Nesting distributions of Galápagos Boobies: an apparent case of amensalism

Howard M. Townsend, Kathryn P. Huyvaert, Peter J. Hodum, and David J. Anderson

Where Nazca boobies (Sula granti) and blue-footed boobies (S. nebouxii) nest sympatrically in the Galápagos Archipelago, Nazca boobies nest on coastal sites at cliff edges and ledges, whereas blue-footed boobies nest nearby but farther inland in areas with little topographical relief.  In the absence of Nazca boobies, blue-footed boobies usually nest at coastal sites.  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 hypotheses invoking competition to explain situations like this; however, nesting segregation in this situation is apparently caused by attacks of non-breeding adult Nazca boobies on blue-footed booby nestlings, not on resource-based competition.  The behavioral interaction injures nestlings and ultimately prevents them from fledging.  The interaction does not result in any discernible benefits (i.e., increase in fecundity or survival) for the adult Nazca boobies, so it is best described as an amensal interaction (0/-). This interaction apparently provides a complete explanation of the observed nesting segregation. 
nestdist.jpg (17419 bytes)
Distribution of nest sites of Nazca and blue-footed boobies. 
The farthest inland Nazca booby (NZBO) sites are represented by the thick solid line.  Transects with blue-footed boobies (BFBO) at nests are solid blocks.   The thin solid line represents the point at which vegetation was too thick for booby nesting.
windtemp.jpg (18274 bytes)
November 1993 wind speeds and operative temperatures at typical nest sites of Nazca (NZBO) and blue-footed boobies (BFBO) at 1000h, 1200h, and 1400h.

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