Seed dispersal but not seed germination facilitated by seabirds: seed ecology of Cook’s scurvy grass

Lepidium oleraceum (Brassicaceae) is a threatened New Zealand plant closely associated with seabird colony environments. We hypothesise that this spatial coincidence arises from seabird dispersal of L. oleraceum seed and/or guano stimulating germination. To test the possibility of seabird epizoochory, we used laboratory studies to examine L. oleraceum seed adhesion and tolerance to seawater immersion. To test for stimulation of germination by guano we measured seed germination in increasing concentrations of seabird guano or inorganic fertiliser (to separate a nutrient effect from a possible effect from some other unknown constituent). Seeds developed a sticky mucilage layer after 50 seconds in freshwater and 50% were able to adhere to dry paper for over 24 months. A higher proportion of seeds germinated after soaking in freshwater (91% ± 3.1%) than seawater (44% ± 4.9%), and seawater germination was comparable to unsoaked controls (46% ± 3.1%). A comparison of germination rates under different concentrations of guano and fertiliser showed no significant differences associated with treatments except germination was inhibited above 1 gNL-1. We conclude that seabirds are probable dispersers of L. oleraceum seed through external attachment, but germination is not promoted by guano presence