Lecture 12 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

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

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

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

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

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

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

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

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

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

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

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

14

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

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

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

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

19

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

21

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

23