[Odonata-l] Morphology and niche overlap

[Richard Rowe]

At 09:18 PM 30/03/2006, Roy van Grunsven wrote:
>Testing the niche theory is very difficult. It is always possible that 
>species differ in a characteristic you did not check, no matter how well 
>you've looked.
>
>In dragonflies the larvae are the real playing field, that's where the 
>species compete. Looking at habitat preference a lot of species can be 
>separated on pH, oxygen concentration and different sediments or 
>vegetation needed. But if two species are in the same pond does not mean 
>they automatically compete, they can be in different places (in clusters 
>of plants or in the open) active at different times of the day or year or 
>have different food preferences.
>An example of this is A. juncea and A. subarctica. These two are very 
>alike, both as adults and larvae. They often co occur and (at least in The 
>Netherlands) A. juncea is much more common. When looking more closely the 
>larvae of A. subarctica live in sphagnum mats and have an advantage over 
>juncea in this specific microhabitat. Probably competition with juncea 
>keeps it restricted to these sites but it does allow them to coexist.
>
>Another aspect is temporal change. A lot of species are weak competitors 
>and are out competed all the time if a stronger competitor arrives. But in 
>new sites (after drought for instance) they have a temporal safe haven 
>until out competed again. So they can coexist although it's temporary.
>
>All in all we can not really answer this question. Maybe we'll get the 
>data to show differences but will never have enough data to show that 
>there is no difference between two species.
>
>
>Roy van Grunsven.

We need to go back to the original papers:

Hutchinson, G. E. 1959. Homage to Santa Rosalia, or why are there so many 
kinds of animals? American Naturalist 93:145-159.

and

Lewin, R. 1983. Santa Rosalia was a goat. Science 221:636-639

Once a highly multi-dimensional niche is recognised the problem becomes 
ill-conditioned as nobody can find how many dimensions are involved 
let-alone establish a separation or lack thereoff in whatever dimension, or 
combination of dimensions, is/are of consequence.  In some species 
combinations, where for example there is intense, direct inter-specific 
aggression then a mechanism for displacement exists, but the mechanism is 
so obvious as to be intellectually trivial.

And 'competition' may not be stable ... consider Frank Suhling and team's 
work on populations in desert ponds in Namibia where lotteries seemingly 
dominate the system ...

Richard Rowe


Dr Richard Rowe
Zoology & Tropical Ecology
School of Tropical Biology
James Cook University
Townsville 4811
AUSTRALIA

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