SUNLIGHT

Exposure to sunlight appears to be an underappreciated aspect of habitat. Among non-living features of habitat, it is water and temperature that usually come to mind. This note attempts to show that ignoring sunlight is probably a mistake. Indeed sunlight, it appears, is often the main aspect of habitat that determines the distribution of animals and plants. I recently came across a discussion of the role if sunlight in Helena Cronin’s book, The Ant and the Peacock (1991), pp91-92. There she discusses the findings of such distinguished zoologists as Arthur Cain and P. M. Sheppard, on the degree of exposure to sunlight in determining the distribution of the iconic snail Cepea nemoralis. She goes on to show that Alfred Russell Wallace himself took a deep interest in the role of sunlight. Their work together provides context, which I had not at first fully appreciated, for my own effort to identify habitat selection in the two rain-pool dwelling species of chironomid midges, Chironomus imicola or Chironomus pulcher, Life in the Puddle (1988). Why are some rain pools consistently occupied by either C. pulcher or C. imicola, presumably for thousands or even millions of years? Evidence is accumulating that it is the degree of exposure to sunlight responsible here too. Some typical rain pools recently filled by rain, on rock surfaces in Africa are shown above.