Spatial and temporal patterns of predation on seeds of the surfgrass Phyllospadix torreyi

Spatial and temporal patterns of predation on seeds of the seagrass Phyllos padix torreyi S. Watson were quantified at four sites near Santa Barbara, C alifornia, USA. Over a period of four flowering seasons during 1995 to 1998 , monthly patterns of seed fall and intensity of seed predation were simila r among sites, but were temporally quite variable. Abundance of dispersed s eeds varied greatly both among seasons and years. Within any one year, seed s were present in the environment every month, but they peaked in abundance during the fall months following the annual flowering period. Seeds were m ore abundant during the earlier years of the study. The intensity of predat ion steadily increased throughout the study period, from a low of less than or equal to 10% seeds consumed during 1995 to similar to 50% consumption b y 1997, and it was not correlated with abundance of seeds in the environmen t. Pre-dispersal seed loss also was estimated in two flowering seasons by c ounting the numbers of seeds consumed prior to release from the plant, and was relatively low (< 15%). No differences were detected among the four stu dy sites in patterns of pre-dispersal loss. Field surveys were done at two sites to identify potential seed predators. Three of the most abundant spec ies identified in these surveys were tested in the laboratory to determine size-specific patterns of feeding activity and to assess which early life s tages of P. torreyi (fruits within spadices, dehisced fruits, seedlings) we re most vulnerable to predation. All three of the early life stages were co nsumed. The crabs Pugettia producta (Randall) and Pachygrapsus crassipes Ra ndall were much more voracious predators than the isopod Idotea resecata St impson.