Species interactions: competition

Plant Ecology

Species interactions

• a major component of ecology• all species interact with other species

• types of interactions• mutualism (+/+)• commensalism (+/0)• neutralism (0/0)• antagonism (+/–)• amensalism (0/–)• competition (–/–)

Competition

• competition: an interaction between species that reduces the fitness of both species

• usually caused by shared use of a limited resource

• nutrients• water• space local scale competition• light

• pollinators broad scale competition• seed dispersers (animals are mobile)

Competition reduces fitness

• competition can reduce:• biomass• growth rate• survival• reproduction

Size and density

• plant growth can be highly plastic• changes based on environmental conditions

Asymmetric competition

• asymmetric competition: when species 1 has a large negative effect on species 2, and species 2 has a smaller negative effect on species 1

Self-thinning

• self-thinning: plant mortality due to competition in crowded conditions

Competition for light

• sunlight is unlimited

• but plants overhead can block out over 99% of light

• mechanism to compete for sunlight: overgrowing or overtopping neighbors

• short herbaceous plants shrubs trees

• vines overgrowing/ strangling trees

Apparent competition

• apparent competition: density-dependent negative interactions between species that might appear to be due to competition for resources, but are actually due to a shared predator or herbivore

• species A and species B grow together• the high density of plants attracts herbivores• both species suffer from the other’s presence, but suffer

because of herbivory, not competition for resources

Competitive hierarchies

• “superior competitors” and “inferior competitors”• rankings may change with varying conditions

• promotes coexistence and biodiversity

Quantifying competition

• relative competition index (RCI):

RCI =𝑃𝑚𝑜𝑛𝑜𝑐𝑢𝑙𝑡𝑢𝑟𝑒 − 𝑃𝑚𝑖𝑥𝑡𝑢𝑟𝑒

𝑃𝑚𝑜𝑛𝑜𝑐𝑢𝑙𝑡𝑢𝑟𝑒

• absolute competition index (ACI): ACI = 𝑃𝑚𝑜𝑛𝑜𝑐𝑢𝑙𝑡𝑢𝑟𝑒 − 𝑃𝑚𝑖𝑥𝑡𝑢𝑟𝑒

• log response ratio (LRR):

RCI = ln𝑃𝑚𝑖𝑥𝑡𝑢𝑟𝑒

𝑃𝑚𝑜𝑛𝑜𝑐𝑢𝑙𝑡𝑢𝑟𝑒

Experimental methods for studying competition

• substitutive design: vary the frequencies of two competitors, while keeping the total density constant

Ex: (# of plants of species A and B)10 A 7 A 5 A 3 A 0 A0 B 3 B 5 B 7 B 10 B

• disadvantage: plants are grown at a fixed density, we don’t know how plants perform at other densities

Experimental methods for studying competition

• additive design: manipulate the total density of neighbors, while keeping the target species at a constant density

Ex: (# of plants of species A and B)10 A 10 A 10 A 10 A 10 A0 B 5 B 10 B 15 B 20 B

• disadvantage: results can confound density with species proportions

Experimental methods for studying competition

• additive series design: both densities and frequencies are varied

Ex: (# of plants of species A and B)5 A 5 A 5A 10 A 10 A 10 A 15 A 15 A 15 A0 B 5 B 10 B 0 B 10 B 20 B 0 B 15 B 30 B

• disadvantage: requires a lot of time and effort

Root and shoot competition experiment

Studying competition in the field

• Most common method:

• remove all or some neighbors of a target individual

• heath bedstraw: sandy soils

• limestone bedstraw: limestone soils

Competition example: 2 species of bedstraw

sandy soil limestone soil

HB LB both HB LB both

Competition example: South African shrubs

Example: South African shrubs

green: within native rangegray: outside of native range

Example: South African shrubs

When is competition important?

• hypothesis: competition is intense in nutrient-rich habitats

• In unproductive habitats, success depends on ability to withstand stress (ex: low nutrients, drought, cold) rather than competition

• (but evidence is mixed)

• hypothesis: competition is less important when disturbance is frequent

Tilman’s resource ratio model

• R* model (single species)

• R*: the minimum level of a resource that an organism needs to survive

low hiResource Level

R*

at equilibriumB = DC = S

R* model (2 species)

• Which species wins?• (which species is the better competitor?)

low hiResource Level

R*a R*b

Ba < DaBb < Db

C < S

Ba > DaBb < Db

C > S

Ba > DaBb > Db

C > S

Tilman’s model: competition for 2 limiting resources

• Zero Net Growth Isocline (ZNGI)• an isocline is a line made up of all combination of points

that share the same value

• a ZNGI is the line at which the population growth rate of a species is = 0• made up of different combinations R1 and R2

• ZNGIs vary by species

Tilman’s model: ZNGIs

R1

R2

R1

R2

Substitutable resourcese.g., Pizza Company & Pizza Hut

Non- substitutable resourcese.g., light and nitrogen

Consumption vectors

• consumption = consume = eat (or use)

R1

R2

Consumption of R1Consumption of R2Total consumption

Does this species consume more R1 or R2?

Supply vectors• supply vectors: input of resources into the system• S = resource supply point = combined resources in

the system (in environment + in living organisms)

S

R2

R1

Equilibrium

• equilibrium resource levels (E) occur where the consumption vectors and the supply vectors are equal and opposite

E

R1

R2

When the resource level is at E then B = D and C = S. This is a stable equilibrium.

2 resources, 2 species

R1

R2 Species B

Species A

S

Who can survive?

2 resources, 2 speciesWhat is required for coexistence?

• In order for both species to coexist there needs to be a resource level in the environment at which the growth rate of both species is equal to zero.

• This can only happen if the ZNGIs intersect

R1

R2

2 resources, 2 speciesExtending the consumption vectors

R1

R2Species B

Species A

A

B

1 2 34

5

6

What happens if S is in zone 1? 2? 6?

When is coexistence even imaginable?

Both species must be able to grow to have any chance of co-existence

Thus, coexistence is only potentially possible if S falls in regions 3, 4, or 5

2 resources, 2 speciesPredicting competition results

R1

R2Species B

Species A

A

B

1 2 34

5

6

What happens if S is in zone 3? 5?

Stable equilibrium: each species limits itself more than the other

R1

R2Species B

Species A

A

B

1 2 34

5

6

Unstable equilibrium: each species limits the other more than itself

R1

R2Species B

Species A

B

A

1 2 34

5

6