Marine Predator-Prey Relationships: Study Says Prey Density More Important Than Total Biomass
But the marine food chain's top predators are common prey for the most deadly hunters of all—humans. When top predator species are. Marine resource managers often gauge the health of species based on overall biomass, but a new study of predator-prey relationships in the. Read about different predator-prey relationships on the reef. . to a wide variety of specialized habitats, gobies have become the most diverse marine fish family.
Armies of grazing starfish leave a wake of destruction in their path, killing up to 95 percent of the hard corals in an area. They detect prey using an array of finely tuned senses, including electrical current detection. Rows of razor-sharp teeth and powerful jaws allow them to crack though even the thick carapace shell of full-grown sea turtles. One common defense against predators is a protective covering, such as a shell.
Another is to flee the predator. During its evolution, the green sea turtle Chelonia mydas sacrificed speed in favor of a thick, heavy shell carapace.
The carapace acts as armor, protecting the turtle's body from the sharp teeth of predators.
But some, like the tiger shark, are powerful enough to bite right through the carapace and kill the turtle. Some species of sea slugs, however, such as Platydoris scabra, have evolved immunity against the toxins of specific sponge families in this case, Microcionidae.
This adaptation benefits the slugs in two ways. First, they don't have to compete with many other organisms for the sponges. The sea slugs can also concentrate the sponge toxins to foil their own predators -- at least until the slugs' predators also evolve immunity to the toxins.
Sea sponges, such as those of the Microcionidae family, have escaped predation by all but a few species because they produce foul-tasting and sometimes toxic compounds.
These compounds evolved as chemical weapons for use against other sponges, as well as against fouling organisms creatures that grow on top of other creatures, thus decreasing their fitness -- their defensive function was just a lucky side effect. But some predators, such as sea slugs, have evolved resistance to the toxins and even use those toxins against their own predators.
Like many predators, they have evolved as extremely fast swimmers, with streamlined, torpedo-like bodies. And they are efficient killers, using conical, razor-sharp teeth to quickly rip prey to shreds. In addition, they are resistant to the toxin found in the bodies of many of their prey, such as parrotfish. Named for their bright colors and beak-like mouths, parrotfish Scaridae family. Using their beaks and two pairs of crushing jaws, parrotfish are marvelously adapted for crunching and pulverizing chunks of algae-coated coral.
They digest the algae and excrete the coral as fine sand. Unfortunately, they are poorly equipped to defend themselves against predators, such as barracuda, but some find protection by schooling with better-armored fish. Algae occur in a kaleidoscope of forms and colors on the reef, but they have one main function: Thus they are called "primary producers.
One important algal group, benthic bottom-dwelling algae, rapidly grows over dead coral and other inert objects, providing a grazing yard for herbivores, such as parrotfish.
Their gentle disposition disappears, however, in the presence of another favorite food: When feeding, the butterflyfish turn into vicious predators, darting in to rip off the anemones' fleshy tentacles.
Having evolved resistance to the anemones' toxins, they need only get past clownfish guards to pick off a delicious meal. Packed with miniature toxin-loaded harpoons nematocyststhe tentacles of sea anemones provide an excellent deterrent against almost all would-be predators. Saddled butterflyfish, though, have evolved resistance to the toxins and apparently relish the tentacles.
Still, to grab a meal, the butterflyfish must get past the anemones' second line of defense: One such predator, the smallscale scorpionfish Scorpaenopsis oxycephalaclosely resembles the reef's rocky, algae- and coral-encrusted bottom, where it lies in wait for crustaceans and small fish, such as gobies.
Safely tucked in coral crevices or half-buried in sand and rubble, gobies Gobiidae family maintain a low profile on the reef to avoid predation. In addition, they have evolved independently swiveling eyes that constantly search the water for potential attackers. But their efforts can be foiled by ambush predators, like the smallscale scorpionfish, whose camouflage prevents gobies and other prey from seeing them until it's too late. The two fish benefit by the association; a third fish, however, has evolved to take advantage of them both.
To capture their prey, marine mammals have evolved special sensory abilities e. They have also evolved specialized strategies to capture prey, such as cooperation to corral fish, or the production of curtains of air bubbles used by humpback whales Megaptera novaeangliae to capture herring.The Ocean's Predators : Documentary on the Predator/Prey Relationship in the Ocean
Marine mammals have also evolved specialized feeding behaviors to capture prey that move diumally up and down the water column or to capture prey that move seasonally across broad geographic ranges. This in turn has likely influenced the fife history strategies of marine mammals and their prey. For example, baleen whales feed for about 6 months when plankton are abundant and concentrated in shallow water, and then fast for the remainder of the year when the plankton are too dispersed to make them worth finding.
