4. DISCUSSION
We assessed and monitored genetic diversity within and between
populations of brown trout over 40 years in a total of 27 small alpine
lakes in ten geographically separate systems in central Sweden. We used
genetic data to identify genetically separate populations and detected
29 such populations that were stable over time. Individual populations
occurred in 1-4 alpine lakes within the same water system and at least
12 of the lakes harbored more than one population.
We quantified the genetic diversity within and between populations and
applied recently proposed indicators to track genetic diversity over
time, and we propose that these indicators can aid in biodiversity
monitoring. All populations were located in protected areas where
maintaining biodiversity is a common goal. While we found no general
trend with respect to decrease or increase of genetic diversity among
the 29 populations of the ten water systems we did observe considerably
lower levels of genetic diversity in the two above-tree-line systems. In
one of these systems we also found a decrease of genetic diversity at a
magnitude exceeding the proposed limiting values for within population
diversity change (ΔH indicator).
In contrast, increased genetic diversity was observed in several
separate populations as well as in three population systems (Figure 7).
The effective population size (N e) was generally
low; 20 out of 29 local populations and five out of eight
metapopulations revealed N e<500 which
is below the threshold value proposed for this indicator. The between
population indicator ΔF ST showed significant
increase of divergence, potentially suggesting reduced gene flow in the
system where the ΔH and N e indicators gave
warning signals (Skåarnja 2; Figure 7b), although the change was within
the proposed limiting values. In two systems ΔF STshowed a significant reduction, potentially indicating increased gene
flow, but here too within the proposed limiting values (cf. Figure 3).
Applying the CBD indicators to the present data provides conclusions
consistent with the national indicators but with less detail (Figure
7c).