Extinction debt of species and interactions in a fragmented landscape

Addressing the worldwide biodiversity crisis in fragmented landscapes requires considering both immediate and delayed extinctions—the extinction debt. Assuming a paid debt in long-fragmented habitats, we examined the potentially unpaid debt in recently fragmented habitats as the difference between the current richness of species and interactions and their richness expected from area and connectivity for long-fragmented habitats.

Methods
We selected 14 remnant habitat fragments of Monte dryland in an agricultural region of central-western Argentina. To distinguish between recently and long-fragmented habitats, we defined long-fragmented habitats as fragments that experienced significant changes before 2003, and recently fragmented habitats, after 2003. We measured current areas and connectivities using the 2015 map, while for past areas and connectivities we used the 2003 map. To estimate fragment connectivity, we calculated the sum of the shortest geographical distances between neighboring fragments. To estimate current richness of plants, herbivorous insects, parasitoids, and their trophic interactions in the habitat fragments, we sampled at the centroid of each fragment in the austral spring and summer between 2015 and 2017. We measured plant-herbivore interaction richness as the number of individual interactions of plant-gall, plant-mine, plant-aphid colony, and plant-scale insect observed in the field. Then, we measured the richness of herbivore-parasitoid interactions as the number of individual interactions of gall-parasitoid, mine-parasitoid, aphid colony-parasitoid, and scale insect-parasitoid observed in the laboratory.