Processes responsible for population structuring across spatial and
temporal scales represent key components in understanding speciation and
evolution. We use a hierarchical approach to investigate the degree and
mechanisms of structuring in landlocked and diadromous populations of
the facultatively amphidromous fish Galaxias brevipinnis across various
temporal and spatial scales in southern New Zealand. To determine
long-term structuring, multiple lakes and coastal sites were compared
genetically. Short-term structuring was assessed using otolith
microchemistry for a subset of sites, and behavioural mechanisms driving
population structuring were assessed via larval distributions. Genetic
data show that lakes foster divergence of lake-developing populations
from each other and from coastal stream populations, whereas there is
relatively little structuring within coast or lake populations. However,
otolith analyses indicate that on a shorter time scale, most larvae do
not disperse, i.e. recruitment is local. Thus, lake and coastal
populations show a distinct meta-population structure based on
catchment, in contrast to the prevailing assumption of widespread
dispersal, with implications for management. Most larvae were
distributed in river plumes, suggesting that a simple larval behavioural
mechanism, e.g. positive rheotaxis, may result in larval retention
within catchments and lakes. However, not all larvae were retained in
plumes, creating opportunities for genetic exchange within-lake or among
coastal sites. Genetic divergence of lake populations as a consequence
of landscape and behaviour provides an insight into the potential of G.
brevipinnis to diversify and speciate, when landscape and circumstances
align, and also has implications for the management of this and other
facultatively amphidromous species.
Linking carbon and water cycles using stable isotopes across scales: progress and challenges
