Ecology Exam 2

community

group of species that occur at the same time and place and the interactions that bind them

pairwise interactions

interactions based on: harmed (-), benefited (+), unaffected (0)

types of pairwise interactions

predation, herbivory, parasitism, competition, amensalism, mutualsim, commensalism

predation

(+-) predators benefit, prey do not

herbivory

(+-) plant is harmed, organism feeding benefits

parasitism

(+-) host is harmed

(+-) of pairwise interactions just paired under?

predation

competition

(- -)sharing limiting resources, taking resource from another

amensalism (asymmetric competition)

(- 0) one species harmed, another is completely unaffected (elephants foraging in savannah, damaging plants, affecting elephants in no way)

mutualism

(++) plants and their pollinators

commensalism

(+0) algae in lakes, benefit on back of turtles, does not harm turtle, cattle forging on road

most important biotic processes? most every species involved

predation and competition

interactions affect the what of species?

distribution and abundance

competition

interaction that occurs when organisms of the same or different species use an essential resource that is in short supply (limiting resource)

hyenas chased away by lioness

interspecific, interference

characterizing competition

intraspecific/interspecific, exploitative/interference, symmetric (-,-) or asymmetric (-,0)

are fights to death common?

no, they are rare, effects of competition are subtle

competition impacts?

life history traits, delayed effects on population size (N) or growth (r)

life history traits

any trait related to lx, bx, e, i ,R0, r, k

• body size

• longevity

• age at 1st reproduction

• fecundity

• vigor

• sex ratio (producing females are more costly)

intraspecific competition

• first considered as an ecological force by Thomas Malthus, people are going to run out of resources

• Darwin’s “struggle for existence” emerges from ideas on resource limitation

• affects demography (lx, bx, r) and abundances, and to a lesser extent distributions of species

was interspecific competition studied as well?

no, mostly ignored as an important ecological process until the classic study by Tansley

Tansley’s Bedstraw

studied two species, one likes the acidic soil, one with the basic high soil, G. saxatile,

Tansley Bedstraw findings

grown alone: bedstraws survive on both soil types

grown togtehre: each bedstraw species wins on its native soilW

What does this tell you about the distribution of bedstraws?

• species presence/absence due to interspecific competition

• environment affect outcome

  • basic soils: G. sylvestre wins

  • acidic soils: G. saxatile wins

• present distribution may be due to competiton in past (ghost of competition past)

competition experiments

Gause

• 2 species of paramecium

• liming resource: bacteria

• grew them in test tubes with limiting resource of bacteria

• grown alone:

• low and high resources

• P aurelia has highest k (more efficient at utilizing the same amount of resources)

• being more efficient, can make you a better competitor

• together: P aurelia has a reduction in K

  • causes P. caudatum to be extinct (compeition causes extinction to some species)

Competitive exclusion principle

• species utilizing the same limiting resource cannot coexist together

• one species will competitively exclude the other

  • aka Gause’s principle

corollary

species can coexist if they utilize limiting resources in a different way

Thomas Park beetles

interspecific competition in Tribolium flour beetles

• grew in hot and humid conditions, red flour drove the other species extinct, in cold it was vice versa

Joe Connell barnacles

rocky intertidal; upper zone Balanus scarce and no withstand air, exclude Balanus from middle Chthamalus survives by dislodging them; Balanus is a stronger competitor, Chthamalus is only in upper zone bc it can withstand the air

competitive exclusion

• Tansley’s bedstraw

• Gause’s Paramecium

• Park’s flour beetles

• Connell’s barnacles

experimental conclusions

competitive exclusion, outcome depends on environment, distribution of species strongly influenced by interspecific competition

outcome depends on environment

• Tansley: soil conditions

• Park: temperature and humidity

• Connell: tidal zones

compeititon coefficients

• measures negative effects of one speicies on another

• measures competitive ability

largest competition coefficient is a better competitor, larger conversion factor, stronger competitior bc you use more resources per individual

higher carrying capcity more efficient in using resources

a>1

organism has stronger negative effect on species 1 than it has on itself through intraspecific competition

interspecific competition > strength of intraspecific

b<1

species 1 has a stronger negative effect on itself than on species 2

intraspecific> interspecific competition

dN1/dt

realized population growth rate of species 1

r1N1

exponential growth

k1-N1/k1

intraspecific competition

aN2

interspecfic competition

Model predictions

predictions depend on a,B,k1,k2

• r ha no effect on outcome of competition

four possible outcomes

1) species 1 wins 2) species 2 wins 3) stable coexistence 4) unstable coexistence

