Tiger shark intimidation alters the spatial distribution of their prey and structures the dynamics of the seagrass community. In Shark Bay, dugongs and green sea turtles, which are common tiger shark prey, alter their habitat selection based on the presence of tiger sharks. This creates a trade-off between safety and food quality for the species. Tiger sharks also intimidate and affect the location of species that are relatively rare in their diet, such as bottlenose dolphins and pied cormorants. Dugongs prefer the nutritious seagrass found in the middle of large grassy patches, but it is very difficult to escape from a tiger shark in these locations. When tiger shark abundance is high, dugongs feed on the lower quality seagrass located near a patch’s edge, thereby reducing their risk of predation (1). Dugonongs alter their distribution on a daily basis depending on the number and location of sharks in the area (2). When grazing, dugongs remove the entire seagrass plant, altering the composition and structure of the seagrass meadow, the nutrient content of the plant and the detrital structure of the system (3)(4). By forcing dugongs to change their habitat selection, tiger sharks keep grazing in check that in turn keeps the seagrass at relatively constant levels (5). Tiger sharks are indirectly controlling bottom communities (6).The tiger sharks not only indirectly controls the spatial pattern of dugongs, but they also indirectly control green sea turtles spatial distribution. Green sea turtles feed by removing the top portion of seagrass blades from a specific plot (7)(8). The continued grazing in these plots produces a high quality diet for the turtles, while stimulating rapid growth of the seagrass blades and an increased rate of nutrient recycling (9)(10). In the presence of tiger sharks, healthy green sea turtles were found foraging in lower quality habitat that was safer, while sick or injured green sea turtles risked predation to forage in higher quality habitats (11). The tiger sharks’ influence on green sea turtles was shown to redistribute their grazing patterns, which altered the seagrass community, the chemical composition of the blades and the detrital cycle (12).
In addition to triggering trophic cascades, tiger sharks mediate indirect interactions among their prey species. In fact, some species in Shark Bay affect one another in ways that were never predicted and only do so because they share tiger sharks as a predator. For example, larger tiger shark (greater than 3.5m) show up only with the presences of dugongs and these sharks like to spend time in shallow waters where dugongs and their other favorite prey are found. Because of this, bottlenose dolphins, which are not common prey of tiger sharks and eat fish, almost completely abandon their best foraging grounds in the shallow water. This means that the presence of dugongs actually causes the bottlenose dolphins to leave the areas where dugongs forage (13)(14). Once sharks leave the area, dolphins and pied cormorants are able to occupy all habitats and freely pursue their food (15). Even though tiger sharks do not kill many of these species, the changes they induce in their prey’s behavior are equivalent or greater in magnitude than the effects of direct mortality (16).
These examples show just how interconnected the Shark Bay ecosystem is and suggests that the loss of tiger sharks would have unexpected and negative effects on the rest of the ecosystem.
By: Griffin, E., Miller, K.L., Freitas, B. and Hirshfield, M. July 2008.
