The consequences of dioecy for seed dispersal: Modeling the seed-shadow handicap

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.