MOLECULAR ECOLOGY AND SYSTEMATICS OF SOTALIA DOLPHINS - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 276

When there is enough gene flow across the species range, individuals removed by human-
related factors are replaced from other areas. But when a species is split into different and
isolated populations (i.e. MU), each one evolves independently, since they are not connected
(and replenished) through migration. Besides, independent units harbour exclusive genetic
variation (locally originated or maintained, and not spread to other units due to restricted gene
flow), and it is reasonable to assume that some of that variation may encompass local
adaptations. It is crucial to ensure that genetic diversity is preserved, because it constitutes the
evolutionary potential of the species. Inappropriate management of units may result in the
loss of adaptations, which may jeopardize the short-term viability of some populations, or
even the species as a whole (Frankham, 1996; Solé-Cava, 2000; Crandall et al., 2000).
Therefore, knowledge on the population structure is of paramount importance, as it enhances
the probability of success of management and conservation actions (O'Brien, 1994).
Understanding the social structure of Sotalia dolphins may also help in their
conservation. For instance, if females of either species prove phylopatric, management must
be based on mitochondrial data, even if there is evidence of gene flow with nuclear markers
(Avise, 1995; Dizon et al., 1997). Mitochondrial DNA is maternally inherited, so it depicts
the history and structuring of female lineages. If only males disperse, populations are unlikely
to be recolonized after local extinction, and the most conservative strategy would be to ensure
the persistence of each population detected with mitochondrial data. In addition, if mortality
rates are higher in areas between populations (which has been demonstrated for some
species), that mortality would translate into a higher loss of males compared to females,
causing unequal sex-ratio and reduction of the effective population size and genetic
variability of the species.
The studies cited above provide the first, and most reliable, data for the establishment of
MU for S. guianensis . Before their publication, there was no information on genetics,
demography, morphology, behaviour, bioacoustics, parasites, ecology or contaminants that
could argue for any delimitation of MU for the species, even provisional. That is the present
situation for S. fluviatilis , but it will change in the near future, as the investigation of its
population structure using molecular markers is currently underway.
Many threats to the persistence of both Sotalia species have been identified. However,
the paucity of information on the taxonomy and biology of Sotalia dolphins hindered the
evaluation of their conservation status; hence they are considered ―data deficient‖ by the
Brazilian environmental agency IBAMA (2001) and by IUCN (2008). Some countries took a
precautionary approach and decided to give Sotalia a conservation status: in Colombia and
Venezuela, both species are regarded as ―vulnerable‖ (Rodríguez-Mahecha et al., 2006;
Bolaños-Jímenez et al., 2008), and in Ecuador, S. fluviatilis is listed as ―endangered‖ (Tirira,
2001). With the data now available, environmental agencies need to reassess the conservation
status of both species, especially in Brazil, because that country encompasses over half of the
range of S. guianensis , and most of the distribution of S. fluviatilis.