and euryhaline in respect to salinity, stenohydric and 

 euryhydric in respect to water, stenothermal and 

 eitrythermal in respect to temperature, stenophagic 

 and euryphagic in respect to food, stenoecious and 

 euryoecious in respect to niche or habitat, and so on. 



Law of the minimum 



An organism is seldom, if ever, exposed solely 

 to the effect of a single factor in its environment. On 

 the contrary, an organism is subjected to the simul- 

 taneous action of all factors in its immediate sur- 

 roundings. However, some factors exert more influ- 

 ence than do others, and the attempt to evaluate their 

 relative roles has led to the development of the law 

 of the minimum. 



The first elaboration of this law was made by the 

 German biochemist, Justus von Liebig, in 1840, who 

 stated : 



// one of the participating nutritive constituents 

 of the soil or atmosphere be deficient or wanting or 

 lacking in assimilability , either the plant does not 

 grow or its organs develop only imperfectly. The de- 

 ficient or lacking constituent makes those that are 

 present inactive or lessens their activity. If the de- 

 ficient or lacking constituent he added to the soil or 

 if occurring in insoluble form it be made soluble, then 

 the other nutrients become active (Browne 1942). 



Blackman (1905) developed the more compre- 

 hensive concept of limiting factors when he listed five 

 factors involved in controlling the rate of photosyn- 

 thesis : amount of COo available, amount of HoO 

 available, intensity of solar radiation, amount of 

 chlorophyll present, and temperature of the chloro- 

 plast. Any one of these factors will control the rate 

 of the process if the factor is present in least favor- 

 able amount, or may actually stop it when insufficient, 



320 



16 20 24 28 32 36 40 



TEMPERATURE, °C 



FIG. 2-4 The relation between maximum respiration rate, tem- 

 perature, and oxygen tension (mm Hg as shown by values in 

 the graph) in young goldfish acclimated to each temperature 

 before measurements were taken (Fry 1947). 



even though all other factors occur in abundance. 

 The same principle applies to animal functions. 



Since the rate of a process may be controlled by 

 too great an amount of a substance, such as heat, as 

 well as by too small an amount, and since the pres- 

 ence or abundance of an organism may be limited by 

 a variety of environmental factors, biotic as well as 

 chemical and physical, and since the limiting effect 

 may be due to two or more interacting factors rather 

 than a single isolated one (Shelford 1952), the laiv 

 of the minimum may be restated in broad ecological 

 terms, as follows : the functioning of an organism is 

 controlled or limited by that essential environmental 

 factor or combination of factors present in the least 

 favorable amount. The factors may not be continu- 

 ously effective but only at some critical period during 

 the year or perhaps only during some critical year in 

 a climatic cycle (Taylor 1934). 



BEHAVIOR RESPONSES 



Orientation 



Behavior responses to changes in environ- 

 mental factors can usually be detected immediately 

 as turning or locomotor activities on the part of the 

 organisms (Fraenkel and Gunn 1940). These move- 

 ments tend to take the organism away from points of 

 danger and into more favorable locations, or to per- 

 form sotne task essential to existence, or to reproduc- 

 tion. If the movement involves curvature or a turn- 

 ing movement either toward or away from the source 

 of stimulus, the movement is called a tropism. Motile 

 organisms frequently respond by actual locomotion 

 toward or away from the stimulus rather than mere 

 turning, and such guided or directed locomotor move- 

 ments are called taxes. When the movements of the 

 animal are random in direction, and there is no im- 

 mediate orientation to the source of stimulus, but the 

 frequency of turning or speed of the movements is 

 dependent on the intensity of stimulation, such re- 

 sponses are termed kineses. As the result of kineses 

 an animal may arrive by chance in a favorable en- 

 vironment, by which the intensity of the stimulus is 

 reduced or entirely eliminated. To identify the stim- 

 ulus to which the organism is responding, the fol- 

 lowing prefixes are employed : thermo-, tempera- 

 ture ; photo-, light: geo-. gravity; hydro-, moisture; 

 chemo-, chemicals ; thigmo-. contact ; baro-, pressure ; 

 rlico. current; and galvano-. electricity. 



Jacques Loeb, during the period 1888-1918, vig- 

 orously maintained that all tropisms and taxes of 

 organisms were mechanical, automatic, and explain- 

 able in simple concepts of physics and chemistry. 



. . . the overzvhelming majority of organisms 

 have a bilaterallv s\nnmetrical structure. . . . Nor- 



Background 



