BIOL 251 Midterm 3

Community & Interactions

Ecological Community is an assemblage of pop of various species living close enough for potential interaction

Community interactions are classified by whether they help, harm, or have no effect on the species involved

• Interspecific interactions can affect the survival and reproduction of each species

• Effects can be positive (+), negative (-), or no effect (0) on fitness.

Community Interactions Table

Competition

Competition: the interaction b/w 2 orgs using the same limited resources.

• Both individuals are negatively affected — directly or indirectly

• Experience reduced fitness

• Can be a driving forces of adaptation & evolution

Plants are most often competing for water, light, nutrients, or physical space.

Animals are most often competing food, territory, and for mates.

Direct vs Indirect Competition

Direct Competition

• interference competition: competing individuals (usually of the same species) directly interact in an attempt to reduce the other's access to resources

• e.g. allelopathy: chemical warfare among plants

• e.g. territoriality among animals (requires mobility)

Indirect Competition → mediated through a 3rd party

• resource competition: consumption of a resource by one individual ends up depriving another individual of that resource

• e.g. preemption: individuals prevent others from using a location by arriving first

Fundamental vs Realized Niche

Competition occurs because of niche overlap among individuals​

Niche: the set of conditions and resources needed by a species for survival and reproduction​

Hutchinson’s Niches:

• Fundamental Niche: complete set of env conditions in which the species can potential survive and reproduce

• Realized Niche: set of env conditions in which the species is actually found in nature, a subset of the specie’s fundamental niche

Elton’s niche description is defined by its so-called trophic position (essentially where it sits in the food web) and the functional attributes of the species that allowed it to specialize on those foods.

Competitive Exclusion Principle + Example

Competition Exclusion Principle: no 2 species can occupy the exact same niche. If they do, the better competitor (grow or reproduce more quickly) will always win.

Gause’s Law: a tale of 2 Paramecium species

P. aurelia and P. caudatum follow the same trends when grown separately as they have the same K. But when grown together, the larger P. caudatum had an advantage at first, but the smaller and faster P. aurelia quickly overtook it.

P. aurelia pop kept growing while P. caudatum’s pop shrank. P. aurelia reached its K and P. caudatum vanished. They competed for limited resources.

Interspecific vs Intraspecific

Competition can be intraspecific or interspecific

• Interspecific is b/w individuals of diff species that require the same limiting resource due to some niche overlap

• Intraspecific is b/w individuals of the same species — if competition b/w individuals of the same species for a limiting resource is strong, then individual performance is subject to negative density dependent growth, survival and/or reproduction

  • the mechanism through which resource limitation constrains pop growth & b/c individuals of the same species have greater niche overlap, intraspecific interaction is strong

Population Growth

• w/ unlimited resources, a pop will grow at its intrinsic rate of growth r.

  • modeled w/ the exponential growth equation: dN / dt = rN.

• When resources are finite, density dependent factors constrain the growth rate as N approaches K, so pop growth drops below r, reaching ~0 at K.

• For logistic growth: increase in competition = decrease in resource abundance and K

Interspecific Competition

• Occurs when individuals of one species limit the resources available to another species

  • Typically, negative outcome for both species, but depends on their relative competitive ability

• When growing in competition w/ other species, K is affected for at least the weaker competitor species.

• Eq pop size: species i ceases to change pop size (Ni).

• Competing pop are at eq pop size when dN1 / dt = dN2 / dt = 0

  • occurs when Ni < Ki in interspecific competition

Competitive Exclusion

The competitive exclusion states that 2 species w/ the same niche cannot coexist.

Strong competition leads to competitive exclusion of the weaker competitive species. Competitive exclusion is a result of asymmetric competition.

• Asymmetric competition – one species suffers a much greater fitness decline than the other species​ (but both -/-)

• Symmetric competition – each of the interacting species experiences a roughly equal decrease in fitness​

• Weaker competitor squeezed out of portion of its fundamental niche to its realized niche

Competition Coefficients

Competition coefficients: the per capita effect on the growth of one species (1) of a second species (2) = α_1,2

• First coefficient = species is being affect by second coefficient = doing the effecting

• Used to quantify the relative competitive strength of individuals of a diff species vs. a member of the same species.

The closer α_1,2 = 1, the more species 2 has a competitive effect on species 1 like species 1 has on itself

Basically, α_1,2 is a conversion factor for species 2 making it equivalent in competitive effect to individuals from species 1.

Lotka-Volterra Competition Equations

Used to estimate equilibrium pop sizes of 2 competing species.

• If α_1,2 < 1, effect of interspecific competition is less than the effect of intraspecific competition on species 1

• If α_1,2 > 1, effect of interspecific competition is greater than the effect of intraspecific competition on species 1

Phase Planes I

It helps visualize which species would win a competitive interaction by plotting N of both species against each other.

• If steady state does NOT depend on initial Ns, then it is “globally stable”.

