Competition between mutualists : the role of differential flower abscission in yuccas

Competition among yucca moths occurs because retention of flowers is resource limited: the more flowers that are visited the lower the probability that a visited flower will become a fruit. However, an unusual mechanism causes asymmetric competition between two species of yucca moths, with the asymmetry being based upon differential retention/abscission of flowers. Flowers containing eggs of yucca moths that oviposit through the carpel wall ( = deeps) are more likely to abscise when other flowers on the same plant contain only the eggs of yucca moths that oviposit into the surface of the carpel wall ( = shallows). The advantage of shallows over deeps suggests that shallows should be able to invade sites occupied by deeps, and outcompete deeps. However, proportions of deeps and shallows on Yucca kanabensis have been relatively stable for the last 12 years, and shallows have failed to invade sites with only deeps. We examined four factors that could affect the ability of deeps to coexist with shallows. 1) The competitive advantage of shallows over deeps was greatest on plants with intermediate levels of visitation. With low visitation few pollinated flowers abscised, competition was weak, and the differential success of shallows was weak. With high visitation intra- and interspecific competition was strong, but the differential success was low because most flowers received eggs from both deeps and shallows. 2) There was a partial seasonal segregation of deeps and shallows, with deeps emerging earlier than shallows, thereby increasing the probability of some deeps occurring on plants with relatively few shallows. 3) Although independent aggregation of deeps and shallows at the level of plants could promote coexistence by generating plants where deeps encounter few shallows, association of deeps and shallows was positive. 4) Deeps aggregated among flowers within plants much less than did shallows, which decreases the number of flowers with only shallow ovipositions and decreases the opportunity for asymmetric competition.