Elsevier

Biological Conservation

Volume 142, Issue 2, February 2009, Pages 424-431
Biological Conservation

Evidence-based culling of a facultative predator: Efficacy and efficiency components

https://doi.org/10.1016/j.biocon.2008.11.004Get rights and content

Abstract

Human activities have greatly modified predator–prey dynamics within seabird communities by favouring a rapid increase in density of large predatory gulls. To counteract such a subsidized growth, conservation agencies perform massive random culling programs, which generally fail to restore the original predator–prey relationship. We used long-term individual-based information to evaluate the effects of a selective culling of a top seabird predator, the yellow-legged gull (Larus michahellis), on the predatory pressure, survival and reproductive success probabilities of a secondary prey, the vulnerable European storm-petrel (Hydrobates pelagicus). The selective removal of only 16 gulls in 3 years led to a reduction of ca. 65% in the number of petrels killed, and to a relative increase in their survival and breeding success probabilities of 16% and 23%, respectively. Our results show that only a few specialised predators were responsible for the bulk of the impact on a secondary prey and that the removal of those specialised individuals was an effective and efficient way to improve prey demographic parameters.

Introduction

Seabird communities are characterized by networks of multiple interactions among species; many of these have been much altered in recent times by human induced changes in marine resources (Stenhouse and Montevecchi, 1999, Solé and Montoya, 2001, Votier et al., 2004b). These alterations have modified predator–prey dynamics within seabird communities owing to the negative effects of fisheries on fish stocks and by the increase of alternative food supplies, such as urban tips and fishery discards, which subsidize predators (Votier et al., 2004b, Furness et al., 2007). Gulls are the classic example of species affected by these deep and rapid changes; over the last century their populations have increased substantially (Oro et al., 1995, Thibault et al., 1996, Duhem et al., 2008) up to the point that large gulls are currently perceived as a pest by wildlife managers, for a large number of reasons, including their impact on smaller and threatened syntopic species (Feare, 1991, Vidal et al., 1998, Finney et al., 2003; though see Oro and Martínez-Abraín, 2007). As a consequence, many conservation agencies have set up culling programs to control gull populations, which typically consist of systematic removal of large numbers of eggs, chicks or breeding adults (e.g. Blokpoel and Spaans, 1991). These programs are generally conducted at the local population level assuming that all individuals equally contribute to, or are equally responsible for, the conservation problem identified. This assumption is rarely supported and in most cases the systematic culling ends up being inefficient and unjustified (Oro and Martínez-Abraín, 2007). On the contrary, Brooks and Lebreton (2001) showed that the optimal harvest strategy to limit the number of yellow-legged gull (Larus michahellis) breeding pairs is to cull those individuals with the highest site/state-specific reproductive values. Likewise, the impact of predators may be only due to a few individuals specialised on a particular food item. Individual specialisation in diet is well documented in many animal taxa such as fish (e.g. Svanback and Persson, 2004), mammals (e.g. Estes et al., 2003) and birds (e.g. Spear, 1993, Hario, 1994, Guillemette and Brousseau, 2001, Martínez-Abraín et al., 2003, Oro et al., 2005). In these cases, the optimal harvesting strategy would be to eliminate only specific individuals, regardless of their reproductive value, since a small number of specialist individuals may account for a significant proportion of particular prey consumed (Votier et al., 2004c). If effective, this strategy would minimize the cost of the control program and the impact of the predator leaving its total numbers almost untouched.

The most common facultative predator of seabird species in the Mediterranean basin is the yellow-legged gull (Burger and Schreiber, 2002). The species preys occasionally on eggs, chicks and adults of other seabirds, and is the target of systematic culling programs in many Mediterranean regions (Oro and Martínez-Abraín, 2007). The Mediterranean storm-petrel (Hydrobates pelagicus melitensis), a vulnerable seabird, is among the potential prey of yellow-legged gulls (Mínguez, 2004). Previous research showed that yellow-legged gulls preyed upon both breeding and immature storm-petrels and that this mortality was additive to other causes of mortality (Walmsley, 1986, Zotier et al., 1992, Borg et al., 1995, Adam and Booth, 2001, Oro et al., 2005). As gulls prey upon adult breeding storm-petrels, the potential impact on population viability is high since a whole set of life-history traits, such as low annual productivity, long reproductive cycles, delayed reproductive maturity and low adult mortality, make petrels particularly vulnerable to factors affecting adult survival (Warham, 1990, Saether and Bakke, 2000). Adult storm-petrels are an occasional component of the yellow-legged gull diet, and it may well be that only a few gulls have learnt how to exploit this secondary food resource (Oro et al., 2005). In this case, a systematic culling is likely to be an inefficient and improper measure of control.

Here we simultaneously evaluate the effects of a selective culling of a top seabird predator on the survival, reproductive success and predatory pressure of a secondary prey. We report an experimental study on the evidence provided by an ad hoc research program on a western Mediterranean Island, holding both a storm-petrel and a yellow-legged gull colony, to encourage evidenced-based conservation (Pullin et al., 2004). The burrowing nesting habits of petrels make it difficult to obtain reliable estimates of population abundance, and an insight into its population dynamics can only be obtained by the estimate of demographic parameters from individual-based data (Oro et al., 2004). We tested the hypothesis that selective culling triggered a decrease in predation and, in turn, an increase in petrel survival and/or reproduction parameters. Using detailed information on marked individuals we then estimated the impact of predation on these parameters to experimentally evaluate the demographic consequence of gull removal.

