4/15: In-Depth Notes on Predator-Prey Interactions (REC)(DOC)

Introduction to Predator-Prey Interactions

• Focus on the concept of costs and benefits in species interactions.

• Understanding fitness losses or gains as a result of these interactions.

Predator-Prey Framework

• Interactions primarily depicted as animals eating other animals, but herbivory (plants as prey) is also significant.

• Plants have evolved defense mechanisms to avoid being consumed.

Isle Royale Case Study

• Location: Isle Royale, Lake Superior, isolated park suitable for research.

• Moose Population: Significant presence, largest in the deer family, averaging 1,200 - 1,500 pounds.

• Behavior: Generally docile, forage on aquatic vegetation in warmer months.

• Population Dynamics: Historical changes from 3,000 in early 1900s to fluctuating numbers below 500.

  • Influencing Factors: Predator pressure, food availability, impacts of environmental changes (e.g., fires).

Fire Impact on Moose

• 1936 Fire: Burned areas of low-lying vegetation preferred by moose.

• Recovery of nutritious young plants post-fire, benefiting the moose.

Introduction of Wolves

• Wolves (% population): Colonized Isle Royale mid-1900s, likely crossed an ice bridge.

• Hunting Strategy: Uses pack dynamics, communication and strategic movements when hunting moose.

• Predator-Prey Dynamics: Wolf populations rise post-moose population increase, leading to eventual decline of moose.

Lotka-Volterra Model

• Demonstrates predator-prey interactions as cycles.

• Prey population directly influences predator population sizes and vice versa.

• Simplification of ecological realities but provides foundational understanding of dynamics.

Prey Defense Mechanisms

• Prey adapt with various strategies to avoid predation:

  • Secondary Metabolites: Chemical defenses in plants (e.g., tannins in oak trees).

  • Constitutive vs Inducible Defenses: Continuous presence (thorns) versus responsive mechanisms (armadillo curling).

  • Crypsis and Aposematic Coloration: Camouflage (mantises, flounders) vs bright colors signaling toxicity (Asian lady beetle).

• Mimicry: Includes Müllerian (toxic species sharing warning) and Batesian (harmless mimicking toxic).

  • Examples include monarch and viceroy butterflies, and the versatile mimic octopus.

Co-evolutionary Arms Race

• Predators evolve tactics to bypass prey defenses (e.g., blue jays pre-preparing caterpillars).

Conclusion

• Complexity of predator-prey interactions emphasizes the dynamic nature of ecological relationships.

• Understanding these dynamics is crucial for broader ecological knowledge and conservation efforts.

Lecture Week 13 (4/15)

Interspecific Exploitation (predator/prey)

I. Introduction

A. Some of the strongest links between populations are those between herbivore and

plant, between predator and prey, and between parasite or pathogen and host. The

commonality among these interactions is that the interaction increases the fitness of

one individual (herbivore, predator, parasite, and pathogen), while reducing the

fitness of the other (plant, prey, and host, respectively). In total, these relationships

are often called exploitative interactions, and are summarized by a + / - designated

relationship.

B. Although there are somewhere near 10 million different species on the planet, the

number of exploitative interactions between species (not between individuals) is

much, much greater. The reason for this is that all species on the planet are food for a

number of other species and host to a variety of parasites and pathogens. For

example, K.E. Havens (1994) estimated that the known 500 species in Lake

Okeechobee, Florida are linked by approximately 25,000 exploitative interactions.

That’s 50x the number of species!

C. Of the various exploitative relationships, three are frequently grouped together under

the general description of Consumption: herbivory, parasitism, and predation.

Exploitation provides much of the detail in the mosaic of nature. This lecture

attempts to describe some of that detail by covering the three areas of consumption.

D. Case study of the moose on Island Royale.

E. Population graphs of the moose and wolves follow a specific pattern. The two graphs

look similar, but the wolf graph occurs out of phase with that of the moose. This

pattern is similar to that predicted by the Lotka-Volterra Model of Predator-Prey

Interactions. This model predicts that as prey increase, they should become more

available to predators. Predator numbers should then be able to increase to the point

where they consume prey at a greater rate than prey are being produced. The result is

a decrease in prey numbers. However, since predator reproduction takes place after

prey are incorporated into the predator body, predator numbers continue to increase

beyond the time prey numbers decrease (this is an example of population inertia.).

Predator numbers decline due to the lack of food. Eventually, predator numbers

decrease to the point where the prey population can rebound and begin to grow again.

When the prey population is large enough, predators start finding prey at a rate great

enough to cause an increase in predator numbers, and the whole cycle repeats itself.

