Effects of invasive rats and burrowing seabirds on seeds and seedlings on New Zealand islands

Rats (Rattus rattus, Rattus norvegicus, Rattusexulans) are important invaders on islands. They altervegetation indirectly by preying on burrowing seabirds.These seabirds affect vegetation through nutrient inputsfrom sea to land and physical disturbance through tramplingand burrowing. Rats also directly affect vegetationthough consumption of seeds and seedlings. Seedlingcommunities on northern New Zealand islands differ incomposition and densities among islands which have neverbeen invaded by rats, are currently invaded by rats, or fromwhich rats have been eradicated. We conducted experimentalinvestigations to determine the mechanisms drivingthese patterns. When the physical disturbance of seabirdswas removed, in soils collected from islands and insideexclosures, seedling densities increased with seabird burrowdensity. For example, seedling densities inside exclosureswere 10 times greater than those outside. Thus thenegative effects of seabirds on seedlings, by trampling anduprooting, overwhelm the potentially beneficial effects ofhigh levels of seed germination, seedling emergence, andpossibly seed production, which result from seed burial andnutrient additions. Potential seedling density was reducedon an island where rats were present, germination of seedsfrom soils of this island was approximately half that foundon other islands, but on this island seedling density insideexclosures was 7 times the density outside. Although thetotal negative effects of seabirds and rats on seedlingdensities are similar (reduced seedling density), the differencesin mechanisms and life stages affected result invery different filters on the plant community.