Ecology/Predation and Herbivory

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Chapter 11. Predation and Herbivory

Introduction: Predation
Ultimately, the source of energy for all life originates from the sun. Plants utilize the sun's energy, animals eat plants and utilize the plants' energy, and some animals eat other animals and utilize their energy. The food chain is a cycle of predation, and although it is necessary for life to exist, it has to have limits. Prey develop defenses against their predators, and predators strive to overcome such obstacles. The balance between survival of prey and predator is part of the reason that our ecosystem is so diverse.

It is easy to think of predation in the context of common sense. Predator eats prey. However, ecologically, predation is defined as any interaction between two organisms that results in a flow of energy between them. This definition is applicable to both plants and animals. There are four commonly recognized types of predation: (1) carnivory, (2) herbivory, (3) parasitism, and (4) mutualism. Each type of predation can by categorized based on whether or not it results in the death of the prey. Carnivory is lethal to the prey, while herbivory and parasitism may or may not be lethal to the prey. Mutualism is not lethal to either predator or prey but, rather, benefits both organisms.



Carnivory
Carnivory is usually the first type of predation that comes to mind when thinking about relationships between predator and prey. Carnivory takes place when a predator consumes meat rather than plants and consequently kills its prey. Organisms that eat meat are accordingly called carnivores. Some types of carnivores do not need to eat meat in order to survive but do so anyway. Obligatory carnivores, on the other hand, cannot survive without meat in their diet. Hypercarnivores are an extreme example of obligatory carnivores and are able to eat only meat due to restricted digestive capabilities. Even though we sometimes associate carnivores with larger animals, carnivorous habits can occur in plants and fungi that feed on insects or microscopic invertebrates. Therefore, the concept of meat in the definition of carnivory is open to interpretation.

Carbone et al. (2007) used mathematical models to predict prey size limitations for carnivorous mammals. These limitations can be divided into two dietary groups: (1) small-bodied species, which feed on prey smaller than themselves, and (2) large-bodied species, which feed on prey of approximately the same size. A daily energy expenditure model was developed that incorporates the energy cost of hunting per unit of body mass. The resulting predictions were compared to daily energy intake estimates for 32 species. The results showed consistent differences in energy and time budgets between small and large-bodied species. Specifically, it was shown that energy expenditure by the predator increased with increasing prey size. That is, large-bodied carnivores expend more energy hunting their prey. Carnivores have, therefore, been assigned a maximum possible body mass of one ton. Even though the largest extant carnivore, the polar bear, weighs only about one half of one ton, there are documented extinct species that tipped the scales at nearly one ton. Interestingly, herbivores can grow to be much larger than carnivores, as they don't need to hunt down their prey and often aren't limited in the way that carnivores are. There are extinct herbivores that weighed in at nearly 15 tons!

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Modeling Herbivory
For monophagous herbivores, we use the same model as the Lotka-Volterra Predator-Prey model.


 * dP/dt = βαNP - mP

For polyphagous herbivores, assuming a constant rate of herbivory,


 * dN/dt = rN(1 - N/K) - h, where h is a constant

In this population, N is dependent upon r and k, where r is the growth rate of N and k is the carrying capacity.

Parasitism and Mutualism


Parasitism takes place when one organism (parasite) benefits at the expense of another (host). This type of interaction usually harms the host but, unlike carnivory, does not always result in the death of the host. In most cases, the parasite is much smaller than the host and has a much faster reproductive rate. The tapeworm is an example of a parasite that can thrive in the human digestive system, where it uses nutrients that are important to its host. This often results in weight loss.

Interestingly, it has been proposed that sleep evolved as a defense against parasitism. Studies have been done in which different mammals slept for varying amounts of time. White blood cell counts were subsequently compared, and parasitism rates for different mammals were analyzed. Based on the data, there was a strong correlation between sleep and immune system strength (white blood cell count). Also, mammals that slept longer were shown to be less prone to parasitic infection. It is questionable whether or not sleep evolved directly to ward off parasitism, but it is clear that sleep has an important role in boosting the human immune system.

Mutualism is similar to parasitism except that the host is not harmed. In fact, both species benefit from the interaction. One example of this type of interaction is called the endosymbiotic theory. The endosymbiotic theory suggests that, at some point in the past, an anaerobic cell engulfed an aerobic bacterium, and a mutualistic relationship resulted. That is, the cell provided food and shelter for the bacterium, and the bacterium made additional energy available to the cell. This mutualistic interaction proved so beneficial that natural selection favored these cells. The bacteria that were engulfed are now commonly known as mitochondria. A similar scenario involving a eukaryotic cell (already possessing mitochondria) engulfing a photosynthetic cyanobacterium is hypothesized to have resulted in chloroplasts.