II. Consumption

A. Herbivores.

Generally, herbivory is the consumption of plant material by an animal that may

either reduce the fitness of the plant (+/-) or act to increase the fitness of the plant

(+/+). The latter, known as mutualism, involves eating fruits and dispersing seeds, or

eating nectar and dispersing pollen, and will be covered in more detail in a later

lecture. Exploitative herbivory can include an animal eating many different plant

parts (seeds, fruit, flowers, leaves and stems, bark, roots, sap, or any smaller division

of these), and most herbivores will restrict their foraging to specific areas of a plant.

Unless otherwise indicated, our use of herbivory in this lecture refers to the general

consumption of plants by animals, regardless of which part of the plant is involved.

1. Food is generally more available for herbivores than it is for carnivores. That

is, plants are more available as food than animals are (This concept will be

covered in a later lecture on energy flow in ecosystems.).

2. Generally, high-energy plant foods like seeds, fruit, and flowers are the least

available food types, while those plant parts that have the least amount of

accessible energy (wood, bark, branches) or moderate amounts of energy

(leaves, grasses, algae) are more abundant.

3. The digestive tracks of herbivores tend to be of greater length than those of

carnivores, due to the need to digest more complex components of plant

material (See plant defenses below).

B. Carnivores

Although herbivory is a form of predation, typically, when people think of Predator-

Prey interactions they think of one animal eating another animal. Carnivory is the

killing and consumption of other animals. The prey may be partially or entirely

consumed. Although carnivores must eat, and do so frequently, seeing predation in

action is really quite a rare event. As a rule, predators only hunt and eat when they

are confident that they are not being pursued by other animals themselves. The

witnessing of a predator in action typically requires that you remain unnoticed by the

predator and their prey.

1. Animals represent a higher quality food item (protein, fat) than plants. The

mechanical and chemical breakdown of animal tissue demands fewer and

more simple digestive specializations, as reflected in the relatively lower

biomass of carnivore digestive systems in comparison to herbivore digestive

systems.

2. Specializations for carnivory include those required for catching, subduing and

killing prey, such as claws and talons, refined senses and large brains,

specialized teeth, venom glands, and extreme morphological specializations

for speed and prey capture involving leaping/grasping limbs.

3. While pursuing prey, predators typically utilize a specific search image (visual

and chemical focus on the most abundant/rewarding prey type while ignoring

nearby, but, otherwise, good prey for at least as long as the focal prey is the

most economical to pursue). The utilization of a search image makes a

predator more successful at finding specific prey, often allowing them to

notice very small parts of their prey when they are hidden or camouflaged.

Searching for several different prey types at once leads to confusion by the

predator, and a lack of success.

C. Parasites.

The one form of consumption that does not typically lead to the death of an individual

is parasitism. Parasitism is the relationship where one individual, the parasite, lives

in (endoparasite) or on (ectoparasite) another organism, the host. The host is not

normally killed, but does usually suffer some reduction in fitness. Actually, it is not

in the parasite’s best interest to kill its host, since it may depend on it for food and

protection for extended periods.

1. Frequently, students use the terms parasite and parasitoid interchangeably.

However, the two are not the same. Parasites do not generally kill their hosts

(although the host may die due to the presence of parasites if the host is already

weakened from previous interactions, or if the parasite causes some secondary

infection or threat). Parasitoids behave like parasites, but typically kill their

host in the end. The most prevalent group of parasitoids comes in the form of

insects that lay their eggs in the bodies of other insects, and the parasitoid

larvae consume their host as they grow and develop.

III. Defenses

A. Introduction

Prey are not simply there to be eaten by predators. Prey are as active in their predator

avoidance and defenses as predators are in their pursuit. The fact that their lives

depend on their success in avoiding predators, indicates that prey species should have

evolved ways to avoid being eaten. Additionally, the evolution of prey defenses is

not solely the realm of animals. Plants have also evolved a variety of defense

strategies. Behavioral ecologists often make the distinction between standing (or

constitutive) defenses and inducible defenses. Standing defenses are those that are

permanently present within the prey, regardless of whether a predator is present.

Inducible defenses are those that appear as a result of predator pressure, whether

directly (e.g., herbivore eating a plant), or indirectly (e.g., their presence in the

environment).

B. Plant defenses.

Between 15 and 70% of all terrestrial plants are eaten by herbivores on an annual

basis, depending on the ecosystem measured. The broad range of consumption, and

the fact that less than 100% of plants are eaten, is primarily because of plant defenses.

As in all traits, though, there are costs in possessing defense strategies.

