Alejandro Martínez-Abraín

Ecological and evolutionary implications of food subsidies from humans

Human activities are the main current driver of global change. From hunter-gatherers through to Neolithic societies-and particularly in contemporary industrialised countries-humans have (voluntarily or involuntarily) provided other animals with food, often with a high spatio-temporal predictability. Nowadays, as much as 30-40% of all food produced in Earth is wasted. We argue here that predictable anthropogenic food subsidies (PAFS) provided historically by humans to animals has shaped many communities and ecosystems as we see them nowadays. PAFS improve individual fitness triggering population increases of opportunistic species, which may affect communities, food webs and ecosystems by altering processes such as competition, predator-prey interactions and nutrient transfer between biotopes and ecosystems. We also show that PAFS decrease temporal population variability, increase resilience of opportunistic species and reduce community diversity. Recent environmental policies, such as the regulation of dumps or the ban of fishing discards, constitute natural experiments that should improve our understanding of the role of food supply in a range of ecological and evolutionary processes at the ecosystem level. Comparison of subsidised and non-subsidised ecosystems can help predict changes in diversity and the related ecosystem services that have suffered the impact of other global change agents.

Influence of food availability on demography and local population dynamics in a long-lived seabird.

Few studies have addressed the effects of food availability as a proximate factor affecting local adult survival in long–lived organisms and their consequences at local population dynamics. We used capture–recapture analysis of resightings of 10 birth cohorts of ringed Audouins gulls, Larus audouinii, to estimate adult survival and dispersal (both emigration and immigration). For the first time, permanent emigration (the transient effect in capture–recapture analysis) was modelled for the whole population and not only for the newly marked birds. Gulls exploit to a large extent fishes discarded from trawlers, and a trawling moratorium established since 1991 has decreased food supply for the colony. This was used as a natural experiment of food availability to assess its effects on adult survival and emigration. These and other demographic parameters were used in a projection modelling to assess the probabilities of extinction of the colony under two scenarios of lower and higher food availability. Food availability (together with the age of individuals) influenced emigration probabilities, but not adult survival, which was estimated at 0.91 (s.e. = 0.02). When food was in shorter supply during the chick–rearing period, emigration was very high (ca. 65%) for younger breeders, although this rate decreased sharply with age. Probabilities of extinction were very high when food availability was low, and when environmental stochasticity was introduced, and only stochastic immigration from the outside seemed to prevent extinction. The results highlight the importance of dispersal processes in the population dynamics of long–lived organisms.

Influence of density dependence on predator-prey seabird interactions at large spatio-temporal scales.

Theoretical investigations of competitive dynamics have noted that numbers of predator and prey influence each other. However, few empirical studies have demonstrated how a life-history trait of the prey (such as fecundity) can be affected simultaneously by its own density and the density of predators. For instance, density dependence can reduce fecundity with increasing number of prey, while inverse density dependence or Allee effects may occur especially when the prey is a social organism. Here we analysed an intraguild predator–prey system of two seabird species at a large spatio-temporal scale. As expected, we found that fecundity of prey was negatively affected by predator density. Nevertheless, fecundity of prey also increased nonlinearly with its own density and strikingly with the prey–predator ratio. Small groups of prey were probably not able to defend their nests especially against large number of predators. At the highest prey densities (i.e. when anti-predator strategies should be most efficient), prey fecundity also lowered, suggesting the appearance of density dependence mediated by food competition. Allee effects and density dependence occurred across a broad range of population sizes of both the prey and the predator at several local populations facing different ecological environments.

Integration of invasive Opuntia spp. by native and alien seed dispersers in the Mediterranean area and the Canary Islands

The success of many alien plant species depends on mutualistic relationships with other species. We describe the assemblage of seed dispersers on three species of alien Opuntia invading Mediterranean and Macaronesian habitats, and examine the quality of such plant-animal interactions. We identified vertebrates consuming O. maxima, O. dillenii and O. stricta fruits by direct observation and collecting droppings and pellets. Phenology of the alien species, as well as that of coexisting native species, was monitored for an entire year. Germination tests of ingested and non-ingested seeds were performed both in the greenhouse and in the field. Seed coat thickness and viability were also measured for all treatments. A great variety of taxa, including reptiles, birds and mammals actively participate in the seed dispersal of Opuntia. Phenology of Opuntia fruits in Menorca and Tenerife overlaps with only a few native fleshy-fruited plants present in the study areas, which suggests an advantage for the invader. Most seeds germinated during the second year of the experiment, independently of the effect produced by the dispersers’ guts. We found great variation in the germination percentage of Opuntia after gut passage and in the effects of ingestion on seed coat thickness. Seed viability was somewhat reduced after gut passage compared to manually depulped seeds. Our results show how different Opuntia species are integrated into native communities by means of mutualistic interactions, with both native and alien dispersers. Although with heterogeneous effects, either type of disperser potentially contributes to the spread of these alien cacti in the recipient areas.

