Demographic rates reveal the benefits of protected areas in a long-lived migratory bird

Abstract

Recent studies have suggested that protected areas often fail to conserve target species. However, the efficacy of terrestrial protected areas is difficult to measure, especially for highly vagile species like migratory birds that may move between protected and unprotected areas throughout their lives. Here, we use a 30-y dataset of detailed demographic data from a migratory waterbird, the Whooper swan (Cygnus cygnus), to assess the value of nature reserves (NRs). We assess how demographic rates vary at sites with varying levels of protection and how they are influenced by movements between sites. Swans had a lower breeding probability when wintering inside NRs than outside but better survival for all age classes, generating a 30-fold higher annual growth rate within NRs. There was also a net movement of individuals from NRs to non-NRs. By combining these demographic rates and estimates of movement (into and out of NRs) into population projection models, we show that the NRs should help to double the population of swans wintering in the United Kingdom by 2030. These results highlight the major effect that spatial management can have on species conservation, even when the areas protected are relatively small and only used during short periods of the life cycle.

Keywords: demography; migration; protected area.

Fig. 1.

Whooper swans’ (A) survival and (B) encounter rates within and outside nature reserves, and (C) movement rate out of and into NR. Note, to account for trap dependence, we considered different encounter probabilities for trap-aware adults (TA) and trap-unaware adults (TU). Black tick marks denote posterior means, gray tick marks denote 95% credible intervals and the extent of the gray polygons shows the extent of the posterior distribution.

Fig. 2.

Whooper swans’ breeding probability with age for (A) individuals within and outside nature reserves (NR) during the two preceding winters, and (B) for individuals moving from outside nature reserves in the previous winter to inside nature reserves (into NR) and vice versa (out of NR). Solid lines denote the posterior mean and colored polygons the extent of the 95% credible intervals.

Fig. 3.

Bayesian population projection model with demographic stochasticity and parameter uncertainty to model the changes (A) within and outside nature reserves (NRs), and (B) in the total Whooper swan population size, and in a counterfactual scenario where NRs had never been set up. Dots represent actual population numbers from census data. Thick lines show the posterior mean and each thin line shows one iteration of the model. (C) Cross-sectional view of the demographic projections in (B) comparing the mean (±80% CIs, grey ticks) between the total population size and the scenario where NRs had never been set up, for the years 2020, 2025, and 2030. (D) Elasticity of the demographic parameters from the projections where swans were allowed to move between NRs and non-NRs.