8.2: Exploitative Interactions

Exploitative interactions include the diversity of interactions in which the fitness of one individual is enhanced at the expense of another, such as between a predator and its prey. This interaction benefits the fitness of the predator who gets a meal (+), while the prey suffers a strong fitness cost as it does not survive the interaction (-). Not all exploitative interactions have such extreme fitness consequences, however. Take for example, the interaction between a mosquito and a human. The mosquito experiences a fitness benefit through obtaining a meal, while the fitness cost to the human (assuming no diseases were transferred such as malaria) in the form of insignificant blood loss and an annoying itchy bump is quite minimal. Thus, exploitative interactions exist over a range of fitness benefits and costs and include a diversity of interactions such as those found between herbivores and plants, between predators and prey, and between parasites, parasitoids, pathogens, and their hosts.

Predation

Populations of predator and prey can have significant effects on one other. Predators can reduce the population sizes of their prey which can in turn affect other members of the communities. Predators and prey can become locked into evolutionary arms races where both predatory and prey experience strong selective pressure to either capture prey, or escape predation.

Definition: Predation

A Predation is a biological interaction where one species, the predator, kills and eats members of another species, its prey.

Populations of predators and prey in a community are not constant over time: in most cases, they vary in cycles that appear to be related. The most often cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), using nearly 200 year-old trapping data from North American forests (Figure \(\PageIndex{1}\)). This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, they kill so many hares that hare population begins to decline, followed by a decline in the lynx population because of scarcity of food. When the lynx population is low, the hare population size begins to increase due, at least in part, to low predation pressure, starting the cycle anew.

 

Figure \(\PageIndex{1}\): The cycling of lynx and snowshoe hare populations in Northern Ontario is an example of predator-prey dynamics. (CC-BY via OpenStax

Some researchers question the idea that predation models entirely control the population cycling of the two species. More recent studies have pointed to undefined density-dependent factors as being important in the cycling, in addition to predation. One possibility is that the cycling is inherent in the hare population due to density-dependent effects such as lower fecundity (maternal stress) caused by crowding when the hare population gets too dense. The hare cycling would then induce the cycling of the lynx because it is the lynxes’ major food source. The more we study communities, the more complexities we find, allowing ecologists to derive more accurate and sophisticated models of population dynamics.

Parasitoids represent a special kind of predator. Parasitoids lay their eggs on or inside other animals, especially insects, and the eggs hatch into larvae that consume the host individual from the inside, eventually killing it. Most parasitoid species are wasps and flies. 

Herbivory

Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. Herbivory, reduces the fitness of the plants being consumed, but is often not fatal. Still many plants have developed mechanisms to defend against herbivory such as spines, or toxic chemicals.

Definition: Herbivory

The consumption of one species (a plant) by another species (an animal) as a food source. The plant suffers a fitness cost, but often survives the interaction.

Defense Mechanisms against Predation and Herbivory

Predation and predator avoidance are strong selective agents. Any heritable character that allows an individual of a prey population to better evade its predators will be represented in greater numbers in later generations. Likewise, traits that allow a predator to more efficiently locate and capture its prey will lead to a greater number of offspring and an increase in the commonness of the trait within the population. Such ecological relationships between specific populations lead to adaptations that are driven by reciprocal evolutionary responses in those populations. Species have evolved numerous mechanisms to escape predation and herbivory (the consumption of plants for food). Defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of armor in animals or thorns in plants, discourage predation and herbivory by discouraging physical contact (Figure \(\PageIndex{2}\) a and b). Many animals produce or obtain chemical defenses from plants and store them to prevent predation. Many plant species produce secondary plant compounds that serve no function for the plant except that they are toxic to animals and discourage consumption. For example, the foxglove produces several compounds, including digitalis, that are extremely toxic when eaten (Figure \(\PageIndex{2}\) c).

Figure \(\PageIndex{2}\): The (a) honey locust tree (Gleditsia triacanthos) uses thorns, a mechanical defense, against herbivores, while the (b) Florida red-bellied turtle (Pseudemys nelsoni) uses its shell as a mechanical defense against predators. (c) Foxglove (Digitalis sp.) uses a chemical defense: toxins produced by the plant can cause nausea, vomiting, hallucinations, convulsions, or death when consumed. (d) The North American millipede (Narceus americanus) uses both mechanical and chemical defenses: when threatened, the millipede curls into a defensive ball and produces a noxious substance that irritates eyes and skin. (credit a: modification of work by Huw Williams; credit b: modification of work by “JamieS93”/Flickr; credit c: modification of work by Philip Jägenstedt; credit d: modification of work by Cory Zanker). (CC-BY; via OpenStax)

Many species use physical appearance, such as body shape and coloration, to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig which makes it very hard to see when stationary against a background of real twigs (Figure \(\PageIndex{3}\) a). In another example, the chameleon can, within limitations, change its color to match its surroundings (Figure \(\PageIndex{3}\) b). Both of these are examples of camouflage, or avoiding detection by blending in with the background. There are many behavioral adaptations to avoid or confuse predators. Playing dead and traveling in large groups, like schools of fish or flocks of birds, are both behaviors that reduce the risk of being eaten.

