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Biology

Fear and Floating in the Atlantic

 

Article: Hammerschlag, Neil, et al. “Evaluating the landscape of fear between apex predatory sharks and mobile sea turtles across a large dynamic seascape.” Ecology (2015).

http://dx.doi.org/10.1890/14-2113.1

Background:

The landscape of fear has been proposed as a unifying model in ecology, providing the missing link in understanding predator-prey dynamics and describing how animals move and interact within their landscape. All prey must be able to feed and survive within their home boundaries as well as the boundaries of their predators. Basically put, the landscape of fear model suggests that when prey move from low predation risk areas to high predation risk areas they alter their behavior to avoid predation. This behavioral change can have ecosystem-wide implications by directly and indirectly impacting other organisms within the ecosystem and altering the trophic cascade. Evidence of the landscape of fear exists in many systems, but typically, they are well contained. For example, grasshoppers have been observed to alter their feeding behavior in the presence of spiders, switching from a high-quality (but risky) food source to a low-quality (but safe) food source (Fig. 1). However, this model may not hold up when it comes to highly migratory animals that cover a lot of ground, simply due to the inconsistent and diminished pressure from predators.

Fig. 1: A grasshopper must decide between risking predation and eating a preferred food (Bob Handelman).

Fig. 1: A grasshopper must decide between risking predation and eating a preferred food (Bob Handelman).

Researchers from the University of Miami decided to test the model using migration data from sea turtles and their shark predators. These test species are ideal because their home ranges overlap and their behaviors can be documented. If you’re a sea turtle, you need to spend a fair amount of time at the surface of the water; after all, you breathe air. But if you’re in the presence of a predator, the more time you spend at the surface, the more at risk you are. As a result, sea turtles have to balance the need for air with the risk of predation. By looking at overall movement and depth for both predator and prey individuals, researchers hope to determine if predation risk influences prey behavior supporting the landscape of fear model.

The Study:

The Atlantic coast of the United States is home to one of the largest nesting populations of loggerhead turtles (Caretta caretta) (Fig. 2). These turtles migrate seasonally, feeding in northern coastal waters in the summer and southern offshore waters in the winter. One of their common predators, the tiger shark (Galeocerdo cuvier) (Fig. 3), inhabits tropical and temperate waters of the US Atlantic coast, overlapping the home range of loggerhead turtles. Both of these species have been well studied, and satellite-tracking data exists for both.

Fig. 2: A loggerhead turtle (Caretta caretta) (telegraph.co.uk).

Fig. 2: A loggerhead turtle (Caretta caretta) (telegraph.co.uk).

Fig. 3: A tiger shark (Galeocerdo cuvier).

Fig. 3: A tiger shark (Galeocerdo cuvier).

 

 

 

 

 

 

 

 

In 2010, 31 tiger sharks were tagged off the coast of Florida and the Bahamas. Their movement was observed through 2013. Loggerhead turtles were tracked from 1998 to 2008, with 68 female individuals tagged. First, researchers put together kernel density estimates, which are essentially heat maps for both species, allowing ranges and densities to be outlined and overlap to be determined (Fig. 4). Overlapping ranges represent high predation areas for turtles. Based on when the tracking devices transmit data, scientists pieced together how long individual sharks and turtles were at the surface. The kernel density estimates where species’ ranges overlapped were combined with the surfacing data to determine shark-turtle interactions. Basically, if a turtle and a shark were both found to be at the surface at a location within the overlapped range, one would expect an interaction. If turtles rarely surface in this region, this would indicate behavioral avoidance.

Fig. 4: This figure shows the kernel densities for tiger sharks (A,D), loggerhead turtles (B,E), and the areas of highest overlap (C,F). Maps are also broken down by summer and winter seasons.

Fig. 4: This figure shows the kernel densities for tiger sharks (A,D), loggerhead turtles (B,E), and the areas of highest overlap (C,F). Maps are also broken down by summer and winter seasons.

If the landscape of fear model fit the relationship between loggerheads and tiger sharks, the authors expected that in areas of high overlap, turtles would reduce the frequency and duration of their surface visits. However, researchers observed the opposite: loggerheads were surfacing just as much in their overlapped range as they were elsewhere (Fig. 5). While sea turtles didn’t alter their behavior due to shark presence, turtle presence may have had an impact on shark behavior. Sharks were observed to spend more time in sub-surface waters when they overlapped with sea turtles. This change in shark behavior could result in high predation rates as sharks likely spend time below the surface in order to ambush the turtles at the surface.

Fig. 5: This figure shows the behavior of both loggerhead turtles and tiger sharks based on how much overlap their is between the home ranges. Turtles appear to surface more when overlap is highest, while sharks dip below the surface presumably to gain a hunting advantage.

Fig. 5: This figure shows the behavior of both loggerhead turtles and tiger sharks based on how much overlap their is between the home ranges. Turtles appear to surface more when overlap is highest, while sharks dip below the surface presumably to gain a hunting advantage.

The Significance:

Well, it looks like the landscape of fear model doesn’t fit for these species, and ultimately there are exceptions to rules. But this study was the first to investigate highly mobile species in this light. While sea turtles don’t alter their own behavior where they more frequently encounter sharks, they might be influencing their predators’ behavior. This study shows that sea turtle behavior and distribution is more directly influenced by their need for high-quality feeding and nesting grounds than their fear of predation. There is value in understanding what governs the behavior and interaction between two migratory species. Although the landscape of fear model does not explain behavior, results of this study can help in our understanding of the ecology of these species and aid in conservation efforts.

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