The ecology of Cook’s scurvy grass (Lepidium oleraceum s.s.) and its relationship with seabirds

Abstract

Seabird colonies influence surrounding vegetation through nutrient enrichment, disturbance and seed dispersal which favours ornithocoprophilous plants. One New Zealand endemic seabird-associated plant is Lepidium oleraceum s.s, a threatened coastal cress which has experienced on-going decline for at least the last century. One explanation for decline is reduced suitable habitat due to declines in seabirds. This thesis aims to understand the general ecology and investigate whether seabird declines are a likely driver of L. oleraceum declines. Wild L. oleraceum populations on Stephens, North Brother, Matariki and Mahuki Islands were studied to look at habitat characteristics, plant characteristics and population distribution, dynamics and structure. Associated vegetation was determined based on herbarium records, island visits and vegetation plots. Soil samples were collected adjacent to L. oleraceum individuals to look at the composition and distribution of the soil seed bank. Germination experiments in relation to salt and nutrients were undertaken along with tests involving seed mucilage. The impact of guano and salt spray on growth was tested with shade house experiments. Lepidium oleraceum occurs on seabird-enriched soils and close to seabird burrows but consistently separated from gannet nests. Population fluctuations did not relate to climatic variables so the decline observed on Mahuki may relate to rodent eradication. Each island had populations with seedling recruitment, and some populations seemed to develop from local founders. Associated plants in the vegetation and seed bank were either halophytes or ruderal exotics and the genera Coprosma, Disphyma, Senecio and Bromus were most common. Lepidium oleraceum seed generally occurred within 0.5m of “parent” plants. Concentrated guano inhibited germination and seed retained viability after four weeks in saltwater. Seed mucilage developed in five minutes, dried after 20 minutes and adhered for days, indicating external dispersal by seabirds is possible. Increasing guano resulted in elevated growth and flowering up to a critical concentration, after which it had little effect because excess nutrients were inhibitory. Twelve hours of salt spray “storm” did not impact growth but caused stress-induced flowering. Overall this indicates seabirds are important to the ecology of L. oleraceum and its decline is likely related to concurrent declines in seabirds.