Lecture 14 Community Interactions

Types of Interactions Within A Community• Competition• Predation• Symbiosis: two (or more) kinds of organisms live

together in close association– Three kinds:

• Mutualism – Both participating species benefit • Commensalism – One species benefits and the other neither

benefits nor is harmed• Parasitism – One species benefits while the other is harmed

• Competition is the struggle of two organisms to use the same resource ie. share same niche~ any use .. of a resource by one species reducing its availability to

another species

– Interspecific competition between species– Intraspecific competition within species

• Outcome varies:– One species may be eliminated– Both may persist but at decreased population levels

– Niche is divided– Fundamental niche– Realized niche

Interspecific competition and relatedness

•Darwin – greater competition between related spp.

•Many exceptions – convergence/food in webs:

– Species of intertidal– Species feeding on krill– Species feeding on inverts of forest litter

• Competitive Exclusion• In the 1930s, G.F. Gause studied interspecific competition

among three species of Paramecium– P. aurelia; P. caudatum; P. bursaria– All three grew well alone in culture tubes

• However, P. caudatum declined to extinction when grown with P. aurelia

• Gause formulated the principle of competitive exclusion– No two species with the same niche can coexist

What parameter (think about evolutionary processes) results in survival of one species, to the exclusion of another (or others)?

Is one competitor always eliminated from the habitat?

– The two shared the same realized niche and P. aurelia was better competitor

• P. caudatum and P. bursaria were able to coexist

• Gause’s principle of competitive exclusion can be restated– No two species can occupy the same niche indefinitely

• When niches overlap, two outcomes are possible– Competitive exclusion or resource partitioning

– The two have different realized niches and thus avoid competition

• Persistent competition is rare in natural communities– Either one

species drives the other to extinction

– Or natural selection reduces the competition between them

Resource Partitioning

• Five species of warblers subdivided a niche to avoid direct competition with one another

• Asymmetric Competition• Results in division of niche realized niche of each

– Determined by competion/competator– each exists in microhabitat

• Connell study of interspecific competition between Chthamalus stellatus and Balanus balanides

Mutualism

• Interactions between individuals of different species that benefit both partners.– Facultative Mutualism occurs when a species can

live without its mutualistic partner.– Obligate Mutualism occurs when a species is

dependent on a mutualistic relationship.

• Ants and Aphids

– Ants transport the aphids and protect them from predators

– Aphids provide the ants with food in the form of continuously excreted “honeydew”

• Animal – Animal Mutualism

Ants and Bullshorn Acacia

• Herbivores attempting to forage on accacia plants occupied by accacia ants are met by a large number of fast, agile, highly-aggressive defenders.

• Ant Benefits:– Thorns provide living space.– Foliar nectaries provide sugar.– Beltian bodies are a source of oils and

protein.

Mycorrhizae

• Plant – Fungus mutualistic relationship

• Fungus benefits from carbohydrate nutrition provided by plant

• Plant benefit– Fungus provides increased access to water

and soil nutrients– In many cases plants cannot effectively

become established without mycorrhizal association

Plant Performance and Mycorrhizal Fungi

• Two most common types of mycorrhizae:– Arbuscular mycorrhizal fungi (AMF)

• Produces arbuscules - site of exchange between plants and fungi, hyphae - fungal filaments, and vesicles - energy storage organs.

– Ectomycorrhizae (ECM)• Forms mantle around roots - important in increasing

plant access to phosphorus and other immobile nutrients.

Animals and Evolution of Flowering Plants – Mutualism and Coevolution

Two levels:1. Movement of male gametophyte plant

(pollen)• Wind – random, not efficient• Coevolution with pollinators

• Movement of pollen more reliable

2. Dispersal –• Heavy seed – reserves for developing plant• Efficient dispersal relies on mechanical

transport by animals

Parasitic Relationships– Host– Parasite types:

• Based on size:– Microparasite– Macroparasite– parasitioids

• Based on living within or on outside of host:– Ectoparasite– Endoparasite

• Relationship with Host:– Obligate parasites– Facultative parasites

• Parasite Life Cycle– may involve multiple hosts:

• Definitive host: supports maturation of parasite• Intermediate host: harbor developmental phase(s)

– Sometimes several

– Vector• Alternate hosts• Reservoir host(s) – alternate hosts

• Parasite Impacts on Host:

• Balanced host-parasite relationship – tolerant– Host survives – often with less vigor– Parasite multiplies

• Balance altered:– High host mortality– Possibly decreased parasite multiplication

• Reduced host fitness

• Altered host behavior

• Brood parasitism – common in birds – within

species – non-obligate

• Obligate brood parasites:– Cuckoo– Cowbird

• Host species react by ejecting eggs of parasite from next

• May be severely affected if behaviors not evolved

– Kirkland’s warbler

Cuckoo

Meadow pipit