UCLA

As human-induced global change impacts nearly every ecosystem worldwide, there is increasing concern for the degradation of foundation species. These species play key roles in ecosystem functioning by facilitating other species and supporting community structure through amelioration of harsh conditions and/or provision of habitat structure. Human alteration of forces controlling foundation species’ abundance and dominance, such as nutrient limitation and herbivory, can result in not only the decline of foundation species, but also a shift in spatial dominance to other, often less desirable species. Replacement species are often fast growing and ephemeral, but there are cases in which “secondary” foundation species establish and expand. Secondary foundation species are dependent on the original (primary) foundation species to establish and persist. I investigated the influence human manipulation of nutrients has on dominance of a primary (a temperate seagrass) and secondary (a coral reef macroalga) foundation species, and the possible consequences to trophic structure in two of the most threatened marine systems worldwide.

In chapter 1, I demonstrated that anthropogenic increases in nutrients can indirectly, through stimulating algal blooms, drive declines in a primary foundation species, the temperate seagrass, Zostera marina. It is widely recognized that these bloom-forming algal species tend to grow quickly in response to nutrients, and can negatively affect seagrasses by inhibiting light and changing flow and sediment conditions. However, the abundance of macroalgae that can be tolerated by seagrass beds and the possible impacts to epiphytes, which are an important trophic resource, are unclear. To address this knowledge gap, I conducted two caging experiments in a Z. marina bed in Bodega Harbor, California where I maintained six densities of Ulva and Gracilariopsis, two common bloom-forming algal species worldwide. Both species caused declines in seagrass health by decreasing shoot density and epiphyte abundance. Further, reduced epiphyte load suggests that not only the habitat structure afforded by Z. marina is degraded by macroalgal enrichment, but that there will be trophic consequences through a decrease in resources. Changes to bottom-up control cause significant impacts on this foundation species.

In chapters 2-3, I demonstrated that anthropogenic increases in nutrients may facilitate dominance by Turbinaria ornata, a secondary foundation species of brown algae that has been expanding both in range and habitat usage, likely at the expense of the primary foundation coral species. Coral reefs are typically overgrown by the same sorts of ephemeral, fast growing macroalgae commonly found in seagrass beds. In contrast, I established that T. ornata is a persistent foundation species as it facilitated epiphytes, other macroalgae, and herbivorous fish in a field study on fringing reefs in Mo’orea, French Polynesia. However, as T. ornata is dependent on the hard substrate created by corals to establish persistent aggregations, it is a secondary, not a primary foundation species, and shifts to this community will ultimately result in reef degradation. To understand mechanisms driving the expansion of T. ornata, I examined the role of nutrients and herbivory in facilitating T. ornata through a combination of field and mesocosm experiments. My results showed a unique interaction whereby enhanced physical defences with nutrient enrichment release T. ornata from herbivore control, which may allow expansion of T. ornata to habitats where it is usually controlled by high herbivory. Thus, on coral reefs in the South Pacific, I found a secondary macroalgal species was likely being indirectly facilitated by human impacts. In this case nutrient enrichment by humans caused an expansion of a secondary foundation species indirectly, through weakening of control by herbivory.

Overall, my results demonstrate that human alterations of nutrient supplies can cause both degradation of primary and facilitation of secondary foundation species. In some cases, the facilitation of a secondary foundation species may result in direct replacement of primary foundation species, which may prove to be difficult to reverse, as these species are, by definition, more persistent once established.