Ecolrl 



NMATQDE ECOLOGY 

 C. Overgaard Nielsen 



The title provides a very spacious framework, and since I cannot possibly 

 cover the whole field of nematode ecology within the time at my disposal, 

 I propose to restrict myself to a few selected aspects. 



Most of the papers which have been given so far at this meeting have been 

 rather detailed and technical, which is in agreement with the purpose of 

 the meeting. I shall follow a somewhat different line and only talk 

 about generalities. 



Vlhat I am attempting to do here is to present the general physiological 

 background against which I would like the practical nanatologist to con- 

 sider the activity of nematodes. When I say nematodes, I mean primarily 

 free-living soil nematodes, but I hope to present some generalities at a 

 level which also applies to many problems connected with plant parasites. 



First, I would like to emphasize that in essence soil nematodes are fresli- 

 water organisms, although they show many adaptations to the conditions 

 prevailing in the water phase of the soil. These adaptations show up 

 when we consider the distribution of nematode species throughout an 

 imaginary transect from river bottom through water meadow, up the slope 

 of the river valley to the high ground, and ending in a moss cushion on 

 a tree trunk. The distribution of species may be as follows, where the 

 figures replace the names of species: 



species nvanbers 



In river bottom 1-UO 



In water meadow 5-50 



In soil on high ground 3^-75 



In moss cushion on tree trunk 75-76 



When we consider habitats with very different moisture regimes, the 

 species spectrum is seen to change. In going from the bottom layers of 

 the river to the water meadow, four species have disappeared but ten 

 newcaners have appeared (the actual figures are, of course, chosen quite 

 arbitrarily in order to illustrate a principle) . 



Why this changing spectrum? Do some nematodes prefer wet stations and 

 others, dry? One often comes across such statements. The explanation 

 is entirely different: All nematodes require the presence of available 

 water in order to be active, but some have developed the ability to 

 survive periodical desiccations. That is what malces them true soil 

 inhabitants J they are not seriously affected by droughts but spend such 

 periods in a state of dormancy. This category of nematodes is repre- 

 sented by species ntmbers 3^-76, and they are the ones I am going to 

 speak of here. The nematodes predominant in soils with a stable and 

 rich water supply have been studied by Dr. Hirschmann, and I can refer 



