300 LIVES OF PLANTS AND ANIMALS 



food for other plants lacking chloroplr^ll, while some algae and 

 fungi actually live together in such a way as to be of mutual benefit 

 to each other. The larger plants may shelter the smaller ones, 

 protecting them from wind and storm, while the trees provide 

 humus which holds the moisture in the ground, giving it off slowly 

 to other plants. Animals scatter seeds far and wide, and man may 

 even start entire colonies in new localities. 



Practical Exercise 12. Describe some plant or animal community you 

 have seen. What forms of life are associated together? 



Could you have a plant community in the laboratory or school yard ? What 

 conditions would you expect to find? What plants Living together? 



Self-Testixg Exercise 



Conditions of (1) (2) (3) and 



(4) are the chief factors which (5) what plants and animals 



will live together in (6). Life in such (7) is not 



all (8) but a mutual give and take. The animals and plants 



best (9) to live under such (10) crowd out the 



(11). 



PROBLEM VII. WHAT IS AN ECOLOGICAL SUCCESSION? 



Changes in environment cause changes in life. Changes are 

 always taking place in plant and animal communities. Some- 

 times these changes are brought about artificially, as when a forest 

 fire sweeps a country or man introduces water by irrigation into a 

 desert region. But always there are changes going on, which 

 cause plant and animal associations to change in a given locality 

 and often to move to new localities. Most of these changes are 

 very slow, so that we rarely notice them. Here is an example 

 quoted by Elton : A hole in a beech tree was first used by an owl 

 as a nest ; then with the growth of the tree the hole became 

 smaller and was used by starlings. Later it became too small for 

 them to enter, and the hollow was filled by a wasps' nest. 



How plants invade new areas. New areas are tenanted by 

 plants in a similar manner. Alter the burning over of a forest, 

 we find a new generation of plants springing up, often quite unlike 

 the former occupants of the soil. First come the fireweed and 



