Crotalus atrox

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Crotalus atrox
Crotalus molossus
Crotalus scutulatus
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Species diversity and disturbance

• In general, species are adapted to average
  environmental conditions.
• However, all environments have periodic
  disturbances varying in frequency and intensity.
• Disturbances affect species diversity.
• Principle intermediate levels of disturbance
  increases species diversity.

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• Intertidal zone communities. H1 (Connell) --
  both high and low levels of disturbance will
  reduce diversity.

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The test of the hypothesis

• Effects of disturbance on algae and invertebrate
  diversity
• Intertidal rocks (substrate)
• Disturbance large waves from winter storms.
• Level of disturbance depends on wave size and
  rock size.
• Small rocks (much disturbance) large rocks
  (little disturbance).
• Rocks supporting greatest diversity of species
  were those subjected to intermediate levels of
  disturbance.
• These were intermediate sized rocks.

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Disturbance and Diversity in the Intertidal Zone
Explanation Low frequency disturbance time
for dominant species to displace other
species. High frequency disturbance difficult
for all but a few species to colonize the rocks.
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Disturbance and diversity N.A. temperate
grasslands

• Biotic sources of disturbance
• Burrowing mammals e.g., prairie dogs gophers
• Until 1919 occupancy of c. 40 million ha
• Extensive burrow systems (towns) with a dynamic
  border and various ages of mounds piles of
  excavated soil around burrow entrances
• open to colonization by r-selected species.
• Intermediate levels of disturbance prevents
  either colonizers or dominant plants for
  exclusion.
• Extermination programs simplify plant communities.

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Community Organization

• 1. Trophic levels (functional organization)
• Patterns of energy flow who eats whom
• Each community will have a pattern of energy
  transfer called a food web.

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Antarctic pelagic food web
Subdivisions 1. Top predators 2. Basal
species 3. Intermediate species
Links
Levels
85 sp.
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• Food web generalizations
• 1. Food chains are short.
• Mean trophic levels from top predator to
  producers is ca. 5. (10 max.)

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• 2. On average, the number of connections (links)
  among species increases as species richness
  increases.
• e.g., an estuary in NE Scotland
• 95 species and 5,518 food chain links

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Boreal forest (N Canada)
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• 3. Proportions of predators are nearly constant
  regardless of web size.
• Approximately 1 predator 2-3 prey species.

92 freshwater invertebrate food
webs prey/predator 21 3.51
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• Food webs
• Possible interactions ( species)2
• Connectance actual interactions / possible
  interactions
• 10/49 0.20

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Community Organization

• 2. Guilds (functional organization)
• Guild a group of species exploiting a common
  energy source in a similar fashion.
• Ecological units -- not taxonomic units.
• i.e., functionally equivalent species are
  interchangeable (can replace one another in the
  guild).
• Number of functional roles in a community is less
  than the number of species.
• so, relative species abundance can change without
  affecting total biomass of the guild.

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What is 1. a parasitoid? 2. a hyperparsitoid?
Guild
Guilds
Resource
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Food Web Structure and Species Diversity

• A community may be controlled by a single
  species.
• Its activities determine the nature of the
  community.
• Such species keystone species.
• A keystone species will reduce the likelihood of
  competitive exclusion.
• Decreasing competition increases the number of
  species that can coexist.
• E.g., intertidal community
• Experimental removal of a sea-star predator

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Response, decline in species richness 5 years
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How to recognize a keystone species

• Exceptionally strong effects on community
  structure despite low biomass

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Keystone species
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2nd level Predators
1st level Predators
Herbivores
Trophic cascade effect on primary production
Reference