Introduced species are frequently implicated in population declines and extinctions of native species. In some instances, the interactions between introduced and native species have been well studied and strong inferential links have been established. However, in many more cases, evidence linking introduced species to native declines is weak and multiple other drivers of decline are equally plausible. Well-designed, replicated experiments based on a priori hypotheses at appropriate temporal and spatial scales provide the most unambiguous test of hypotheses regarding the role of species interactions. However, such experiments are not always feasible for logistical or political reasons. In the absence of experiments, rigorous analysis of vital rates (e.g., colonization and extinction or survival and growth) provide stronger inferences and quicker rates of learning than focusing on state variables (e.g., occupancy or abundance). Here, we focus on two case studies to illustrate how occupancy dynamics or population dynamics of interacting species can be modelled using detection/nondetection or mark-recapture data.
Joint modelling of territorial occupancy of barred and Northern spotted owls in the Pacific Northwest of the USA using two-species dynamic occupancy models illustrates that territorial extinction probabilities of both species are elevated in jointly occupied areas. However, the effects of competition on territorial extinction are greater for the endangered Northern spotted owl. Joint modelling of population dynamics or adult rainbow trout and juvenile humpback chub in Grand Canyon using mark-recapture data for both species suggests the survival probabilities of juvenile humpback chub are depressed when rainbow trout abundances are higher. Invading species (barred owls and rainbow trout) significantly decrease the detection or capture probabilities of endangered resident species in both case studies. We briefly discuss how results of joint dynamic modelling can be used to predict the future state (occupancy or abundance) of endangered species under various management alternatives.