D— Continued 



dispersal— Movement of individuals or their 

 disseminants or propagules (seed, spores, 

 larvae) out of a population or area. 



disphotic zone— The dimly lit zone in 

 bodies of water where there is insuffi- 

 cient light for photosynthesis. 



diversity— Refers to the number of differ- 

 ent kinds of species in an area. 



diversity indices— Various formulas convert 

 numbers and importance of species, and 



numbers of individuals into dimension- 

 less values which indicate variety. 



dominants— A species or group of species 

 which largely control the energy flow 

 and strongly affect the environment 

 within a community or association. 



dystrophic lake-A lake which is low in 

 productivity and contains brown -stained, 

 low pH waters which have a high organic 

 content. 



--E 



ecdysis— The shedding of the old exoskele- 

 ton in arthropods. 



ecological energetics— The cycling of energy 

 through trophic levels. Some energy is 

 lost when transferred from one trophic 

 level to the next higher. 



ecological indicators— Species or associa- 

 tions which indicate certain ecological 

 conditions such as soil, type, climate, 

 temperature or salinity. 



ecological succession— See succession. 



ecological valency— Refers to the range of 

 environmental variation tolerated by a 

 species. 



ecosystem-The living organisms and the 

 nonliving environment interacting in a 

 given area. See also biocoenosis and 

 biogeocoenosis. 



ecotone— A transitional area between two 

 or more communities. See also edge 

 effect. 



ecotope-Physical environment of a habi- 

 tat. See also biotope. 



edaphic— A term referring to the soil 

 conditions or types as ecological factors. 



edge effect — The change increase (usually 

 increase) in species diversity and density 

 in the transitional area between com- 

 munities. See also ecotone. 



eltonian pyramids— Refers to the concept 

 of representing trophic levels as layers 

 based on numbers of individuals or mass, 

 thus forming a pyramid. Primary pro- 

 ducers create the greatest mass and 

 carnivores the least. See also pyramids. 



emergent vegetation — Plants which al- 

 though rooted in shallow water have 

 most of their growth above the water. 



endemic— Native to a specific geographic 

 area. 



endogenous— Originating or growing from a 

 specific geographic area. 



energetics, ecological— See ecological ener- 

 getics. 



epilimnion— In lakes, the layer of water 

 above the thermocline. It is the area 

 where main primary productivity takes 

 place. 



