Global Defaunation and Plant Invasion: cascading effects on seagrass ecosystem services
Ecosystems have been providing our society with useful services to sustain us in our livelihood, survival and health for as long as we exist. Seagrass ecosystems are especially successful in carbon storage, sediment stabilization, and providing food and habitat for fauna (Nordlund et al., 2016). It is important to conserve these ecosystems in order to maintain its high value.
Human impact is changing the seagrass landscape. One of the biggest impacts of humans on the marine ecosystem is defaunation, the removal of (predatory) fish and large herbivores such as manatees and turtles, that has been ongoing for the past centuries. These large vertebrates have a big impact – either direct or indirect – on foundation species. For instance, moderate turtle grazing can increase plant productivity, but overgrazing by turtles can lead to a collapse of a seagrass meadows (Christianen et al., 2014). In case of an abundant herbivore community, sharks can come to the rescue and either prey on the turtles or create ‘landscapes of fear’ that turtles will avoid and where seagrass can grow (Wirsing et al., 2007). Thus, defaunation likely induces strong alterations in ecosystem functioning and the services they provide.
The introduction of exotic species, through increased globalization, is another impact that can have far-reaching consequences on ecosystem services. The spread of invasive species leads to novel ecosystems, where plants and herbivores occur in combinations that are unfamiliar to each other (Williams, 2007). The resulting effect on herbivory rates, food web interactions and ecosystem services are unknown. The PhD project of Fee Smulders will focus on how human impact through defaunation and invasive plant introduction affects ecosystem services in seagrass ecosystems.
Lac Bay on Bonaire is home to extensive seagrass meadows, dominated by turtlegrass (Thalassia testudinum). This bay provides one of the most important foraging grounds for juvenile green turtles in the Caribbean. The invasive seagrass Halophila stipulacea, native to the Red Sea and the Western Indian Ocean, settled on Bonaire in 2010 and has been increasing throughout the bay ever since (Smulders et al., 2017). Green turtle leaf grazing seems to modify the rate and spatial extent of this invasive species' expansion, due to grazing preferences, and increased space for settlement (Figure 1, Christianen et al., 2018). Defaunation of e.g. predatory sharks could limit the top-down control on sea turtles. This may cause an increase in grazing pressure, and in combination with the fast-growing invasive H. stipulacea may explain the decline of native T. testudinumwe observe in Lac Bay. The ecological effects of this invasion are still largely unknown.
The invasive H. stipulacea is likely to become the dominant species in Lac Bay, and therefore this project aims to quantify and compare the ecosystem services of T. testudinumand H. stipulacea. In this way, potential changes in ecosystem services will be unraveled under the projected species shift. In addition, competition between the native and exotic seagrass species will be investigated. Assessing the impact of the invasion on the ecosystem services of Lac Bay is important for future management and conservation of this protected nature area.
This year, we are collaborating with researchers at 14 other sites in the Caribbean. At all 15 sites, an exclosure experiment will be carried out. We will investigate the effects of nutrient addition and (various levels of) grazing on T. testudinumseagrass structure and function. This large-scale project will give insights in to the tropicalization of turtlegrass. Gradients in grazing intensity, light and temperature may explain differences in ecosystem services across latitudes. Close collaboration with other seagrass scientists will facilitate knowledge exchange across the habitat range of this important seagrass species.