Ecological Archives E091-202-A2

Adrian C. Stier and Craig W. Osenberg. 2010. Propagule redirection: Habitat availability reduces colonization and increases recruitment in reef fishes. Ecology 91:2826–2832.

Appendix B. Recruitment model, parameterization and projections.

We started with the standard formulation of the Beverton-Holt recruitment function (e.g. Osenberg et al. 2002):

(B.1)

where Dt is the density of recruits to a site (i.e., settlers who survive t days); D0 is the initial density of settlers to that site, α is the per capita density-independent mortality rate, and β is the per capita density-dependent mortality rate. Because we were interested in the separate effects of habitat (h) and numbers (N), per se, we re-expressed density (D = N/h) to obtain:

(B.2)

Notice that the parameters have the same meanings and units in Eq. B.2 as in Eq. B.1 (i..e, α: 1/day, t: day; β: corals / fish / day). We then parameterized the Beverton-Holt model (fitting α and β) using data from previous studies of density dependence conducted on one of the focal species, D. flavicaudus (Holbrook and Schmitt 2002, Schmitt and Holbrook 2007). The corals used by Schmitt and Holbrook (2007) were approximately twice the areal dimension of ours, so we parameterized the model by doubling their coral numbers to reflect the difference in areal cover of corals in the two studies (i.e., h is expressed as the number of corals of the size used in our study).

Recruitment patterns of newly settled reef fishes, including D. flavicaudus, are driven by high mortality within the first few days following their arrival to the reef (Almany and Webster 2006, Schmitt and Holbrook 2007). We therefore calculated the expected number of survivors after two days on the reef (i.e., “recruitment”). This time scale is the same as the one used to generate the data with which we parameterized the model (Schmitt and Holbrook 2007). We assumed that density dependence was most strong among fish that settled within 2 weeks of one another, and therefore used the total number of settlers per coral observed in our 28 day (i.e., + 2 weeks) field study. We then compared the levels of settlement and recruitment in a system with low habitat availability (h = 1) to one with high habitat availability (h = 2.75); this 2.75 fold difference in habitat availability is within the range of habitat losses observed in coral reef systems (e.g. Connell et al. 2004, Bruno and Selig 2007). Expected settlement intensities were taken from our field experiment (low vs. high habitat availability) for D. flavicaudus, but we also examined the results under much lower and higher settlement intensities to better generalize the results.

LITERATURE CITED

Almany, G. R. and M. S. Webster. 2006. The predation gauntlet: early post-settlement mortality in reef fishes. Coral Reefs 25:19–22.

Bruno, J. F. and E. R. Selig. 2007. Regional decline of coral cover in the Indo-Pacific: timing, extent, and subregional comparisons. PLoS ONE 2:e711, 711–718.

Connell, J. H., T. E. Hughes, C. C. Wallace, J. E. Tanner, K. E. Harms, and A. M. Kerr. 2004. A long-term study of competition and diversity of corals. Ecological Monographs 74:179–210.

Holbrook, S. J. and R. J. Schmitt. 2002. Competition for shelter space causes density-dependent predation mortality in damselfishes. Ecology 83:2855–2868.

Osenberg, C. W., C. M. St. Mary, R. J. Schmitt, S. J. Holbrook, P. Chesson, and B. Byrn. 2002. Rethinking ecological inference: density dependence in reef fishes. Ecology Letters 5:715–721.

Schmitt, R. J. and S. J. Holbrook. 2007. The scale and cause of spatial heterogeneity in strength of temporal density dependence. Ecology 88:1241–1249.


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