Herbivory is a widespread but specialized interaction

Herbivory is a widespread interaction, with the vast majority of herbivores being insects. Of those herbivorous insects, more than 90 percent are specialists that feed on just one or a few, often taxonomically related, plant species. Generalist herbivores, in contrast, feed on as many as hundreds of unrelated plant species. Vertebrate herbivores are usually generalists; a cow grazing in a pasture, for example, can consume many different plant species in a single afternoon. There are exceptions to this pattern, however. Australian koalas famously feed exclusively on the foliage of eucalyptus trees, and the diet of giant pandas is made up almost entirely of bamboo.

Herbivores, particularly insects, generally consume only parts of their food plants and usually do not kill them. In most natural ecosystems, insects rarely remove more than a small percentage of the plant biomass. For that reason, some ecologists have questioned the ability of herbivores to exert strong selection pressure on plant traits. Mortality is not, however, the only form of selection that leads to evolutionary change; herbivores can reduce plant fitness if the plants they attack produce fewer offspring.

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investigating life

The Lionfish King

experiment

Original Paper: Albins, M. A. and P. J. Lyons. 2012. Invasive red lionfish Pterois volitans blow directed jets of water at prey fish. Marine Ecology Progress Series 448: 1–5.

Mark Albins and his colleagues asked what feeding behaviors the invasive lionfish Pterois volitans might use to be a successful predator on Atlantic and Caribbean coral-reef fish. They conducted field observations of lionfish feeding on small reef fish and noticed that lionfish typically face their prey, flaring their pectoral fins and moving very slowly within striking distance of the prey. During the approach, lionfish produced a strong, pulsed jet of water from their mouths and directed it toward the prey. To investigate this novel feeding behavior, Albins and his colleagues conducted feeding trials on lionfish and native goby prey.

image

work with the data

In addition to the aquarium experiments, the researchers conducted field observations of the feeding behavior of lionfishes in different locations—their native Pacific Ocean and their non-native Atlantic Ocean. They found that, in the Atlantic Ocean, most successful captures by lionfishes did not require them to blow jets of water at their prey (only 18 percent of the captures involved this behavior). However, in the Pacific Ocean, they found that 56 percent of successful captures by lionfishes involved them blowing jets of water at their prey. The researchers also reported that an extensive literature search showed that blowing jets of water at prey appears to be unique to lionfishes.

QUESTIONS

Question 1

Refer to Method step 3 in the experiment. Why do you suppose the researchers conducted lionfish aquarium feeding trials with free-swimming fish prey? Alternatively, what did the containers containing the fish prey afford the researchers?

The researchers used both methods to collect data for two different purposes. In the open aquarium trials, they wanted to observe the behavior of the lionfish and its prey in a relatively natural setting to see what strategies the lionfish used to capture and eat the prey. They learned that the lionfish would capture the prey when the prey turned and oriented its head toward the head of the lionfish. This headfirst orientation allowed the lionfish to more easily capture its prey.
In the container trials, the researchers wanted to observe the behavior of the lionfish, particularly the production of water-jet pulses from its mouth, in a setting where the lionfish could see, but not capture or eat, the prey. In this way the researchers could measure the number of water jets produced and the maximum distance a water jet could travel, to get an idea of what behaviors a lionfish might employ when its prey was hard to catch (in this case, because the prey was in an inaccessible container).

Question 2

Given what you know about the feeding behavior of lionfishes, would you expect them to produce more or fewer water jets to capture fish prey in the Atlantic Ocean compared with the Pacific Ocean? Would the distance the water jet had to travel be shorter or longer for the Atlantic Ocean prey compared with the Pacific Ocean prey?

Given that fish prey in the Atlantic Ocean are more naive to lionfish predatory behavior than fish prey in the Pacific Ocean, it would make sense that lionfishes would need to produce fewer and closer jets of water to capture their fish prey in the Atlantic Ocean compared with the Pacific Ocean. The Pacific Ocean fish should be harder to catch and thus require more water jets produced from a greater distance from the prey.

Question 3

Based on all the lionfish feeding behavior information presented here, what is the most plausible hypothesis to explain why lionfishes are such effective predators on coral-reef fishes in the Atlantic Ocean compared with the Pacific Ocean?

The field observations suggest that Pacific Ocean lionfishes need to resort to the water jet–blowing behavior to catch their prey more often than Atlantic Ocean lionfishes do. This pattern could be explained by the hypothesis that fish prey in the Atlantic Ocean are more naive to lionfishes and thus easier to catch. Atlantic Ocean fish prey are less likely to take appropriate evasive actions or require the use of the water jet–blowing behavior to confuse or disorient them. In addition, it is likely that the production of water jets is metabolically costly, and that lionfishes would use this behavior only when the cost of producing it is outweighed by the advantages it confers. Presumably this would be more likely to be the case for prey in the Pacific Ocean than for prey in the Atlantic Ocean.

Media Clip 55.2 Lionfish “Strike” Again!

www.life11e.com/mc55.2

A similar work with the data exercise may be assigned in LaunchPad.

PLANT DEFENSES AGAINST HERBIVORES Plant chemistry is one of the principal defense mechanisms against herbivores. Plants in the mustard family (Brassicaceae) offer just one example of the strategy of producing secondary metabolites as a mechanism to reduce herbivory. The amazing variety of secondary metabolites produced by plants to defend themselves against herbivores is considered in Key Concept 38.2.

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Some plants and algae protect themselves by being physically difficult to eat. For example, thorns and spines are highly effective deterrents to browsing vertebrate herbivores. Coralline algae contain calcium carbonate within their tissues, which deters many marine herbivores. An exception is some sea urchin species that have powerful articulated jaws and teeth and long coiling guts that can process less nutritious chalky tissue.

RECIPROCAL ADAPTATIONS IN HERBIVORES AND PLANTS A spectacular variety of adaptations to plant defenses has evolved in herbivores. Many herbivores circumvent plant defenses by behavioral means. For example, the secondary metabolites produced by a plant called St. John's wort (Hypericum perforatum) require exposure to sunlight for optimal toxicity, so some insects that feed on this plant roll its leaves into a light-impervious cylinder and feed in comfort in the dark. Large herbivores such as deer and horses graze on a wide variety of plant species, minimizing their exposure to any particular defensive chemical. Long-lived and with relatively good memories, they can learn to avoid plants with an unpleasant taste.

Unlike large mammalian herbivores, caterpillars and many other insect herbivores may spend their entire lives feeding on a single individual plant. Such exclusive diets are associated with highly specialized detoxification systems. The diamondback moth caterpillar eats plants in the cabbage family, which are rich in toxic mustard oil glycosides. In its gut is an enzyme that breaks down the glycosides into harmless by-products.

Some herbivores take resistance a step further by storing, or sequestering, plant toxins in specialized organs or tissues that are insensitive to those toxins. This strategy also makes the sequestered chemicals available for defense against the herbivores’ own enemies. The caterpillar of the monarch butterfly, for example, is insensitive to the neurotoxic glycosides in its milkweed host plants, but most of its enemies, including insect-eating birds, cannot tolerate these compounds.