Download Lecture 8 - Community Interactions and Niche Diversity

BIO 120
Field Natural History
Spring
LECTURE 8
Community Interactions and Niche Diversity
I.
Introduction to Community interactions.
A. The distribution of many species is often determined by interspecific interactions. These are
interactions with other species.
1. Organisms have evolved in response to the abiotic environment (abiotic – non-living),
and the biotic environment. (biotic – living).
2. Biotic interaction can lead to coevolution. This is the reciprocal evolutionary adaptation
of two species.
3. Example: Zebra butterfly and Passionflower.
a. Passion flowers are toxic.
b. Zebra butterfly adapts. It has enzymes that break down harmful compounds.
c. Female zebra butterflies avoid passion flower leaves that already have eggs on them.
d. Passionflower leaves evolve yellow spots that look like eggs to deter egg-laying
zebra butterflies.
II.
Concept of Habitat and Niche.
A. Habitat corresponds to the organism’s address, and the niche is its profession.
B. The extreme importance of a seemingly small niche – dung beetles.
1. Dung beetles imported to Australia from Africa to solve the problem of too many cow
flops AND, they significantly reduced the bush fly population.
III.
Interactions among organisms.
A. A simple way to remember the outcome of interactions between organisms is the mnemonic
device of paired signs (+/ −).
B. Predation.
1. Predation: (+ / −) Best known when a member of one species (predator) eats a
member of another species (prey). Predation also includes:
a. Parasitism- a predator (parasite) that lives in or on its host. In this arrangement, the
parasite benefits while the harm is done to the host.
b. Parasites can be categorized as microparasites land macroparasites.
i. Microparasites include those organisms that cause disease.
ii. Macroparasites includes parasitic worms and insects.
iii. Insect parasites are called parasitoids.
2. The effects of predation on population sizes of both species are entirely predictable.
a. Predation increases the growth of the predator population . The higher the predation,
the better fed the predators. This leads to higher birth rates and lower mortality.
That’s the plus part.
b. Predation decreases the size of the prey population, or at least slows its growth (even
if prey are not killed, they are weakened) lower birth rate, higher death rate. That’s
the negative.
BS 120
Field Natural History
Spring
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C. Herbivory – When an animal eats a plant. This includes insects. About half the insect species
in temperate regions are herbivorous. Also includes mammals, ranging from meadow mice to
elephants.
1. Two main types of herbivorya. Grazing or browsing- part of the plant is consumed, but plant is not killed.
b. When the whole plant is consumed or killed, herbivory is predation.
c. The use of seeds by animals is a form of predation- it removes and individual from
the population.
2. Herbivory can greatly reduce plant populations
a. White-tail deer browse on oak seedlings.
b. The number of plant species in a field has been shown to increase when protected
from insect herbivores.
c. Imported, leaf eating beetles control introduced weed from Europe – St. John’s
Wort.
• Klamath weed now restricted to shadier sites, where beetles tend to avoid laying
eggs.
3. If there are so many herbivores, then why is the world still green?
a. Herbivores may have self-regulatory mechanisms that prevent them from destroying
their food supply.
b. Other control mechanisms, such as predation, keep herbivore populations low.
c. All that is green may not be edible.
4. Plants have evolved an array of defenses against herbivores.
a. Structural defense - thorns. Rose bushes. Blackberries.
b. Chemical defenses - stinging nettles - release acetylcholine, a neurotransmitter.
c. More chemical defenses -Garlic mustard- mustard oils (glucosinolates).
d. Other chemicals- Nicotine, morphine and caffeine are alkaloids found in plants.
Peppermint oil and catnip contain terpenoids. These compounds not only taste bad,
but also can reduce insect growth.
D. Symbiosis - a term that encompasses a variety of interactions between two species. The most
important implication of this term is that the two species have a close association.
Based on this definition, parasitism is considered a symbiotic relationship. BUT, it makes
more sense to group it with predation.
1. Commensalism (+ / 0) A commensal lives on or around individuals of some other
species (usually called hosts) and derives benefit from that association. The host suffers
no negative affects, nor seems to benefit.
a. Example: Egrets that follow grazing cattle.
b. Example: in the broader sense burrowing owls use old prairie dog burrows as nest
sites.
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Field Natural History
Spring 2000
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2. Mutualism (+/ +) These are interactions where both species benefit.
Mutualistic relationships require the evolution of adaptations in both participating species,
since changes in either species are likely to affect the survival and reproduction of the
other.
3. Some mutualistic relationships are non-symbiotic- the two organisms do not live in close
contact.
a. The bee gets nectar and pollen (both important food sources), plants get effective
pollen transfer (ensures reproduction).
b. Some associations can become obligatory. The only pollinator of the yucca is the
yucca moth. Moth's only food source (both adult and larvae) is yucca pollen and
seeds.
4. Other mutualistic relationships are symbiotic- i.e. the two organisms live in close
contact.
a. Lichen - association of fungus and alga. (The singular form)
b. Fred fungi to a ‘lichen’ to Alice algae. (The plural form)
5. Interesting to consider how such associations arise.
a. Consider bees: they feed on plant tissues (and energetically expensive ones, at that).
That's predation. In effect, the plants co-opt the bees into working for them. They
turn a negative effect into a positive effect.
b. Alga's perspective is that fungus is a parasite. Algae leak more food in presence of
fungi than when alone.
Fungal view - it provides shelter to algae.
E. Competition (− / −) Two species use the same limited resource or harm one another while
seeking a resource.
1. Definitions.
a. Short- Competition results in mutually adverse effects to organisms that utilize a
common resource in short supply.
b. Longer - Competition is an interaction between individuals, brought about by a shared
requirement for a resource in limited supply, and leading to a reduction in
survivorship, growth and/or reproduction of the competing individuals.
2. Competition occurs among and between species.
a. Intraspecific competition - competition between individuals of the same species.
b. Interspecific competition - competition between individuals of different species.
3. An important distinction between intraspecific and interspecific competition is the
difference in their population consequences.
a. Intraspecific competition leads to the stable regulation of population size within the
limits imposed by the environment.
b. Interspecific competition may cause the extinction of one of the competing
populations.
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Field Natural History
Spring
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4. The main consequence of competition between two species is competitive exclusion.
a. Two species compete only if their niches overlap.
b. Two species with similar requirements cannot coexist in the same community. The
more efficient species should drive other to extinction.
c. Test with 2 closely related Paramecium. P. aurelia and P. caudatum.
d. Result provided key evidence for the competitive exclusion principle -'two species
cannot coexist on the same limiting resource."
e. Limiting resource is a requirement of the definition, since competition expresses
itself only when consumption depresses resources and thereby limits growth.
IV. Other ways that species can coexist.
A. Resource partitioning – using the same resource in different ways.
1. Five species of migratory warbler appeared to be foraging for the same prey in the same
spruce tree.
2. Each warbler concentrated more than half of its feeding time in zones not used by the
other 4 species.
3. Because the zones are different, they are inhabited by different insects. So all five
warblers do not rely entirely on the same types of insects.
4. Note that all five species occupy the same habitat – they just utilize it differently.
B. Another result of competition is character displacement.
1. When two species share a characteristic (that is, it overlaps) live together in the same
environment, the characteristics tend to diverge.
2. Good examples are finches of the Galapagos, which display character displacement in
beaks sizes and hence, size of seeds they eat.