Ecological Interactions: Predation, Competition, and Mutualism

Introduced species

Species that are introduced to a region of the world where it has not historically existed, generally by human activity, either accidentally or on purpose.

Invasive species

Introduced species that spreads rapidly and has negative effects on other species, human recreation, or human economies.

Biological control

Introducing species to help control the abundance of another species.

Benefits of biocontrol

Environmental safety because agents are typically non-toxic to humans, animals, and plants, and it is cost-effective once established, promoting natural ecosystem balance.

Unintended consequences of biocontrol

Nontarget effects where sometimes non-target species are affected, unanticipated spread, resistance development, and long-term ecological changes.

Mesopredator

Relatively small carnivores that consume herbivores and are not at the top of food chains.

Top predators

Predators that consume both herbivores and mesopredators and are at the top of the food chain with no natural predators of their own.

Functional response curve

The relationship between prey density and an individual predator's rate of food consumption, showing how many prey are available and how much the predator eats.

I response

Results in a constant proportion of prey being consumed prior to satiation, with predator's prey consumption rate increasing linearly with prey density until satiation.

II response

Results in a decreasing proportion of prey being consumed as the predator's rate of prey consumption begins to slow as prey density increases and then plateaus when satiation occurs.

III response

Predator exhibits low prey consumption under low prey densities, rapid consumption under moderate prey densities, and slowing prey consumption under high densities, forming an S curve.

Functional response vs. numerical response

Functional response describes how one predator changes its rate of prey consumption as prey density increases, while numerical response describes how the number of predators in an area changes as prey density increases.

Lotka-Volterra model

Model of predator-prey interactions that incorporates oscillations in the abundances of predator and prey, foundational for understanding these dynamics.

Prey population (N)

The population of prey in the Lotka-Volterra model.

Predator population (P)

The population of predators in the Lotka-Volterra model.

Prey growth

Prey grows exponentially unless eaten by predators.

Predator decline

Predators starve and decline when prey becomes scarce.

Prey recovery

With fewer predators, prey population recovers.

Cycle repeats

The cycle of predator and prey populations continues to oscillate.

Example of functional response

A wolf eats more deer when more deer are around.

Example of numerical response

More hawks move into a field full of mice as prey density increases.

Equilibrium Isocline

Population size of one species that causes the population of another species to be stable, aka zero growth isocline.

Joint Population Trajectory

Shows how predator and prey change over time, together.

Equilibrium Point

Shows where both species are at equilibrium isoclines at the same time, predator and prey population growth both equal zero.

Predation and Herbivory

Favor the evolution of defenses.

Behavioral Defenses

Examples include alarm calling, spatial avoidance, temporal avoidance, herding/flocking, thanatosis/playing dead, mobbing, distraction displays.

Crypsis

A physical defense, ability of an organism to blend into its environment, aka camouflage.

Structural Defenses

Built in, physical traits reducing chances of being attacked, such as spines/quills, shells, thick skin/scales, camouflage/crypsis, false eyes/warning shapes.

Warning Coloration

Aposematism; using bright/bold colors or patterns to signal predators of toxicity.

Batesian Mimicry

Palatable species evolve warning coloration that resembles unpalatable species, diluting the message of aposematism.

Mullerian Mimicry

When several unpalatable species evolve a similar pattern of warning coloration, reinforcing the message of aposematism.

Aggressive Mimicry

Predator/parasite mimics harmless/appealing organism to lure prey, e.g., orchid mantis.

Coevolution

When two or more species affect each other's evolution; the fitness of one organism affects the fitness of another.

Horizontal Transmission

Parasite moves between individuals other than parents and offspring, spreading quickly through populations.

Vertical Transmission

When a parasite is transmitted from parent to offspring during birth or reproduction.

Reservoir Hosts

Species that harbor parasites without severe symptoms, acting as a source of ongoing infection.

Vector

Living carriers, often insects, that transmit parasites between hosts.

Counterattacks

Parasite counterattacks by evolving resistance to immune responses, hiding inside host cells, changing surface proteins, or manipulating host behavior.

SIR Model

Simplest model of infectious disease transmission incorporating immunity.

Susceptible (S)

Individuals who can be infected.

Infected (I)

Individuals currently infected and infectious.

Recovered (R)

Individuals who recovered, are immune, or are dead.

Infection Dynamics

Susceptibles contact infected individuals, become infected; over time, infected individuals recover or die, reducing active infections.

Initial Infection Growth

When infection is introduced, there is an initial rapid growth in the number of infected individuals.

Population density

High density increases transmission risk.

Resistance

Reduces infection rate.

Tolerance

Reduces harm from infection but not infection rate.

Ectoparasite

Parasite that lives on the outside of an organism, e.g., lice.

Endoparasite

Parasite that lives inside an organism, e.g., prions, bacteria, protozoans.

Intraspecific competition

Competition within a species, e.g., male deer vs male deer.

Interspecific competition

Competition between different species, e.g., deer vs rabbit.

