4.3.2: Antagonistic Interactions

Unit 4.3.2 - Antagonistic Interactions

• Please read and watch the following Learning Resources
• Reading the material for understanding, and taking notes during videos, will take approximately 1 hour.
• Optional Activities are embedded.
• Bolded terms are located at the end of the unit in the Glossary. There is also a Unit Summary at the end of the Unit. 
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Learning Objectives

• Explain the predator-prey cycle
• Give examples of defenses against predation and herbivory
• Describe the competitive exclusion principle
• Identify adaptations of pathogens

Introduction to Antagonistic Interactions

Video

Recognize predation and the many ways prey organisms have developed to avoid it in this 10-minute video.
Question after watching: What is meant when Hank says that predator-prey interactions are an "evolutionary arms race"?

Predation

Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviors that includes parasitism and micropredation (which usually do not kill the host) and parasitoidism (which always does, eventually). 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

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.

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.

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. Zoologists generally call this a form of parasitism, though conventionally parasites are thought not to 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.

Predator-Prey Cycles

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 the 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.

The idea that the population cycling of the two species is entirely controlled by predation models has come under question. More recent studies have pointed to undefined density-dependent factors as being important in cycling, in addition to predation. One possibility is that 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 communities are studied, the more complexities are found, allowing ecologists to derive more accurate and sophisticated models of population dynamics.

Optional Activity \(\PageIndex{1}\)

Pick from the statements below to best describe a predator-prey cycle:

1. Prey increase in numbers, causing an increase in the predator population, which, in turn, causes a downturn in prey numbers, and leads to a downturn in predator numbers, and then the cycle repeats.

2. The number of prey is directly related to the number of predators so that the two populations remain at the same ratio even though the total population numbers fluctuate.

3. Increasing prey numbers trigger decreases in predator numbers, which eventually causes a decrease in prey numbers as predators become too sparse, and then the cycle repeats.

4. A prey population undergoes a cyclic increasing and decreasing fluctuation in size as its predator population undergoes the same cycle but in a mirror-image relationship.

Answer

a. Prey increase in numbers, causing an increase in the predator population, which, in turn, causes a downturn in prey numbers, and leads to a downturn in predator numbers, and then the cycle repeats.

Herbivory

Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. Herbivory describes the consumption of plants by insects and other animals, and it is another interspecific relationship that affects populations. 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. 

Unlike animals, most plants cannot outrun predators or use mimicry to hide from hungry animals. Coevolution and phylogenetic correlation between herbivores and plants are important aspects of the influence of herbivore and plant interactions on communities and ecosystem functioning, especially in regard to herbivorous insects. This is apparent in the adaptations plants develop to tolerate and/or defend from insect herbivory and the responses of herbivores to overcome these adaptations. Some plants have developed 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 evolution of antagonistic and mutualistic plant-herbivore interactions do not exclude one another and may co-occur.

Defense Mechanisms against Predation and Herbivory

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 and Chemical Defenses

Mechanical defenses, such as the presence of thorns on plants or the hard shell of 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 \(\PageIndex{2}\) shows some organisms’ defenses against predation and herbivory.

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)

Crypsis

Many species use their 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 change its color to match its surroundings (Figure \(\PageIndex{3}\)b). Both of these are examples of crypsis, or avoiding detection by blending in with the background.

a

b

Figure \(\PageIndex{3}\): (a) The tropical walking stick and (b) the chameleon 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)

Warning Coloration

Some species use     . This type of defensive mechanism is called aposematic coloration, or warning coloration (Figure \(\PageIndex{4}\)). 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 chemical, 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. 

Figure \(\PageIndex{4}\): (a) The strawberry poison dart frog (Oophaga pumilio) uses aposematic coloration to warn predators that it is toxic, while the (b) striped skunk (Mephitis mephitis) 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)

Mimicry

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 \(\PageIndex{5}\)).

a

b

Figure \(\PageIndex{5}\): Batesian mimicry occurs when a harmless species mimics the coloration of a harmful species, as is seen with the (a) bumblebee and (b) bee-like robber fly. (credit a, b: modification of work by Cory Zanker)

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. 

