Eating between the lines: functional feeding response of bonnetheads (Sphyrna tiburo)
Mobile mesopredators can have strong influences in the structuring of marine food webs in shallow coastal ecosystems. Few quantitative estimates of the effects of predation on prey populations by highly mobile mesopredators exist, yet they are necessary to evaluate the impact mesopredators have on prey resources. Quantifying a predator’s functional response provides valuable information on how a predator’s per capita consumption rate of prey can influence community structure and prey populations. We examined the functional response of bonnetheads (Sphyrna tiburo) to one of its few natural prey items brown shrimp (Farfantepenaeus aztecus) to determine if per capita feeding rates would increase with increases in prey density. We simulated natural conditions in outdoor mesocosms and offered live prey at varying densities. The functional response of a predator can take one of three forms: linear (type I), rise to an asymptote (type II), and a sigmoid shape (type III). Bonnetheads consumed prey proportional to prey density and demonstrated a response that was in between a type I (R 2 = 0.847, p < 0.01) and type II (R 2 = 0.877, p < 0.01) functional response. Bonnetheads showed an increased rate of consumption with initial low prey densities but high densities of available prey did not consistently result in increased consumption rates. At the maximum prey density offered, bonnetheads consumed less prey than at intermediate densities. Bonnetheads may not have as strong of an influence on prey populations at high prey densities compared to predation impacts at low densities. Bonnethead per capita feeding rates did not conform to a type I, type II, or type III functional response and our results suggest that bonnetheads follow a functional response continuum more closely. This study highlights the importance and difficulty in understanding the feeding ecology of highly mobile mesopredators.
KeywordsSmall coastal shark Feeding ecology Predation Predator–prey dynamics Elasmobranch
This work was funded through the University of South Alabama and the Dauphin Island Sea Lab. The authors thank Matt Ajemian, Suni Chutkan, Brian Klimek and members of the Fisheries Ecology Laboratory for support in the field and Brian Cabral at the Dauphin Island Sea Lab for support at the mesocosm facility. We thank members of Discovery Hall Programs at the DISL for aid and cooperation in the collection of specimens. This manuscript was improved by insightful comments and suggestions by John Carlson, William Driggers III, Marcus Drymon, Ken Heck Jr., and three anonymous reviewers. This research is approved under the Institutional Animal Care and Use Committee (IACUC) protocol #11014.
- Clark E, von Schmidt K (1965) Sharks of the central gulf coast of Florida. Bull Mar Sci 15:13–83Google Scholar
- Compagno LJV (1984) FAO Species catalogue. Vol. 4. Sharks of the world: an annotated and illustrated catalogue of shark species known to date. Part 2. Carcharhiniformes. FAO Fish Synop 125:251–655Google Scholar
- Cortés E, Manire CA, Hueter RE (1996) Diet, feeding habits, and diel feeding chronology of the bonnethead shark, Sphyrna tiburo, in Southwest Florida. Bull Mar Sci 58:353–367Google Scholar
- Hassell MP (1978) Arthropod predator–prey systems. Princeton University Press, PrincetonGoogle Scholar
- R Core Team (2012) R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org/
- Tyminski JP, Cortés E, Manire CA, Hueter RE (1999) Gastric evacuation and estimates of daily ration in the bonnethead shark, Sphyrna tiburo. American Society of Ichthyologists and Herpetologists. 79th Annual Meeting, Pennsylvania State University, University Park, June 24–30Google Scholar
- Wetherbee BM, Cortés E (2004) Food consumption and feeding habits. In: Musick JA, Carrier JC, Heithaus M (eds) Biology of sharks and their relatives. CRC Press, Boca Raton, pp 223–244Google Scholar