Consumption: Predation and Herbivory

Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is distinct from scavenging on dead prey, though many predators also scavenge. Predation and herbivory overlap because seed predators and destructive frugivores kill their “prey.” The concept of predation is broad, defined differently in different contexts, and includes a wide variety of feeding methods

Predation

Perhaps the classical example of species interaction is predation: the consumption of prey by its predator. Nature shows on television highlight the drama of one living organism killing another. Predators are adapted and often highly specialized for hunting, with acute senses such as vision, hearing, or smell. Many predatory animals, both vertebrate and invertebrate, have sharp claws or jaws to grip, kill, and cut up their prey. Other adaptations include stealth and aggressive mimicry that improve hunting efficiency. When prey is detected, the predator assesses whether to attack it. Predators may actively search for or pursue prey (pursuit predation) or sit and wait for prey (ambush predation), often concealed, prior to attack. If the attack is successful, the predator kills the prey, removes any inedible parts like the shell or spines, and eats it.

Modeling Predator/Prey Dynamics

Population sizes 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 14.1). Lotka and Volterra independently proposed in the 1920s a mathematical model for the population dynamics of a predator and prey that would explain this observation. These Lotka-Volterra predator-prey equations have since become an iconic model of mathematical biology. The Lotka-Volterra model predicts that 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 to low predation pressure, starting the cycle anew. This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population.

Like all mathematical models, the Lotka-Volterra model of predator-prey dynamics makes a number of assumptions that must be met in order for the model’s predictions to be observed. These assumptions include that the number of prey grow exponentially in the absence of predators (there is unlimited food available to the prey), and that the number of predators decay exponentially in the absence of prey (predators must eat prey or starve). Contact between predators and prey increases the number of predators and decreases the number of prey.

The Lotka-Volterra model is one of the earliest predator-prey models to be based on sound mathematical principles. It forms the basis of many models used today in the analysis of population dynamics. Unfortunately, in its original form Lotka-Volterra has some significant problems. It predicts that there is no stable equilibrium point, but rather that the predator and prey populations will cycle endlessly. While this cycling has been observed in some situations in nature (such as the historical trapping data from lynxes and hares), it is not very common.

The Lotka-Volterra by itself is not sufficient to model many predator-prey systems because it assumes that the only factors influencing predator and prey populations are the population sizes of the predators and prey. However, there are other density-dependent factors that are important in influencing prey population size, 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.

Herbivory

Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs are known as primary consumers. Herbivory is usually limited to animals that eat plants. Fungi, bacteria, and protists that feed on living plants are usually termed plant pathogens (plant diseases), while fungi and microbes that feed on dead plants are described as saprotrophs. Flowering plants that obtain nutrition from other living plants are usually termed parasitic plants.

Two herbivore feeding strategies are grazing (e.g. cows) and browsing (e.g. moose). For a terrestrial mammal to be called a grazer, at least 90% of the forage has to be grass, and for a browser at least 90% tree leaves and twigs. An intermediate feeding strategy is called “mixed-feeding”. In their daily need to take up energy from forage, herbivores of different body masses may be selective in choosing their food. “Selective” means that herbivores may choose their forage source depending on environmental conditions such as season or food availability, but also that they may choose high-quality (and consequently highly nutritious) forage before lower quality. The latter especially is determined by the body mass of the herbivore, with small herbivores selecting for high-quality forage, and with increasing body mass animals are less selective.

Defense Mechanisms against Predation and Herbivory

Predation has a powerful selective effect on prey, and prey evolve anti-predator adaptations such as warning coloration, alarm and other calls, camouflage, copying (mimicry) of well-defended species, and defensive spines and chemicals. Sometimes predator and prey find themselves in an evolutionary arms race, a cycle of adaptations and counter-adaptations. Unlike animals, most plants cannot outrun predators. However, plants have evolved a variety of mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.

The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten. Figure 14.2 shows some organisms’ defenses against predation and herbivory.

