[Taxacom] How frequent should you collect for a good representation of insect fauna?

[Doug Yanega]

Soowon Cho wrote:

>I have collected insects
>in my university for a year: with four different collecting methods at four
>sites in every other week, and the data tells me that on average only about
>30% of the insect species collected at a collecting are collected again at
>the next collecting.

You say the results surprised you, but I would only be surprised at 
this if malaise or berlese trap samples gave that result; for nearly 
every other collecting method I'm familiar with, the proportion of 
taxa sampled relative to the number present is generally so low that 
you will commonly fail to collect many taxa that *could* have been 
caught - because so much of what is present occurs in such low 
numbers that they're unlikely to be sampled except by a very robust 
sampling protocol. When a large proportion of the fauna is "rare", 
the result is a lot of "false negatives" in each collecting event, 
inflating the apparent differences between successive samples not 
because of genuine differences in species composition, but because 
the odds of sampling the same *rare* taxa in successive intervals is 
extremely low.

Comparisons between different collecting techniques will further 
inflate the differences (I work with pan traps frequently, and it's 
been found that you can place yellow, blue, and white traps right 
next to one another and still have about 30% of the taxa trapped by 
each color not replicated by the other colors). While a technique 
like malaise trapping can give a fair overlap from sample to sample, 
other collecting techniques performed alongside malaise trapping can 
still give very different results (pan traps, for example, collect 
MANY more bee species than malaise traps), so combining results will 
give very different patterns from just using malaise traps alone.

The funny thing is that I don't really know how often folks publish 
discussions specifically referring to the nature of sampling 
protocols and analysis; most of what I'm familiar with is from 
first-hand experience and discussions with other collectors, and I'd 
have a hard time giving actual citations. That does remind me, though 
- this is something to bring up at this year's meeting of the 
Entomological Collections Network, to see about putting together a 
website listing references discussing sampling techniques, if no one 
else already has a similar website in place somewhere.

Peace,
-- 

Doug Yanega        Dept. of Entomology         Entomology Research Museum
Univ. of California, Riverside, CA 92521-0314        skype: dyanega
phone: (951) 827-4315 (standard disclaimer: opinions are mine, not UCR's)
              http://cache.ucr.edu/~heraty/yanega.html
   "There are some enterprises in which a careful disorderliness
         is the true method" - Herman Melville, Moby Dick, Chap. 82