Complex interactions between remnant shape and the mating system strongly influence reproductive output and progeny performance in fragmented populations of a bird-pollinated shrub

Habitat fragmentation can have large and negative impacts on mating, reproduction and fitness within remnant plant populations. Linear strips of native vegetation are common occurrences in agricultural and revegetated landscapes, yet the impacts of this geometry on aspects of population viability are little studied. Population shape affects the configuration and abundance of mates and therefore could influence patterns of mating, seed production and progeny fitness. Here, we investigate whether population shape, as a key aspect of fragmentation, has an impact on reproduction or progeny performance in the shrub Banksia sphaerocarpa var. caesia and whether impacts reflect variation in the mating system. Plants in linear populations were larger and produced more inflorescences and cones, but had substantially smaller seeds and produced seedlings with lower survival rates. Among-plant and among-population variation in seed size was strongly related to paternal diversity, with larger seeds produced in seed crops with a greater diversity of paternal contribution. Smaller seeds had lower germination and produced smaller seedlings with lower rates of survival. We also found significant inbreeding depression for seedling dry weight. Our results highlight the complex and multi-dimensional nature of species’ responses to fragmentation. Further, they suggest that remnant shape and the diversity of paternal contribution can have important effects on the reproduction, fitness and viability of plant populations, but are overlooked by most fragmentation studies.