Ecology/Predation and Herbivory

Chapter 11. Predation and Herbivory

Types of Predation

The skunk is an example of a prey who uses its chemical defense to scare off predators. The skunk releases a toxin or noxious compound that has a foul smell/odor to it

There are several different types of predation. One type is known as carnivory which is lethal to the prey. Another type is herbivory which may not be lethal to the prey. The third type of predation is known as parasitism which is a specialized form of predation that is lethal or non-lethal to the prey. The final type of predation is known as mutualism which is non-lethal and both organisms benefit rather than just one organism benefitting over the other.

Predation can shape prey populations' responses. The prey develop an anti-predation strategy and the predators evolve strategies to overcome prey defenses. This is known as the red-queen theory. This plays an important role in evolution and predator-prey interactions.

There are several different categories of prey defenses- (1) Chemical defense in which the organisms produce toxins or noxious compounds that may taste or smell bad. All of these organisms are aposematic which means that they have markings or colorings on them that signal to the predator that they are toxic and non-palatable (taste bad). Continuing on with prey defenses:(2) Intimidation/threat displays can be used to make the organisms appear larger than they really are, hissing and growling may also be used to display aggression, (3) Fighting which occurs when the organism fights back against the predator, (4) Crypsis/camouflage which is when organisms use their shape and their coloring to blend into their environment, (5) Escape/flee which is when the organism escapes from the predator, (6) Mimicry which is when an organism looks like something else that the predator will want to avoid (the organism mimics the body shape and coloration of another organism), (7) Satiation and masting which occurs when the organism isn't hungry anymore because it has eaten all that it can, (8) Armor which serves as protection (things such as the shells on turtles, spine or needles on a pufferfish, or thorns on a plant), (9) Startle response which is when the prey startles the predator long enough to escape (example would be a toad shooting blood from its eyes or an animal imitating a loud sound), and (10) Indirection which includes lizard tails and squid ink.

The prey defense of mimicry can be divided into two main categories. The first category is known as the Batesian Mimicry which is when a palatable species (species that taste good) imitate a non-palatable species (species that taste bad). Batesian mimicry only works if the palatable species population is smaller than the non-palatable species population. An example of Batesian mimicry would be South American butterflies. The other type of mimicry is Mullerian Mimicry which occurs when both species are non-palatable and converge coloration and patterns. This mimicry serves as a reinforcement. The South American butterflies are also an example of Mullerian mimicry.

Read Wikipedia:en:Mimicry

Herbivory

File:Plants diversity.jpg

Commonly consumed plants by herbivores

Herbivory is a type of predation in which animals/organisms consume autotrophs such as plants, algae, and photosynthesizing bacteria. Herbivory is a term commonly used to describe animals that consume plants. Herbivores can be divided into two main groups: monophagous and polyphagous. Monophagous herbivores are organisms that eat 1 species of plant exclusively. Because they eat only one species, the survival of these organisms is dependent on the survival of this primary food source. Also, these herbivores are immune to the plant's defenses, both mechanical and chemical. For example the Monarch Butterfly that feeds on milkweed is immune to its toxic defenses. Other examples of monophagous herbivores are the Giant Panda whose diet consists of 99% bamboo and Koala Bears who feed on Eucalyptus leaves. Herbivores can also be further divided into several sub groups which are frugivors, meaning they eat primarily fruit, folivors, which eat leaves, nectarivores, which feed on nectar. Herbivory can occur above and below the ground. Any plant part above the ground can be consumed as well as the roots and tubers below the ground.

Herbivory has an impact on a habitats health, structure and diversity of the plant and soil communities as well as the productivity of important crops. There are several positive impacts of herbivory. One positive impact is that low level herbivory can remove aging roots and leaves, allowing new growth of young roots and shoots. The new roots and shoots that grow provide better nutrients for absorption and reproduction. Herbivores also contribute their plants feces which cause plant parts to fall to the ground enriching the soil. This increases the chances of successful seedling growth. For example ants may take seeds back to their nest. Ants nest are usually rich in nutrients and surrounded by water. These are favorable growth conditions for plants. Herbivores also prune plants which allow light to shine through and making it easier for seeds to fall from a parent plant.

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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

The plant (prey) population N is controlled by m/βα, which are predator terms. This results in a linear horizontal growth of N, so that the change in r (growth rate of N) does not affect the size of N. An increase in r, however, does bring about an increase in P, the predator population. The plant population, N, is m/βα.

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.

Plant Defenses

Plants have evolved defenses against herbivory. There are three main categories of defenses that plants use. The first defensive mechanism is mechanical defense, which includes physical features such as thorns and needles. The second type of defenses are chemical defenses, which are comprised of five subroups. The first chemical response is secondary metabolites. This type defense involves not allowing the herbivore to use 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 , and a couple of plants that take advantage of this 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 plants 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 defenses is repellants. These plants usually contain one of the chemicals terpenes or amines. One type of plants that uses these methods are citronella plants. The third type of plant defenses is masting. This involves producing more progency then 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.

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{d N}{d t} = r N - α N P

\frac{d P}{d t} = β α N P - m P

In this model:

N=Prey Species Population

P=Predator Population

α

=Predation Coefficient(consists of searching, catching, and consuming)

β

=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 $\frac{d N}{d t}$ or $\frac{d P}{d t}$ is set equal to zero. Then the equation is is solved for P or N. To solve for the prey's ZGI you solve for P when $\frac{d N}{d t} = 0$ . To solve for the predator's ZGI you solve for N when $\frac{d P}{d t} = 0$ .

\frac{d N}{d t} = 0

r N - α N P = 0

r N = α N P

r = α P

P = \frac{r}{α}

In order to maintain prey population at equlibrium, the predator population must increase or the must become more effecient( $α$ increases) predators. The larger $α$ 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 $α β$ . Also the more effecient P (higher $α β$ ) the fewer N needed. When you plot the prey's ZGI with N as the horizontal axis and P(r/ $α$ ) as the horizontal axis you get a horizontal line. When you plot the predator's ZGI on the same graph (N=m/ $β α$ ) you get a vertical line. When you plot the vectors for all different populations you get a stable limit cycle, which also leads stable oscialltions 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 mathematition 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.

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 1720's the Canadian government and the Hudson Bay Company have been keeping meticulous records on the riseand fall of Lynx and Hare numbers. Every eight to ten years the cycle can be observed.