Compensatory immigration counteracts contrasting conservation strategies of wolverines (Gulo gulo) within Scandinavia
Introduction
Population properties, such as density or survival rates, are not homogenously distributed in space, as they frequently vary as a consequence of the uneven spatial distribution of resources and mortality risks (Kareiva, 1990, Thomas and Kunin, 1999). When populations exhibit different local mortality rates this might result in a source–sink dynamic, where individuals emigrating from the areas with lower mortality to those with higher mortality. This process is known as compensatory immigration (Pulliam, 1988, Turgeon and Kramer, 2012).
While it has received much attention in meta-population theory (Hanski, 1996), compensatory immigration can also play a role in continuous populations (McCullough, 1996, Turgeon and Kramer, 2012), with important consequences for their management and conservation. It can jeopardize the effectiveness of alien species control, when more individuals of the undesired species immigrate into a given area after removal (Mack et al., 2000); it can increase disease transmission and prevalence by increasing individual mobility (Donnelly et al., 2006); and it can threaten the persistence of small populations confined to protected areas, when a gradient in mortality rates exists between the inside and the outside (Gundersen et al., 2001, Woodroffe and Ginsberg, 1998).
In conservation programs aimed at wide ranging species such as large carnivores or marine mammals, there is often a mismatch between the scale of population processes and that at which management actions are implemented (Trouwborst, 2010). This is an additional source of spatial variation in vital rates. Portions of the same population often fall within different administrative units or states, in which different regulations apply, so that the same species can be fully protected or heavily harvested on the two sides of a border. In such a case, different individuals living close in space within the same population can experience different mortality risk patterns, costs associated with reproduction, and local intra-specific competition, depending on where they live with respect to the artificial lines that mark the shift between one conservation regime and the other. Such a sharp gradient can initiate compensatory immigration and other source–sink dynamics between different portions of the same population, so that conservation actions implemented in one administrative unit can generate unintended and undesired consequences on the demography of the species in other administrative units. When contrasting conservation goals exist between bordering geographic areas, compensatory immigration can compromise both goals (Cooley et al., 2009, Gundersen et al., 2001, Robinson et al., 2008).
Wolverines (Gulo gulo) and their contrasting management regimes in Scandinavia are one such case. They are fully protected in Sweden under the European Union's Habitats Directive 92/43, in which they are listed as strictly protected (annex IV). In contrast, they are subject to intensive lethal control in Norway, which is not part of the European Union and hence not bound to the same set of regulations (Swenson and Andrén, 2005). Sweden has set a national goal of about 550 individuals (Reinfeldt, 2013), which was reached at the end of the 2000s (Fig. 1b). In Norway the national management goal is set at about 250 individuals (Ministry of Environment, 2003), but the population has remained above this goal for the last decade (Flagstad et al., 2013). This happened despite the fact that Norwegian management authorities have steadily increased control pressure during the last 10 years (Fig. 1a). There is no geographic barrier between the two portions of the population, but the consequences of such contrasting management regimes on the demography of the species have not yet been explored.
Here we present an analysis of the spatio-temporal dynamics of wolverine survival and trans-boundary movement patterns in the southern part of the Scandinavian population over a 12 year period between 2002 and 2013. Our aim was to evaluate the consequences of the contrasting management regimes in Norway and Sweden, both on the species demography and on the efficiency of management actions. The study was based on the hypothesis that the contrasting management regimes in the two bordering countries are creating a source–sink dynamic. We also hypothesized that such dynamics are reinforced by the extent of the harvest pressure in Norway, and that compensatory immigration counteracts, at least partially, the effort by the Norwegian management authorities to reduce population size in their country.
Under the compensatory immigration hypothesis, we expected the following predictions to be fulfilled:
- 1.
Wolverines experience a higher mortality rate in Norway than in Sweden;
- 2.
Immigration from Sweden to Norway is higher than in the opposite direction;
- 3.
Immigration rates across the Norway–Sweden border are positively correlated with the intensity of the harvest pressure in Norway;
- 4.
A proportion of the wolverines removed in Norway each year is compensated by immigration from Sweden, thus reducing the effective harvest rate.
Section snippets
Study area and data collection
For our study we used non-invasive genetic sampling data collected between 2002 and 2013 in Southern Scandinavia, south of the E14 highway that separates the northern and the southern portions of the Scandinavian wolverine population (Fig. 2). Such partition is mainly an administrative one, as different sampling regimes have been used north and south of the highway during the study period, but it also marks the main geographic barrier to wolverine movement in this part of the Scandinavian
Results
The best supported capture–recapture model included a year, sex, and country effect on the detection probability (model 1 in Table 1). Male wolverines had a higher detection probability than females in both countries. Average detection probability across years, countries, and sexes was 0.60 (95% CIs = 0.57–0.62), and it was similar in the two countries (Norway: p̂ = 0.58, 95% CIs = 0.54–0.62; Sweden: p̂ = 0.56, 95% CIs = 0.50–0.62). Males had in average a higher detection probability (p̂ = 0.70, 95% CIs =
Discussion
In the southern portion of the Scandinavian wolverine population Sweden acts as a source and Norway as a sink. This has not been taken into account so far when planning and evaluating conservation and management strategies in the two countries. All the conditions expected under the compensatory immigration hypothesis were confirmed. First, wolverines living in Norway experienced a higher mortality than the ones living in Sweden, and this difference in survival has been increasing over the last
Acknowledgments
This study is based on data from the Norwegian Large Predator Monitoring Programs in Norway and Sweden, financed by the Directorate for Nature Management in Norway and the Environmental Protection Agency in Sweden. The study would not have been possible without the enormous sampling effort by dedicated wardens from SNO and NVV. Data analysis was funded by the Research Council of Norway and the Norwegian Environment Agency (152782/S30).
References (34)
Large-scale noninvasive genetic monitoring of wolverines using scats reveals density dependent adult survival
Biol. Conserv.
(2010)- Bischof, R., Brøseth, H., Gimenez, O., Wildlife in a politically divided world: insularism inflates estimates of brown...
- et al.
Implementation uncertainty when using recreational hunting to manage carnivores
J. Appl. Ecol.
(2012) - et al.
Yngleregistreringer av jerv i Norge i 2013
A theory of combined analysis of ring recovery and recapture data
- et al.
Model Selection and Multimodel Inference
(2002) - et al.
Program E-SURGE: a software application for fitting multievent models
- et al.
U-CARE: utilities for performing goodness of fit tests and manipulating capture–recapture data
Ecography
(2009) - et al.
Source populations in carnivore management: cougar demography and emigration in a lightly hunted population
Anim. Conserv.
(2009) - et al.
Positive and negative effects of widespread badger culling on tuberculosis in cattle
Nature
(2006)