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7 - The distribution and conservation of birds of coastal salt marshes
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- By Russell Greenberg, Smithsonian Conservation Biology Institute, National Zoological Park, Washington, DC, USA, Augusto Cardoni, Naturales Universidad Nacional de Mar del Plata, Bruno J. Ens, SOVON Dutch Centre for Field Ornithology, SOVON-Texel, Den Burg, Texel, The Netherlands, Xiaojing Gan, Massey University, Juan Pablo Isacch, Naturales Universidad Nacional de Mar del Plata, Kees Koffijberg, SOVON Dutch Centre for Field Ornithology, Nijmegen, The Netherlands, Richard Loyn, Arthur Rylah Institute for Environmental Research, Heidelberg, VIC, Australia
- Edited by Brooke Maslo, Rutgers University, New Jersey, Julie L. Lockwood, Rutgers University, New Jersey
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- Book:
- Coastal Conservation
- Published online:
- 05 June 2014
- Print publication:
- 27 March 2014, pp 180-242
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Summary
Introduction
Salt and brackish coastal marshes (coastal salt marsh) are distributed thinly along the mid- to high-latitude coastlines of all the major continents except Antarctica. Where coastlines are protected and supplied with a source of sediment, grasses and shrubs colonize and stabilize the substrate, paving the way for further marsh accretion. Salt marshes form along lagoons protected by barrier islands, at the mouths of river deltas and along the edges of protected estuaries. Salt marshes are widely distributed, but account for a small amount of land cover. Although precise quantification of the current extent of salt marsh is lacking, an estimate of 60 000 km2 seems reasonable (Greenberg et al., 2006b). Salt marsh vegetation is replaced by mangrove forest between 32°N and 40°S or coexists with it at higher tidal levels (see Chapter 2). Whatever the exact amount of extant salt marsh, it is clear that it is a fraction of what existed even a century ago. The direct and indirect effects of human activity are particularly acute for salt marshes, as most of the human population lives on or near the coasts or within the watershed that feeds the estuaries where marsh grows (Rickey & Anderson, 2004).
Salt marshes show a great deal of similarity throughout the world in their simple vegetative structure punctuated by tidal sloughs and their low floristic diversity. In general, they are dominated by one to a few species of salt-tolerant grasses and shrubs (mostly of the Chenopodiaceae), often showing distinct zonation associated with the frequency of tidal inundation and salinity (Figure 7.1). However, marshes along the different continental shorelines are unique, showing differences in the dominant plant taxa, source of the colonizing fauna, specifics of the tidal regime, frequency of storm disturbance, and the tremendous variation in human activity and use. While similar to the eye, even within a region subtle differences in marsh structure give rise to distinct biotic assemblages (Figure 7.2).
Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates
- John D. Goss-Custard, Andrew D. West, Michael G. Yates, Richard W. G. Caldow, Richard A. Stillman, Louise Bardsley, Juan Castilla, Macarena Castro, Volker Dierschke, Sarah. E. A. Le. V. dit Durell, Goetz Eichhorn, Bruno J. Ens, Klaus-Michael Exo, P. U. Udayangani-Fernando, Peter N. Ferns, Philip A. R. Hockey, Jennifer A. Gill, Ian Johnstone, Bozena Kalejta-Summers, Jose A. Masero, Francisco Moreira, Rajarathina Velu Nagarajan, Ian P. F. Owens, Cristian Pacheco, Alejandro Perez-Hurtado, Danny Rogers, Gregor Scheiffarth, Humphrey Sitters, William J. Sutherland, Patrick Triplet, Dave H. Worrall1, Yuri Zharikov, Leo Zwarts, Richard A. Pettifor
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- Journal:
- Biological Reviews / Volume 81 / Issue 4 / November 2006
- Published online by Cambridge University Press:
- 24 July 2006, pp. 501-529
- Print publication:
- November 2006
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As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free-living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual-based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro-invertebrates. Intake rate is measured as the ash-free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.
A review of 30 functional responses showed that intake rate in free-living shorebirds varied independently of prey density over a wide range, with the asymptote being reached at very low prey densities (<150/m−2). Accordingly, most of the many studies of shorebird intake rate have probably been conducted at or near the asymptote of the functional response, suggesting that equations that predict intake rate should also predict the asymptote.
A multivariate analysis of 468 ‘spot’ estimates of intake rates from 26 shorebirds identified ten variables, representing prey and shorebird characteristics, that accounted for 81% of the variance in logarithm-transformed intake rate. But four-variables accounted for almost as much (77.3%), these being bird size, prey size, whether the bird was an oystercatcher Haematopus ostralegus eating mussels Mytilus edulis, or breeding. The four variable equation under-predicted, on average, the observed 30 estimates of the asymptote by 11.6%, but this discrepancy was reduced to 0.2% when two suspect estimates from one early study in the 1960s were removed. The equation therefore predicted the observed asymptote very successfully in 93% of cases.
We conclude that the asymptote can be reliably predicted from just four easily measured variables. Indeed, if the birds are not breeding and are not oystercatchers eating mussels, reliable predictions can be obtained using just two variables, bird and prey sizes. A multivariate analysis of 23 estimates of the half-asymptote constant suggested they were smaller when prey were small but greater when the birds were large, especially in oystercatchers. The resulting equation could be used to predict the half-asymptote constant, but its predictive power has yet to be tested.
As well as predicting the asymptote of the functional response, the equations will enable research workers engaged in many areas of shorebird ecology and behaviour to estimate intake rate without the need for conventional time-consuming field studies, including species for which it has not yet proved possible to measure intake rate in the field.