DISCUSSION
We investigated the habitat fragmentation-related effects (e.g., the
consequences of reduced size and increased isolation) on the demographic
and genetic processes shaping the metapopulation of two epiphyllous
bryophyte species with contrasting mating systems. We showed that
fragmentation induces marked effects on the functional connectivity of
both plant populations in less than half a century of experimental size
and connectivity reduction. The long-term reduced population density in
small patches (1- and 10-ha) (Sierra et al., 2019b), has led to
accelerated genotypic differentiation (genetic drift) compared to the
populations in large patches (100-ha fragment and continuous forests).
This result supports our hypothesis (H1) that spatial variation in patch
size and isolation are important determinants of the metapopulation
genetic process (stochastic loss of genotypes) in a fragmented forest,
despite the notion of bryophytes as being highly vagile organisms
(Vanderpoorten et al., 2019)
We observed that species’ mating system confers different sensitivity to
the effects of habitat fragmentation, where a shift in the genetic
structure (diversity and allele frequency: FST )
of the unisexual plant was less negatively impacted by forest
fragmentation than of the bisexual counterpart, contrary to our
expectations (H2). However, this should be taken with caution since is
expected that genetic diversity meltdown will take a longer time to be
apparent than changes in allele frequency in response to habitat
fragmentation (Lowe et al., 2005). In the case of the unisexual species
the observed differentiation in 1-ha might indicate initial signs of
diversity loss in the following generations under the current landscape
fragmented scenario.
The metapopulation migration network of the two species shows
differences in functional connectivity, with the unisexual species more
inter-connected among patches than the bisexual species, contrary to our
predictions (H3). For R. flaccida (unisexual) we observed a
patchy metapopulation with a highly interconnected migration network,
with all patches exchanging migrants among them, irrespective of size
and isolation. However, positive isolation by distances in small patches
suggests that migrants are from nearby source populations. WhileC. surinamnesis (bisexual) present symmetrical migration only
between small patches, and no sign of isolation by distance among
populations. Stochastic extinctions are expected to increase in small
patches, but genetic diversity might be maintained through colonization
from nearby source populations if functional connectivity is maintained
(Auffret et al., 2017). Despite the patch size effect on the
metapopulation demography and genetics of both epiphyllous bryophyte
species, small patches remain valuable for conservation assessment, as
they can act as a source of migrants to other forest fragments and
continuous forests, as observed for Macronesian oceanic islands (Laenen
et al., 2011; Patiño et al., 2015).