THE RELATIONSHIP BETWEEN POPULATION-DENSITY AND BODY-SIZE - THE ROLE OF EXTINCTION AND MOBILITY

The relationship between abundance and body size is the subject of con siderable debate in ecology. Here we present new data on the relations hip between population density and body mass for flightless birds. Unl ike flying birds, flightless birds (and mammals) show a strong negativ e relationship between population density and body mass. Density decre ases as the -1.40 power of body mass, which is significantly steeper t han in flying birds. This difference is mainly due to a real absence o f small species with low population densities in flightless birds. Sma ll populations of small-bodied species may not be able to persist unle ss sustained by immigration. The high mobility of flying birds may all ow even small local populations of small-bodied species to persist thr ough supply of immigrants, and thus can explain the weak relationship between abundance and body size for this group compared to that of fli ghtless birds (and mammals).