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
– The two shared the
same realized niche and
P. aurelia was better
competitor
• 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?
• P. caudatum and P. bursaria
were able to coexist
– The two have different
realized niches and thus
avoid competition
• 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
Resource
Partitioning
• Persistent
competition is
rare in natural
communities
– Either one
species drives
the other to
extinction
– Or natural
selection
reduces the
competition
between them
• 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.
• Animal – Animal Mutualism
• Ants and Aphids
– Aphids provide the ants
with food in the form of
continuously excreted
“honeydew”
– Ants transport the aphids and protect them from predators
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
Meadow pipit
– 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