1. Mechanical defenses

a. Tough epidermis: of seed shells; bark on branches and trunks

b. Entanglement devices: thick waxy cuticle and plant hair on leaves

and stems entangles and deters small herbivores

c. Piercing devices

i. -Cutting edges in “cutgrass”, a wetland species (silica)

ii. -Spines and needles in many deciduous plants and cacti where

plants act as a source of food and water, because available

water is a limited resource.

d. Polymers within tissues: Cellulose and Lignin are indigestible by

herbivores on their own and could prevent herbivory (but see Counter

Adaptations below).

2. Chemical defenses

a. Known as secondary metabolites, plant chemical defenses are

derivatives of existing plant metabolism that deter herbivory.

Secondary metabolites are evolved to specifically deter the common

herbivores of the plant population, and thus, may not affect other

herbivores that did not co-evolve with the plant. For example, all of

the spices we use in cooking (except sugar and salt), like cinnamon,

ginger, paprika, and oregano include chemicals that plants evolved to

avoid being eaten. However, we find them (at least in small

quantities) tasty. Most of these spices, if presented to the herbivores

that evolved with the plants in question, would be avoided by the

herbivore.

b. Examples:

i. Phenolics (eg. tannins, and flavonoids): may reduce

protein digestion, slow growth and block cell division.

ii. Terpenes (eg. mints and citrus): may block ion

transmission across membranes, cause dermatitis, and

interfere with animal hormone action.

iii. Alkaloids (eg. morphine, codeine, caffeine, and

cocaine): may block ion channels in membranes, interfere

with neurotransmission, inhibit enzymes, and cause

dizziness and vomiting.

C. Animal defenses

1. Mechanical/behavioral defenses of prey

a. Retaliation: Porcupine quills, sting ray spine, Zebra kick

b. Startling behavior: Under wing of some moths with hind wing

eye spots looking like owl face

c. Deflection of attack to non-vital area: 4-eyed butterfly fish

(eyespots on tail)

d. Large size: elephant, buffalo

e. Death feigning: most carnivores avoid eating aged flesh to avoid

toxins/disease of rotted flesh – Hog nose snake, opossum

f. Fleeing: quickly as deer, antelope; Retreat to burrow in ground

hogs

g. Crypsis: becoming hard to locate

h. Clustering: Safety in numbers and confusion effect

2. Chemical defenses

a. Nausea induction: skunk scent, hydrogen cyanide production by

lady bugs

b. Ex. Monarch butterfly.

D. Host defenses

Consider the Nasonia vitripennis case study.

IV. Counter adaptations

A. Herbivores

1. Selection of certain plant parts that may have fewer defense mechanisms.

a. Example gray squirrels eating acorns.

2. Tough tongue, mouth and gut, as with cactus-eating tortoises & wood rats,

and giraffe who eat the prickly underside and outer crown leaves of Acacia

trees.

3. Mastication apparatus and grinding mills to breakdown tough seed coats and

resistant tissue: grinding teeth, continually growing teeth, gizzards in birds,

regurgitation of cud and re-mastication in many hoofed mammals and rodents)

4. Countering chemical defense

a. Cellulase evolution: many bacteria, protists and fungi (decomposers of

plant products) have evolved the enzyme cellulase to breakdown

cellulose (a defense product) into highly utilizable simple sugars

(glucose).

b. Alkaline pH: Ex. Spongy moth caterpillars have an alkaline (high pH)

stomach, which keeps tannins from binding to protein and making the

protein unavailable to the herbivore. Consequently, spongy moths eat

oak leaves and hemlock bark, both high in tannins, and suffer no loss

in protein digestion.

5. Form a mutualism with another species that can digest the defense:

Internal gut symbiosis of many animals with bacteria and protists that can

break down cellulose. Ex. Ruminant herbivores, hoofed mammals, rodents, a

few birds and termites have microbial partners (protists, bacteria) in their

stomachs and/or caeca that breakdown cellulose into glucose.

6. Eat sub-toxic amounts of plants (some primate leaf eaters) and then move on

to another plant species (easy to do in the tropics with a rich diversity of plant

species)

7. Eat plant before induced defenses are activated, and then abandon the plant.

Gypsy moth larvae can eat enough oak leaf material to develop into older

larvae with elevated gut pH levels before oak trees increase levels of tannins

in their leaves.

B. Carnivores

1. Overcoming prey mobility

a. Social carnivory (lions)

b. Pursuit: short distance (cats) or long distance (wolves) dichotomy

of adaptations

c. Luring (angler fish lure and snapping turtle tongue)

d. Sit and wait (praying mantis) and surprise pounce/grasp/bite