56 U. S. BUEEAU OF VISHERIES. 



oscillation of only 1 foot twice repeated during the summer of 1919 

 produced an appreciable weakening effect on the vegetation of a pond 

 near Media.'^ 



Among information and opinions furnished by others, an excerpt 

 is quoted with permission from a letter from Clyde B. Terrell, who 

 has had much experience in developing aquatic game preserves : 



Where it is possible tu periodically lower and raise the water level, as you 

 mentioned in your letter, this is possibly as good a method of destroying the 

 vegetation as any. In our work of planting feeding grounds for ducks, fish, 

 and game we have often had to remove undesirable growths to make room 

 for tlie more desirable plants to provide food. * * * j know from personal 

 experience that it is difhcult to make any kind of vegetation grow where tlie 

 water is constantly raised and lowered, especially if one can effect the change 

 of deptli of water of 8 or 10 feet. * * * x have noticed that the establisli- 

 ment of aquatic growth was difficult where the water was raised and lowered 

 by dams. I have two places in mind, one on the Coosa River, near Talladega 

 Springs, Ala., and the other at Lake Delta, near Rome, N. Y. 



The ecological principles involved are manifest. The fixed pond 

 margin flora is a complex made up of three principal associations, the 

 submerged aquatic, the aquatic emergent, and the landward marsh 

 plants. These are in constant and severe competition, but as each 

 (species grows best under certain optimum conditions of depth, bot- 

 tom, soil, etc., the zones which they occupy remain clearly defined 

 so long as the level remains constant, but when the water level fluc- 

 tuates these optimum conditions shift and the zones become ill defined 

 and overlapping. If the level be varied at such periods that optimum 

 conditions for few or none of the plants are maintained for a time 

 sufficiently long to enable them to become well established at the new 

 levels, not only is competition increased, but nearly every component 

 of the flora is weakened by unfavorable conditions of changing 

 severity. It is to be expected, therefore, that the flora as an aggregate 

 will deteriorate. The details of the process are familiar to every 

 field biologist and need not be dwelt upon here. 



Inasmuch as different species of plants differ greatly in their 

 adaptability and powers of resistance to this treatment, what is most 

 needed is tliat the frequency, length, and season of the periods neces- 

 sary to effective control of particular associations and species should 

 be worked out experimentally. A few long periods of considerable 

 amplitude of change will prove more effective than many short ones 

 of little change as the adaptive factor of safety of most plants will 

 carry them through the latter. A whole association of plants has 

 become adapted to and thrives under diurnal tidal fluctuations. But 

 there is an experimentally determinable limit of endurance. The 

 more the physiological factor of safety is strained the more the plant 

 will suffer. What is harmful to one type of plant will prove bene- 

 ficial to another, and reversal of conditions should take place before 

 the replacing types have become well established. These compensat- 

 ing effects will prove the best index for determining the number and 

 lengih of ]ieriocls and the range of oscillation necessary to secure the 

 most complete control. Probably some plants, especially perennials, 

 will require supplementary measures. 



If persons desirous of effecting mosquito control in ponds by means 

 of fishes would employ this method wherever posf5ible, a suitabh^ 



* In a paper publishod while this report was in press Ileadlee (1921, p. 172) reconi- 

 meuds dropping the level of water about 12 inches in .liine and returninc: it to the former 

 level at the close of the season as a means of opening the plant barrier to fishes. 



