Consequences of mating system and dispersal potential
Species with different reproductive strategies might present different
sensitivity to small patch sizes and to isolation to maintain genetic
diversity (Honnay et al., 2005; Obbard et al., 2006; Nazareno et al.,
2013). Bisexual bryophytes are presumed to have high fertilization rates
and consequently higher spore production, which ensure successful
colonization (Laenen et al., 2016). In contrast, unisexual species will
present lower dispersal potential as the probability of fertilization
will depend on that male and female plants are found in the same colony
(Snäll et al., 2004; Maciel-Silva et al., 2012; Alvarenga et al., 2016).
We observed high migration of R. flaccida among patches across
the fragmented landscape irrespective of patch size. Functional
connectivity between patches recovered in migration network analysis
explained the ability of the species to maintain genetic diversity
despite population density decline. Colonization rates of R.
flaccida (unisexual), albeit low, were sufficient to curtail genetic
drift as suggested by Zartman et al., (2006). On the other hand, the
maintenance of genetic diversity observed in R. flaccida in small
patches could be attributed to other two reasons. Firstly, genetically
diverse populations have been maintained in small patches (although
declining) due to their adaptative potential to cope with environmental
change (Lowe et al., 2005); which is corroborated by the maintenance of
linkage disequilibrium in small forest fragments (Zartman et al., 2006).
Second, the standing genetic variation has not yet been lost through
stochastic demographic events due to constant local recruitment from
asexual propagules when sexual reproduction is limited (Honnay &
Bossuyt, 2005). Limited sexual reproduction has been reported forR. flaccida majorly during the dry season, which is expected to
exacerbate under future climate change scenarios (Zartman et al., 2015)
affecting long-term demographic stability (Scott et al., 2021). Clonal
reproduction represents an alternative recruitment strategy when
environmental conditions suppress sexual reproduction in unisexual
species, maintaining genotypes before fragmentation effects become
apparent (Honnay & Bossuyt, 2005). In bryophytes, as observed herein
for R. flaccida , asexual reproduction plays an important role in
maintaining genetic diversity over time by preventing local genotype
extinction (Honnay & Bossuyt, 2005) and assuring successful sexual
reproduction (Alvarenga et al., 2016). Evidence from unisexual
bryophytes found that genetic diversity is maintained when species
stakes on asexual reproduction for their survival (Pohjamo et al., 2008;
Wang et al., 2013; Holá et al., 2015; Escolástico-Ortiz et al., 2023),
but contrasting evidence was found for a hornwort species (Alonso-Garcia
et al., 2020).
Regarding C. surinamensis (bisexual), we expected a higher
dispersal potential due to the higher probability to produce sporophytes
since both androecial and gynoecia branches are produced in the same
colony (Maciel-Silva et al., 2012; Laenen et al., 2016). Contrary to our
expectations (H3), this species shows restricted dispersal among small
patches (1- and 10-ha) where significantly lower genetic diversity and
significant changes in allele frequency were observed. The stochastic
genetic loss with declining populations of C. surinamensis in
small patches, suggests an extinction debt has been already paid during
the 40 years of patch isolation. A possible explanation for the lower
dispersal potential would be that the genus Cololejeunea presents
low spore output per capsule (250–900 spores) when compared to other
genera in the family Lejeuneaceae (He & Zhu, 2011). Even if the
sporophyte frequency of the bisexual and unisexual species studied here
are similar in a population, the lower spore output per capsule ofC. surinamensis might limit the dispersal across the landscape
(Snäll et al., 2004). Demographic surveys of epiphyllous bryophytes
showed that species differ in the threshold size to initiate sexual
expression, where higher size requirements might limit sporophyte
frequency under unsuitable environmental conditions (Zartman et al.,
2015; Alvarenga et al., 2016). Environmental conditions in degraded
forest fragments might affect reproductive performance, by limiting
sexual expression and sporophyte frequency of colonies growing in
unsuitable habitat conditions (Maciel-Silva et al., 2012), which will
further determine the assembly rate of individuals and genotypes (Sierra
et al., 2019a; Mežaka et al., 2019).