Decadal-scale variation in diet forecasts persistently poor breeding under ocean warming in a tropical seabird.

Climate change effects on population dynamics of natural populations are well documented at higher latitudes, where relatively rapid warming illuminates cause-effect relationships, but not in the tropics and especially the marine tropics, where warming has been slow. Here we forecast the indirect effect of ocean warming on a top predator, Nazca boobies in the equatorial Galápagos Islands, where rising water temperature is expected to exceed the upper thermal tolerance of a key prey item in the future, severely reducing its availability within the boobies' foraging envelope. From 1983 to 1997 boobies ate mostly sardines, a densely aggregated, highly nutritious food. From 1997 until the present, flying fish, a lower quality food, replaced sardines. Breeding success under the poor diet fell dramatically, causing the population growth rate to fall below 1, indicating a shrinking population. Population growth may not recover: rapid future warming is predicted around Galápagos, usually exceeding the upper lethal temperature and maximum spawning temperature of sardines within 100 years, displacing them permanently from the boobies' island-constrained foraging range. This provides rare evidence of the effect of ocean warming on a tropical marine vertebrate.

Assessment of costs of reproduction in a pelagic seabird using multistate mark-recapture models.

We used a long-term population band-resight survey database, a parallel reproduction database, and multistate mark-recapture analysis to assess the costs of reproduction, a keystone concept of life-history evolution, in Nazca boobies (Sula granti) from Punta Cevallos, Isla Española, Galápagos, Ecuador. We used eight years of resight and breeding data to compare models that included sex- and state-specific survival probabilities and probabilities of transition between reproductive states using multistate mark-recapture models. Models that included state-specific effects were compared with models lacking such effects to evaluate costs of reproduction. The top model, optimizing the trade-off of model simplicity and fit to the data using the Akaike Information Criterion (AIC), showed evidence of a temporally varying survival cost of reproduction: nonbreeders showed higher annual survival than breeders did in some years. Because increasing investment among breeders showed no negative association with survival and subsequent breeding success, this evidence indicates a cost to both males and females of initiating, but not of continuing, a reproductive attempt. In some cases, breeders reaching the highest reproductive state (fledging an offspring) showed higher survival or subsequent breeding success than did failed breeders, consistent with differences in overall quality that promote both survival and reproduction. Although a male-biased adult sex ratio was observed in this population of Nazca boobies, models of state- and sex-specific survival and transition probabilities were not supported, indicating that males and females do not incur different costs of reproduction, and that the observed sex ratio bias is not due to sex-specific adult mortality.

Nesting distributions of Galápagos boobies (Aves: Sulidae): an apparent case of amensalism.

Blue-footed boobies (Sula nebouxii) in the Galápagos Islands nest at coastal sites such as cliff edges if Nazca boobies (S. granti) are absent. However, if sympatric with nesting Nazca boobies, they nest nearby, but farther inland, in areas with little topographical relief. Nazca boobies nest at the coastal sites whether blue-footed boobies are present or not. The segregated nesting pattern of these two species offers a model system to investigate factors influencing community structure. We tested a non-interactive hypothesis, in which different fundamental niches generate the non-overlapping distributions, and an interactive hypothesis, in which the two fundamental niches overlap and an interaction between the two species causes the segregation. Data on three factors considered as likely parameters differentiating fundamental niches (nest microclimate, nature of the nesting substrate, and ease of taking flight from nest sites) failed to support the non-interactive hypothesis. These results suggest that the two species have indistinguishable fundamental niches with respect to these parameters, but different realized niches. Researchers studying resource partitioning by ecologically similar species often only consider competition (a "-/-" interaction) to explain situations like this, ignoring the more parsimonious amensal (0/-) possibility. Nesting segregation in this situation is apparently caused by attacks of non-breeding adult Nazca boobies on blue-footed booby nestlings, injuring nestlings and ultimately preventing them from fledging. The interaction does not result in any discernible costs or benefits (i.e., effects on fecundity or survival) for the adult Nazca boobies, so it is best described as an amensal interaction. This interaction provides a sufficient explanation of the observed nesting segregation, and precludes present competition for nesting space.