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 14.3a). In another example, the chameleon can, within limitations, change its color to match its surroundings (Figure 14.3b). 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.

Some species use coloration as a way of warning predators that they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemicals, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration (Figure 14.4).

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 14.5).

In Müllerian mimicry, multiple species share the same warning coloration, but all of them actually have defenses. Figure 14.6 shows a variety of foul-tasting butterflies with similar coloration.

Parasitism

Parasitism is a symbiotic between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. Like predation, parasitism is a type of consumer-resource interaction, but unlike predators, parasites are typically much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. Parasites of animals are highly specialized, and reproduce at a faster rate than their hosts. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the malaria-causing Plasmodium species, and fleas.

Parasitism describes a relationship in which one member of the association benefits at the expense of the other. So, while the parasite itself benefits, the host organism is harmed. This is in contrast to commensalism in which one member benefits without affecting the other. The parasite typically weakens the host as it consumes resources that the host would normally benefit from. The parasite usually does not directly kill the host, but could indirectly contribute to the host’s death by weakening it and robbing it of resources.

Parasites are classified in a variety of overlapping categories, based on their interactions with their hosts and on their life-cycles, which are sometimes very complex. An obligate parasite depends completely on the host to complete its life cycle and cannot survive without the host. In contrast, a facultative parasite benefits from parasitizing from a host but does not require its host for survival. An endoparasite lives inside the host’s body; an ectoparasite lives outside, on the host’s surface. Mesoparasites are in between endoparasites and ectoparasites—they enter an opening in the host’s body and remain partly embedded there. Some parasites can be generalists, feeding on a wide range of hosts, but many parasites are specialists and extremely host-specific.

The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat is consumed. The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is eating, and may grow to be over 50 ft long by adding segments. The parasite moves from species to species in a cycle, making two hosts necessary to complete its life cycle.

Many parasites that has multiple hosts are trophically-transmitted between their hosts. Trophically-transmitted parasites are transmitted by being eaten by a host. In their juvenile stages they infect a host termed the intermediate host. The intermediate host is a host the parasite can infect, but cannot complete its reproduction inside of. When the intermediate-host animal is eaten by a predator, the definitive host, the parasite survives the digestion process and matures into an adult; some live as intestinal parasites. When parasites have multiple hosts, the definitive host is the host that they can reproduce inside of. Many trophically-transmitted parasites modify the behavior of their intermediate hosts, increasing their chances of being eaten by a predator.

Connection to Evolution

Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae (Figure 14.7a), and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia. Plasmodium falciparum and P. vivax are the most common causative agents of malaria, and P. falciparum is the most deadly (Figure 14.7b). When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1%. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.

 

In Southeast Asia, Africa, and South America, P. falciparum has developed resistance to the anti-malarial drugs chloroquine, mefloquine, and sulfadoxine-pyrimethamine. P. falciparum, which is haploid during the life stage in which it is infectious to humans, has evolved multiple drug-resistant mutant alleles of the dhps gene. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P. falciparum needs only one drug-resistant allele to express this trait.

In Southeast Asia, different sulfadoxine-resistant alleles of the dhps gene are localized to different geographic regions. This is a common evolutionary phenomenon that occurs because drug-resistant mutants arise in a population and interbreed with other P. falciparum isolates in close proximity. Sulfadoxine-resistant parasites cause considerable human hardship in regions where this drug is widely used as an over-the-counter malaria remedy. As is common with pathogens that multiply to large numbers within an infection cycle, P. falciparum evolves relatively rapidly (over a decade or so) in response to the selective pressure of commonly used anti-malarial drugs. For this reason, scientists must constantly work to develop new drugs or drug combinations to combat the worldwide malaria burden (Summit Vinayak et al., 2010).

Sumiti Vinayak, et al., “Origin and Evolution of Sulfadoxine Resistant Plasmodium falciparum,” Public Library of Science Pathogens 6, no. 3 (2010): e1000830, doi:10.1371/journal.ppat.1000830.

