Graphic Assessments

by Carl Strang

The amphibian trapping season at Mayslake Forest Preserve is done for the year. A painful back strain stopped me a week early (I mention this so you’ll know why blog postings may be thin for a time). The main purpose of the trapping effort is to determine which amphibian species occur on the preserve, particularly salamanders which, unlike frogs and toads, are silent and hidden most of the year. As I posted earlier, in this second year the traps did indeed reveal the presence of tiger salamanders. In addition, though, the traps have caught a variety of larger invertebrates.

One example is the predaceous diving beetle Dytiscus verticalis. I photographed this one on its back to show the brown color on the underside of the abdomen, a species identification feature.

As time goes on, the traps continue to catch animals, but new species appear less and less often. This can be rendered graphically.

This graph combines the two years’ data. I keep records of all individuals caught, which allows me to see how new species have appeared as the total catch has grown.

As you can see, there appears to be a leveling off at the current total of 11 species. Keep in mind that this is not the total number of animal species in the stream corridor marsh, just those that can be caught in the traps. Such data usually are converted to logarithms in ecological analyses.

The same data as in the previous graph, converted to their logarithms.

Again there seems to be a leveling-off, but not so dramatic. Another approach to species richness is to consider the accumulation of species according to sampling effort.

Here the same species are shown being added as the trap-days accumulate.

This graph reveals something significant. Can you see how there appear to be two episodes of the species count leveling, at 4 and at 11 species? That first leveling represents the latter part of last year’s trapping. If I had been content with those results, I would have drawn the conclusion that the marsh contains around 4 species susceptible to these traps. But this year’s results were different, adding not only the tiger salamander but a few new invertebrates as well as two frog species. Also, I did not catch two of the species I caught last year. Here is the same data set converted to logs.

This graph gives a hint of why the logarithmic conversion is done. It takes out some of the misleading tendencies that appeared in the unconverted data.

From this final graph I conclude that I need to continue trapping at least another year. I will sense that I have an idea of the marsh’s species richness when the log-log graph of trapping effort levels off. Even without this graphical analysis I would have wanted to continue trapping, however. Something the graphs don’t show is the dramatic difference in the kinds of animals caught each year. Where last year the sampling was dominated by predaceous diving beetles, this year there were few of them but a lot of white river crayfish. Clearly there are some dynamic changes occurring between years, and I want to see what future years hold.

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2 Comments

  1. April 12, 2013 at 3:37 pm

    Interesting content! However, I believe that your Dytiscus verticalis is a Hydrophilus (Hydrophilidae).

    • natureinquiries said,

      April 14, 2013 at 6:25 am

      Thanks, Wollfram,
      Apparently I have only that one photo of the individual in question. It must have appeared identical to verticalis on its dorsal side, and the highly convex abdomen would seem to point to Dytiscus, but I see that the head area does look different from other photos of unambiguous Dytiscus spp. I’ll need to be more alert to this possibility in the future.
      Regards,
      Carl


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