General Criteria for Coexistence

a and b are < 1

model assumptions

• logistic growth

• linear density dependence

• a, b, k1, k2, r1, r2 constant

• closed population

• the supply of resources to the system is constant

• no other mortality factors

• model provides a reasonable description for 2 competing species

coexistence of competitors

desert plants; different root structures;occupy different niches

niches

role of an organism within the community

relationshiop of a species to all aspects of the abiotic environment

how did niches revise Gause’s corollary?

competing species can coexist by occupying different niches

niche conept

Hutchinson; based on activity (tolerance) range for an environmental variable; range of something an organism can survive and reproducee

each abiotic variable represents?

1 dimension (abiotic variable)

fundamental niche

the activity range over all possible (n) dimensions for abiotic factors

niche dimensions often have?

optimal dimensions

realized niche

portion of the fundamental niche that is occupied by a species in the presence of biotic factors (how predators, etc can change niche)

predators

negative effect, diminish the niche; cause the fundamental niche to compress (realized niche when predators are present, reduce the size of the fundamental niche; realized niche is generally smaller)

competitors

can prevent a species from occupying a space it normally could; reduce niche; negative effect, diminish the niche

mutualists

postive effect, can expand the niche, realized niche is bigger than the fundamental niche

generally even with the prescence of mutualists

the realized niche is smaller just due to the competitors and predators

problems with realized niche

only one organism to know for sure the realized niche (humans)

• a complete quantification of the niche of an organism is nearly impossible

• a species’ niche is not fixed, but can vary with

a species’ niche can vary with

• age or life stage

• time (ex: changing climate)

• space

solutions on the provlems with the realized niche

measure only most important dimensions (ex: those affecting resource use)

overlap in realized niches

sparrows,

tells about

• niche breadth (measure of range of the resources they use (the line in the picture)

if x axis represents a limiting resource,

competition should occur among individuals in area of overlap

greater overlap means

greater competition

competitive exlcusion in the field; intentional/accidental introductions

introduced to a new area; fire ants into the US

• areas where fire ants were present, native species were 70% lower than when they were present

  • total number of fire ants is 90% lower in areas, excluding native ant species

competitive exclusion example

cuban brown anole; introduced brown with green; changed its niche, green anoles declined and shifted perches higher above ground

why do introduced species appear to be so damaging to native species?

NEED TO KNOW ANSWER FOR THE EXAM

if resources are limited,

species should modify their behavior to minimize overlap in resource use and reduce the negative effects of competition

• alter their niche to reduce effects of competitionn

niche partitioning

the division of a limitng resource by 2 or more species that share it; reduce likelihood for competition

niche partioning

MacArthur’s Warbler birds

• feed on insects in conifers

• not eating the same part of the tree

• avoiding overlap in resource use

• species that use the same limiting resources can partition to all surivive and live in the same place

• if eliminate one species, the other would be able to expand its niche

Warbler

• where black throat was absent, the yellow would expand its forging up the tree, suggest that the species are niche partitioning

• competitive release is going on

competitive release

when a species is released from competiton, expanding their niche to the other species’ niche

slimy salamanders

found them on different elevation, the lower salamander was able to expand its niche up the mountain

coexistence is due to?

niche partioning; little overlap in resource use, little competition

spiny mice

niche partioning; predators hunt during day; remove the one who is noctunral, the other one switches to forage at night time

