Evaluating the effectiveness of predator control: the non-native red fox as a case study

Non-native vertebrate predators pose a severe threat to many native species , and a variety of management programs are aimed at reducing predator effec ts. We sought to assess the effects of predator-control programs by analyzi ng changes in prey and predator populations based on data commonly collecte d in these programs. Me ( examined data from a predator-control program tha t primarily targets The introduced red fox (Vulpes vulpes regalis) in centr al California. Red foxes negatively affect populations of native waterbirds , particularly tbe endangered California Clapper Rail (Rallus longirostris obsoletus). Using a combination of matrix population modeling, simple diffe rence equations, and statistical analysis, we analyzed data on removed pred ators and monitored prey populations, Past control efforts succeeded in dep ressing fox numbers in local areas over 3-month intervals, and there was a significant, positive relationship between the growth rate of local Clapper Rail populations and the successful trapping of red foxes in the preceding year. By modeling tbe effect of different-fox-removal rates, we found that a stable or declining population could be achieved by removing a minimum o f 50% of the adults and 25% of the juveniles. Under trapping rates of 50-70 %, the proportion of the fox population composed of immigrants averaged 20- 52%. In contrast to the current management approach, elasticity analyses su ggested that changes in adult survival rates had relatively little cf(ect o n long-term population growth. Overall, our approach indicated that predato r control was effective in the short term, but for longer-term success it), nay be necessary, to redirect efforts to control juvenile and immigrant fox es. Our analytical approach) is potentially useful for evaluating current c ontrol programs aimed at reducing the effects of predators on native specie s.