03 competition facilitation mutualism

[Competition] Definition of competition

Any use or defense of a resource by one individual that reduces its availability to other individuals

[Competition] Resource

Something critical for growth, survival and reproduction; limitation reduces individual fitness and population growth

[Competition] Where is competition strongest?

Between individuals of the same species (intraspecific)

[Competition] Intraspecific competition and logistic growth

Intraspecific competition drives density dependence seen in logistic growth curves

[Competition] Tansley's experiment

Common garden experiment on two related plants (Galium) on different soil pH; also a competition experiment

[Competition] Three lessons of Tansley's experiment

Presence of one species can affect another; outcome depends on environmental conditions; present segregation may result from past competition

[Competition] Competition as a selective pressure

Individuals with high resource overlap have lower fitness, selecting for divergence

[Competition] Character displacement

Evolutionary divergence in traits between competing species over evolutionary time to reduce overlap

[Competition] Character displacement is what type of selection?

Disruptive selection

[Competition] Character displacement example

Anole lizards (Stroud et al. 2024)

[Competition] Interference competition

Direct competition between individuals (e.g., aggressive encounters, lion territoriality)

[Competition] Exploitation competition

Indirect competition via reduction of a shared limiting resource (e.g., wildebeest competing for forage)

[Competition] Exploitation competition as an indirect effect

Consumer A has a negative effect on the resource, which has a positive effect on consumer B, giving A a negative indirect effect on B

[Competition] Competitive exclusion (Gause) [Extra]

Two species limited by the same resource cannot stably coexist; the better competitor wins unless niches differ

[Facilitation] Facilitation

One-way positive nonconsumptive interaction between species

[Facilitation] Tree canopy and fire example

Canopies reduce fire intensity, so small trees under canopies survive better than those in the open

[Facilitation] Stress gradient hypothesis

Facilitation becomes more common, and competition less important, as environmental stress increases

[Facilitation] Callaway et al. 2002

Alpine plants: relative neighbor effect (RNE) negative (competition) at low elevation, positive (facilitation) at high elevation

[Facilitation] RNE

Relative neighbor effect on target plant biomass

[Facilitation] Where are positive interactions more likely?

In stressful environments

[Mutualism] Mutualism

Reciprocal positive effects (+/+) between two species

[Mutualism] Mutualism examples

Rhizobia-legumes, plants-pollinators, oxpeckers-ungulates, yuccas-yucca moths, plants-mycorrhizae, ants-acacias

[Mutualism] How do we prove a mutualism?

Measure costs and benefits to both partners, using manipulative experiments

[Mutualism] Kiers et al. 2011: carbon result

Plants transferred more carbon (14C sucrose) to mycorrhizal fungi that provided more phosphorus

[Mutualism] Kiers et al. 2011: phosphorus result

Fungi transferred more phosphorus (33P) to plant roots that provided more carbon

[Mutualism] Kiers et al.: overall conclusion

Reciprocal rewards: partners trade more with the partner that gives more

[Mutualism] Why radioactive isotopes (14C, 33P)?

Used as tracers to follow where C and P move and quantify transfer between partners

[Mutualism] Oxpeckers and cattle (Weeks 2000)

Oxpeckers don't significantly reduce tick loads but increase wound counts, so may be parasites rather than mutualists

[Mutualism] Yucca-yucca moth

Moths pollinate but lay eggs in flowers; yuccas abort flowers when moths lay too many eggs

[Mutualism] Sanction/cheating in yucca-moth system

Flower abortion punishes cheating moths and stabilizes the mutualism

[Mutualism] van der Heijden 1998

Mycorrhizae and rhizobia effects extend beyond partners to whole communities and ecosystems

[Mutualism] Empty forest problem (Terborgh 2008)

Forests depleted of vertebrate seed dispersers have far fewer tree saplings

[Mutualism] Other non-trophic interactions

Dispersal of genetic material or propagules; modification of the abiotic environment, habitat or landscape

[Mutualism] Take-home messages

Competition is for limiting resources; positive interactions occur, more in stress; mutualisms need costs and benefits measured; cheating and breakdown can occur