Mod 10 (Ch 14) - Competition

updated for spring 2026, included on exam 3

community

populations of different species living and potentially interacting in a certain area at a given time

different species interactions (5 types) and how they impact the survival (Sx) and fecundity (Fx) of each individual involved

competition (-/-)

mutualism (+/+)

predation (+/-)

herbivory (+/-)

parasitism/pathogens (+/-)

how competition impacts survival (Sx) and fecundity (Fx) of each individual/population involved

Sx and Fx of both populations are decreased

neither population benefits

how mutualism impacts survival (Sx) and fecundity (Fx) of each individual/population involved

Sx and Fx of both populations are increased

both populations benefit

how predation impacts survival (Sx) and fecundity (Fx) of each individual/population involved

Sx and Fx of one population are increased (the predator)

Sx and Fx of the other population are decreased (the prey)

one population benefits, one population is harmed

how herbivory impacts survival (Sx) and fecundity (Fx) of each individual/population involved

Sx and Fx of one population are increased (the herbivore)

Sx and Fx of the other population are decreased (the plant)

one population benefits, one population is harmed

how parasitism and pathogens impact survival (Sx) and fecundity (Fx) of each individual/population involved

Sx and Fx of one population are increased (the parasite/pathogen)

Sx and Fx of the other population are decreased (the host)

one population benefits, one population is harmed

competition

interaction between two or more individuals both requiring the same limited resource

when that resource is used/defended by one individual, the survival or reproduction of the other is reduced

two types: intraspecific and interspecific

usually reduces Sx and Fx of both individuals (but not always)

subdivisions: interference and exploitative

resource

anything in the environment that is used or consumed, and thus reduced

e.g. food, oxygen (in a limited environment), space, water

limited resource

a resources that has higher demand than availability

Liebig’s law of the minimum

most limited resource = limiting resource

limiting resource ultimately determines population growth rate

(idea “developed” by Justus von Liebig, but thought of by Karl Sprengel before that)

example of competition between diatoms

2 spp of diatoms: Asterionella and Synerda

both compete for silica (silica is needed for their cell walls)

when both put together, Synerda is a better competitor (can persist at lower silica levels) → Asterionella is driven to extinction

Synerda is still impacted, just not as much

example of competition reducing Sx and Fx of two individuals

Brown & Davidson

2 species, ant and rodent, both eat same limiting resource (same size seeds)

population sizes of both species were affected when in the presence of the other; changed when the other was removed (both decreased in number when competition was present, rodents’ size also decreased when competition was present)

seed densities were reduced to the same level whether together or alone, so each spp was eating less than they could when competition was present (each spp eats as much as they can)

interference competition

one subdivision of competition

direct interactions

individuals fight for access to limiting resources

e.g. direct fight between hyena and vulture over a carcass, allelopathy

can occur in both interspecific and intraspecific competition

allelopathy

chemical warfare between plants

e.g. eucalyptus and melaleuca trees have flammable oils; increase fire frequency/intensity → reduce potential competitors

e.g. plants secreting chemicals that keep other plants away from their root space

exploitative competition

one subdivision of competition

indirect interactions

affect others from accessing resources by consuming the resources first

e.g. roots crowding out other roots; faster-growing roots get more water

e.g. plants growing for sunlight; plants below get little sunlight

ecological niche

range of abiotic and biotic conditions within which individuals of a species can survive, grow, and reproduce

arises from competition limiting distribution and abundance

e.g. what they eat (type, size, where found), where they live (location, climate), when they are active (day, night)

can be fundamental or realized

fundamental niche

ALL area in which an individual/species can live

all areas where a species has the potential to use all of certain resources

realized niche

the limited part of area in the fundamental niche where an individual/species can ACTUALLY use because of interactions/competition with other species

example of fundamental vs realized niche

study performed by Connell in 1961

Chthalamus and Balanus barnacles in rocky intertidal zones of Scotland

both live on rocky intertidal zones

Cthalamus is very desiccation-tolerant, can occur in regions exposed from low tide until high tide