• Can be rep by a phase portrait

• Where dN₁/dt = 0, N₁ is unchanging

  • r₁ = 0, N₁ = 0, and N₁ = -α_1,2N₂ + K₁

Isocline of 0 pop growth for species 1.

Phase Planes II

When the zero growth isoclines do not intersect, stable coexistence is not possible - the species w/ the isocline furthest from the og will exclude the other

When the zero growth isoclines intersect, a stable coexistence is possible → tend toward their eq pop sizes.

Could also have competitive exclusion, w/ outcome dependent on initial pop size

Phase Planes II Pic

Competition in the Real World

• Lots of evidence that competition has a big negative effect on species across a range of trophic levels in the real world too

• Some trophic levels are more affected by comp than others

• Exp that caged-in pop yielded stronger effect of comp - spatial partitioning

• One mechanism that can explain coexistence despite comp has to do w/ spatial and temporal variations prevent competitive exclusion

Coexistence

What enables ecologically similar species to coexist?

1. Env heterogeneity in time

2. Spatial uneven distribution of resources

3. Predators’ mediate competition

Exploitation Interactions

Exploitation interactions benefit the actor (species A) to the determinant of the recipient (species B)

• Parasitism — animal lives in or on its prey, may or may not kill it

  • Not technically predators

• Herbivory — animal eats a plant

• Predation — animal consumes another animal, killing it

Parasitism

• Depend on 1+ host to complete their life cycle

  • high degree of specialization to host or even host organ

  • High specialization allows high diversity

• Selection pressure balances virulence and host survival (but see parasitoids)

  • Parasites — rarely lethal

  • Parasitoids — eat host from the inside out, host is usually fully consumed and goes from larvae to adult size and emerges from host to reinfect (i.e. wasps)

• Transmission b/w hosts can be passive, active or host-to-host

• Complex life cycles w/ diff mobility/behaviors/resource needs

• Can have broad ecological impacts

Ectoparasites vs Endoparasites

Endoparasites → found inside hosts & have to worry about the host’s immune system (a trade off as they are protected from the outside environment)

Cleaner wrasse specialize on eating ectoparasites → coral reefs w/ cleaner wrasse have fewer fish w/ ectoparasites than reed w/o cleaner wrasse

• Ectoparasites avoid the host’s immune system

• More exposed to predation & the harsh outside world

Herbivory

Most herbivores don’t kill the whole plants, so not truly “predation of plants”

• Grazers — specialize on herbaceous plants

• Browers — specialize on woody plant (leaves, bark, twigs)

• Granivores — specialize on seeds

• Frugivores — specialize on fruits

Grazers & Browers’ plants are high in cellulose. Require mutualism w/ symbiotic microbes to digest the cellulose.

Hindgut fermenter eats the food only once while ruminants eat it and then rechew it again. So, food chewed twice.

Defense Against Herbivory

Plants can evolve defenses against herbivory

• Mechanical → Developing structures like thorns that make it harder for animals to eat them.

• Chemical → Producing chemicals that are noxious or poisonous to herbivores.

• Nutritional → Growing structures that are less nutritious for grazers (have less N and P) so predators eat others that are more nutritious

• Tolerance → Adaptions to regrow quickly after being grazed.

Defense expression can also be an acclimation

Predator/ Herbivory Mediated Coexistence

• Beetle herbivory suppresses the growth by the dominant plant species (golden rod)

• W/o the beetles (by adding pesticides), golden rod outcompete the other plants and plant diversity drops

• Counter-intuitive, but herbivory sometimes maintains plant biodiversity

Predation

4 modes of predation

• Stalk → hiding behind trees and brush until they can get close enough to pounce on a hare (leopard vs ostrich b/c leopards cannot run for long distances)

• Pursuit → where the predator chases prey over a distance (wolf vs hare)

• Ambush → where the predator hides and waits in one spot until prey comes along

• Random Encounter → where the predator and prey meet by chance (i.e. spiders and flies)

Species can be flexible in the type of strategy they use to kill their prey

Defense Against Predation

• Chemical → Producing chemicals that are noxious or poisonous to predators

• Physical → Developing physical barriers to predators (i.e. shells)

• Aposematism → Warning colors, sounds, or other characteristics to alert predators that this prey will not be tasty

• Crypsis → Camouflaged colors, shapes, and other ways of hiding from predators (i.e. bugs that look like sticks)

• Mimicry → Looking, sounding, or in other ways mimicking a species that predators avoid

• Behavioral → Behaving in ways that minimize risk from predation

The Predator Encounter Term

Modify the exponential growth eq to include a predator encounter term

• Encounters w/ a predator reduces the rate of increase in the prey pop

• More predators = more encounters

• Fewer prey = fewer encounters

The Prey Consumption Rate

Search efficiency or attack rate can vary by predator, w/ some being more efficient predators than others.

