Short communicationWhen immigration mask threats: The rescue effect of a Scopoli's shearwater colony in the Western Mediterranean as a case study
Introduction
Survival, reproduction and dispersal processes (emigration and immigration) drive population dynamics and viability (Hanski, 1999, Morris and Doak, 2002). Whereas local survival and recruitment (accession to reproduction) determine the intrinsic viability of a local population (Oro, 2013, Sanz-Aguilar et al., 2009, Sanz-Aguilar et al., 2014, Sanz-Aguilar et al., 2015), dispersal is the key process driving the dynamics of spatially structured populations and metapopulations (Hanski, 1999, Hill et al., 2002). Demographic differences among local populations can arise from heterogeneous habitat quality, in which populations habiting sub-optimal habitats or threatened differently by anthropogenic factors may not be able to balance local mortality by reproduction, being sinks (Pulliam, 1988, Dias, 1996). Source-sinks dynamics seem common in nature, but additive impacts from the global change can alter those dynamics and exacerbate extinction probabilities at sinks, posing a challenge for the conservation of endangered populations (Liu et al., 2011). Population viability of long-lived species is highly sensitive to changes in adult survival (Sæther and Bakke, 2000). However, populations can compensate for adult mortality by density-dependent changes in other demographic parameters, such as fecundity and/or recruitment (Boyce et al., 1999, Gaillard et al., 1998). Recruitment has been identified as a key factor responsible for population fluctuations (or stability) in several long-lived species (Servanty et al., 2011, Tenan et al., 2014, Votier et al., 2008). For example, populations of wild boar Sus scrofa, common guillemots Uria aalge or kittiwakes Rissa tridactyla subject to high adult mortality, buffered the loss of breeders by recruiting at earlier ages or increasing recruitment rates (Porter and Coulson, 1987, Servanty et al., 2011, Votier et al., 2008). However, sometimes, reproduction and local recruitment alone cannot buffer the loss of breeders and immigration is needed to maintain a local population (Lieury et al., 2015, Sanz-Aguilar et al., 2014). Dispersal from source populations with demographic excess permits the maintenance of sink populations (Fernández-Chacón et al., 2013), which although unviable alone, they contribute to increase metapopulation size and time to extinction (Pulliam, 1988, Howe et al., 1991, Liu et al., 2011). The correct identification of whether a breeding population is maintained by local recruitment (demographic stability or excess) or by immigration (demographic deficit) is of special importance to identify source and sink populations and design adequate conservation actions (Howe et al., 1991, Dias, 1996, Liu et al., 2011, Oro, 2013). Identifying source and sink populations is a complicate task because it requires long-term demographic studies for estimating dispersal between patches and also because sinks can become sources (and vice-versa) if environmental conditions change (Dias, 1996, Liu et al., 2011).
In a recent study, Tenan et al. (2014) showed that the stability of a Scopoli's shearwater Calonectris diomedea colony in the western Mediterranean (Pantaleu colony, Balearic Archipelago, 39°34′N, 2°21′E) was achieved through recruitment processes buffering the loss of adult individuals by mortality. However, whether those recruits maintaining the population are local or foreign individuals dispersing from other populations remains unknown. The Scopoli's shearwater is a long-lived bird with low fecundity, delayed recruitment and high adult survival, largely affected by oceanographic processes (SOI index) and fisheries bycatch (Genovart et al., 2013a, Igual et al., 2009, Jenouvrier et al., 2009, Ramos et al., 2012). Unlike other seabirds like gulls, in which natal and breeding dispersal are very common (Fernández-Chacón et al., 2013, Sanz-Aguilar et al., 2014), petrels and shearwaters behave faithfully to their breeding sites (Igual et al., 2007, Sanz-Aguilar et al., 2011) and local individuals mostly recruit very close to their natal site (Bonadonna and Sanz-Aguilar, 2012, Gómez-Díaz et al., 2009) but see (Martínez-Abraín et al., 2002). The importance of dispersal in these species is, however, very difficult to evaluate due to their hypogeous breeding habits and because only a very small proportion of birds are ringed, and in a reduced number of colonies (typically in small ones). In fact, genetic studies pointed to a non-negligible role of dispersal in the Scopoli's shearwater (Genovart et al., 2013b, Ramírez et al., 2013).
Exhaustive nest and individual monitoring during a long-term program (2001–2014) at Pantaleu, facilitated by the relatively small size of the islet, 2.5 Ha, allowed a robust estimate of local demographic parameters (Genovart et al., 2013a, Sanz-Aguilar et al., 2011, Sanz-Aguilar et al., 2016) and provided reliable data on temporal variation in breeding numbers (Tenan et al., 2014). Here, we developed robust population models and integrated census and demographic data to disentangle the importance of local recruitment and immigration in the dynamics of the population. In particular, we compared the observed breeding numbers over the study period with the breeding numbers projected under different scenarios of immigration combined with the local estimates of survival, recruitment, breeding propensity (sabbatical) and breeding success. This comparison allowed the quantification of the reproductive potential of the population to balance local mortality, and thus its identification as a sink or source.
Section snippets
Methods
We calculated the observed mean population growth rate λobs as the geometric mean of the annual observed growth rate (No. nests occupied t + 1/No. nests occupied t) (Morris and Doak, 2002).
Age-stage structured deterministic and stochastic matrix population models were built to forecast deterministic and stochastic population dynamics for the studied population (Caswell, 2001, Morris and Doak, 2002) using the package POPBIO in the software R (Stubben and Milligan, 2007, Team, 2005). The model
Results
The breeding population of Scopoli's shearwater at the Pantaleu colony was almost stable during the study period, λobs = 0.989 (Fig. 1), whereas the deterministic population model not considering immigration predicted a sharp decline (λ = 0.907, ~ 10% annual decrease). The stable age distribution indicated that the population should be primarily composed of breeders older than 8 years (32.4%; Fig. A1.2. Appendix 1), which are the individuals with the highest reproductive value (10.7%; Fig. A1.3.
Discussion
Mediterranean populations of Scopoli's shearwater and Atlantic populations of Cory's shearwater (C. borealis) are similarly affected by large scale oceanographic variables during wintering (Genovart et al., 2013a, Jenouvrier et al., 2009, Ramos et al., 2012). Additionally, the species is severely affected by important local threats in foraging areas during the breeding season, such as bycatch in longline fisheries (Barcelona et al., 2010, Belda and Sanchez, 2001, Laneri et al., 2010, Ramos et
Acknowledgements
We would like to thank those who helped with fieldwork over the years. Research funds were provided by the Spanish Ministries of Science, Economy and Competitiveness (refs. BOS2003-01960, CGL2006-04325/BOS, CGL2009-08298, CGL2013-42203-R).
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