Prey Defenses


Predation can result in the development of anti-predation strategies by prey populations. Consequently, predator populations develop their own strategies to overcome prey defenses. This phenomenon, known as the red-queen theory, plays an important role in evolution and predator-prey interactions.

Prey defenses can be divided into several different categories. Many organisms utilize chemical defenses in the form of toxic or noxious secretions that cause them to taste or smell bad. Most of these organisms exhibit aposematic coloring patterns, which means that they are characterized by distinct markings or bright colorings that signal to the predator that they are toxic and non-palatable (unpleasant tasting). Due to the success of aposematic organisms, organisms that are not noxious, and therefore vulnerable to predation, may exhibit similar coloring patterns to aposematic organisms in order to falsely repel their predators. This is another type of defense called Batesian mimicry. Batesian mimicry occurs when a palatable species imitates a non-palatable species. This type of mimicry is only successful if the population of the palatable species is smaller than the population of the non-palatable species. Müllerian mimicry occurs when two non-palatable species converge their coloration and patterns for reinforced protection. South American butterflies provide an example of both Batesian and Müllerian mimicry.

For more information, * Read Mimicry

Prey also use coloring patterns to blend into their surroundings. Crypsis or camouflage is a means by which organisms use their shape and their coloring to blend into their environment rather than standing out. Clearly, color is an important defense that can be used in multiple ways to protect prey from their predators.

Fighting, escaping, and armor are three other possible prey responses. Some prey fight back against their predators as a form of self defense. Organisms may also try to escape or flee from their predators by using a startle response or indirection. During this type of defense, the prey perform an action that startles or distracts the predator long enough for them to escape. For example, some toads shoot blood from their eyes, and other animals imitate loud sounds. Armor is a form of physical protection. Things such as the shells on turtles, spines or needles on a pufferfish, and thorns on a plant are examples of different types of armor.

Other organisms employ intimidation or threat techniques that make them appear larger than they really are. Hissing and growling are examples of intimidation techniques that display aggression and create the illusion of an increased threat to predators. Satiation and masting are intertwined ideas that result in the survival of some prey at the expense of others. Masting is more commonly thought of as herding and occurs when organisms travel in very large groups. Because of the size of the groups, predators can only consume so many of them, resulting in the survival of some prey, but not all.

Predation
The predation hypothesis states that predators reduce prey. Fewer prey liberates resources, which in turn are used by other species, which increases species richness. According to Holling (1973), populations of predators and prey often mirror each other. When there is an influx in prey, predator populations tend to rise as well due to an abundance of food. As prey diversity increases, it decreases the likelihood of any single species being overly predated. Therefore this also leads to species richness due to predation. Yoshida et al. supports this idea, and adds that predator density also has a huge impact on species richness. If there is a low predator density, then prey have less of a need to adapt. However, as predator density increases, it forces prey to evolve and to become more resistant to predation, thus forcing more species diversity. Predation also includes the concept of keystone species, or a species that the community is dependent upon, which maintatins stability of the system and increases species richness. Keystone species are often predators, e.g. the African elephant, Loxodonta africana, a keystone species of the savannas. One problem this hypothesis encounters is what drives predator diversity.
 * Read Predation
 * Read Keystone species

The African elephant maintains its habitat by uprooting small trees, which left alone would overtake the savannas and turn them from a grassland into a forest or shrub thicket. By maintaining the grasslands, the elephant ensures the survival of herds of grazing antelope, and the antelope's predators, such as the prides of lions.

The first predator-prey models were proposed in the 1920s by Lotka and Volterra and predict oscillations in populations of predators and prey. Their model was expanded to include heterogeneity so the model could predict actual predator-prey behavior. (Huffaker 1958)
 * Read Lotka-Volterra Equation

Mimicry

 * read Mimicry

Batesian Mimicry
As a defense against predation some species have evolved to mimic the coloration and patterning of species that are poisonous or less palatable. This type of defense mechanism was first recognized by the English naturalist Henry Bates in 1862.

Müllerian mimicry
Two species that are non-palatable converge on a similar pattern or coloration. This is known as Müllerian mimicry and serves as a reinforcing warning to the predator.

Plant defenses
In their fight for survival, plants have evolved defenses against herbivory. There are three main categories of defense that plants use. The first defensive mechanism is mechanical defense, which includes physical features such as thorns and needles. The second type of plant defense is masting, which involves producing more progeny than any predators can consume. An example of this is Oak and Beach trees, which release so many cones that they can not all be consumed by predators. The final type of defense is chemical defense, which is comprised of five subgroups. The first chemical responses are secondary metabolites. This type defense prevents the herbivore from using its central metabolism to digest the plant. The second type of chemical defense is producing unpalatable taste which keeps predators from consuming the plant. These plants normally contain terpenes and phenolics. Some plants that take advantage of this mechanism are milkweed and mustards. The third type of chemical defense is inhibiting absorption. These plants do not allow the predators digestive enzymes to get absorb nutrients during digestion. The more of the plant that the predator eats the greater the effect will be. An example of plants that use this method are tannins and cumerins. The fourth chemical defense is a plant's ability to make toxins. These toxins are usually alkaloids and cyanides. Deadly nightshade is a plant that uses a toxin to protect itself from any predator. The last type of chemical defense is repellants. These plants usually contain one of the chemicals terpenes or amines. Citronella plants take advantage of this method.


