Causes and consequences of outbreaks of a nonindigenous species in kelp beds.

摘要

In rocky subtidal ecosystems in the western North Atlantic, high abundances of a nonindigenous epiphytic bryozoan (Membranipora membranacea) exacerbate seasonal defoliation of kelps. By removing the competitively dominant species, outbreaks of M. membranacea facilitate the invasion of other benthic organisms. The primary objective of this thesis is to identify and examine factors controlling the population dynamics of M. membranacea. First, spatial and temporal patterns in key population metrics (e.g. settlement, recruitment, growth, colony abundance, and colony percent cover on each of 3 species of kelp, as well as larval distributions in the water column) were quantified in relation to physical factors, particularly temperature. Second, to predict how variations in temperature affect population dynamics, relationships between temperature and each of settlement and growth were incorporated into an individual-based model. Larval abundance was higher in the surface mixed-layer than below the pycnocline in a stratified water column, suggesting shoreward transport of larvae concurrent with downwelling. Settlement and recruitment were an order of magnitude higher following an unusually warm winter, and were associated with a 70% decrease in canopy cover of kelp the following autumn. Slight differences in temporal and spatial patterns of colony cover among the 3 kelp species were attributed to species-specific variation in both bryozoan settlement and the robustness of kelp blades. Bryozoan growth rates increased with colony size and temperature, and were higher in the field than in the laboratory. In a modeling simulation, the timing of onset of population growth increased linearly, and colony abundance and cover on kelp blades increased non-linearly, each with increases in temperature. Temperature in summer had the greatest effect on bryozoan coverage on kelp blades, whereas temperature in winter had the greatest effect on the timing of onset of population growth. The results indicate that through a combination of increased and earlier larval supply, settlement and recruitment, and faster colony growth, outbreaks of M. membranacea may increase in frequency and intensity as a consequence of warming water temperatures. Through the associated loss of kelp, this may have considerable implications for shallow rocky subtidal communities.