Colonization and population growth of Yellow‐legged Gull Larus cachinnans in southeastern Poland: causes and influence on native species

Abstract

The Yellow-legged Gull Larus cachinnans was first recorded in Poland in the 1980s. We analysed the probable factors responsible for its successful colonization of new areas. We also expected that such a large species should affect populations of other colonial waterbirds. We studied the breeding and feeding ecology in the largest inland colony of the Yellowlegged Gull in Poland, located in a sedimentation basin near Tarnów (southeastern Poland). The first breeding pair was recorded in 1992 and the population reached 177 pairs in 2001. The population growth rate in this colony, of about 58% per year, fits an exponential model. Nine localities with breeding pairs have been found recently in southern Poland and we now estimate the total population size to be 200-250 pairs. The large clutch size, and high hatching and breeding success in the Tarnów colony suggest that food was plentiful. Food items were frequently found at the nests. Fish, mainly Carp Cyprinus carpio , were the predominant food items delivered to chicks; however, there was more refuse brought to nests during the incubation stage. Immigration probably caused the growth of the colony studied, although our calculations have shown that natal productivity alone is sufficient to maintain this population. The study showed that the growing population of Yellow-legged Gull might cause considerable reduction in the population sizes of some of the native waterbird species.Many hypotheses attempt to explain the range size and distribution of species . It is believed that populations in the centre of the range occupy habitat of better quality and have a higher intrinsic rate of growth than those at the range margin (Lawton 1993). However, as populations of some species have increased rapidly, others have become vulnerable to extinction, and this is often associated with boundary range changes. Finding the factors that affect these changes is important in understanding the processes occurring in populations at a species' geographical boundary; this may be crucial for predicting the effects of population growth in invasive species, and may be helpful for conservation of endangered species. Yet few studies have addressed these issues.There are essentially two possible explanations for range-size changes. First, range size may increase when high productivity occurs in central source populations, which causes intensive immigration into sink habitats located mainly at the range edge (Pulliam 1988). In this situation, range size relates to the source populations' productivity at the range centre, and range size should extend during periods of higher productivity. However, when the productivity of source populations drops, this should cause a reduction in range size because sink populations at the range boundary are unable to sustain themselves alone. Alternatively, sink habitats at the range edge may become source habitats. This is possible when individuals living in the sink habitats adapt to local conditions, or the habitat quality changes. In such cases, range size s...

An invasive gull displaces native waterbirds to breeding habitats more exposed to native predators

“…In most cases, invasion of a species is an unpredictable process and in the late 1990s the invasion of Caspian gulls had just started. The colony in Tarnów was one of the largest at that time and one of the few in Poland (Skórka et al 2005;Lenda et al 2010). Other colonies were also much smaller (Lenda et al 2010).…”

“…The population size of the invasive Caspian gull was monitored by the same methods and was the subject of a more detailed population study (Skórka et al 2005. We collected data on the number of breeding Caspian gulls since 1992 (when the species first colonized the reservoir).…”

“…Its colonies consist of up to several thousand breeding pairs along the coast or on marine islands and several hundred breeding pairs on inland water reservoirs. During the breeding season, which lasts from the end of March to the beginning of June at inland reservoirs, the colonies are located predominantly on either islets or shores (Skórka et al 2005;Lenda et al 2010). The availability of islets is a factor that limits the local population sizes of many waterbirds (Amat 1985;Skórka et al 2006;Kajzer et al 2012).…”

“…Because the Caspian gull is primarily a seabird that breeds on coastal islands in its native geographical range, it is alien to inland waterbodies. The main reasons underlying this extensive expansion were vast manmade alterations in the environment that facilitated both high reproductive success and their spread into the interior of the continent (Fasola et al 1993;Jonsson 1998;Skórka et al 2005;Lenda et al 2010). This species utilizes humanrelated food resources in newly colonized areas, mostly fish farms and refuse tips (Lenda et al 2010).…”

“…Its population has grown rapidly and expanded both north and west, mainly along large river valleys into the interior of the European continent (Filchagov 1996;Jonsson 1998;Lenda et al 2010). The first breeding pairs in Poland were recorded in the Middle Vistula valley in the late 1980s, with inland reservoirs in Southern Poland being colonized a few years later (Faber et al 2001;Skórka et al 2005). The Caspian gull is a large (56-68 cm long, with a 140-150 cm wingspan and a body mass of 700-1,600 g) colonial waterbird (Malling Olsen and Larsson 2004).…”