Limited migration from physiological refugia constrains the rescue of native gastropods facing an invasive predator

Biological invasions have caused the loss of freshwater biodiversity
worldwide. The interplay between adaptive responses and demographic
characteristics of populations impacted by invasions is expected to be
important for their resilience, but the interaction between these factors
is poorly understood. The freshwater gastropod Amnicola limosus is native
to the Upper St. Lawrence River and distributed along a water calcium
concentration gradient within which high-calcium habitats are impacted by
an invasive predator fish (Neogobius melanostomus, round goby), whereas
low-calcium habitats provide refuges for the gastropods from the invasive
predator. Our objectives were to 1) test for adaptation of A. limosus to
the invasive predator and the low calcium habitats, and 2) investigate if
migrant gastropods could move from refuge populations to declining invaded
populations (i.e., demographic rescue), which could also help maintain
genetic diversity through gene flow (i.e., genetic rescue). We conducted a
laboratory reciprocal transplant of wild F0 A. limosus sourced from the
two habitat types (high calcium/invaded and low calcium/refuge) to measure
adult survival and fecundity in home and transplant treatments of water
calcium concentration (low/high) and round goby cue (present/absent). We
then applied pooled whole-genome sequencing of twelve gastropod
populations from across the calcium/invasion gradient. We identified
patterns of life-history traits and genetic differentiation across the
habitats that are consistent with local adaptation to low calcium
concentrations in refuge populations and to round goby predation in
invaded populations. We also detected restricted gene flow from the
low-calcium refugia towards high-calcium invaded populations, implying
that the potential for demographic and genetic rescue is limited by
natural dispersal. Our study highlights the importance of considering the
potentially conflicting effects of local adaptation and gene flow for the
resilience of populations coping with invasive predators.