A new paper published today in BioScience by researchers from University of Miami’s R. J. Dunlap Marine Conservation Program, and The Dept. of Applied Ecology at NC State has just come out discussing extinction proneness in wild populations. The paper uses hammerhead sharks as a model, or case study, to argue that suites of highly derived adaptations make taxa more vulnerable to extinction at the hands of humans.
That specific attributes contribute differentially to extinction risk is not a new idea. A broad and deep literature on extinction risk shows that life-history traits are good predictors of extinction risk. Basically, slow growing, long-lived, and low fecundity species are highly susceptible to overharvest, for example, tortoises, whales, and rhinoceri, simply because their populations grow very, very slowly. This slow growth makes rebounding from overharvest difficult.
What is new in this paper, is that we invoke a variety of specialized traits – including locomotor, behavioral, and reproductive adaptations as critical contributors to the vulnerability and decline of species.
Hammerhead sharks have unique adaptations that in suite exacerbate their risk of extinction:
High performance swimming capacity – Hammerheads are fast, agile swimmers. This increases extinction risk in several ways. First, the high swimming performance of hammerhead sharks comes with a large physiological cost…they are easily exhausted. Exhaustion due to overexertion is a problem when hammerheads are captured and fought on hook and line because the probability of death after release is very high. New regulations in Florida specifically address this by banning ‘landing’ of hammerhead sharks on beaches. These sharks simply die after capture. Another locomotor adaptation of hammerheads are those long dorsal and pectoral fins which presumably aid in stability and agility at during bouts of high activity foraging. These fins make hammerheads high-value targets for ‘fin fisheries’.
Aggregation – Several species of hammerheads regularly form large groups that are thought to be related to reproduction. These large groups can number hundreds or perhaps thousands of sharks, often in well-known places. Unfortunately, that makes these sharks susceptible to overharvest by targeting fishing.
Reproduction – We know very little about reproduction in hammerhead sharks, but there is a fair bit of evidence that females move to shallow coastal bays (and tidal creeks) to give birth. This brings both gravid females and pups into a whole host of threats including fishing pressure.
In concert with their low reproductive rates, these adaptations make hammerheads highly susceptible to overharvest. Indeed, hammerhead populations have declined precipitously all over the world.
But we also make another suggestion, which I think has distinct value, which is that management and conservation strategies could also capitalize on specific adaptations. For example, Lanthanide hooks emit weak magnetic fields detectible by and detestable to shark sensory systems and could be used to deter by-catch of sharks. Employing tools and strategies that make use of specific adaptations may be a viable conservation measure and are worth exploring.
Also of note, much of the data used to frame this essay were collected from sharks caught in or around The Bahamas.
Enjoy the read…and the cover of this month’s BioScience.
Gallagher AJ, Hammerschlag N, Shiffman DS, and Giery ST. 2014. Evolved for Extinction: The Cost and Conservation Implications of Specialization in Hammerhead Sharks. BioScience 64(7):619-624. (link to PDF)
The interactions between the evolutionary history of species and contemporary changes in their environment can result in both positive and negative outcomes for fitness and survival. Sharks are one the oldest groups of all extant vertebrates but, today, are among the most threatened globally, primarily because of destructive fishing practices. Hammerhead sharks (Sphyrnidae) exhibit extremely specialized traits and complex behaviors that have increased their vulnerability to human exploitation, which impedes conservation efforts. By bringing together published data on aspects of hammerhead shark phylogeny, morphology, biology, physiology, and ecology, we argue that the same novel adaptations that have historically contributed to evolutionary success have become maladaptive under current levels and modes of exploitation. Therefore, we suggest that future management be made in light of—rather than in spite of—the unique evolutionary and ecological traits possessed by hammerhead sharks, because similar patterns are threatening other taxa with high extinction risk.