1. Lecture 12 Community Interactions

2. 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

3. Interspecific competition between species
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 3

4. 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 4

5. 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 5

6. 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? 6

7. 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 7

8. 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 8

9. 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 9

10. 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. 10

11. 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 11

12. 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. 12

13. Plant – Fungus mutualistic relationship
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 13

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15. 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. 15

16. Animals and Evolution of Flowering Plants – Mutualism and Coevolution
Two levels:
Movement of male gametophyte plant (pollen)
Wind – random, not efficient
Coevolution with pollinators
Movement of pollen more reliable
Dispersal –
Heavy seed – reserves for developing plant
Efficient dispersal relies on mechanical transport by animals 16

17. 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 17

18. Parasite Life Cycle may involve multiple hosts: Vector
Definitive host: supports maturation of parasite
Intermediate host: harbor developmental phase(s)
Sometimes several
Vector
Alternate hosts
Reservoir host(s) – alternate hosts 18

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20. 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 20

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22. Obligate brood parasites:
Cuckoo
Meadow pipit
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 22

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