Tuesday, December 18, 2012

Heithaus & al - fantastic!

"Research" in Shark Bay - don't try this at home!

And talking about non-consumptive risk effects of Sharks.

This is fantastic, seminal stuff!
Alas, 15 years of research results in Shark Bay (check out the list of Sharks and Rays!) are way to exhaustive to be summarized in a blog post, so I really do invite you to spend those 25 bucks and buy the PdF - or could David and Andrew maybe venture a synopsis hint hint?

Here's a great video.

And here's a teaser from the paper itself.

The best-studied species with respect to multiple layers of anti-predator behaviour in Shark Bay is the dugong. 
In addition to modifying their use of habitats (Wirsing et al. 2007a), microhabitats (Wirsing et al. 2007b) and patches (Heithaus et al. 2007b), they also modify their feeding modes, diving behaviour and duration of behavioural bouts in response to predation. These responses, however, tend to be concentrated in high-risk, shallow habitats and are less pronounced, or even absent, in safer, deep waters. 

As tiger shark numbers increase, dugongs largely switch their foraging behaviour in shallow habitats from excavation, which allows them to access more nutritious seagrass rhizomes but at the cost of creating large plumes of sediment that could mask the approach of a tiger shark, to cropping the less nutritious leaves of A. antarctica, a feeding mode that does not inhibit anti-predator vigilance (Wirsing et al. 2007c). 

Dugongs observed using the excavation feeding tactic make more and shorter dives during periods of high tiger shark abundance than during periods of low shark abundance, whereas dugongs using cropping do not change their diving behaviour as tiger shark catch rates vary (Wirsing et al. 2011). 
This change in diving behaviour is likely to allow excavating dugongs to be vigilant for tiger sharks more often by rising above sediment clouds.

A final anti-predator behaviour identified in dugongs is modification of behavioural sequencing. 
Dugongs in dangerous shallow habitats, but not in deep habitats, more frequently switch between foraging and traveling, and resting and traveling, during periods of high shark abundance (Wirsing and Heithaus 2012). This sequencing adjustment results in individual dugongs avoiding long, continuous bouts of foraging and resting, during which their capacity to detect sharks is inhibited. 

Interestingly, the multiple layers of anti-predator behaviour in dugongs closely mirror those of elk (Cervus elaphus) foraging under the risk from grey wolves (Canis lupus) (Wirsing and Ripple 2011).

Is this way cool or what!
Anyway, enjoy the paper - required reading if you want to be able to talk about top-down effects other than the usual, and rather suspect, trophic cascades.

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