• Search efficiency coefficient of the predator is a

a times the predator encounter term gives the Prey Consumption Rate

The Predator Population Growth & Dynamics

Model the predator’s pop dynamics

• Prey consumption rate

• add in the conservation coefficient (b) for how many preys a predator must consume before it has accumulated enough resources/energy to have offspring

  • larger predators needs more prey to make another large predator

dN_predator / dt = abN_preyN_predator

• we also need to account for predator mortality (m)

dN_predator / dt = abN_preyN_predator - mN_predator

Waveforms

A waveform has 2 key properties

1. The amplitude: the difference between the peak of the cycle and the trough between cycles

2. ​The period: the amount of time from one peak to the next peak in the cycle, or one trough to the next trough.​

Phase Planes

Phase planes represent interspecific relationships - how pop sizes of predators and prey change relative to one another.

• # of individuals in the predator pop on the Y-axis

• # of individuals in the prey pop on the X-axis

The dimensions (height and width) of the “circle” tells us about the amplitude of the two waveforms in the predator-prey population cycle, ​but the phase plane does not tell us anything about the period​

Phase Portraits

The amp of the phase plane b/w predator and prey pop size will depend on the starting pop sizes.

A phase portrait shows whether the predator and prey pop will increase or decrease in the next timestep, given diff starting conditions

Varies b/w max and min

• Prey will shrink when predators > -25

• Predators will shrink when prey < -50

Zero Growth Isoclines

Lines representing the pop size at which the growth rate is 0.

• above the line = pop shrinks

• below the line = op increase

Depending on where the initial population sizes plot out around the zero growth isoclines, we can predict what the populations will do in the next time step.​

• Bottom left - Prey increase, predators decrease​

• Upper right – Prey decrease, predators increase​

• Bottom right – prey and predators both increase​

• Upper left – Prey and Predators both decrease​

If the starting pop are at the intersection b/w the 2 zero growth isoclines , neither pop will grow or shrink so the cycle won’t either

Lotka-Volterra Predatory-Prey Model Assumptions

• Prey and predator pop are growing exponentially - no density dependence

• All predator and prey individuals have an equal prob of encounters - the pop are well mixed and evenly spread out

• Closed system - no i or e of predators or prey

• no other cause of death for the prey than being consumed by predator

• Prey is the only food source for predator - no prey switching or subsides

• No prey handling time (h), as predator catches and consumes prey instantaneously

Pop stabilize

• Introducing a carrying capacity causes the amp of the pop cycles to dampen until they stabilize

• The lower the carrying capacity (K), the higher the density dependence

• The lower the carrying capacity, the faster the pop stabilize

• As they stabilize, they’re no longer neutral stable

Predator Self-Limitation

Intraspecific competition among predators. The stronger the self-limitation, the more the isocline will bend to the right. ​

• N_predators stabilize at a lower value

• N_prey stabilize at a higher value

Refugia

• assumptions of well-mixed, evenly distributed prey and predators, so that encounters are equally probable for any individual in the pop

• Refugi benefit prey at low N_prey, helping them avoid predation

• Predators are not equally efficient

• Prey may have refugia from predators, which protects small prey pop

Functional Response of Predator

• C.S. Holling (1959) described how a predator’s feeding rate changes w/ prey density

• Generally, as prey become more abundant, the predator’s feeding rate increases

  • Type I: random encounter predators + no handling time

  • Type II: specialists w/ handling time

  • Type II: prey-switching generalists or prey w/ refugia

Functional Response of Predators — Type I

Lotka-Volterra model gives pop-level responses of the predator and prey pop

• Predation rate is the same for all predators in the pop: aN_preyN_predator

• Assuming each predator will eat prey in direct proportion to how many prey are nearby

• Assume at one point, predation rate caps out b/c predator becomes full, so it is a straight line before plateauing to a flat line

Type I example

• Web spinning spiders will catch all the prey that are trapped in their web

• As prey density increases, the # of encounters goes up and so the avg # of prey per predator goes up proportionately

• Linear slope = type I functional response

Functional Response of Predators — Type II

Functional Response of Predators — Type III

• Ex of type II functional response

  • red fox consuming deer as fawns

  • seals consuming salmon

• Efficiencies at higher prey density — prey switching generalist predator behavior

  • learned hunting techniques can be mastered

  • search image — w/ repeated encounters, predator can improve their ability to detect a prey type

• At low densities, prey may avoid predation w/ refugia

C-Evolution Defined

Co-evolution: the product of reciprocal selection, were species living in close associated adapt in response to selective pressure from each other

• diffuse (aka guild coevolution)

• specific

Co-evolutionary arms race: specific case of co-evolution, where in an exploitative relationship, NS favors

• traits that increase the success of the exploiter species

• traits that decrease the vulnerability of the exploited species

• yields an ongoing struggle b/w competing sets of co-evolution traits that develop escalating adaptations and counter-adaptations against each other, resembling the geopolitical concept of an arms race

Directional Reciprocal Selection

• the newt

Red Queen Hypothesis

• The faster the mutation rate, the faster a species can evolve

• In a battle of reciprocal selection, the faster evolving species will typically outlast the pop of the slower evolving species

• Faster rates of mutation associated w/ sexual reproduction are the main explanation for why species reproduce sexually, despite all the costs