402 



POPULATIONS 



8 mm.- 



4 mm- 



FIRST 

 SECOND 



% CLEAVED 



FIRST — 

 SECOND- 



7oCLEAVED- 



■ 52.25- 

 80.50- 

 -99- 



-58.25-60.25 

 -85.83-90.25 

 -99 100 



Fig. 140. The effect of crowding on the 

 rate of cleavage of the eggs of the sea urchin. 

 Arbacia. Figures below the diagrams, unless 

 otherwise indicated, give time in minutes. 

 ( Modified from Allee and Evans. ) 



had liigh statistical probability of being 

 real. Essentially similar results have been 

 reported for four other genera of sea ur- 

 chins. An optimal population size for 

 rapidity of early cleavage of sea urchin 

 eggs is clearly demonstrated; the exact 

 number of eggs in such a population de- 

 pends on many variables. Similar relations 

 hold in populations of frogs' eggs (Mer- 

 win, 1945), and in a related field, optimal 

 populations, somewhat larger than the pos- 

 sible minimum, are well established for 

 asexual reproduction in Protozoa (p. 357) 

 (Robertson, 1927; Petersen, 1929; Cause, 

 1934; Kidder, 1941; Johnson, 1941; and 

 Mast and Pace, 1946). 



The rate of reproduction is relatively 

 low in oversmall colonies of several species 

 of birds. Small collections of terns are less 

 successful than larger ones, perhaps be- 

 cause a tern colony must reach a consider- 

 able size before the birds can form a mob 

 large and active enough to frighten ma- 

 rauding gulls away from their nests. Dar- 

 ling (1938) has focussed attention on the 

 possible functional significance of the num- 

 bers present in breeding groups of birds by 

 his observations on herring gulls (Lams a. 

 argentatus) and lesser black-backed gulls 

 (L. fiiscus affinis). Darling recognizes 



three effects of colony size upon the breed- 

 ing activities of these birds. The larger 

 colonies show an earlier onset of laying, a 

 greater synchronization of breeding, and a 

 higher reproductive success. He and others 

 have found that oversmall groups of cer- 

 tain colonial nesting birds do not breed at 

 all. 



Small numbers in a breeding colony of 

 gannets {Sula hassana) result in abnor- 

 mally ineflBcient breeding (Fisher and 

 Vevers, 1944). There is an average of over 

 four years' time after a new breeding lo- 

 caUty of fulmars {Fulmarus g. glacialis) 

 has been first populated before egg laying 

 occurs (Fisher and Waterson, 1941). The 

 possibility that a minimum threshold of 

 numbers is necessary for producing off- 

 spring is suggested by these data. Larger 

 colonies of yellow-headed blackbirds (Xan- 

 thocephalus xanthocephalus) have a higher 

 percentage of reproductive success than 

 smaller ones in the same region (Fautin, 

 1941). 



Not all birds show these tendencies. Pen- 

 guin colonies (Pijgoscelis papua and 

 Eudyptes cristatus), ranging in size from 

 thirty to about 800,000 pairs, gave no con- 

 clusive evidence of earlier egg-laying or 

 of a shorter egg-laying span in the larger 

 when compared with smaller colonies (Rob- 

 erts, 1940). An intensive study of the 

 eastern red-wing (Agelahis p. phoeniceus) 

 yielded only limited indications of group 

 relations in their breeding colonies com- 

 parable to those reported by Darling for 

 terns and gulls (H. M. Smith, 1943). 



The population density at which adverse 

 effects of undercrowding occur varies 

 greatly in different animals and in different 

 habitats for the same species. It is low with 

 bobwhite quail, since these birds have high 

 ability to announce their presence to others 

 of their kind. The situation is different for 

 the muskrat {Ondatra zibethicus). One 

 creek that Errington (1940, 1943) observed 

 in Iowa has a carrying capacity of three 

 to four pairs of muskrats per mile. Over- 

 crowding becomes apparent above this 

 population density. In this instance the 

 maximal rate of increase was attained by 

 two to three pairs per mile. In regions in 

 which muskrats are new invaders, there is 

 often a lag of some years before the full 

 reproductive rate is reached, even though 

 no observable changes occur other than 

 those brought about by muskrat occupancy. 