Variations on parasitism

Classic parasites live in or on the host’s body. However, there are many interactions also described as parasitism where the parasite lives in close relationship with the host, but not directly in or on it.

Social parasitism

Social parasites take advantage of interspecific interactions between members of eusocial animals such as ants, termites, and bumblebees. An example is the large blue butterfly, Phengaris arion. Larvae (caterpillars) of this species mimic ants by emitting ant queen pheromones. Ant workers find larvae and care for them as they would their queen; they take it to an inner chamber in their nest and feed it. Another exampe is a bumblebee, Bombus bohemicus, which invades the hives of other bees and takes over reproduction while their young are raised by host workers.

Brood Parasitism

In brood parasitism, the hosts act as parents as they raise the young as their own. Brood parasites include birds in different families such as cowbirds, whydahs, cuckoos, and black-headed ducks. These birds do not build nests of their own, but leave their eggs in nests of other species. The eggs of some brood parasites mimic those of their hosts, while some cowbird eggs have tough shells, making them hard for the hosts to kill by piercing. The adult female European cuckoo further mimics a predator, the European sparrowhawk, giving her time to lay her eggs in the host’s nest unobserved.

Parasitoidism

Some relationships that result in the prey’s death are not generally called predation. A parasitoid, such as an ichneumon wasp, lays its eggs in or on its host; the eggs hatch into larvae, which eat the host, and it inevitably dies. This is distinct from classic parasitism because parasites do not directly kill their hosts. A predator can be defined to differ from a parasitoid in that it has many prey, captured over its lifetime, where a parasitoid’s larva has just one, or at least has its food supply provisioned for it on just one occasion.

Most parasitoids are parasitoid wasps or other hymenopterans. They can be divided into two groups, idiobionts and koinobionts, differing in their treatment of their hosts.

Idiobiont parasitoids sting their often large prey on capture, either killing them outright or paralyzing them immediately. The immobilized prey is then carried to a nest, sometimes alongside other prey if it is not large enough to support a parasitoid throughout its development. An egg is laid on top of the prey and the nest is then sealed. The parasitoid develops rapidly through its larval and pupal stages, feeding on the provisions left for it.

Koinobiont parasitoids, which include flies as well as wasps, lay their eggs inside young hosts, usually larvae. These are allowed to go on growing, so the host and parasitoid develop together for an extended period, ending when the parasitoids emerge as adults, leaving the prey dead, eaten from inside. Some koinobionts regulate their host’s development, for example preventing it from pupating or making it molt whenever the parasitoid is ready to molt. They may do this by producing hormones that mimic the host’s molting hormones (ecdysteroids), or by regulating the host’s endocrine system.

References

Adapted from

Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/details/books/biology-2e

and

Chasnov (2022). The Lotka-Volterra Predator-Prey Model. LibreTexts. Retrieved from https://math.libretexts.org/Bookshelves/Applied_Mathematics/Mathematical_Biology_(Chasnov)/01%3A_Population_Dynamics/1.04%3A_The_Lotka-Volterra_Predator-Prey_Model

and

Holmberg, T. J. (2023). Antagonistic Interactions. LibreTexts. Retrieved from https://bio.libretexts.org/Sandboxes/tholmberg_at_nwcc.edu/BIOL_1213/04%3A_Unit_4%3A_-_Ecology/4.03%3A_Community_Ecology/4.3.2%3A_Antagonistic_Interactions

and

Various authors. (2022). Ecology for All!. LibreTexts. Retrieved from https://bio.libretexts.org/Courses/Gettysburg_College/01%3A_Ecology_for_All/16%3A_Antagonistic_Interactions/16.04%3A_Parasitism#:~:text=Parasitoids%20are%20insects%20which%20sooner,dipterans%20such%20as%20phorid%20flies.