Distribution and Reproductive Characteristics of Nonindigenous and Invasive Marine Algae in the Hawaiian Islands

Abstract

Quantitative and qualitative surveys were conducted on five of the main Hawaiian Islands to determine the current distribution of nonindigenous algae and to assess the level of impact that these algal species pose to Hawai'i's marine ecosystems. Maps were generated to examine the spread of these organisms from initial sites of introduction and to assimilate information regarding habitat characteristics that appear to make some sites more susceptible to invasion than others. Blooms of native invasive algae were also documented when encountered. The potential for vegetative propagation via fragmentation was examined experimentally as a mode of reproduction for four of the most common species of nonindigenous algae in Hawai'i. This research has demonstrated that each of these algal species currently has a distinctive distribution and reproductive strategies appear to vary among species. More research is needed to further understand the competitive strategies and unique ecological characteristics that allow these nonindigenous species to become highly successful in the Hawaiian Islands.

Healthy coral reef ecosystems are often dominated by reef-building corals and coralline algae, with macroalgae and algal turfs typically restricted to areas of reefs that are relatively less accessible to herbivores. On reefs subjected to anthropogenic disturbances such as increased terrestrial nutrients or the removal of grazers, however, algal growth rates may exceed grazing rates and result in overgrowth of corals and other benthic invertebrates (Hatcher and Larkum 1983, Littler and Littler 1984, Steven and Larkum 1993, Smith et al. 2001, Stimson et al. 2001). The long-term consequences of these phase shifts from coral to algal dominance may include the loss of biodiversity, a decrease in the intrinsic value of the reef, changes in the community structure of the reef fishes dependent upon corals for habitat and shelter, and erosion of the physical structure of the reef (Hughes 1994). Phase shifts involving both indigenous and nonindigenous algae have been documented in Hawai'i but have not been thoroughly studied. Thus, documenting the nature and characteristics of these problems before invasive algal species become ecological dominants on Hawai'i's reefs is crucial.

Blooms of both indigenous and nonindigenous marine algae have become common in the Hawaiian Islands over the last several decades (Russell 1987, 1992, Stimson et al. 1996, Rodgers and Cox 1998). In tropical regions, blooms of indigenous algae have often been tied to reductions in grazing intensity and increases in anthropogenically derived nutrient levels (Miller et al. 1999, McClanahan et al. 2001, McCook et al. 2001, Smith et al. 2001, Stimson et al. 2001, Thacker et al. 2001). However, the mechanisms driving the abundance and success of nonindigenous algae worldwide remain unclear and may be the result of a number of interacting factors.

The introduction of nonindigenous algae in the marine environment has been, and continues to be, a devastating issue in relation to the health and stability of nearshore ecosystems. The introduction and impacts of nonindigenous algae such as Caulerpa taxifolia (Vahl) C. Agardh in the Mediterranean, Codium fragile (Sur.) Hariot subsp. tomentosoides (van Goor) Silva in New England and New Zealand, Sargassum muticum (Yendo) Fensholt in Europe and Mexico, and Undaria pinnatifida (Harvey) Suringar in Australia, New Zealand, and Europe have been widely documented (Hanisak 1980, Carlton and Scanlon 1985, Espinoza 1990, Meinesz et al. 1993, Trowbridge 1995, Bellan-Santini et al. 1996, Critchley et al. 1997, Ferrer et al. 1997, Andrew and Viejo 1998, Campbell and Burridge 1998, Curiel et al. 1998, Karlsson and Loo 1999, Stuart et al. 1999, Schaffelke et al. 2000). In the Tropics, nonindigenous marine plants pose threats to both coral-dominated habitats and sea grass beds and have the potential to reduce biodiversity and substantially alter the structure of reef ecosystems (Maragos et al. 1996, Critchley et al. 1997, Den Hartog 1997).

The success of these nonindigenous algae may be the result of a variety of factors including chemical or physical defense from herbivory and diverse physiological characteristics that lead to rapid growth rates (Borowitzka 1981, Duffy and Hay 1990, Holmlund et al. 1990, Hay et al. 1994...