Figure \(\PageIndex{3}\): The tropical walking stick (a) and the chameleon (b) use body shape and/or coloration to prevent detection by predators. (credit a: modification of work by Linda Tanner; credit b: modification of work by Frank Vassen) (CC-BY; via OpenStax)

Some species use coloration as a way of warning predators that they are distasteful or poisonous. For example, the monarch butterfly caterpillar sequesters poisons from its food (plants and milkweeds) to make itself poisonous or distasteful to potential predators. The caterpillar is bright yellow and black to advertise its toxicity. The caterpillar is also able to pass the sequestered toxins on to the adult monarch, which is also dramatically colored black and red as a warning to potential predators. Fire-bellied toads produce toxins that make them distasteful to their potential predators. They have bright red or orange coloration on their bellies, which they display to a potential predator to advertise their poisonous nature and discourage an attack. These are only two examples of warning coloration, which is a relatively common adaptation. Warning coloration only works if a predator uses eyesight to locate prey and can learn—a naïve predator must experience the negative consequences of eating one before it will avoid other similarly colored individuals (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): The strawberry poison dart frog (Oophaga pumilio) (a) uses aposematic coloration to warn predators that it is toxic, while the striped skunk (Mephitis mephitis) (b) uses aposematic coloration to warn predators of the unpleasant odor it produces. (credit a: modification of work by Jay Iwasaki; credit b: modification of work by Dan Dzurisin) (CC-BY; via OpenStax)

While some predators learn to avoid eating certain potential prey because of their coloration, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless ones, even though they do not have the same level of physical or chemical defenses against predation as the organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous insects, thereby discouraging predation (Figure \(\PageIndex{5}\)).

 

Figure \(\PageIndex{5}\): One form of mimicry is when a harmless species mimics the coloration of a harmful species, as is seen with the (a) wasp (Polistes sp.) and the (b) hoverfly (Syrphus sp.). (credit: modification of work by Tom Ings) (CC-BY; via OpenStax)

 

In Müllerian mimicry, multiple species share the same warning coloration, but all of them actually have defenses. Figure \(\PageIndex{6}\) shows a variety of foul-tasting butterflies with similar coloration. In Emsleyan/Mertensian mimicry, a deadly prey mimics a less dangerous one, such as the venomous coral snake mimicking the nonvenomous milk snake. This type of mimicry is extremely rare and more difficult to understand than the previous two types. For this type of mimicry to work, it is essential that eating the milk snake has unpleasant but not fatal consequences. Then, these predators learn not to eat snakes with this coloration, protecting the coral snake as well. If the snake were fatal to the predator, there would be no opportunity for the predator to learn not to eat it, and the benefit for the less toxic species would disappear.

Figure \(\PageIndex{6}\): Several unpleasant-tasting Heliconius butterfly species share a similar color pattern with better-tasting varieties, an example of mimicry. (credit: Joron M, Papa R, Beltrán M, Chamberlain N, Mavárez J, et al.)) (CC-BY; via OpenStax)

Parasitism

A parasite is an organism that feeds off another without immediately killing the organism it is feeding on. In this relationship, the parasite benefits, but the organism being fed upon, the host, is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. Parasites may kill their hosts, but there is usually selection to slow down this process to allow the parasite time to complete its reproductive cycle before it or its offspring are able to spread to another host.

The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm causes disease in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed (Figure \(\PageIndex{7}\)). The tapeworm can live inside the intestine of the host for several years, benefiting from the host’s food, and it may grow to be over 50 feet long by adding segments. The parasite moves from one host species to a second host species in order to complete its life cycle. Plasmodium falciparum is another parasite: the protists that cause malaria, a significant disease in many parts of the world. Living inside human liver and red blood cells, the organism reproduces asexually in the human host and then sexually in the gut of blood-feeding mosquitoes to complete its life cycle. Thus malaria is spread from human to mosquito and back to human, one of many arthropod-borne infectious diseases of humans.

Figure \(\PageIndex{7}\): This diagram shows the life cycle of the tapeworm, a human worm parasite. (credit: modification of work by CDC). (CC-BY; via OpenStax)

Parasites infect many types of organisms, including other animals and plants. For example, the fleas and roundworms are common dog parasites. Plants can be infected by fungi, bacteria, and viruses; there are also parasitic plants that parasitic other plants. Even bacteria can parasitized by viruses called bacteriophages. 

Attribution

This page is a modified derivative of:

• 45.6 Community Ecology; CC-BY via OpenStax
• 19.4 Community Ecology; CC-BY via OpenStax