Deconstructing myths on large gulls and their impact on threatened sympatric waterbirds

Abstract

Owing to increasing population trends and facultative predatory habits, large gulls have been identified as significant agents of change in the alteration of many ecological communities. Often, they are perceived as negatively impacting the population trends of most sympatric waterbirds. Consequently, culling programs have been implemented to remove adults, chicks and eggs intensively. Here, we review the interactions recorded in the literature between the yellow-legged gull Larus michahellis and 10 sympatric waterbirds in the Mediterranean region, all threatened and classified as species of conservation concern. We also used 177 long-term population trends derived from previous studies to study the population dynamics of these species and the culling effort performed. We show that gulls negatively affected survival, fecundity, foraging ecology and nesting habitat availability for many species. However, the annual population growth rates of most sympatric waterbirds showed positive values, even at sites where culling has yet to be initiated and local yellow-legged gull populations are large and increasing. Our results suggest clearly that population increase has not been exclusive of yellow-legged gulls, especially at the regional level. Yet, growth rates of both yellow-legged gulls and sympatric waterbirds were positively associated. Strikingly, the population extinction rate was similar between colonies of yellowlegged gulls and those of sympatric species. Thus, evidence exists to state that the success of gull control programs is relatively low in the long term. We recommend that conservation agencies heed several basic principles of population and community ecology before initiating control, for instance that (1) yellow-legged gulls have bred historically with other bird species and have likely developed defensive mechanisms against this predator and (2) populations of large gulls are regulated by density-dependent mechanisms in both space and time. Incoming European environmental policies on fishing discards and rubbish management should control more naturally and efficiently the density of large gulls and the composition of seabird communities in the long term.

Feeding ecology of yellow-legged gulls Larus michahellis in the western Mediterranean: a comparative assessment using conventional and isotopic methods

“…In most cases it is considered a pest because of its interactions with human populations in airports, cities, arable lands or fisheries. In addition, it interacts with other species, usually under protection, which can be disturbed, predated and displaced by the gulls from their breeding areas (Vidal et al 1998, Oro & Martínez-Abraín 2007.…”

“…In most cases it is considered a pest because of its interactions with human populations in airports, cities, arable lands or fisheries. In addition, it interacts with other species, usually under protection, which can be disturbed, predated and displaced by the gulls from their breeding areas (Vidal et al 1998, Oro & Martínez-Abraín 2007.On the other hand, owing to its abundance, wide distribution and opportunistic feeding habits, the yellowlegged gull might also be used to measure the effects of pollution in the Mediterranean area. This is another important aspect of applied ecology in the case of this species, as it benefits from the same resources consumed by humans, as indicated by its use of refuse dumps and fishery discards.…”

Population control of an overabundant species achieved through consecutive anthropogenic perturbations

“…A large number of studies have tackled mitigation and management measures, such as direct culling aimed at reducing the ecological and social consequences of overabundant species , Baker and Harris 2006, Merrill et al 2006, Beasley et al 2013. However, although these measures have been implemented worldwide, they have often turned out to be expensive and/or ineffective (Baker and Harris 2006, Oro and Martı´nez-Abraı´n 2007, Warburton and Norton 2009. The main flaw is that management measures often aim to mitigate the consequences of the human alteration rather than avert the occurrence of the alteration itself.…”

“…Because seabirds are long-lived species, the reduction of adult survival is expected to have important population consequences (Saether et al 1996). However, direct culling had little effect on gull numbers because processes such as earlier recruitment or density-dependent higher fertility and immigration (Brooks and Lebreton 2001) were likely enhanced by high food availability, which might have boosted gulls' resilience to this type of perturbation (Oro and Martı´nez-Abraı´n 2007). Although food limitation has some impact on breeding performance, it is not expected to be particularly useful in controlling long-lived species; nevertheless, a long-term limitation of resources might be the key to controlling overabundant species by reducing recruitment (Oro and Martı´nez-Abraı´n 2007).…”

Research priorities for seabirds: improving conservation and management in the 21st century

“…Although predatory interactions among seabird species are less well studied than competitive interactions, they do occur (Gilchrist 1999, Votier et al 2004, Oro et al 2006a, Oro & Martínez-Abraín 2007. Large opportunistic species (e.g.…”