Transcript Community Structure

Community Structure
BIOL400
12 October 2015
Abundance of
Species in Communities:
Preston’s Log-Normal
Bell Curve
Fig. 19.1 p. 379
6814 moths of 197 species from Rothamsted, England
(not shown: a species with 1799 captures)
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
Fig. 19.3 p. 380
Snakes of Panama
British birds
HANDOUT—Preston 1962
Factors
Promoting
Community
Diversity
A) General Latitudinal Trend
 Species
diversity is greatest in tropical
areas and declines toward either pole
Fig. 19.6 p. 382
Fig. 19.7 p. 383
Fig. 19.9 p. 384
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
C) Spatial Heterogeneity
 More
spatial heterogeneity of habitat =
more niches to be filled
 Predicts less overlap in resource use in
tropics
HANDOUT
Longenecker, 2007
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
Fig. 19.23 p. 396
HANDOUT—Sousa 1979
Fig. 19.24 p. 397

The "tail" of
lowered
abundance on
one side or the
other may not
exist


Tide-pool data
support
hypothesis
Emergentsubstrate data
do not
Fig. 19.25A p. 397
Fig. 19.25B p. 397
Regulation of
Species’
Abundances in
Ecosystems
Regulation of Species’
Abundances in Ecosystems
 Ecologists
generalize about effects of
species on one another in nutrientvegetation-herbivore-carnivore 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
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
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
Fig. 21.11 p. 436
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
Table 21.3 p. 437

In most studies,
herbivore removal
had strong
positive effect on
plants

Supports MS
model
Two Additional Models
 Top-down


NVHC
aka “Trophic Cascade”
 Bottom-up

Regulation
Regulation
NVHC
Fig. 21.14 p. 439
Fig. 21.15 p. 439
Top-down regulation in
Zion National Park, Utah
Fig. 21.16 p. 440
Cougars common
Cougars rare
HANDOUT
Hebblewhite et al. (2005)
Table 21.4 p. 441
Food Webs and
Their Linkages
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
Fig. 20.7 p. 408

Food chains are
short

Data on 95
species in an
estuary in
Scotland (5518
links)
Fig. 20.8 p. 409

Food chains
shorten at
lower
productivity

Experimental
tree holes that
varied in
added leaf
litter (100%,
10%, or 1%)
Fig. 20.5 p. 407

Connectance = proportion of all total possible
links that occur = 10/72 = 0.20

Generally ~0.14 regardless of web’s diversity
Two Alternate Hypotheses
 Constant

Connectance
L is some constant proportion of S2, which is
the maximum possible L
• L=S2: each species is linked to every other
species, including itself via cannibalism
 Link-Species

Scaling
Posits linear relationship of L with S
HANDOUT—Martinez (1992)
Constant Connectance:
Supported by Exponent of 1.54?
S1
S1.54
Link-Species Scaling
Regression
20
50
100
101
413
1,202
S2
Constant Connectance
400
2,500
10,000
Diversity-Stability
Hypothesis
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
Fig. 20.20 p. 419
168 experimental plots of MN prairie
Community stability = Mean/SD for late-summer biomass over 10 years
Fig. 20.21 p. 419
Resistance measures changes in abundance of plant species