Balanus is less tolerant to desiccation

competition → Cthalamus is limited to areas where Balanus cannot occur

tl;dr: fundamental niches of both spp overlap, but Balanus is a better competitor in lower areas → realized niches do not overlap (see picture)

to test that this distribution was truly due to competition, Connell tested Chthalamus (normally in the upper zone) to the lower zone with and without Balanus → Chthalamus survived fine in the lower zone without Balanus, thus its distribution is limited by competition with Balanus

competitive exclusion principle

two species cannot coexist indefinitely on the same limiting resource (i.e. with substantial niche overlap)

when niches overlap substantially, two outcomes: competitive exclusion of 1 species (extirpation/local extinction), or coexistence via resource (niche) partitioning

A.G. Tansley tested it in 1917 with 2 species of Galium plant

Gause tested in lab in 1934

A.G. Tansley’s 1917 test of the competitive exclusion principle

observed 2 closely related species of Galium, which generally don’t occur together (G. saxatile and G. sylvestre)

each spp. grown alone can grow on either soil, but survival and growth were higher on their preferred soil (G. saxatile on soil A, G. sylvestre on soil A)

when grown together, spp. on nonpreferred soil has reduced survival and growth (goes locally extinct)

G. saxatile dominated growth and G. sylvestre went locally extinct on soil A

G. sylvestre dominated growth and G. saxatile went locally extinct on soil B

competitive exclusion

when the inferior competitor goes locally extinct (extirpates) when niches overlap substantially

e.g. Galium in Tansley’s experiment

coexistence via resource (niche) partitioning

similar species can coexist if they share (partition) the resources such that their niches no longer completely overlap

i.e. natural selection drives competing species into different patterns of resource use and thus different niches (each spp adapts in the opposite direction)

allows each spp to use a subset of the resource (e.g. instead of two spp eating medium seeds, one chooses smaller seeds and one chooses larger seeds)

partitioning can happen by specialization on a:

physical attribute (particular size, color, etc.), spatial attribute (choose resources occurring in a specific location), temporal attribute (use resources only during a specific time)

examples of changes from adaptive evolution that allow specialization and niche partitioning

morphology: shorebird beak shapes changing to specialize on insects at different depths and different sized insects

morphology: anoles specializing on insects in different parts of a tree and different sized insects, thanks to differently shaped legs and toes

physiology: Fowler’s toad and spring peeper breed at slightly offset times. Tadpoles hatch in different months, so don’t compete for food

behavior: Fowler’s toads and spring peepers (same as above)

behavioral differentiation due to adaptive evolution vs learned response

morphological/physiological differentiation occurs via adaptive evolution, not easily changed (is fixed and innate)

behavioral differentiation may be due to adaptive evolution OR a learned response (is not fixed)

e.g. common spiny mouse (nocturnal) vs golden spiny mouse (diurnal) both coexist in rocks near the Dead Sea. When nocturnal spp. was removed, after a few months the diurnal spp. could be found at night. → learned response, not adaptive evolution, resulted in differentiation, which led to partitioning of niches

character displacement

type of niche partitioning that occurs only where two species occur together

the 2 spp differ only when together; when alone, they exhibit the same characteristics

e.g. Darwin’s finches, morphological differentiation (beak sizes)

(see example 14.17 in book: chipmunk spp. in mountain ranges)

Lotka-Volterra competition model of competition

developed in 1920s (both developed the same model independently but at the same time)

2 models for population growth rates of 2 competitors

modification of logistic equation

considers both intraspecific and interspecific competition in reducing a population’s growth rate

how close N is to K still determines growth rate

has some differences from logistic growth

differences of the Lotka-Volterra model from logistic growth

adding individuals of another spp. may also bring a population closer to K and affect its growth rate

adding an individual from your own spp. affects your population growth rate differently than adding an individual of a different spp.

what everything in the Lotka-Volterra competition model represents

1 = spp. 1

2 = spp. 2

r = intrinsic rate of increase (i.e. how fast the population can grow)

N = population size

K = carrying capacity

t = time

α = competitive effect of spp. 2 on 1 (affecting spp 1)

beta = competitive effect of spp. 1 on 2 (affecting spp 2)

adding an individual of your own spp. vs another spp. in the Lotka-Volterra competition model

if α=1: adding an individual of spp. 1 or 2 has the same effect on population growth rate

if α<1: adding an individual of spp. 2 has less effect than adding an individual of spp. 1 (if spp. 1 is your own spp. and spp. 2 is the competing spp.)

if α>1: adding an individual of spp. 2 has a greater effect than adding an individual of spp. 1

if α=0: no competition; equation reduces to the logistic equation

same if beta replaces α

addition of an individual from either spp brings you closer to carrying capacity