Large and small distribution areas

[Doug Yanega]

>I have the impression that in "megadiverse" groups, such as spiders
>(40,000 known species), a large proportion of the known species has
>rather limited distribution areas and a rather smaller number of species
>has worldwide distribution. On the other hand, in some small groups,
>such as Myxomycetes (1000 species) or Tardigrades (600 species) a large
>proportion of the known species seem to have a worldwide or at least
>very large distribution area.
>My questions:
>1) Is this true?
>2) Is it a general phenomenon?
>3) If so, why?

Among insects, there is a pattern that gives some relevant
information: when you have a group that disperses easily and has
relatively low host specificity, then you get very low proportions of
endemicity, and a trend towards very large distributions. This is
most evident among the really tiny wind-blown insects: mymarids,
thrips, aphids, etc. It's a lot harder to find species in these
groups with small, localized distributions, and when you do, almost
invariably it's a taxon with extreme host specificity, or in an
isolated habitat (e.g., sand dunes, mountain peaks, oceanic islands).
When you think about it, aphids and thrips aren't nearly as diverse
as one might expect, given their ecological roles - sure, they've got
more than 1000 species worldwide ( the entire Order Thysanoptera has
under 5K, I'm not sure about aphids), but they're *insects*, after
all - a group where single families can easily have 30-60K species
worldwide.
Do myxomycetes and tardigardes have low habitat specificity and
easily-dispersed wind-blown life stages? If so, they fit the pattern.

Peace,
--

Doug Yanega        Dept. of Entomology         Entomology Research Museum
Univ. of California - Riverside, Riverside, CA 92521
phone: (909) 787-4315 (standard disclaimer: opinions are mine, not UCR's)
            http://entmuseum9.ucr.edu/staff/yanega.html
   "There are some enterprises in which a careful disorderliness
         is the true method" - Herman Melville, Moby Dick, Chap. 82