1974 ] 
Shapiro — Pieris napi Populations 
365 
of niche differentiation, competitive exclusion, and species packing. 
Emmel (1974) reported three species of inflorescence feeders ( Pieris 
protodice Boisduval and LeConte, A. sara and A. lanceolata ) on 
Arabis glabra in Riverside County, 2 June 1973. Shapiro (1974) 
described five- and six-species Pierid assemblages in two high Sierran 
localities and concluded that competition is reduced by behavioral 
mechanisms (habitat selection) . 
Pieris napi occurs with up to six other Crucifer-feeding Pierids 
at the four localities described above. Based on adult collections, the 
Crucifer-feeding Pierid faunas of the four localities are (inflorescence 
feeders are marked “I”; others primarily leaf feeders) : 
San Andreas Reservoir: P. napi, P. rapae , A. sara (I), Euchloe 
ausonides Lucas ( I ) . 
Mix and Gates Canyons: P. napi , P. rapae, P. sisymbrii Boisduval 
( I ? ) , A . sara ( I ) , E. ausonides ( I ) . 
Washington: P. napi, P. rapae, A. sara (I), A. lanceolata (I), 
E. ausonides (I) . 
Lang Crossing: P. napi , P. rapae, P. sisymbrii (I?), A. sara (I), 
A. lanceolata (I), E. ausonides (I), E. hyantis Edwards (I). 
Not all of these breed in the same microhabitats. At Lang Cross- 
ing, for example, P. sisymbrii and E. hyantis are found only in 
exposed rocky situations and appear to breed only on Streptanthus, 
and are thus not in competition with the woodland species. In 
examining the fauna of particular plants in particular habitats, the 
division of Pierids into a leaf-feeding and an inflorescence/silique 
feeding guild seems paramount. At Mix and Gates Canyons, where 
Barb area is obviously in “short supply” and frequently completely 
defoliated, at least one species from each guild (P. napi, A. sara) 
can occur on this plant. At Lang Crossing the combined visible 
impact of four species — two of each guild — on A rabis glabra is 
so small that it is tempting to speculate that the populations are 
regulated by other factors below the level at which interspecific 
competition would be significant. 
Despite the potential for interspecific competition, at each of the 
study areas one plant received the bulk of the attention from Pierids 
while another appeared largely or wholly unutilized. It remains to 
be seen whether this reflects nutritional or toxicological unsuitability 
of certain Crucifer species. (In unpublished laboratory studies, 
Shapiro and F. Slansky ( pers . comm.) have found variation in the 
suitability of native and weedy Crucifers as hosts of Pieris rapae 
