BIOLOGY OF THE SEA-HOW THEY LIVE 33 



docs not mean literally that the green turtle is to be found all over the tropics. 

 What it does mean is that this turtle will be found only in the particular habitat 

 of the tropics to which it is adapted, namely shallow beds of marine plants near 

 rocks and coral. Second, the number of individuals of a species to be found in 

 suitable habitats varies greatly with food preference, size, enemies, disease, 

 reproductive capacity, and a number of other factors. Basically, however, density 

 comes down to carrying capacity, a measure of the limit of the numbers of 

 individuals the en\'ironment can support, which is dependent mainly on food 

 supply. 



There are certain psychological checks that prevent carrying capacity from 

 being exceeded. Territoriality is the chief of these. This means that a breeding 

 pair of a species sets up and defends a certain area where other members of 

 the species are not tolerated. If breeding space is all taken, individuals that 

 do not find adequate territory do not breed. In species that do not set up 

 territories, carrving capacity is limited chiefly by food. 



Carrving capacitv is not static. For instance, in summer in north boreal seas, 

 long davlight hours and cool viscous waters support huge amounts of plankton 

 so that the carrying capacity for whales and fishes greatly increases at that 

 time. Therefore, the number of a species found in any area depends upon 

 the carrving capacitv for the species in that area at a particular time and is 

 regulated largely by food and the psychological factors. 



FOOD AND FOOD CHAINS 



Since the adaptive niche of animals is largely defined by their place in the 

 food chain, it is important that this subject be understood if the life of the 

 sea is to be appreciated. Elton (1935) is mainly responsible for focusing attention 

 on this subject. 



There would be no food in the sea and no life on earth were it not for 

 chlorophvll and the reaction of photosynthesis. Chlorophyll is present in the 

 great majoritv of plants and in some plantlike animals such as flagellates, and 

 is a catalyst in utilizing the sun's light energy in the following reaction: 



Light energy + carbon dioxide + water ^ ^ sugars and starches + oxygen 



Basically, the food chain involves the following relationship which is common 

 to all communities (with the possible exception of the deep sea, to be discussed 

 later in this section) : 



r ^1 photosynthesis i eaten by i ^ ^- • i l i,- 



energy of the sun 1 > plants ^ plant-eatmg animals or rierbivores 



I eaten by 



flesh-eating animals or carnivores 



This is an oversimplified diagram, of course. For instance, the omnivores, which 

 eat both plants and flesh, are not included. Neither are the detritus- or debris- 

 eaters which eat dead organic matter from the bottom. Furthermore, there are 

 many grades of carnivores, from the plankton feeders (whales, mackerels) to 



