Nutrient availability induces community shifts in seagrass meadows grazed by turtles

Thalassia testudinum

In the Caribbean, green turtles graze seagrass meadows dominated by — through rotational grazing, resulting in the creation of grazed and recovering (abandoned) patches surrounded by ungrazed seagrasses.

grazing pressure

At increasing —, the total biomass of T. testudinum declined, whereas that of early-successional increased.

late-successional

T. testudinum is a — species that maintains sediment nutrient concentrations at levels below the requirements of early-successional species when dominant.

Medium-; Long-term

— and — grazing promoted a shift in community species composition.

Ammonium

— concentrations were highest in the patches of Medium-term (9.2 + 0.8 μM) and Long-term grazing levels (11.0 + 2.2 μM) and were lowest in the control areas (4.6 + 1.5 μM).

resource availability; herbivory

Changes in the species composition of seagrass communities have been primarily attributed to bottom-up control mechanisms, such as — with top-down mechanisms, such as —, only playing a minor role.

green turtles

Herbivores, particularly large herbivores such as —, can alter plant species composition and community structure.

selective grazing

Large herbivores may alter plant communities through numerous mechanisms, such as — which alters competitive interactions among plant species

ungulates

In grasslands for example,— directly influence the nitrogen cycle by adding nitrogen through urine and feces, but more importantly they indirectly affect decomposition processes in soil through changes in plant litter.

Resource heterogeneity created by ungulate grazing may alter the competition for resources, leading to an increase in plant species diversity in grazed areas

Thalassia testudinum

In the Caribbean, climax seagrass communities are dominated by the robust late-successional species — (slower-growing), which is the preferred food source for green turtles.

grazing patches

Green turtles create — within T. testudinum meadows which are easily recognized as they graze seagrass leaves in specific areas or patches that they maintain and revisit.

nitrogen; lignin

Continuous grazing on T. testudinum increases — content in the leaves and reduces their — content, which improves the quality of the food

Rotational; internal carbohydrate depletion

— grazing by green turtles decreases the above-ground community biomass, particularly that of T. testudinum. Grazed patches are maintained for 1–2 years after they are abandoned most likely because of reduced leaf growth due to — and the turtles do not return to abandoned patches.

Abandoned

— patches show thinner and shorter leaves of T. testudinum, with sparser seagrass shoots than ungrazed nearby meadows

light; nutrients

Seagrass community structure and dynamics are highly influenced by competition for — and —.

succession

Instead, nutrient availability in Caribbean seagrass meadows is fundamental in the process of — described by Williams (1990).

more abundant

We hypothesize that early-successional rhizophytic algae and seagrass species (faster-growing) will become — if nutrient availability changes under increasing grazing pressure.

1. ungrazed (control)

2. previously grazed (recovery)

In what two previous conditions were the experimental patches created?

Control patches

— were established in ungrazed sections of the seagrass bed without clipping.

True

T/F. Simulated turtle grazing increased food quality, as nitrogen (N) content in T. testudinum leaves increased

True

T/F. Phosphorus (P) content instead did not change among grazing levels.