554 



THE COMMUNITY 



particular species, gives a partial picture 

 of total activity at the community level. 

 This is dramatic in the tropical rain forest, 

 where the volume of sound at dawn and 

 dusk, when the nocturnal and diurnal 

 faunas are in the process of alternately ex- 

 changing roles, is both varied and relatively 

 great (Figs. 189 and 190). 



Although the crepuscular flashing of fire- 

 flies (Craig, 1917; Hess, 1920; Buck, 1935, 

 1937, 1937a), the chorus of howler mon- 

 keys (C. R. Carpenter, 1934), and the shrill 

 stridulations of seventeen-year cicadas and 

 of tree crickets (Frost, 1942) appear to be 

 integrated responses of bioluminescence or 

 sonification, the integration apparently is 

 supplied by the external environment. In 

 these examples of group activity a given 

 sequence of flashes, or of sound production, 

 may be initiated on the stimulus supplied 

 by a leader, but there is no evidence that 

 the animals coordinate to join their fellows 

 in activity as a response, primarily to an in- 

 ner, or endogenous, rhythm. The point is 

 important in the sense that the initial stim- 

 ulus, to a leaderless group, or to the group 

 leader is, manifested in the environment, and 

 hence represents a community coordination. 



Vertical diel migrations of animals 

 through the strata of communities are 

 among the best-substantiated phenomena of 

 community activity. Such migrations take 

 place in marine, fresh-water, and terrestrial 

 communities. As we have seen, they are 

 subject to seasonal and local weather con- 

 ditions, and, with few exceptions, the fre- 

 quency and amplitude of such a vertical 

 movement is under the control of the oper- 

 ating diel factor complex. 



Where they have been investigated, the 

 majority of pelagic marine animals per- 

 form periodic vertical migrations. These in- 

 volve an upward movement from deeper 

 water at the approach of evening, and a 

 downward movement from the surface to 

 deeper water just before, or at, sunrise. The 

 influences regarded as chiefly responsible 

 for these rhythmic migrations are ( 1 ) light, 

 (2) temperature, (3) hunger, (4) salinity, 

 (5) gravity. Light is regarded as the chief 

 motivating factor, but there is doubt 

 whether this influence operates through its 

 change in intensity, or by its absolute in- 

 tensity (Clarke, 1933; 'Johnson, 1938). 

 There is also the possibility that light oper- 

 ates indirectly, by modification of the re- 



sponse to such factors as gravity, food, and 

 temperature. Upward movement may be a 

 searcliing reaction of animals that feed 

 upon the phytoplankton. These latter are 

 concentrated in the euphotic layer, and 

 such movements have been regarded pri- 

 marily as food-procuring reactions in 

 fresh-water zooplankton, at times of 

 the diel cycle when light intensities are not 

 prohibitive (Worthington, 1931), Again, 

 gravity is a relatively invariable factor that 

 operates constantly upon plankton, and we 

 have noted numerous devices employed by 

 aquatic organisms to offset or minimize this 

 force. Swimming upward is a reaction often 

 correlated with negative geotaxis, and 

 it has been suggested that light intensity 

 may be the stimulus that affects a change 

 in sign of geotaxis (Esterly, 1917; Clarke, 

 1934). 



Vertical movements may be regarded as 

 complex shifts in population densities with- 

 in the community, motivated internally 

 by hunger, and controlled directly or 

 indirectly by the periodic rhythm in fight. 

 In general terms, planktonic animals sink, 

 or swim downward, when the intensity of 

 light, or the change in intensity, is such 

 that the organismal response is geopositive. 

 At depths, with gradual weakening of the 

 stimulus of light, a threshold is reached. 

 Here the downward movement is arrested, 

 and a maximum diurnal population density 

 occurs, usually at the middle of the day. At 

 the end of the day the reverse sequence of 

 phenomena occurs. Lowering light intensity 

 reverses the geotactic reaction, and the pop- 

 ulation swims upward. Feeding usually 

 takes place at or relatively near the sea sur- 

 face, and there is a maximum nocturnal 

 population density usually near the middle 

 of the night. This sequence of events is il- 

 lustrated in Figure 195, in which diel move- 

 ments are stylized for three North Atlantic 

 plankton species, a medusa (Cosmetira 

 pilosella), a copepod (Calanus finmarchi- 

 ctis), and a mysid crustacean {Leptomijsis 

 gracilis). Such movements in zooplankton 

 involve nocturnal feeding upon the phy- 

 toplankton that inhabit the surface stratum; 

 in turn, larger nektonic animals, like the 

 herring, feed upon zooplankton. 



In such vertical movements the upper 

 component may encounter a sharp tempera- 

 ture gradient, or thermocline, that may pre- 

 vent further ascent (Sverdrup, Johnson, 



