size of first sexual maturation and its capability of spawning along the whole hydrologic cycle (apparently not associated to a specific environmental cue) may also facilitate the establishment of populations into new environments. Moreover, the behavior of mimicking dead leaves, which is mainly performed by juveniles, may lessen predation pressures. Under an eco-evolutionary perspective, the traits highlighted in this work may buffer selective pressures experienced by populations in different biotic and/or abiotic conditions, which may also favor the increasing of the geographical range by allowing the evolutionary lineage to remain similar even in disconnected and/or striking different environments.
Density-dependent regulation is a necessary process for the long-term persistence of populations. Nevertheless, ecologists still debate whether floodplain fish populations are subject to density-dependent dynamics or mostly regulated by the density-independent seasonal flooding. We surveyed Jewel tetra (Hyphessonbrycon eques) populations during 4 years in the floodplain lagoons of the Cuiabá River (Brazil) in order to determine if recruitment, growth, and survival are subject to density-dependent effects. We showed that population dynamics of this species are influenced by the seasonal drought, which affects the various life stages of the cohort in different ways. The flood had positive effects on the strength of recruitment, and indirectly affected the probability of survival of recruits during the following dry season as well as on the extinction probability of local populations. Additionally, by facilitating the dispersal of pre-larvae and juvenile recruits, it enables the re-establishment of locally extinct populations during the preceding dry season. Therefore, flooding is essential for the persistence of the species at the regional scale. On the other hand, the limiting conditions during the dry season cause high mortality rates among juveniles. More importantly, the mortality during the dry season is density-dependent and causes changes in population size structure and extinction of local populations, which will affect recruitment in the following year. Our results change the traditional belief that flood alone control population dynamics of fish dwelling in floodplains of larger rivers but a longer survey is required to better understand the effects of drought.
Following flooding peaks in the Beni River, a massive upstream migration event involving juvenile pencil catfish (Trichomycterus barbouri) or chipi chipi is described for the first time. The annual migration begins in the floodplains of the Beni River, where enormous schools of juveniles form to travel upstream through the straits of the last foothills of the Andes into Andean foothill forest streams and rivers. Observations and local knowledge suggest a migration distance of at least 370 km over an average of 32 days in February and March with an average speed of 12 km/day. The migrating juveniles weigh less than 0.38 g and measure less than 33 mm in standard length. As such, considering body length and body weight to distance travelled ratios they are one have one of the greatest migration efforts of any freshwater fish. Local people harvest juveniles across the migration route, but especially in Rurrenabaque, where they are considered a seasonal dish. This scientific revelation highlights the Amazon as a place where natural phenomena are still being discovered, described and documented in an era when hydroelectric infrastructure threatens the ecology of many aquatic ecosystems.
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