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Other Interspecific Interactions
Chapter 7
Interspecific Interactions

Symbiosis intimate
association
between
individuals of
different species,
in which one lives
on or in the other
Interspecific Interactions
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Commensals “guests” - neither
harmful nor
beneficial to the host
Use surface of the
host as a place to live
Interspecific Interactions
Epiphytes - bromeliads,
Spanish moss
Interspecific Interactions
Interspecific Interactions
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Mutualism relationship of benefit
to both organisms
Enhanced growth,
survival, reproduction
in presence of each
other (mirror-image
of competition?)
Interspecific Interactions
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Obligate for each
(required)
Facultative for each
(helpful, but not
required)
Mixture
Culture of crops/livestock
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Humans and domesticated plants/animals
Ants and fungus
Pollination
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Insects, birds, bats as pollinators
Reproduction for plant, food reward for pollinator
Gut Inhabitants
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Cattle rumen with bacteria
Termite gut with protozoans, bacteria
Mycorrhizae
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Fungus and root tissue
Fungus increases water, nutrient uptake
Plant root supplies organic carbon
Algae & Animals
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Hydra with Chlorella
Coral with dinoflagellates (side effect of
photosynthesis is precipitation of calcium
carbonate)
Lichens
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Fungus and algae
Fungus absorbs water, nutrients
Algae photosynthesizes, provides organic carbon
(algae often in obligate relationship)
Interspecific Interactions
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Parasitism - obtains
nutrients from one or
few hosts
Normally causes harm,
but not death
Often includes
pathogens (diseasecausing), viruses
Superabundant - > half
of species on earth
Microparasites


Bacteria, viruses, protozoa, fungi
Multiply directly within host (usually within
cells)
Microparasites

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Transmitted directly from host to host (VD,
influenza)
Transmitted by vector (some other animal)
(malaria, sleeping sickness)
Macroparasites
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Flatworms, roundworms, insects
Grow in/on host, produce infective stage that
leaves, live within body cavities or intercellularly
Macroparasites
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Transmitted directly (intestinal nematodes, lice,
plant fungi)
Transmitted indirectly (tapeworms, flukes)
Transmission
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Transmission rate depends on host density
Rate increases with density (susceptible hosts
only - genetics)
Distribution
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Distribution is clumped (site-specific)
Few hosts have large numbers, most have none
High intensity of infection, low prevalence
Response of hosts
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Die in whole or in part
Biotrophic parasites require living hosts
Necrotrophic parasites prefer dead hosts
(pioneering decomposers)
Plant hosts

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Infected cells die immediately - hypersensitivity
Surrounding cells produce phytoalexins to
prevent spread of parasites
Invertebrate hosts
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Phagocytic cells engulf foreign particles
Vertebrate hosts
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Immune responses - several types of killer cells with
“memory”
Inhibits future infection by same things
Response most effective for bacteria, viruses
Response least strong for macroparasites, protozoans
Bottom Line
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Reduced survival, growth, fecundity, competitive
ability of host
Some evidence that parasites may be chief factors
controlling populations of some organisms (e.g.,
humans)