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Species interactions
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Species interactions
• Embedded within the realm of population and community ecology– Goal: understand how populations of
interacting species reduce and/or regulate each other population density
– Importance: they affect the abundance and distribution of species; they also provide the basis to understand how communities are assembled
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Interspecific Interactions
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Competition
• Competition usually refers to interspecificcompetition for limiting resources
• Ecological niche: set of habitat requirements; sum total of a species’ use of the biotic and abiotic resources in its environment
• Competitive exclusion principle: no two species can coexist having the same ecological niche
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Species c
SpeciesB
Re s
o urc
e 2
Species A
Resource 1
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Competitive exclusion principle
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Competition theory: How can species coexist?
• Niche differentiation occurs through resource partitioning reducing overlap in the habitat/resources used– Fundamental niche: the combination of
conditions that a species occupies in the absence of competitors
– Realized niche: the portion of resources or areas used when competition occurs
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Resource partitioning in a group of lizards
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Intertidal competitors
Chthamalusin upperintertidal zone
Mean tidal level
Balanusin lowerintertidal zone
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COMPETITION EXPERIMENT
Upperintertidal
Lower intertidal
1. Transplant rockscontaining youngChthamalus tolower intertidal.
2. Let Balanuscolonize the rocks.
3. Remove Balanusfrom one-half ofeach rock.Monitor survivalof Chthamaluson both sides.
On which side of the rocks doChthamalus survive better?
Chthamalus
Balanus
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Fundamental/realized niches
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Predation/Parasitism/Herbivory
• Predation: individuals (predator) catch and consume other individuals (prey) therefore removing them from the population
• Parasitism: individuals consume parts of a living prey organism, or host. It can decrease host fecundity and increase probability of host dying
• Herbivory: individuals eat whole plants or parts of plants; herbivores act either as predators or parasites
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Predator adaptations• Acute senses to locate prey; claws, teeth, fangs,
stingers, poison to catch and subdue prey; digestive tract modified for specific resource. Examples:– Snakes have heat sensing organs to locate prey;
poison to kill the prey– Insects can locate appropriate plants by using
chemical sensors; they have mouthparts adapted for shredding vegetation
– Digestive tract of herbivores greatly elongated to digest plant materials
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Prey Adaptations
• Plant defenses: chemical toxins, spines, thorns. Examples:– Morphine from opium poppy, nicotine from
tobacco plant, mescaline from peyote cactus• Animal defenses: hiding, escaping, self-
defense, crypsis, physical or chemical defenses, aposematic coloration– Examples: various insects achieve crypsis by
resembling sticks, leaves or flower parts
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Crypsis: Poor-will (left), lizard (right)
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Aposematic (warning) coloration in a poisonous blue frog
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Mimicry
• Batesian: palatable animals mimic the appearance of noxious unpalatable animals. Prey fools predator into sensing it is unpalatable
• Müllerian: unpalatable animals resemble one another. Prey produces cues to enhance learning of bad experience by predators
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Batesian mimicry
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Müllerian mimicry: Cuckoo bee (left), yellow jacket (right)
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Adaptations in parasites
• Transmission: accomplish dispersal through hostile environments via complicated life cycles; some parasites modify behaviors of host to enhance transmission
• Virulence: chemical depressors of host immune system (AIDS virus), mimic host proteins (trypanosomes), coat themselves with hosts’ proteins (schistosomes)
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Birds that prey on snails arethe next host for the parasite
Infected snails move to opensunny areas; tentacles wiggle.
Uninfected snails stay in shadedareas; tentacles do not wiggle.
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Host adaptations
• Resistance: immunological response through inflammation and antibody production
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Mutualism
• Interspecific interaction that benefits both species
• They sometimes require the co-evolution of adaptations in participating species
• Examples: Plant pollination by hummingbirds or bees, mutualism between leaf-cutter ants and fungi, mutualism between fishes
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Mutualism between ants and fungus
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Mutualism between fish
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Costs and benefits of mutualism• Each partner is out to do the best it can by
obtaining what it needs from its mutualistat the lowest possible cost to itself
• The net benefit of a mutualism can be measured by subtracting the costs of the interaction against the benefits, in terms of offspring production; the cost-benefit equation may change through time
• Examples: Insect pollinators and nectar producing plants, treehopper and ants
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Treehopper excreting honeydew, which is harvested by ants
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100
80
60
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Plantswithants
Plantswithoutants
20 25 30 5 10 20
AugustJuly
Ave
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ngtr
eeho
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ntWhich treatment contained more treehoppers?
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Commensalism
• Interaction that benefits only one of the species involved in interaction, while the other one is unaffected by the interaction
• Examples:– Cowbirds and cattle egrets increase their
feeding rates by feeding on insects flushed out by grazing bison, cattle, horses and other hervibores