1. Effective evidence-based conservation requires full quantification of the impacts of targeted management interventions on focal populations. Such impacts may extend beyond target individuals to also affect demographic rates of non-target conspecifics (e.g. different age classes). However, such collateral (i.e. unplanned) impacts are rarely evaluated, despite their potential to substantially alter conservation outcomes. Subsequent management decisions may then be poorly informed or erroneous.
2. We used 15 years of individual-based demographic data in a “before-after control-impact” (BACI) analysis to quantify collateral demographic impacts of a targeted multi-year supplementary feeding programme designed to increase sub-adult survival and hence viability of a small, threatened red-billed chough (Pyrrhocorax pyrrhocorax) population. Specifically, we assessed whether the intervention also affected adult survival and reproductive success, and whether such collateral effects were themselves sufficient to stabilise population size and hence achieve short-term conservation aims.
3. The probabilities of adult survival and successful reproduction increased substantially between the “before-feeding” and “during-feeding” periods in those choughs associated with supplementary feeding, but not otherwise. Overall breeding success (i.e. number of chicks fledged per occupied territory) also tended to increase, even though brood sizes did not increase. These relationships, which were detectible only through BACI analyses, suggest that supplementary feeding targeted at sub-adults had unplanned positive impacts on adult demographic rates.
4. Deterministic matrix models designed to project population growth demonstrate that these estimated collateral effects were sufficient to make a substantial contribution to increasing population growth rate and achieving short-term population stability.
5. Synthesis and applications: Our results indicate substantial positive collateral impacts of a targeted supplementary feeding intervention on population viability, despite no a priori expectation that the non-target adults were food-limited. This case-study illustrates how thorough assessment of collateral impacts of targeted interventions can affect assessment of short-term efficacy and reveal new opportunities for future interventions, thereby informing subsequent management decisions.
- adaptive management
- adult survival
- annual reproductive success
- before-after control-impact
- evidence-based conservation
- population growth rate
- supplementary feeding