I'm evaluating trophic resources available for shorebirds on an intertidal mudflat. I used core samples (diameter: 10 cm, depth 3 cm. ) to obtain abundances and calculate densities of preys available and accesible for small calidrine sandpipers (Western, Least and Semipalmated Sandpiper).
With every sample I counted the individuals and obtained abundance data, but while reading some papers I notticed that invertebrate density is always expressed as ind/m2 instead of ind/m3. We are not sampling areas but volumes, is it right to express it as individuals per unit area? Why not to use volume? Is it right to compare densities from different works even when the didn't use the same core diameter and depth?
My opinion is that we shouldn't compare data from different studies because core depth is different on every work (p.e: 3 cm, 10 cm, 30 cm depth) and different burying depth of benthic organisms affects the abundances and subsecuently the density of individuals on every sample, and it wouldn't be possible to compare even if i express my data as density per volume unit for the same reason: volumes are dependent of core depth and diameter and because I haven't found works with abundances expresed in function of volume.
How should I deal with those issues?
I will appreciate all of your help and opinions.
I have worked as a marine benthic ecologist for over 30 years and have never seen abundance described by the volume of sediment in a sample, although there is a movement to sample benthos only to depth of 10 cm. It is an intriguing idea.
I work primarily in Puget Sound (Washington State, USA) in the marine subtidal region and we typically use either a modified van Veen sampler or some type of box core. These can be heavy devices weighing up to 40 Kg. The heaver the device the greater the penetration. A van Veen sampler has a maximum depth of 17 cm and a box core, depending on type can sample 40 cm or more. In a compact sand substrate neither of these can penetrate much past 5 cm. What we are looking for is the biological active zone. In a compact sand environment that zone can be as small as the upper 2 or 3 cm. As the percentage of silt and clay increases so does the penetration depth but the majority of species numbers and abundance will still be in the upper 5 to 10 cm.
In terms of comparison you cannot or should not compare a sand environment to a silt clay environment; they house different species/communities and the biologically active zone will be different. Different samplers can have different surface areas and ecologists can extrapolate benthic abundance to a per meter square area for comparison between different samples with the same or similar habitat types (sediment grain size, total organic carbon). In a way we may be accounting for sampler volume. However we do not extrapolate species numbers to a per meter square area because of the patchy distribution of species.
I did a quick internet search for “species abundance by sample volume" and after looking through 12 pages, I could not find any citations.
Good Luck and take care,
Pete
I am associated with the Chesapeake Bay Monitoring Program at ODU. One of the major components of that program is a benthic monitoring program that has been ongoing since 1985 at 21 fixed stations. Pete is correct in that estimates of densities are typically made on an areal basis. This probably reflects the difficulty in obtaining consistent volumes of samples more than anything which is reflective of differences in sediment type as Pete suggests. I do take issue with his assertion that comparisons between sediment type cannot be made. In fact, I don't think he actually meant to say this. Benthic ecologists actually spend a great deal of time comparing communities within and between sediment types and salinity regimes and expounding on various relationships between such things. If your core samples are of a consistent volume, there's no reason not to express them in terms of volume. Also you might consider, obviously, looking at the depth partition of your organisms. Our program looks at this to some limited degree but there is considerable early work in the literature about it. Another aspect you may probably consider for shorebirds are the meiofauna which are typically neglected. They are organisms in the sand grained size. They might be important for smaller birds. I don't know.
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