Effective Population Size and Demography of the Rare Flightless Galapagos Cormorant

The population structure and effective population size of the Galapagos Cormorant (Compsohalieus [Nannopterum] harrisi), a rare seabird endemic to the Galapogos Islands, Ecuador, were investigated using standard demographic techniques and data collected over nine years (1970‐1973, 1977, 1978, and 1989‐1992). The study population was located around Punta Espinosa, Fernandina Island, where cormorants bred at seven traditional places scattered along 9 km of bare rocky coastline. This population, averaging 96 ± 13 adults (mean ± 1 SD) (47 ± 8 males; 50 ± 7 females), is an estimated 10% of the total number (≈980) distributed along 350 km of coastline around Fernandina and the western coast of Isabela Island. The Galapagos Cormorant's restricted distribution and small population size probably result from its adaptation to rich upwelling systems where food is highly reliable and abundant. This adaptation apparently is also shaping the population's general breeding ecology. The population has high potential fecundity and is probably regulated through food competition by density‐dependent processes which affect reproductive success and juvenile recruitment more than adult survival. Demographic analyses and direct counts along the species' entire range showed that the population is stable both in its demographic structure (e.g., sex ratio and age structure) and number. The population is characterized by a high and relatively constant annual adult survival, long life‐span, and a generally low and variable reproductive success and annual productivity. The serial polyandrous mating system apparently has little effect on these demographic traits. Dispersal data indicated that gene flow is extensive (≈28 km per generation) and that the whole population is distributed in at least 10 relatively discrete demes or neighborhoods. The demographic analysis yielded an expected among‐deme genetic variance component (Fₛ ₜ) of 0.049 (4.9%), which implies that the population is not genetically substructured to a large degree. Thus, for analyses of the effect of genetic drift, the whole population can be considered as a single panmictic unit. However, the exceptionally small total population size, and its even smaller effective population number (Nₑ ≈650), predicts a rate of loss of neutral genetic variation (ΔF = 1/2 Nₑ) of 0.0008 (0.08%) per generation, equivalent to 0.006% per year. The population is therefore expected to be highly homozygous (G = 0.997). These population genetic traits should be taken into account for conservation purposes. Although after a crash (e.g., in 1982) the population may rapidly recover in number and otherwise remain relatively stable, its genetically effective size barely reaches the minimum number judged to be required for long‐term persistence in the face of genetic and environmental stochasticity. Moreover, the recurrent human disturbance of the habitat where cormorants find their prey, mainly through alteration of the sea bottom for the capture of sea cucumbers, may severely affect the survival of this endemic species, one of the world's rarest seabirds.