Chapter 53: Community Ecology

1. Chapter 53: Community Ecology Community Assemblage of populations of different species living close enough for potential interaction Interspecific interactions Relationship of an organism with other species in its community Types: Competition Predation Herbivory Symbiosis Parasitism Mutualism Commensalism disease

2. Interspecific interactions are -/-. • Competitive exclusion: when 2 species compete for a resource that is in short supply it can lead to the elimination of one of those species Ecological niche • sum total of species’ use of biotic & abiotic resources in its environment • Fundamental niche • Theoretical resource use • ideal conditions • Realized niche • actual resources use • real conditions of competition & predation included • **2 species cannot coexist with identical niches

3. Evidence of competition in nature • Resource partitioning • Sympatric species consume slightly different resources such as food • Character displacement • Tendency for traits to be more divergent in sympatric populations than in allopatric populations of the same 2 species • Parasitism (+/-) • Predator lives on or in host but seldom kills host • Endoparasite examples: tapeworm, flukes • Ectoparasite examples: European cuckoos laying eggs in nest of another species • Parasitoidism (+/-) • Parasite kills host • Example: wasp lays eggs in host, larva hatch, feed, & eventually kill host • Disease (+/-) • Pathogen=microscopic parasite that causes lethal harm

4. Symbiosis: close association between host & symbiont • Commensalism (+/0) • Symbiont benefits from host while host is uneffected • Few absolute examples • Cattle egret feeds on insects that grazing cattle flush out of grass • Mutualism (+/+) • Interaction benefits both species • Examples: nitrogen-fixing bacteria on legumes, cellulose digesting microorganisms in digestive tracts of termites & cows, specific nectar pollinating insects of specific flowers • Coevolution • Change in one species acts as a selective force on another species • Counter-adaptations of the second species in turn affects selection in the first species

5. Predation & Herbivory (+/-) • Predator eats its prey • Adaptations for stalking prey: • Acute senses • Heat sensors, chemical sensors, sharp eyesight • Structures • Claws, teeth, fangs, stingers, poisons • Speed & agility • camouflage • Prey/Plant defenses include • Structural • Thorns, spines • Chemical • Toxic or bitter taste

6. Passive hiding • Cryptic coloration • Shape of animal, deceptive markings (i.e. fake eyes) • Aposematic coloration • Bright coloration as a warning of toxicity • Mimicry • Batesian- edible species resembles inedible species • Mullerian- two inedible species resemble each other • Use by predator to lure prey- example… snapping turtle wags tongue like a worm to attract fish

7. **Community structure is dependant on species diversity & trophic structure; is also effected greatly by certain species in the community • Species diversity • The variety of different organisms that make up the community • Components • Species richness • Total number of different species in the community • Relative abundance • Proportion of each species in the community

8. Trophic structure • Food chains • Feeding relationships between organisms in the community • Food webs • Linked food chains • Limits to food chain length • Energetic hypothesis • Food chain is limited by inefficiency of energy transfer along the chain (only 10% passed on to next trophic level) • Dynamic stability hypothesis • Long food chains are less stable than shorter food chains • a result of the magnification of population fluctuations at higher trophic levels (top predators more likely to go extinct) • Variable environments=shorter food chains

9. Species with a large impact on community structure • Dominant species • Species in the community with the most biomass or abundance • Keystone species • Species that play a pivotal ecological role or niche • Foundation species • Species that causes physical changes in the environment that affects the structure of the community • Ex. beavers

10. Non-equilibrium model • Communities are constantly changing • Disturbance influences species diversity & composition • Disturbance • event that changes a community through removal of organisms &/or altering resource availability • Examples: storm, fire, drought, human activity* *largest/widespread agent of change • Intermediate disturbance hypothesis • Moderate levels of disturbance can create conditions that encourage species diversity by opening up new niches • low levels of disturbance lower diversity because species are out-competed • high levels of disturbance reduce diversity because of stress to species

11. Ecological succession • Disturbed area is colonized by a variety of species which are replaced by other species which are again replaced…. • Primary succession • Lifeless area without soil begins being inhabited by variety of species • Dominant species are often autotrophic prokaryotes, lichens, & mosses to start replaced by grasses, shrubs, & trees • Secondary succession • Existing community is cleared by a disturbance that leaves the soil intact • Ex. Species inhabiting a forest after a fire

12. Biogeographic features affect community biodiversity • Geographic location • Species diversity is higher at equatorial region compared to seasonally disturbed higher latitudes • Evapotranspiration rate is higher at equatorial regions • Size • Larger geographic areas have a greater number of species as long as all other factors are equal

13. Models for community organization • Bottom-up model • Suggests alteration of biomass at lower trophic levels will influence the higher trophic levels • Ex. Adding nutrients to soil increases plants, then herbivores then carnivores • Top-down model/trophic cascade model • Postulates that predation controls community organization • Ex. Removing lake predator increases herbivores which decreases plants which increases nutrients

14. Hypothesis for community structure • Interactive hypothesis • Community is an assemblage of closely linked species having mandatory biotic interactions that cause the community to function as an integral unit • Individual hypothesis • Community is a chance assemblage of species found in an area because of similar abiotic requirements • Generally accepted by plant ecologists • Models for individual hypothesis: • Rivet model • Most species in a community are associated tightly with others in its community in a web of life • Redundancy model • Web of life is loose • Increase or decrease in one species has little effect on other species because there are redundant species to fill voids