Compromise between seabird enjoyment and disturbance : the role of observed and observers

SUMMARY Natural areas are increasingly visited by people, and urban human visitors expect to watch wildlife as close as possible, but this may have associated disturbance costs. Here, effects of number of visitors and bird density on flight initiation distance (FID) as a proxy of disturbance vulnerability were evaluated in the large ground-nesting yellow-legged gull, Larus michahellis. Mean FID decreased with increasing number of visitors and with increasing gull densities, suggesting that (1) ground-nesting gulls habituate to massive human presence, while retaining their antipredatory mechanisms, and (2) dense groups of gulls were more reluctant to fly away. This density effect may be due to the increased risk of clutch predation by conspecifics at high densities and, if so, FID is a reliable metric of disturbance vulnerability in groundnesting gulls. In conclusion, set-back distances are specific to local populations and it is unnecessary to ban or restrict human visits to ground-nesting gull colonies; redistributing visits, taking into account both the number of visitors and gull density, is preferable.

Atlantic Cory’s Shearwaters Breeding in a Colony of Mediterranean Cory’s Shearwaters

Abstract We report on the breeding of several individuals of Atlantic Cory’s Shearwater (Calonectris diomedea borealis) in a small colony of Mediterranean Cory’s Shearwater (C. d. diomedea) in the western Mediterranean (Columbretes Islands, northeastern Spain). We briefly discuss the potential role of Atlantic Cory’s Shearwaters in gene flow at small and declining Mediterranean colonies.

The dynamics of a colonization event in the European shag: the roles of immigration and demographic stochasticity

-We developed both deterministic and stochastic (through Monte Carlo simulations) matrix projection models to account for the relative roles of local recruitment and immigration in the formation and growth of a new colony of the European Shag (Phalacrocorax aristotelis) in the western Mediterranean (Columbretes Islands). The best fit of observed and simulated growth curves of the colony was obtained by introducing two immigration waves and taking into account demographic stochasticity. Immigration and local recruitment of the founder population alone cannot explain the observed growth of the colony. Immigrants might come from a large colony in Majorca Island, approximately 160 km. away, that declined markedly at the same time that the Columbretes colony was established. According to a sensitivity and elasticity analysis, changes in ?4 year old adult survival have the largest impact on the population growth rate (X). Received 31 March 2000, accepted 1OJune 2000.

Unforeseen effects of ecosystem restoration on yellow-legged gulls in a small western Mediterranean island

SUMMARY A conservation project aimed at ecosystem restoration had several unforeseen effects on a colony of the yellow-legged gull Larus michahellis in a small western Mediterranean island (Benidorm Island). The project included regulation of massive tourist visits to help restore the soil and autochthonous vegetation. However, gulls habituated rapidly to regulation of tourist activities, as nests located either close to or far from the main trail showed a similar hatching success. The quiet conditions produced by regulation seemingly facilitated a rapid colony increase. Partial removal of alien vegetation (Opuntia maxima) showed that gulls had a preference for sites with high vegetation cover because the growth of the colony was proportionally larger in well-vegetated plots. The pricking of a large number of gull eggs surprisingly coincided with a high reproductive success compared to the previous year, although indicators of food availability remained constant between years and the colonyhaddecreasedinnumbers.Untreatednestswere probably more successful because territory size for chicksincreasedandintraspecificpredationdecreased. Extremecaremustbetakenwhenplanningecosystemwide management on islands with yellow-legged gull colonies, or other gull species locally considered as pests, to prevent unwanted effects.

A note on the potential role of philopatry and conspecific attraction as conservation tools in Audouin's Gull Larus audouinii

Summary This short communication reports on the first record of Audouin’s Gulls Larus audouinii breeding in captivity and on the behaviour of some individuals released as fledglings. During the period 1995–2000 a hand-reared male returned to its natal site and responded, year after year, to the translocation of a captive group of Audouin’s Gulls for nesting site selection. The potential role of philopatry and conspecific attraction as conservation tools for this vulnerable seabird after an eight year experiment are discussed.

A long-term macroecological analysis of the recovery of a waterbird metacommunity after site protection

We used the so called “land-bridge island” or “nested-subsets” theory to test the resilience of a highly fragmented and perturbated waterbird metacommunity, after legal protection of 18 wetlands in the western Mediterranean. Sites were monitored during 28 years and two seasons per year. The metacommunity was composed by 44 species during breeding and 67 species during wintering, including shorebirds, ducks, herons, gulls and divers (Podicipedidae). We identified a strong nested pattern. Consistent with the fact that the study system was to a large extent a spatial biogeographical continuous for thousands of years, fragmented only during the last centuries due to human activities. Non-random selective extinction was the most likely historical process creating the nested pattern, operated by the differential carrying capacity (surface-area) of the remaining sites. We also found a positive temporal trend in nestedness and a decreasing trend in species turnover among sites (β-diversity), indicating that sites are increasingly more alike to each other (i.e. increased biotic homogenization). This decreasing trend in β-diversity was explained by an increasing trend in local (α) diversity by range expansion of half the study species. Regional (γ) diversity also increased over time, indicating that colonization from outside the study system also occurred. Overall our results suggest that the study metacommunity is recovering from historical anthropogenic perturbations, showing a high long-term resilience, as expected for highly vagile waterbirds. However, not all waterbird groups contributed equally to the recovery, with most breeding shorebird species and most wintering duck species showing no geographical expansion.