A recovery network leads to the natural recolonization of an archipelago and a potential trailing edge refuge

Rapid environmental change is reshaping ecosystems and driving species
loss globally. Carnivore populations have declined and retracted rapidly
and have been the target of numerous translocation projects. Success,
however, is complicated when these efforts occur in novel ecosystems.
Identifying refuges, locations that are resistant to environmental change,
within a translocation framework should improve population recovery and
persistence. American martens (Martes americana) are the most frequently
translocated carnivore in North America. As elsewhere, martens were
extirpated across much of the Great Lakes region by the 1930s and, despite
multiple translocations beginning in the 1950s, martens remain of regional
conservation concern. Surprisingly, martens were rediscovered in 2014 on
the Apostle Islands of Lake Superior after a putative absence of
>40 years. To identify the source of martens to the islands and
understand connectivity of the reintroduction network, we collected
genetic data on martens from the archipelago and from all regional
reintroduction sites. In total, we genotyped 483 individual martens, 43 of
which inhabited the Apostle Islands (densities 0.42-1.46/km2). Coalescent
analyses supported the contemporary recolonization of the Apostle Islands
with progenitors likely originating from Michigan, which were sourced from
Ontario. We also identified movements by a first-order relative between
the Apostle Islands and the recovery network. We detected some regional
gene flow, but in an unexpected direction: individuals moving from the
islands to the mainland. Our findings suggest that the Apostle Islands
were naturally recolonized by progeny of translocated individuals and now
act as a source back to the reintroduction sites on the mainland. We
suggest that the Apostle Islands, given its protection from disturbance,
complex forest structure, and reduced carnivore competition, will act as a
potential refuge for marten along their trailing range boundary and a
central node for regional recovery. Our work reveals that translocations,
even those occurring along southern range boundaries, can create recovery
networks that function like natural metapopulations. Identifying refuges,
locations that are resistant to environmental change, within these
recovery networks can further improve species recovery, even within novel
environments. Future translocation planning should a priori identify
potential refuges and sources to improve short-term recovery and long-term
persistence.