Data from: Long-term monitoring reveals how pondscape connectivity shapes the early spread of a biological invasion

Biological invasions lead to loss of biodiversity and ecosystem
degradation, and a better understanding of their drivers is urgently
needed. This is particularly true in the initial stages of an invasion,
when the transition from local establishments to regional advancing fronts
occurs. The abundances of the local population, demographic rates, and
spatial conditions for dispersal have been proposed to interact in
determining invasive species expansion, but little empirical evidence has
been accumulated. This study examines the population dynamics of the
American bullfrog (Aquarana catesbeiana), a globally invasive aquatic
anuran, during the initial stages of its invasion process in a pond
landscape within the Pampas grasslands of Uruguay. We evaluated how the
spatial spread of the invasion was influenced by connectivity between
invaded and uninvaded ponds, bullfrog abundance in invaded ponds,
elevation gradients, and pond area. This analysis was based on an 11-year
monitoring program (2012-2022) that captured the onset and initial stage
of spatial expansion. Throughout the study period, the number of invaded
ponds increased at a rate of 7.7 % per year. A model of invasion
probability revealed that connectivity to invaded ponds and the population
status of those ponds were key determinants of spread. Connectivity to
previously invaded ponds interacted with bullfrog abundance to determine
invasion probability. Ponds connected to invaded locations with
intermediate bullfrog abundances showed the highest invasion likelihood,
with colonization odds more than twice those associated with connections
to ponds with low bullfrog abundances, whereas connections to ponds with
high abundances had more moderate effects. Our results highlighted how
ponds with intermediate bullfrog abundance play a crucial role in
facilitating the invasion spread, this is most likely due to higher
propagule release. Ponds with high bullfrog abundance were likely
constrained by density-dependent effects, reducing the survival and
dispersal of metamorphs. Our study highlights that prioritizing bullfrog
eradication in systems with intermediate abundances may be more effective
to prevent the expansion of the invasion. We note that landscape features
and population demography within invaded areas could be more interrelated
than commonly assumed and should be jointly considered in invasive species
management strategies.