 * Read Plant defense against herbivory

Predator-Prey Models
In 1965 Rosenweig and McArthur came up with a predator prey model using linked equations that was a modification of the Lotka-Volterra equation.
 * $$\frac{dN}{dt} = rN-\alpha NP$$


 * $$\frac{dP}{dt} =\beta\alpha NP -mP$$

In this model:
 * N=Prey Species Population
 * P=Predator Population
 * $$\alpha$$=Predation Coefficient(consists of searching, catching, and consuming)
 * $$\beta$$=Conversion Factor
 * m=Mortality Coefficient
 * r=Growth Rate

In order to make more sense of these models we need to look at Zero Growth Isoclines(ZGI) also called a Nullcline. This is where $$\frac{dN}{dt}$$ or $$\frac{dP}{dt}$$ is set equal to zero. Then the equation it is solved for P or N. To solve for the prey's ZGI you solve for P when $$\frac{dN}{dt}=0$$. To solve for the predator's ZGI you solve for N when $$\frac{dP}{dt}=0$$.
 * $$ \frac{dN}{dt}=0$$
 * $$rN-\alpha NP=0$$
 * $$rN=\alpha NP$$
 * $$r=\alpha P$$
 * $$P=\frac{r}{\alpha}$$

In order to maintain prey population size at equilibrium, the predator population must increase or become more efficient ($$\alpha$$ increases) predators. The larger $$\alpha$$, the fewer predators that are needed. To maintain P at equilibrium you need N prey; if m is higher you need more N or an increase in $$\alpha \beta$$. Also, the more efficient P (higher $$\alpha \beta$$), the fewer N needed. When you plot the prey's ZGI with N as the horizontal axis and P(r/$$\alpha$$) as the horizontal axis, you get a horizontal line. When you plot the predator's ZGI on the same graph (N=m/$$\beta \alpha$$) you get a vertical line. When you plot the vectors for all different populations, you get a stable limit cycle, which also leads to stable oscillations on a graph where N and P are plotted against time. This is also well known as the common Predator-Prey cycle.

There are also a few assumptions that are built into this model:
 * 1. Size of N is limited only by P
 * 2. N is the only food source for P
 * 3. P can consume infinite amounts of N (never satiated)
 * 4. N & P encounter each other randomly
 * 5. P & N age structure is invariant

Predator-Prey cycles are an important part of understanding the relationships between predator and prey. The first realization of this cycle was believed to be noticed by an Italian mathematician named Volterra. Volterra's equations are extensively used in the study of species relationships. Volterra observed Adriattic fishing fleets rise and fall with the numbers of fish caught. As the number of fish (prey) increased so did the number of fisherman (predator) and as the fish decreased so did the number of fisherman. This cycle will eventually repeat itself.

It has also become apparent that Predator-Prey relationships are a major driving force behind evolution. As predator density increases, the number of prey consumed also increases. This forces the prey to adapt to avoid consumption. In other words, the individuals who are better at avoiding consumption live on. Since the better-adapted individuals survive to reproduce, they pass on their survival skills to their young. With a prey population that is becoming better at avoiding consumption, the predators must also adapt to become more efficient. The individuals who cannot capture any prey die off, and the better-adapted population lives on to pass on their traits. This continues in an evolutionary predator-prey cycle.

The Canadian Lynx and Snowshoe Hare are prime examples of a predator-prey relationship. The Canadian Lynx are considered a specialized predator: they will try to only feed on the Snowshoe Hare. Since the 1720s the Canadian government and the Hudson Bay Company have been keeping meticulous records of the rise and fall of Lynx and Hare numbers. Every eight to ten years, the cycle can be observed. Another excellent example of these relationships is the predation of lemmings in the high-Arctic tundra in Greenland. The lemmings have 4 main predators: the arctic fox, the snowy owl, the stoat, and the longtailed skua. This is a highly observed relationship because the number of prey has no limits other than predation. This means that they have an unlimited habitat available to them and an unlimited food source. Since there are no other factors contributing to the number of prey, scientists are able to find many relationships between predator and prey, such as which predators are more efficient during high and low prey densities, which predators are more effective during certain seasons, and so on.