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

Optional Activity \(\PageIndex{2}\)

In a region in Texas, biologists observed that two highly venomous snakes with similar markings deter owl predators. Upon closer inspection, the snakes were determined to belong to different genera and species. How would these biologists describe the mimicry in this case?

1. Batesian mimicry, because it involves a nontoxic species that resembles a toxic species.

2. Emsleyan/Mertensian mimicry because an extremely toxic species resembles a less toxic species.

3. Batesian mimicry because it involves an extremely toxic species that resembles a less toxic species.

4. Mullerian mimicry because it involves different species that both produce toxins and display similar warning coloration.

Answer

d. Mullerian mimicry because it involves different species that both produce toxins and display similar warning coloration.

Video

Explore some of the many internal and external defenses that make plants less appealing to herbivores in this 6-minute video.
Question after watching: What are the different physical barriers that plants use to repel microscopic bacteria, fungi, and herbivores?

Competition

Competition can be defined as an interaction between organisms or species, in which both require a resource that is in limited supply (such as food, water, or territory). Competition lowers the fitness of both organisms involved, since the presence of one of the organisms always reduces the amount of the resource available to the other. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition.

According to the competitive exclusion principle, no two species with the same ecological niche can coexist, and the species less suited to compete for resources should either adapt or die out. Competition within and between species for resources plays a critical role in natural selection. Ecologists model competition using the Lotka-Volterra competition model, and use this model to predict the conditions under which two species will coexist or one species outcompetes the other. 

Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. An example of this principle is shown in Figure \(\PageIndex{7}\), with two protozoan species, Paramecium aurelia and Paramecium caudatum. In this famous experiment by Gause (1934), when grown individually in the laboratory, they both thrive. But when they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction. 

This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to occupy different microniches. These organisms coexist by minimizing direct competition.

Figure \(\PageIndex{7}\): Paramecium aurelia and Paramecium caudatum grow well individually, but when they compete for the same resources, the P. aurelia outcompetes the P. caudatum.

In the study of community ecology, competition within and between members of a species is an important biological interaction. Competition is one of many interacting biotic and abiotic factors that affect community structure, species diversity, and population dynamics (shifts in a population over time). Competition within and between species for resources is important in natural selection.

Optional Activity \(\PageIndex{3}\)

Explain how two different species can coexist in the same habitat according to the competitive exclusion principle.

1. Two species can coexist in the same habitat as long as they do not share the same trophic level.

2. Two species can coexist in the same habitat as long as they do not share the same mates.

3. Two species can coexist in the same habitat as long as they do not share the same resources.

4. Two species can coexist in the same habitat as long as they do not share the same life span.

Answer

c. Two species can coexist in the same habitat as long as they do not share the same resources.

Pathogens and Infection

All organisms, from bacteria to trees to humans are capable of getting infections. An infection is the invasion of an organism's body tissues by disease-causing agents (pathogens), the pathogen's multiplication, and the reaction of host tissues to the pathogens and the toxins the pathogens produce. An infectious disease, also known as transmissible disease or communicable disease, is an illness resulting from an infection.

Infections can be caused by a wide range of pathogens, most prominently bacteria and viruses. Hosts can fight infections using an immune system. 

There is a general chain of events that applies to infections, sometimes called the chain of infection (Figure \(\PageIndex{8}\)). The chain of events involves several steps – which include the infectious agent, reservoir, entering a susceptible host, exit, and transmission to new hosts. Each link must be present in chronological order for an infection to develop. Understanding these steps helps health-care workers target the infection and prevent it from occurring in the first place.

Figure \(\PageIndex{8}\): The chain of events that lead to infection.

Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani release a toxin that paralyzes muscles, and Staphylococcus sp. releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example, less than 5% of individuals infected with the polio virus develop the disease. On the other hand, some infectious agents are highly virulent. The prion-causing mad cow disease and Creutzfeldt–Jakob disease invariably kills all animals and people that are infected.

Persistent infections occur because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the Herpes zoaster virus (that causes chickenpox and shingles), which tends to hide in nerves and become reactivated when specific circumstances arise.

Reference

Gause, G.F. 1934. Experimental analysis of vito volterra's mathematical theory of the struggle for existence. Science, 79(2036), 16- 17. https://doi.org/10.1126/science.79.2036.16.b