Flexibility of habitat use in novel environments: insights from a translocation experiment with lesser black-backed gulls

Abstract

Being faced with unknown environments is a concomitant challenge of species' range expansions. Strategies to cope with this challenge include the adaptation to local conditions and a flexibility in resource exploitation. The gulls of the Larus argentatus-fuscus-cachinnans group form a system in which ecological flexibility might have enabled them to expand their range considerably, and to colonize urban environments. However, on a population level both flexibility and local adaptation lead to signatures of differential habitat use in different environments, and these processes are not easily distinguished. Using the lesser black-backed gull (Larus fuscus) as a system, we put both flexibility and local adaptation to a test. We compare habitat use between two spatially separated populations, and use a translocation experiment during which individuals were released into novel environment. The experiment revealed that on a population-level flexibility best explains the differences in habitat use between the two populations. We think that our results suggest that the range expansion and huge success of this species complex could be a result of its broad ecological niche and flexibility in the exploitation of resources. However, this also advises caution when using species distribution models to extrapolate habitat use across space.

Keywords: ecological specialization; flexibility; habitat use; niche comparison; species distributionmodel; translocation.

Figure 1.

Displacement of individual gulls after release. The displacement from the site of release over the first 30 days post release. Individual birds are shown in grey, the median of the group is represented in red. Note that the actual release date differed between the groups owing to the different treatments (table 1).

Figure 2.

Timing of migration. The first day of migration was determined for each individual for which tracking data were available during the migratory period. The boxplot shows the distribution of the timing of autumn migration for the different groups. The boxes represent the 25%, 50% (median) and 75% quartiles. The whiskers show the 1.5-fold interquartile ranges. Black dots represent the raw data for each group.

Figure 3.

General habitat use of lesser black-backed gulls. Shown here are the relative preferences of all treatment groups for terrestrial, marine and freshwater habitats. Unbiased use of these habitat types is represented by the dashed red line. Values above the red line correspond to a positive preference (a relative use of 10 indicates that the bird was observed in a certain habitat 10 times more often than expected from the availability of this habitat type), smaller values correspond to a negative preference. Coloured boxes present the 95% CIs on the mean per treatment group (acquired through 1000-fold bootstrapping), the black bar represents the observed mean, and grey dots represent the raw data.

Figure 4.

Within-group niche overlap between individuals. The niche overlap was calculated between each combination of individuals per group. Coloured boxes present the 95% CIs on the mean per treatment group (acquired through 1000-fold bootstrapping), the black bar represents the observed mean, and grey dots represent the raw data. The respective number of individuals are: Finland: n = 33, Solovki Island: n = 11, Helgoland: n = 9, Kazan: n = 8.

Figure 5.

Results for the niche identity test. The dashed red line shows the observed niche overlap; the histogram represents the expected niche overlap determined by the randomizations. The grey rectangle shows the upper 95% of the distribution.

Figure 6.

Results for the background test. The dashed red line shows the observed niche overlap, the histogram represents the expected niche overlap determined by the randomizations. The grey rectangle marks the 2.5% and 97.5% quantiles of the distribution.