Community Structure

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Community Structure

• BIOL400
• 12 October 2015

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Abundance of Species in CommunitiesPrestons
Log-Normal Bell Curve
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Fig. 19.1 p. 379
6814 moths of 197 species from Rothamsted,
England (not shown a species with 1799
captures)
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Fig. 19.2 p. 380

• Place abundance of species on log2 scale of
  octaves
• Defines a partial bell curve
• Left of the y-axis species too rare to detect
  in the sample

Trees of BCI, Panama
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Fig. 19.3 p. 380
Snakes of Panama
British birds
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HANDOUTPreston 1962
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Factors Promoting CommunityDiversity
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A) General Latitudinal Trend

• Species diversity is greatest in tropical areas
  and declines toward either pole

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Fig. 19.6 p. 382
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Fig. 19.7 p. 383
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Fig. 19.9 p. 384
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B) History

• Older communities have had more time for
  specialization and diversification of species
• Might explain part of the tropics-vs.-
  temperate-zone difference
• Never-glaciated tropics vs. temperate zone that
  was glaciated repeatedly

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C) Spatial Heterogeneity

• More spatial heterogeneity of habitat more
  niches to be filled
• Predicts less overlap in resource use in tropics

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HANDOUTLongenecker, 2007
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D) Intermediate-Disturbance Hypothesis

• Highest diversity of species occurs at
  intermediate frequency of disturbance
• High frequency several species die out due to
  detrimental effects of the disturbance
• Low frequency competitive exclusion occurs as
  species approach carrying capacity of the habitat

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Fig. 19.23 p. 396
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HANDOUTSousa 1979
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Fig. 19.24 p. 397

• The "tail" of lowered abundance on one side or
  the other may not exist
• Tide-pool data support hypothesis
• Emergent-substrate data do not

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Fig. 19.25A p. 397
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Fig. 19.25B p. 397
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Regulation of Species Abundances in Ecosystems
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Regulation of Species Abundances in Ecosystems

• Ecologists generalize about effects of species on
  one another in nutrient-vegetation-herbivore-carni
  vore systems
• 27 (3?3?3) different models of effects can be
  drawn using level-to-level symbols
• ?, ?, and ?? (symbolizing regulation of
  abundance)
• Ex N ? V ?? H ? C

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Fig. 21.12 p. 436

• HSS (1960) Predators limit herbivores, who do
  not limit plants
• Competition (C) is greater among plants and among
  carnivores than among herbivores

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Fig. 21.12 p. 436

• MS (1987) Model incorporates environmental
  stress as possibly trumping competition and
  predation in regulating populations
• Predation becomes progressively more important
  than competition as the environment becomes more
  benign

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Fig. 21.11 p. 436
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Fig. 21.12 p. 436

• HSS (1960) Strong competition among plants
  herbivores compete only weakly and do not
  regulate plants
• MS (1987) Weak competition among plants
  herbivores compete in benign environments and
  regulate plants

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Table 21.3 p. 437

• In most studies, herbivore removal had strong
  positive effect on plants
• Supports MS model

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Two Additional Models

• Top-down Regulation
• N ? V ? H ? C
• aka Trophic Cascade
• Bottom-up Regulation
• N ? V ? H ? C

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Fig. 21.14 p. 439
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Fig. 21.15 p. 439Top-down regulation in Zion
National Park, Utah
Fig. 21.16 p. 440
Cougars rare
Cougars common
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HANDOUTHebblewhite et al. (2005)
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Table 21.4 p. 441
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Food Webs and Their Linkages
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Food Webs and Their Linkages

• Complexity of food webs is limited, primarily by
  inefficiency of energy transfer from one trophic
  level to next
• Generally about 5-20
• Rest lost to heat of metabolism and decomposers

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Fig. 20.7 p. 408

• Food chains are short
• Data on 95 species in an estuary in Scotland
  (5518 links)

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Fig. 20.8 p. 409

• Food chains shorten at lower productivity
• Experimental tree holes that varied in added leaf
  litter (100, 10, or 1)

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Fig. 20.5 p. 407

• Connectance proportion of all total possible
  links that occur 10/72 0.20
• Generally 0.14 regardless of webs diversity

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Two Alternate Hypotheses

• Constant Connectance
• L is some constant proportion of S2, which is the
  maximum possible L
• LS2 each species is linked to every other
• species, including itself via cannibalism
• Link-Species Scaling
• Posits linear relationship of L with S

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HANDOUTMartinez (1992)
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Constant ConnectanceSupported by Exponent of
1.54?

• S1 S1.54 S2
• Link-Species Scaling Regression Constant
  Connectance
• 20 101 400
• 50 413 2,500
• 100 1,202 10,000

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Diversity-Stability Hypothesis
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Diversity-Stability Hypothesis

• Stability resistance to change and rate of
  recovery from change
• Idea that more diverse communities resist and
  recover from major change better than less
  diverse systems
• Some functional redundancy with increased
  diversity of species

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Fig. 20.20 p. 419
168 experimental plots of MN prairie Community
stability Mean/SD for late-summer biomass over
10 years
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Fig. 20.21 p. 419
Resistance measures changes in abundance of plant
species