Estimating the extent of dispersal required to maintain or
restore genetic connectivity and diversity between the subdivided
populations.
Forward time simulations were programmed in PYTHON using the package
SIMUPOP (Peng & Kimmel 2005),
http://simupop.sourceforge.net/]. The aim of these
simulations was to estimate what amount of movement would be needed to
maintain genetic connectivity between the above and below koala
populations located on either side of the newly built rail line. These
simulations randomly selected two koalas (one male, and one female) from
within each of the two populations (above and below) and bred them to
create an offspring for the next generation. This process was then
repeated until the number of offspring matched the number of parents in
each population (therefore, our
simulations maintained a stable population size) for every generation
(Peng & Kimmel 2005). The mating structure of the simulations was set
to mimic the mating system of the impacted koala population where 30%
of the males (Schultz et al. 2020) and 90% of the females (Beyeret al. 2018) mate per generation. To mimic this, each generation
of the simulation only allowed a random 30% of male koalas and 90% of
female koalas to breed and contribute to the genetic make-up of the next
generation of koalas. The simulations had overlapping generations, with
each koala given a 20% change to persist through to the next generation
(up to a maximum of 2 generations). Ten generations were simulated. The
simulations imported the genetic data from the two post-construction
datasets (Dataset3above = 27 koalas and
Dataset3below = 75 koalas) as a starting point. This
shared starting point of the simulations means that genetic
differentiation between each side of the linear transport infrastructure
(above and below), and the genetic diversity within each of those, was
the same at the start of every simulation. Genetic differentiation was
measured at each simulated generation using Fst, Gst and Mutual
Information (Sherwin et al. 2006; Jost 2008; Sherwin 2010;
Sherwin et al. 2017; Jost et al. 2018). Based on the
imported genetic data, the starting point of these connectivity measures
were: Fst = 0.005, Gst = 0.006 and Mutual Information = 0.003. Genetic
diversity was measured at each simulated generation using expected
heterozygosity (He) and Shannon’s information.
We then further simulated 12
dispersal treatments by increasing the number of koalas that dispersed
from each side of the linear transport infrastructure per generation:
(1) zero koalas = no dispersal (2) 1 koala, (3) 2 koalas, (4) 4 koalas,
(5) 6 koalas, (6) 8 koalas, (7) 10 koalas, (8) 12 koalas, (9) 14 koalas,
(10) 16 koalas, (11) 18 koalas, and (12) 20 koalas. In the simulations,
dispersal occurred before breeding. The simulated dispersal between the
subdivided populations caused the population size of each subpopulation
to become equal over time. Each dispersal treatment was simulated for 10
generations and replicated 1000 times.
RESULTS