Recent evidence has suggested that clades of dioecious angiosperms have few
er extant species on average than those of cosexual (hermaphroditic and mon
oecious) relatives. Reasons for the decrease in speciation rates and/or inc
rease in extinction rates are only beginning to be investigated. One possib
ility is that dioecious species suffer a competitive disadvantage with cose
xuals because only half of the individuals in a dioecious population are se
ed bearing. When only females produce seed, offspring will be more spatiall
y clumped and will experience more local resource competition than when eve
ry individual produces seed. We examine two spatially explicit models to de
termine the effect of a reduction in seed dispersers on the invasibility an
d persistence of dioecious populations. Even though dioecious females were
allowed to produce twice as many seeds as cosexuals, our results show that
a reduction in the number of seed dispersers causes a decrease in the abili
ty of dioecious progeny to find uninhabited sites, thus reducing persistenc
e times. These results suggest that the maintenance of dioecy in the presen
ce of hermaphroditic competitors requires a substantial increase in relativ
e fitness and/or a large dispersal advantage of dioecious seeds.