94 



ECOLOGY 



Part II 



Fig. 6.3. Cooperation. Beavers' lodge and winter food storage — a community 

 project. The lodge and passageway to the pond bottom are shown as if cut open 

 and the ice bound pond as if in section. The two beavers working below the water 

 line must frequently come up for air. (Courtesy, Hamilton: American Mammals. 

 New York, McGraw-Hill Book Co., Inc., 1939.) 



CoMMENSALiSM. Meaning at the same table, commensalism was originally 

 applied only to sharing the same food. It is now used for neutral associations 

 which do not seem to affect either partner. A classic example is the sea 

 anemone that rides about on the shell of the hermit crab and thereby gains 

 wider range for forage, but does not eat the same kind of food as its host. Less 

 familiar is the mahout beetle that rides on the head of a worker termite and 

 takes bits of food as it is passed from one termite to another (Fig. 6.4). 



Mutualism. A symbiosis that benefits each partner is mutualism. Honey- 

 bees and many flowering plants aid one another to the point of dependence. 

 Honeybees eat nothing but flower products. And as they collect the nectar 

 and pollen they distribute the latter, usually to flowers of the same kind be- 

 cause they grow together. Thus the bees cross-pollinate them. Many flowers 

 are so formed that they can be pollinated only by insects. In nature the yucca 

 lily (Spanish bayonet) and the yucca moth (Pronuba) are entirely dependent 

 upon each other (Fig. 6.5). The lily is pollinated by the moth, which thrusts 

 a blob of pollen onto the pistil. Thus she effects the fertilization of the ovules 

 and then lays her eggs in the ovary where the larvae can feed on the ovules. 

 The plant does not suffer, for more seeds develop than are eaten by the larvae 

 of the Pronuba. Yucca lilies are native to southern North America but are 

 cultivated farther north, since they are easily pollinated by hand. 



