SELECTIVE ADVANTAGES TO HEMI-PARASITIC ANNUALS, GENUS MELAMPYRUM, OF A SEED-DISPERSAL MUTUALISM INVOLVING ANTS .1. FAVORABLE NEST SITES

Directed dispersal to nest sites that are favorable in several possible regards may be an important factor in the maintenance and, by inference, the evolution of myr-mecochory in Melampyrum. Results of a field study in oak-pine woodlands of the Keweenaw Peninsula of northern Michigan, USA support such a hypothesis. Diaspores were set out at grid points on the forest floor and monitored for ant removal. Nest sites (n=47), which were otherwise undetectable, were located and marked by tracing diaspore-carrying ants to their destination. Additional diaspores were placed at each nest site and then observed until all (n=20) were taken into the nest. Ant-planted diaspores of M. lineare exhibited higher germination success (p<0.0025), and resulted in greater survival to reproduction (p<0.0025) and fecundity (0.005<p<0.01) than randomly planted seeds. Microsites that ants used for diaspore placement had significantly higher light intensities than random microsites. Considering that published reports cite high light intensity as the primary factor benefiting survival and fecundity in M. lineare, this is one means by which ants confer a selective advantage. Effect of cover type chosen by ants for seed burial was also studied. Preferential placement in reindeer lichen microsites (Cladina spp. or Cladonia spp. had 38.3% of ant plantings) was associated with enhanced germination, survival, and fecundity. Since lichen patches often remain unburned during fire events, the study implies that directed dispersal to these microsites plays the additional role of allowing M. lineare to escape destruction by fire. Melampyrum lineare has a dispersal distance curve that resembles that of other myrmecochores, but mean (1.08 m) and range (< 4.5 m) distances are even shorter, suggesting that safe sites for this plant are abundantly available.