• the removal species is a dominant competitor forcing the other one to forage in the daytime

paradox of competitive exclusion

many closely related species co-occur together and have nearly identical niches

• zooplankton

• trees

species not competing

• species well below k (following a disturbance)

• resources abundant (as for new colonists or invasive species)

explanations of paradox of competitive exclusion

1) species not competing

2) fluctuating environments

• dominant doesnt have neough time to extinct the other in the amount of time

3) evolve not to compete

example of competitive release

study with chipmunks

a=1.5, b=0.5 k1<k2, what is the most likely outcome of competitive interaction?

a is for species 2, b species 1, species 2 wins

fire ants example of ?

competitive exclusion

natural selection should favor either:

• increased competitive ability (higher k or a/b)

• reduced negative effects of competition 

trait that can evolve

body size of the falcons; closely related to the size of prey they consume and size of offspring, small flacons eat small prey, big falcons hunt bigger prey; trait related to resource use that can evolve

fitness in the area of the overlap

natural selection should act against phenotypes in overlap; favor divergence in resource use in each species (directional natural selection)

• evolutionary divergence IF genetic basis to the trait associated with resource use

• feeding niche evolve (grow to have less overlap use)

• adaptive evolution

character divergence (character displacement)

evolutionary change in a trait (=character) associated with rsource use tha treduces the effects of competition 

• adaptive evolution

significance of character divergence

• reduces competition

  • explanation for the paradox of competitive exclusion

• lack of competition today could be due to competition in the past (ghost of competition past)

• coevolution occurs through the reciprocal evolution in two competing species

what needs to be proven in studies of character diveregence

1) species differ in resource use

2) resource use related to a particular trait (=character)

3) trait has genetic basis and can evolve

4) lack of overlap in resource use today is due to competition in the past

indirect approach in inferring competition in the past

comparison of:

• niche breadth for a trait associated with resource use when each species occurs alone (allopatry)

sympatry vs allopatry

allopatry: overlap

sympatry: less overlap

in sympatry

• evolve reduced niche overlap (in terms of the trait associated with resource use)

• small niche breadth

in allopatry:

• evolve large niche breadth

• competitive release

Darwin’s finches

• look similar except the bills

• evolve different bill sizes to avoid competition

Rhinoceros Beetles

• occur in allopatry and sympatry

• body size related to food size

beetle species

allopatry, sijmilar size

sympatry, one species is bigger than the other (divererged in body size)

anolis lizards (character divergence)

sympatry: forge on ground vs higher, diverge in niches

allopatry: overlap foraging areas

which trait is evolving? skeletal structure of the lizards

three spined sticklebacks (character divergence)

• body shape genetically based, where they feed

• one smaller body faster to chase, other bigger body at the bottom of pond\

• competitive release - allopatry

first experiment for character divergence

• put divider in pond add the allpatric benthic species with wider niche

• other side with competition

• broad overlap when they are together in body size

• 3 generations later, benthic evolved, no overlap in resource use

significance of stickleback experiment

• natural selection operates on competing species

• allopatric benthic species evolved more benthic habits

  • divergence (and possible speciation) in progress

is character divergence common in nature?

• carnivorous vertebrates most studied

• differences larger in sympatry than allopatry

• suggests character divergence is common

evolution & competition conclusions

• competitors should evolve to reduce competition (coevoltuion)

• evidence is mostly indirect, but strong

• few direct experimental studies

• competition affects species distribution, abundance and evolution

• can help to explain paradox of competitive exclusion

prevalence of competition

• only field studies

• manipulate competitor density

• results

  • intraspecific > interspecific

  • 50-75% show competition

  • most are asymmetric (-,0)

describe an experiment where competition does or doesn’t occur among species? how would you be able to demonstrate that?

• remove a competitor or add one

• competition should have negative effect on fitness, remove a competitior fitness should improve (show if competition is occuring)

• removing a competitor would also have competitive release

• expand its niche when a competitor leaves

• add a competitor, cause negative fitness on the already existing species (lower survivorshiop or reproduction), maybe niche partitioning

hypothetical example to test how competition is occurring in a species

NEED TO KNOW