Section snippets

Study area and predatory–prey dynamics

The study was conducted from 1993 to 2007 at Benidorm Island (6.5 ha; 38°30′N, 0°08′E), a Special Protection Area for the conservation of the European storm-petrel species in the western Mediterranean coast of Spain. Gull numbers on the island have recently increased and evidence of predation on storm-petrels (see below) has encouraged managers to control the yellow-legged gull population.

The yellow-legged gull is a large long-lived gull (average body mass, 800–1500 g, Cramp and Simmons, 1977).

Evidence of gull predation and selective culling

The number of pellets containing petrel remains found during April and May 2004, when the selective culling began, was similar to the one in previous years. However, after the removal in early June of a single gull the evidence of predation decreased drastically (see Fig. 1).This last individual was the male from a pair breeding inside the cave where the petrel colony is located, so their breeding territory was within the petrel colony. In 2005 and 2006 the number of pellets containing petrel

Discussion

Our experimental results showed that (i) predation by gulls affected negatively both adult annual survival probability and breeding success of syntopic petrels, and (ii) after removing specialist gulls, adult survival probabilities and breeding success of storm-petrels greatly and rapidly increased (16% and 23%, respectively). Although 16 gulls were removed during the study period, we believe that the specialist gull pair removed in 2004 that bred within the major petrel colony was responsible

Acknowledgements

We would like to formally acknowledge the many people who participated in the field work over the years. We are indebted to Blanca Sarzo, Llanos de León, the wardens and Environmental Monitoring Service of Benidorm Island (Natural Park Serra Gelada-Generalitat Valenciana). Gonzalo González, Mariano Paracuellos and Marc Bosch provided helpful data on massive gull culling programs costs. AS was supported by a postgraduate grant (Ref. AP2004-1128) of the Spanish Ministry of Science, which also

References (67)

  • M. Bosch

    The effects of culling on attacks by yellow-legged gulls (Larus cachinnans) upon three species of herons

    Colonial Waterbirds

    (1996)
  • M. Bosch et al.

    Short-term effects of culling on the ecology and population dynamics of the yellow-legged gull

    Journal of Applied Ecology

    (2000)
  • J. Burger et al.

    Nest site selection in an expanding population of Herring Gulls

    Journal of Field Ornithology

    (1980)
  • J. Burger et al.

    Biology of Marine Birds

    (2002)
  • K.P. Burnham et al.

    Model selection and multimodel inference a practical information–theoretic approach

    (2002)
  • Choquet, R., Reboulet, A.M., Lebreton, J.D., Gimenez, O., Pradel, R., 2005. U-CARE 2.2. User manual. Montpellier,...
  • R. Choquet et al.

    M-SURGE: new software specifically designed for multistate capture recapture models

    Animal Biodiversity and Conservation

    (2004)
  • J.C. Coulson

    The population dynamics of culling Herring gulls and Lesser black-backed gulls

  • J.C. Coulson et al.

    Changes in the breeding biology of the herring gull (Larus argentatus) induced by size and density of the colony

    Journal of Animal Ecology

    (1982)
  • S. Cramp et al.

    The birds of the Western Palearctic

    (1977)
  • A. De León et al.

    Self-odour recognition in European storm-petrel chicks

    Behaviour

    (2003)
  • Díez, I., Martínez-Abraín, A., 1989. Propuesta de protección ornitológica de la Isla de Benidorm. Unpublished technical...
  • C. Duhem et al.

    Effects of anthropogenic food resources on yellow-legged gull colony size on Mediterranean islands

    Population Ecology

    (2008)
  • J.A. Estes et al.

    Individual variation in prey selection by sea otters: patterns, causes and implications

    Journal of Animal Ecology

    (2003)
  • C.J. Feare

    Control of bird pest populations

  • S.K. Finney et al.

    Reducing the density of breeding gulls influences the pattern of recruitment of immature Atlantic puffins Fratercula arctica to a breeding colony

    Journal of Applied Ecology

    (2003)
  • R.W. Furness et al.

    Influence of management practices and of scavenging seabirds on availability of fisheries discards to benthic scavengers

    Marine Ecology Progress Series

    (2007)
  • M.E. Gompper et al.

    Subsidized predators, landscapes of fear and disarticulated carnivore communities

    Animal Conservation

    (2008)
  • M. Guillemette et al.

    Does culling predatory gulls enhance the productivity of breeding common terns?

    Journal of Applied Ecology

    (2001)
  • M. Hario

    Reproductive-performance of the nominate lesser black-backed gull under the pressure of herring gull predation

    Ornis Fennica

    (1994)
  • Hemery, G., 1980. Dynamique de la population basque française de Pétrels tempête (Hydrobates pelagicus) de 1974 à 1979....
  • J.D. Lebreton et al.

    Modeling survival and testing biological hypotheses using marked animals – A unified approach with case-studies

    Ecological Monographs

    (1992)
  • M.K. Mac Nicholl

    Larid site tenacity and group adherence in relation to habitat

    Auk

    (1975)
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