Resource

Anything an organism consumes or uses that causes an increase in the population growth rate when it becomes more available, such as water, food, light availability, and space.

Renewable resource

Resource that is constantly regenerated.

Nonrenewable resource

Resource that is not regenerated and is gone once used up.

Liebig's Law of the Minimum

Population increases until the supply of the most limiting resource prevents it from increasing further.

Competitive exclusion principle

Two species cannot coexist indefinitely when they are both limited by the same resource.

Logistic model

A model that describes how growth slows as population size approaches carrying capacity (K).

Population size (N)

The number of individuals in a population.

Intrinsic growth rate (r)

The rate at which a population increases in size under ideal conditions.

Carrying capacity (K)

The maximum population size that an environment can sustain.

Competitor equivalents

A measure of how many individuals of one species are equivalent to individuals of another species in terms of competition.

Competition coefficients

Weighted contributions of a competitor's population to how they affect the other species' carrying capacity.

Zero population growth isocline (ZNGI)

The line on a graph where the growth of a species is zero.

Lotka-Volterra model

A model that describes the dynamics of biological systems in which two species compete for the same resources.

Four possible outcomes of competition

1. Species 1 wins (competitive exclusion), 2. Species 2 wins (competitive exclusion), 3. Isoclines cross with species 1 winning depending on initial populations, 4. Coexistence occurs with isoclines crossing and intra greater than inter.

Definite outcomes

Three of four possibilities have definite outcomes, while one outcome is contingent on various factors.

Initial population sizes

The initial population sizes of the two populations affect the outcome.

Initial carrying capacities

The initial carrying capacities of the two populations affect the outcome.

Competition coefficients

The competition coefficient value for a species would be higher if that species were a better competitor for the resource in question.

Lotka-Volterra Competition model

In the Lotka‐Volterra Competition model, carrying capacity for species-1 is determined by the number of individuals of species-1 and the number of individuals of species-2 (the competitor).

Carrying capacity for species-1

Carrying capacity for species-1 is reached when the reproductive rate of species-1 is zero (r1=0).

Carrying capacity and species-2

Carrying capacity for species-1 is reached when the reproductive rate of species-2 is zero (r2=0).

Competition coefficients concept

Competition coefficients convert the number of individuals between the two competing species into equivalents.

Abiotic conditions

Abiotic conditions such as temperature, moisture, and pH can favor one species over the other.

Disturbances

Disturbances like fire, flood, and human activity can reduce the dominance of a strong competitor, allowing weaker competitors to persist or recover.

Predator-mediated coexistence

Predators may reduce the population of the dominant competitor and give less competitive species a chance.

Exploitative competition

Exploitative competition occurs when individuals consume and drive down the abundance of a resource to the point that other individuals cannot persist.

Interference competition

Interference competition occurs when competitors do not immediately consume resources but defend them, interacting with each other.

Allelopathy

Allelopathy is when organisms use chemicals to harm competitors.

Apparent competition

Apparent competition occurs when two species have a negative effect on each other through an enemy, including a predator, parasite, or herbivore.

Mutualisms

Mutualisms can provide water, nutrients, and places to live for various species.

Mycorrhizal fungi

Mycorrhizal fungi attach to plant roots and help absorb water and minerals, while fungi get sugars from the plant.

Nitrogen-fixing bacteria

Nitrogen-fixing bacteria convert N2 gas into usable forms for plants, while bacteria get sugar and shelter.

Fungal partners in mycorrhizae

Fungal partners in mycorrhizae help plants access water in dry soils.

Ants and acacia trees

Acacia trees provide hollow thorns for ants to live in.

Mutualism

A symbiotic relationship where both species benefit.

Corals and zooxanthellae

Coral provides home, algae provides nutrients through photosynthesis.

Ants and acacia

Ants attack herbivores and plants that get too close to acacia.

Cleaner fish/shrimp

Eat parasites off bigger fish and get food/protection.

Fungi in grass tissues

Produce chemicals that turn herbivores away.

Pollination

Bees, butterflies, bats, birds visit flowers to collect nectar/pollen, transferring pollen between flowers.

Seed dispersal

Fruits eaten by animals pass through digestive systems and are dropped elsewhere with fertilizer.

Ants and seed dispersal

Ants carry seeds to nest, eat fatty parts, and leave seed to grow.

Fundamental niche

Mutualism can expand/limit fundamental niche; without mutualist, species might not be able to establish/persist in a habitat.

Keystone mutualisms

Pollinators/seed dispersers can affect plant abundance.

Community stabilization

Mutualism can stabilize communities by increasing reproduction and survival of species.

Energy flow and mutualism

Mutualism can impact energy flow, productivity, and nutrient cycling.

Antagonistic relationships

Mutualisms can turn antagonistic when conditions change.

Mycorrhizal fungi and plants

In nutrient poor soil, plant benefits greatly; in nutrient rich soil, fungi takes more carbon from plant than gives in nutrients.

Cheating in mutualism

Bees bite into sides of flowers to get nectar without pollinating.