Sargassum horneri is a seaweed native to eastern Asia that has recently become established in the coastal waters of southern California and Baja California, Mexico. Currently, little is known about the biology and ecology of this invasive alga in its introduced range. In this dissertation and associated manuscripts, I provide a synopsis of S. horneri distribution, biology and ecology including: invasion chronology, local abundance, life history and dispersal, mechanisms of invasion, impact on native species, and potential for control. In Chapter 1, I present a chronology of the rapid geographic expansion of S. horneri in the eastern Pacific, which I generated from observations of its presence or absence from agency and university scientists in the region. Santa Catalina Island in southern California was one of the first sites of invasion, and the study site I selected for the remainder of my dissertation. In Chapter 2, I use monthly to quarterly surveys over two years to describe the life history of S. horneri and quantify traits capable of promoting its spread and persistence, including habitat affinity, reproductive characteristics, and seasonality in abundance and reproduction. In Chapter 3, I perform grazing assays and examine patterns of co-occurrence between S. horneri and resident algae to evaluate the potential influence of competition, complementarity and herbivory in controlling its invasion. In Chapter 4, I investigate whether S. horneri invasion is a cause or a consequence of ecological change by testing whether disturbance facilitates its colonization and the establishment of reproductive adults, and how its removal influences the biomass and community structure of algal assemblages. In Chapter 5, I assess the feasibility of controlling the spread of S. horneri by investigating the effectiveness of local removal in reducing its population growth and capacity to regenerate from remnant holdfasts. I also quantify the effort required to remove established populations with and without the aid of an underwater suction device.

A general conclusion emerging from this work is that S. horneri is an opportunistic species, but not necessarily a strong competitor. I identified several life history characteristics that likely facilitate its invasion, including a phenology that appears to be offset from most native macroalgae that could minimize competition; the ability to use a wide range of depths and substrates that allows S. horneri to colonize many different environments; high fecundity and local dispersal that promote the persistence of new populations through multiple generations; occupancy of resources underutilized by native species in space and time; and increased deterrence to herbivory relative to native kelps. Colonization and establishment of S. horneri were unaffected by disturbance to the benthic algal community, indicating S. horneri was not limited by space on the reef. Resident algal assemblages likewise did not respond to S. horneri removal except during one year when it achieved extremely high abundance. I found no evidence that S. horneri regenerates from remnant holdfasts, and that removal of adults prior to reproduction can lead to a partial reduction in the subsequent generation relative to unmanipulated areas.

This research provides the first comprehensive assessment of Sargassum horneri distribution, biology and ecology and explains several life history characteristics and ecological interactions that have contributed to its invasion success in the eastern Pacific.