Effect of horseshoe crab spawning density on nest disturbance and exhumation of eggs: A simulation study |
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| Authors: | David R Smith |
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| Institution: | 1.Leetown Science Center,U.S. Geological Survey,Kearneysville |
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| Abstract: | Because the Delaware Bay horseshoe crab (Limulus polyphemus) population is managed to provide for dependent species, such as migratory shorebirds, there is a need to understand the
process of egg exhumation and to predict eggs available to foraging shorebirds. A simple spatial model was used to simulate
horseshoe crab spawning that would occur on a typical Delaware Bay beach during spring tide cycles to quantify density-dependent
nest disturbance. At least 20% of nests and eggs were disturbed for levels of spawning greater than one third of the average
density in Delaware Bay during 2004. Nest disturbance increased approximately linearly as spawning density increased from
one half to twice the 2004 level. As spawning density increased further, the percentage of eggs that were disturbed reached
an asymptote of 70% for densities up to 10 times the density in 2004. Nest disturbance was heaviest in the mid beach zone.
Nest disturbance precedes entrainment and begins the process of exhumation of eggs to surface sediments. Model predictions
were combined with observations from egg surveys to estimate a snap-shot exhumation rate of 5–9% of disturbed eggs. Because
an unknown quantity of eggs were exhumed and removed from the beach prior to the survey, cumulative exhumation rate was likely
to have been higher than the snap-shot estimate. Because egg exhumation is density-dependent, in addition to managing for
a high population size, identification and conservation of beaches where spawning horseshoe crabs concentrate in high densities
(i.e., hot spots) are important steps toward providing a reliable food supply for migratory shorebirds. |
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