Hi Brian - I don't detect a specific question in your post; however, you seem to have a decent handle on the differences between sample and individual based rarefaction. Yes, if the habitats vary in the number of individuals per sample you expect to collect, individual based rarefaction and sample based rarefaction are likely to give different perspectives. In fact, you can use the difference between the two is a measure of "patchiness". Check out:
Gotelli, N. and R. K. Colwell (2001) Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness. *Ecolog= y Letters* 4: 379=96391 Colwell, R. K and J. A. Coddington (1994) Estimating terrestrial biodiversity through extrapolation. *Phil. Trans Roy. Soc. Lond. B* 345: 101=96118. If you haven't seen these two references - you should check them out. Cheers Stephen B. Cox On 8/10/06, Brian D. Campbell <[EMAIL PROTECTED]> wrote: > > A colleague and I are collaborating on study of plant diversity within > agro-ecosystems of the Eastern Ghats, India. One of our main study > objectives is compare species richness of several habitat types which > include both forested and non-forested habitats. Obviously in addition t= o > mean richness per sampling unit (in our case, intercept along 10-m line > transects) we are interested in both a) how many new species are found > with > additional samples, and b) controlling for sample size (no. of transects)= , > what is the expected number of species (rarefaction). Here is where the > confusion arises. When comparing richness of forested and non-forested > habitats by sample based accumulation curves, the two habitat types are > quite similar. Yet, when comparing the habitat types by individual-based > curves, clearly the rate of new species sampled in forests is much > greater. > Hence, an initial interpretation of these data is that near equivalent > richness between these habitat types is a largely a function of abundance > (with greater abundance being in the non-forested habitat). A similar > example would be comparing richness in three forest categories (say for > example, young, mid-age, and old) with equivalent sample sizes in > each. But > lets say the experimenter decided to spend twice as long in one of the > three > treatments, hence over biasing the number of individuals sampled (althoug= h > sample sizes are the same). To make valid comparisons, would one not the= n > be in situation of needing to use individual-based rather than > sample-based > rarefaction? > > Any thoughts are much appreciated. > > Brian D. Campbell > Department of Biology > Queens University > Kingston, Ontario. > Canada. >
