I’ve reached a point in the analysis and writing of my thesis where I can start exploring the ecological concepts behind my results, which inspired me to put up this little post here. In the marine environment, there is often a high degree of overlap when it comes to ecological niches, particularly in the case of predators. While we often think of sharks as the mack daddies of the ocean, in reality any one species usually shares this role with other apex predators like dolphins, large bony fish like swordfish and grouper, and a slew of other sharks. This can be especially extreme in the case of sharks because they often find themselves in competition with intraguild predators (competitors that are capable of eating them, introduced here and expanded here and here). Since sharks are pretty fantastic at killing things, they’re also pretty good at killing each other, which makes inter-predator interactions a powerful influence on where sharks go and what they eat.
This is where resource partitioning comes in.
Resource partitioning is essentially when species stake out the “turf” that they’ll be operating in. This has the effect of reducing competition by lowering the amount of shared resources the two species can compete for. Resource partitioning is usually expressed in predators like sharks by way of competitive exclusion, which means the conflicting species simply don’t occur in the same place all that often. Predatory resource partitioning can also be accomplished by behavioral changes such as feeding on different prey in the presence of the other predator, or modifying foraging routes to avoid conflict. Interestingly, in sharks resource partitioning happens not just between species, but within species, and our hero the spiny dogfish makes a great example.
For a demonstration of how resource partitioning affects shark feeding and distribution, you’ll want to check out Papastamatiou et al. (2006). This paper looks at the distribution and diet of four seemingly disparate shark species in the Hawaiian islands: the tiger, Galapagos, grey reef, and sandbar sharks. Papastamatiou and company observed both the diet and local ranges for all four species, and found that grey reef and sandbar sharks have the highest degree of dietary overlap thanks to a shared taste for fish and squid. That high degree of overlap has a profound effect on the distribution of both species.
The two figures from Papastamatiou et al. (2006) show the relative composition of the four sharks species observed in the study. The grey-shaded portions represent sandbar sharks, black sections represent grey reef sharks, white denotes Galapagos sharks, and striped (naturally) stands for tiger sharks. What becomes strikingly obvious is that grey reef and sandbar sharks almost never co-occur in significant numbers, with the exception of Niihau, which is nestled in the middle ground between the main and northwestern islands. Because the high dietary overlap between grey reef and sandbar sharks could potentially lead to violent and costly conflict, the two species have effectively excluded each other from their respective ranges.
In terms of evolutionary fitness, this strategy is much less costly to both species than rumbling over food on a regular basis. It’s almost like gang territory (I prefer to think in terms of “West Side Story”-style gangs, mainly because it amuses me to think of grey reef and sandbar sharks advancing menacingly towards each other while snapping). The gentlemen in grey suits don’t encroach on the turf of the tan suits, and vice-versa.
Laptikhovsky et al. (2001) show an example of behavioral resource partitioning between, of all things, dogfish and catsharks in the waters around the Falkland Islands. In this area both spiny dogfish and narrowmouth catsharks, a species similar in size and trophic level to spiny dogfish, occupy the same habitats on a regular basis. In this case rather than geographically excluding each other the two shark species feed out of different prey communities. While the dogfish munch on schooling pelagic fishes, the catsharks feed on more benthic species and crustaceans. The only time these species show significant dietary overlap is during the spawning run of Falklands herring, during which there is more than enough prey to go around.
Spiny dogfish also show resource partitioning on an intraspecies basis. Like many sharks, dogfish are known to indulge in a little cannibalism from time to time, so it’s usually in the best interest of the juveniles to stay out of the way of the adults. In contrast to many other shark species, spiny dogfish give birth offshore, and juvenile dogfish will remain down deep on the edge of the continental shelf until they get large enough to move closer to shore and school with their parents.
The distribution maps at right, from Methratta and Link (2007), show that as dogfish increase in size, they begin to move into shallower water. The top map shows neonatal to juvenile dogfish clustered around the shelf break of Georges Bank. Medium dogfish move onto the bank itself, then move farther inshore as they increase in size. The largest dogfish, shown in the bottom map, are practically up on the beach, feeding out of the shallowest, most productive habitats.
By staying deep, juvenile dogfish not only keep from being eaten by their parents, but also avoid having to compete with them for prey. In these deep refuge areas smaller dogfish feed out of an entirely different prey assemblage, to the point that they are assigned to a completely different feeding guild (more small crustaceans, less fish). Small dogfish are quite capable of carving up and consuming bony fish, but because of behavioral resource partitioning actually show an ontogenetic change in diet (which is a fancy way of saying they eat more fish and fewer invertebrates as they get larger).
Interestingly, the turf claimed by adult spiny dogfish has also made them the demographic most readily accessible to fishermen. Due to fishing pressures on adults, fishermen and managers have been encountering more medium-sized dogfish as the smaller dags move in to take their parents’ place in the shallows (mentioned in this post). This provides some evidence that the ontogenetic diet shift in spiny dogfish may have more to do with behavioral resource partitioning than the physical limitations of the smaller sharks.
In conclusion, resource partitioning allows apex predators like sharks (including the mighty spiny dogfish) to coexist peacefully. It’s good to know that even though nature is red in tooth and nail, at least some species can let bygones be bygones.
Laptikhovsky, V.V., Arkhipkin, A.I., & Henderson, A.C. (2001). Feeding habits and dietary overlap in spiny dogfish Squalus acanthias (Squalidae) and narrowmouth catshark Schroederichthys bivius (Scyliorhinidae) Journal of the Marine Biological Association of the United Kingdom, 81, 1015-1018
Methratta, E., & Link, J. (2007). Ontogenetic variation in habitat association for four groundfish species in the Gulf of Maine Georges Bank region Marine Ecology Progress Series, 338, 169-181 DOI: 10.3354/meps338169
Papastamatiou, Y., Wetherbee, B., Lowe, C., & Crow, G. (2006). Distribution and diet of four species of carcharhinid shark in the Hawaiian Islands: evidence for resource partitioning and competitive exclusion Marine Ecology Progress Series, 320, 239-251 DOI: 10.3354/meps320239