Background: 



The General Nature 



of Environmental 



Responses 



Ecology, by definition, deals with the interrela- 

 tions of organisms with each other and with their 

 environment. These interrelations become estab- 

 lished as organisms respond in various ways to con- 

 tacts with one another and with the ever-changing 

 environment. 



The term environment describes, in an unspecific 

 way, the sum total of physical and biotic conditions 

 influencing the responses of organisms. More spe- 

 cifically, the sum of those portions of the hydrosphere, 

 lithosphere, and atmosphere into which life pene- 

 trates is the biosphere. There are no characteristic or 

 permanent inhabitants of the atmosphere, although 

 the air is traversed by many kinds of animals and 

 plant propagules. Of the hydrosphere, there are two 

 major biocycles, the marine and fresh-water ; of the 

 lithosphere there is one, land (Hesse et al. 1931). 



A habitat is a specific set of physical conditions 

 (e.g., space, substratum, climate) that surrounds a 

 single species, a group of species, or a large com- 

 munity (Clements and Shelford 1939). The ultimate 

 division of the biosphere is the niicrohabitat, the most 

 intimately local and immediate set of conditions sur- 

 rounding an organism ; the burrow of a rodent, for 

 instance, or a decaying log. Other individuals or 

 species are considered as part of the community to 

 which the organism belongs and not part of its habitat. 

 The term biotope defines a topographic unit charac- 

 terized by both uniform physical conditions and uni- 

 form plant and animal life. 



In order for organisms to exist they must respond 

 or adjust to the conditions of their environment. 

 The first living organisms probably evolved in the sea 

 and must have possessed very generalized adjust- 

 ments to this relatively uniform and favorable habitat. 

 However, these early organisms had inherent in them 

 the potential for expansion, as they later spread into 

 other and more rigorous habitats, particularly fresh- 

 water and land. As evolution proceeded, organisms 

 became more and more limited in the range of their 

 ability to respond as they became specialized in their 

 adjustments to particular habitats. This led to the 

 great diversification of species that we see at the 

 present time, with each species restricted to its par- 

 ticular niicrohabitat and place in the community. 



Organisms respond to differences or changes in 

 their environment in four principal ways : morpho- 

 logical adaptations, physiological adjustments, be- 

 havior patterns, and community relations. Chapters 

 2 and 3 are a resume of these responses, the general 

 fundamentals of which must be understood before the 

 subtle relations of an organism to its environment 

 that are the substance of ecology can be appreciated. 

 Probably the most important of distinctions be- 

 tween organisms in a consideration of their morpho- 

 logical responses to the environment is whether 

 they are sessile or motile (Shelford 1914). Most 



