Some time ago I posted the following question: > Which is more diverse? Two hectares with an even mixture of > species A and B, or 1 ha of A and 1 ha of B? I was somewhat hesitant in calling this a "riddle" in the subject line. According to the diccionary a riddle requires some ingenuity to answer, what seemed beyond such a simple query. But I did not count on the ingenuity displayed for NOT answering the question! Seriously, I appreciate the response and trust that others will also find interesting and useful the replies collected here. Believe it or not, I had no hidden agenda or second intentions for asking this. No punch line. Just genuinely intrigued and confused by the meaning(s) or interpretation(s) of "diversity", and wandered about its significance in relation to an issue that interests me, multi-purpose stands versus multi-purpose forests (aka zoning, more on that some other time.) The results have been very educational for me. The replies follow, in order of arrival. Two of them were emailed personally to me instead of to the list, so I presume that the authors prefer to remain anonymous. At the end I try to present some conclusions. -------------------------------------------------------------------- From: David South <dsouth@FORESTRY.AUBURN.EDU> or 1 ha of A and 1 ha of B PLUS one individual C? -------------------------------------------------------------------- From: Morwenna Spear <afu030@BANGOR.AC.UK> If the two separate areas of A and B are discrete this combination will be less diverse than the same combination where the two areas are adjacent, i.e. with a third ecosystem on the transition between one area and the next. Perhaps -------------------------------------------------------------------- From: Kjell Pernestal <Kjell.Pernestal@FYSIK.UU.SE> What kind of interaction is there between A and B? -------------------------------------------------------------------- From: Rob Harrison <robh@U.WASHINGTON.EDU> > What kind of interaction is there between A and B? > Kjell Pernestal Also, there is no such thing as a hectare of land with only one or two species. Perhaps one or two tree species, OK, but diversity is much more than the trees on the land. The diversity of species in the soil and that use the area of land come into the equation of "diversity" as well. -------------------------------------------------------------------- From: David Larsen <dlarsen@SYLVAN.SNR.MISSOURI.EDU> We all know that diversity means many things to many people but in terms of standard diversity measures (Simpsons or Shannon-Weiner), two things increase diversity 1. richness (number of categories per sample eg. species per plot) 2. evenness (relative abundance of those categories) As either of these increase diversity increases. By this measure mixed stand are more diverse than the non-mixed. Note that diversity measures do not specify the categories (genera, species, subspecies, etc) to use or what organisms to include or exclude (typically those we can identify). These are judgment of the user of the equations. These distinctions may or may not be important to the concept of diversity but they are the quantitative tools we have to measure diversity. -------------------------------------------------------------------- From: - Oh, let me guess ... By now you have lots of replies essentially telling you that you don't really understand your own question. These people will not, of course, answer the question no matter how much you clarify it. I keep telling them "you decide - now answer the question". The latest dodge around here is that you have alpha, beta, and gamma diversity. Alpha being diversity inside (mixed is more diverse) beta being between (one of each is more diverse) and gamma looking at both at once. By this system, if you have bears in two valleys, go into one valley and shoot them all you have decreased alpha, increased beta and had no effect on gamma diversity. Give these muddle-headed twits no quarter. If they were not surfing on the front of the wave they would not be dangerous, but they are. Sincerity and complexity as a substitute for depth of thinking is not very effective. -------------------------------------------------------------------- From: Nelson Thiffault <nels@MICROTEC.NET> I think that it depends on what kind of "diversity" you think of. Alpha, beta or gamma (ref. Whittaker 1965; 1972) ? -------------------------------------------------------------------- From: - For me, the answer depends on the scale your considering. At some scales, the diversity is identical. However, in some senses, scale is a human-concept. I wonder what scale nature uses? My guess is that for ecological consideration, 2 ha of A+B is almost the same as 1 A and 1 B if they are adjacent. However, 2000 ha vs 1000 + 1000 is a different matter entirely! -------------------------------------------------------------------- My own idea of an answer is not really important, but here it goes anyway. If we are talking about biodiversity (everybody assummed that), I would think that by any reasonable definition there should not be a great difference between the two situations. I stated a hypothetical problem with a grand total of 2 species (not necessarily trees!), assuming that being able to answer simple questions is a prerequisite for answering complex ones. There is probably some "eco-logical" principle that invalidates such a naive assumption :-) If you insist in clouding the issue by thinking in associated flora and fauna, it seems to me that for variety most of the time two ecosystems would be better than one. About landscape diversity, I guess that it depends, literally, on your viewpoint, if from there you can see the wood for the trees or not. --------------------------------------------------------------------
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