Download Community “structure”

Lotic Communities
• What is a community?
A) The Dictionary
A term applied to any grouping of populations of organisms found
living together in a particular environment; essentially, the biotic
component of an ecosystem. The organisms interact (by
competition, predation, mutualism, etc.) and give the community a
structure.
B) The Ideal
All species that interact over a spatial scale that includes the lifetime
range of the widest ranging species and the temporal scale of the
longest-lived species
C) The Fanciful
“Co-occurring species about which it’s interesting to think, talk and
experiment.” (Robert MacArthur)
D) The Pragmatic: “Assemblage”
A group of interacting species defined by the investigator at a
particular spatial and temporal scale.
(e.g., the assmeblage of macroinvertebrates in a riffle)
Community “structure”
• The organization of a biological
assemblage based on the numbers
of individuals within different
taxonomic groups.
– We expect similar community structure
where environmental conditions are
similar.
• Some measures of structure:
– Number of species
– Relative abundances of species
(common to rare)
– Functional roles of species [Fig. 6.14]
Important underlying processes:
• Abiotic environmental conditions (esp. disturbance)
• Biotic interactions (competition, predation)
A goal of community ecology:
• To understand and “predict” community structure
– Are communities repeatable in space?
• Do they show same structure under similar environments?
• Or are they “randomly” vary in response to chance events
and dispersal
– Are communities persistent through time?
• Our perception of a community will change with
scale. That scale may be…
– Taxonomic
• e.g., algae, inverts or fish
– Spatial
• e.g., cobble, riffle, reach, etc. -- within stream or among
streams
– Temporal
• e.g., snapshot, seasonal, annual, etc.
Important aspects of community structure
• (1) Taxonomic perspective: Diversity
– Taxonomic Diversity incorporates 3 aspects of
organization:
• (1) Richness (# species)
• (2) Abundance of species
• (3) Evenness of species abundance
– Converse of evenness is dominance
– A formal measure of diversity
• e.g., the Shannon-Wiener Index:
S
H' = - S
[(n
/N)
*
log
(n
/N)]
i
e
i
i=1
S = # species
ni = abundance of species “i”
N = total abundance
H' = -
S [(n /N) * log
i
e
(ni/N)]
Species I.D.
Comm. 1 Comm. 2 Comm. 3
1
2
3
4
5
50
15
27
5
3
20
20
20
20
20
96
1
1
1
1
Diversity (HХ): 1.24
1.61
0.22
Evenness:
high
very low
low
20 species with 5 each, H’ = 3.00
What gives highest diversity?
---> High Richness and High Evenness
Low diversity and High dominance characterize polluted or naturally stressful habitats.
Some rules about community diversity & richness
• 1) Most species are rare [Fig. 11.3]
Remaining 20% of individuals
comprised of 133 species
(which are thus rare).
15 species comprise
80% of all individuals
Most abundant species,
12% of all individuals
Germany’s Breitenbach has >1000
described species (Table 10.1)
• 2) Species richness
increases with …
Different streams have
different maximum
species richness. Why?
– sampling intensity
• Number individuals sampled
(Fig. 10.4a)
• Number of samples (Fig. 10.4b
shows 4 different streams)
– area sampled
• Species-area relationship
(Figs. 11.2, 11.5)
Mussel richness
increases with river
size.
Area ~ size, complexity
But note variation
among rivers of
similar size (refer to
Fig. 11.1b)
Fish richness
increases with
basin area.
• 3) Evolutionary History
matters! Biogeogrpahy
– Some regions much more speciose
(Fig. 10.2)
– Local communities are potentially
more diverse in regionally diverse
areas
– “Keystone”/Dominant species often
have limited ranges
Region
Native fish species
Tropical S.A.
3000
Europe
250
North America
700
- So. Appalach.
250
- Alaska
180
- California
< 50
Connecting Global to Local Richness
(Tonn’s “filtering” concept)
World fish fauna
Gross physiological
filter
Freshwater fauna
Gross geographic
filter
Continental fauna
Fine geographic
filter
Regional fauna
Climate filter
Local fauna
Local evolution
Fine physiological
filter
Community fauna
Both historical and
contemporary factors
influence distributions of
lotic species
Zoogeography,
Regional conditions
Local conditions,
-Temperature
-Habitat complexity
-Biotic interactions
Local community composition can change over time because species
move.
Species can use habitat
at
multiple scales
For example, fish move to
meet needs at different life
stages or to find refugia
(Fig. 2).
Fausch et al., BioScience 52:483 (2002)
So, what factors regulate Community Structure?
• Via dispersal, species spread
throughout stream networks and
among drainages
– Many barriers to dispersal
• Natural - waterfalls (fish), steep drainage
divides (insects)
• Human - e.g., dams
• Assuming arrival, flourish where
conditions are good
• Two dominant processes that influence
“suitability” and thus regulate …
– biotic interactions
– abiotic factors and chance
Field study showing interplay of spatial and temporal
and biotic and abiotic factors: David Hart (1992,
Oecologia)
Augusta Creek, MI
Observation: Filamentous alga, Cladophora, important habitat
structure, and it occurs along velocity gradient.
- Figure 2
Types of invert communities varies with Cladophora cover
- Table 1 shows positive/negative associations
Crayfish forage on Cladophora, but not > 50
cm/s, because … ?
Caging experiments
show crayfish
controls Cladophora
biomass (Fig. 3)
Other grazers (like Leucotrichia)
can also control Cladophora
(Fig. 4) ... but how?
Why might there
be less algae in
“enclosure” vs.
“open” treatment?
Priority Effect: They must
arrive on bare substrates first
and get established before
Cladophora attains a “size
refuge”. (Weed the Cladophora?)
• What creates bare patches?
DISTURBANCE!
• Who gets there first?
CHANCE!
Current Velocity
< 50 cm/s,
crayfish present
> 50 cm/s,
crayfish absent
Initial Colonization of space by:
Sparse Cladophora
on streambed
Insect Grazers
Sparse Cladophora
on streambed
Cladophora
Dense Cladophora
on streambed
What maintains heterogeneity of