As prey, marine mammals have had to escape aquatic and terrestrial predators. Some species of pinnipeds for example, are particularly vulnerable to predation by bears and wolves while on land, and to predation by killer whales Orcinus orca and sharks while in the water. Thus some species of pinnipeds can reduce their risk of being eaten by aquatic predators by hauling out and resting onshore.
Similarly, species such as Steller sea lions Eumetopias jubatus and northern fur seals Callorhinus ursinus reduce their risk of being eaten by terrestrial predators by breeding and hauling out on offshore rocks and islands where terrestrial predators are absent. Other species, such as ringed seals Pusa hispidagive birth in caverns formed between ice and snow to avoid predation by polar bears Ursus maritimus.
Figure 1 A simplified depiction of the Bering Seafood web: Fish and other cold-blooded species of prey have evolved a number of strategies to increase their chances of survival.
One is cryptic countershading that enables fish to blend in with the bottom when viewed from above, and avoid detection when seen from below against a bright sea surface.
Many species of fish, invertebrates, and zooplankton take refuge from predators in the deep, dark waters during the day and move toward the surface to feed under the cover of night.
Another strategy evoked by the prey of marine mammals is predator swamping, such as large aggregations of spawning salmon and herring that reduce the numerical effect of predators on their prey populations. Schooling is another antipredator behavior that creates confusion through the sheer volume of stimuli from a fleeing school, making it difficult for a marine mammal to actively select and maintain pursuit of single individuals. Scattering and fleeing is yet another option to reduce predation and is used by some prey when attacked by bulk feeders such as baleen whales e.
The line between feeding and fleeing is undoubtedly fine for species of prey and must be continually evaluated by prey to minimize vulnerability to predation. Marine mammals may also have indirectly influenced the evolution of nontargeted species in their ecosystems by consuming the predators of these species. The best example of this is the apparent influence of sea otters Enhidra lutris on kelp and other marine algae.
Most species of marine algae use secondary metabolites to defend against herbivores. However, marine algae in the North Pacific have lower levels of chemical defenses where sea otters occur compared to algae species inhabiting the southern oceans where sea otters are not present.
Sea otter predation on sea urchins and other herbivores may have removed selective pressure for species of marine algae to defend themselves against herbivores. Because secondary metabolites are expensive to produce, this may have allowed algae, like kelp, to radiate and diversify without the added cost of evolving and producing antigrazer compounds. Ecological Time Scales On a shorter time scale than the evolutionary one, predators and prey can directly affect the relative abundance of each other, or they can indirectly affect the abundance of other species.
Their interaction may also affect the physical complexity of the marine environment. Predation by sea otters on sea urchins is probably die best example of how marine mammals can alter ecosystem structure and dynamics. Sea otters were hunted to near extinction in the late s throughout their North Pacific range. Without predation, urchin populations grew unchecked and overgrazed the fleshy algae.
Kelp did not replace the underwater barrens until reintroduced sea otters once again began preying upon sea urchins. Primary production has been estimated to be three times higher in areas where sea otters are present compared to those areas where sea otters are absent, allowing those organisms that feed upon primary production to grow faster and attain larger sizes e.
The increase in primary production may even alter settlement patterns of invertebrates. The kelp also provides habitat for fish and suspension-feeding invertebrates to spawn, grow, and flourish. It can also change water motion and reduce onshore erosion and may even block the shoreward movement of barnacle larvae.
Thus a top predator such as the sea otter can change the structure and dynamics of marine ecosystems. Gray whales Eschrichtius robiistus and walruses Odobenus rosmarus are other species of marine mammals whose foraging behavior can also affect community structure. The feeding pits created by gray whales draw times more scavengers and other invertebrates compared to adjacent sediments.
The disturbed sediments may also help maintain the high abundance of gray whale prey and other early colonizing species. Similarly, walruses turn over bottom substrate in their search for clams and other bivalves.
There is some evidence that they may feed selectively on certain size classes and certain species and that their defecation may result in the redistribution of sediment.
Thus, the interaction of benthic feeding marine mammals with their prey can result in food for scavengers and habitat for other species. Interactions between predators and prey also influence the shapes of their respective life tables i.
In Quebec, Canada, for example, there are a number of freshwater lakes that are home to land-locked harbor seals Phocavitulirw. Studies have found that the trout in these lakes are younger, grow faster, attain smaller sizes, and spawn at younger ages compared to adjacent lakes without seals.
As for marine mammals, they typically have elevated mortality rates during their first few years of life. This is likely due to a number of factors, including their relative vulnerability to predators and their inexperience at capturing prey and securing optimum nutrition. In the Gulf of Alaska and Bering Sea, killer whales have been implicated as a contributing factor, but not the main one, in the decline of Steller sea lions and harbor seals through the s.