Download Guide 33

Guide 33
Communities and Ecosystems
http://www.nature.com/nrg/journal/v7/n7/box/nrg1877_BX1.html
• Overview: What Is a Community?
• A biological community
– Is an assemblage of populations of various species
living close enough for potential interaction
Stream Biota
http://www.d.umn.edu/~seawww/depth/rivers/06.html
http://weblife.org/humanure/chapter3_10.html
1-Biological Crusts 2-Fungi 3-Bacteria 4Protozoa 5-Nematodes 6-Arthropods
http://www.blm.gov/nstc/soil/communities/index.html
Ecological Succession
• Ecological succession
– Is the sequence of community and ecosystem
changes after a disturbance
Primary succession
Occurs where no soil exists when
succession begins
Secondary succession
Begins in an area where soil remains
after a disturbance
Communities can change with time
http://www.biol.vt.edu/faculty/benfield/biol2804/other.htm
• Retreating glaciers
– Provide a valuable field-research opportunity on
succession
Canada
Grand
Pacific Gl.
1940
Alaska
0
1912
1948
1879
1949
1935
Miles
1941
1899
1907
5
1879
1948
1931
1911
1900
1892
1879
1913
1860
Reid Gl.
Johns Hopkins
Gl.
1879
Glacier
Bay
1830
1780
1760
Pleasant Is.
Figure 53.23
McBride glacier retreating
10
• Succession on the moraines in Glacier Bay, Alaska
– Follows a predictable pattern of change in vegetation and
soil characteristics
(a) Pioneer stage, with fireweed dominant
(b) Dryas stage
60
Soil nitrogen (g/m2)
50
40
30
20
10
0
Figure 53.24a–d
Pioneer Dryas Alder Spruce
Successional stage
(d) Nitrogen fixation by Dryas and alder
increases the soil nitrogen content.
(c) Spruce stage
Modern Vegetation Zonation
Alpine Tundra
Spruce/Fir
>1450 m
750-1450 m
Northern Hardwoods 450-750 m
Mixed Hardwoods
<450 m
Speck Pond
• Spruce/Fir Zone.
• Raft tied in the center of
the lake.
Age (years)
versus
depth of
lake bottom
mud.
Determined
by C14
dating
• A community’s interactions include competition,
predation, herbivory, symbiosis, and disease
• Populations are linked by inter-specific
interactions
– That affect the survival and reproduction of the
species engaged in the interaction
• Interspecific interactions
– Can have differing effects on the populations
involved
Predation
• Predation refers to an interaction
– Where one species, the predator, kills and eats the
other, the prey
• Cryptic coloration, or camouflage
– Makes prey difficult to spot
• Aposematic coloration
– Warns predators to stay away from prey
• In Batesian mimicry
– A palatable or harmless species mimics an
unpalatable or harmful model
(b) Green parrot snake
(a) Hawkmoth larva
Mutualism
• Mutualistic symbiosis, or mutualism
– Is an interspecific interaction that benefits both
species
Commensalism
• In commensalism
– One species benefits and the other is not affected
Competition
• Interspecific competition
– Occurs when species compete for a particular
resource that is in short supply
• Strong competition can lead to competitive
exclusion
– The local elimination of one of the two competing
species
Competition
The Green Anole (Anolis carolinensis) is native to the
southern United States. In the 1960's, The Brown Anole
(Anolis sagrei) was introduced from Cuba. The two species
vie for habitat and food resources, and it appears that the
exotic Brown Anole has displaced the native Green Anole in
some physical spaces, such as lower shrubbery and grass.
The Green Anole generally lives higher up in the trees and
foliage than the Brown Anole does. This result of competition
is known as resource partitioning.
The Competitive Exclusion Principle
• The competitive exclusion principle
– States that two species competing for the same
limiting resources cannot coexist in the same place
http://www.saburchill.com/IBbiology/chapters02/035.html
Ecological Niches
• The ecological niche
– Is the total of an organism’s use of the biotic and
abiotic resources in its environment
The niche concept allows restatement of the
competitive exclusion principle:
Two species cannot coexist in a community
if their niches are identical
• However, ecologically similar species can coexist
in a community
– If there are one or more significant difference in
their niches
EXPERIMENT
Ecologist Joseph Connell studied two barnacle
speciesBalanus balanoides and Chthamalus stellatus that have a
stratified distribution on rocks along the coast of Scotland.
RESULTS
When Connell removed Balanus from the lower
strata, the Chthamalus population spread into that area.
High tide
High tide
Chthamalus
Balanus
Chthamalus
realized niche
Chthamalus
fundamental niche
Balanus
realized niche
Ocean
Figure 53.2
Low tide
In nature, Balanus fails to survive high on the rocks because it is
unable to resist desiccation (drying out) during low tides. Its realized
niche is therefore similar to its fundamental niche. In contrast,
Chthamalus is usually concentrated on the upper strata of rocks. To
determine the fundamental of niche of Chthamalus, Connell removed
Balanus from the lower strata.
Ocean
Low tide
CONCLUSION
The spread of Chthamalus when Balanus was
removed indicates that competitive exclusion makes the realized
niche of Chthamalus much smaller than its fundamental niche.
The concept of coevolution was briefly described
by Charles Darwin in On the Origin of Species, and
developed in detail in Fertilization of Orchids. It is
likely that viruses and their hosts may have
coevolved in a number of cases
Garter snake and Rough-skinned newt
Coevolution can occur between predator and prey species as in
the case of the Rough-skinned Newt (Taricha granulosa) and the
common garter snake (Thamnophis sirtalis). In this case, the
newts produce a potent nerve toxin that concentrates in their skin.
Garter snakes have evolved resistance to this toxin through a set
of genetic mutations, and prey upon the newts. The relationship
between these animals has resulted in an evolutionary arms race
that has driven toxin levels in the newt to extreme levels. This is
an example of co-evolution because both organisms changed to
better increase their chance of survival
Character Displacement
• In character displacement
– There is a tendency for characteristics to be more
divergent in sympatric populations of two species than
in allopatric populations of the same two species
G. fortis
G. fuliginosa
Beak
depth
Percentages of individuals in each size class
Santa María, San Cristóbal
40
Sympatric
populations
20
0
Los Hermanos
40
G. fuliginosa,
allopatric
20
0
Daphne
40
20
G. fortis, allopatric
0
8
10
12
Beak depth (mm)
14
16
Keystone Species
• Keystone species
– Are not necessarily abundant in a community
– Exert strong control on a community by their
ecological roles, or niches
• Observation of sea otter populations and their
predation
Otter number
(% max. count)
80
60
40
20
0
(a) Sea otter abundance
400
Grams per
0.25 m2
– Shows the
effect the
otters have
on ocean
communities
100
300
200
100
0
Number per
0.25 m2
(b) Sea urchin biomass
10
8
6
4
2
0
1972
1985
1989
1993
1997
Year
Figure 53.17
Food chain before
killer whale involvement in chain
(c) Total kelp density
Food chain after killer
whales started preying
on otters
What Is Disturbance?
• A disturbance
– Is an event that changes a community
– Removes organisms from a community
– Alters resource availability
• Fire
– Is a significant disturbance in most terrestrial
ecosystems
– Is often a necessity in some communities
(a) Before a controlled burn.
A prairie that has not burned for
several years has a high proportion of detritus (dead grass).
(b) During the burn. The detritus
serves as fuel for fires.
(c) After the burn. Approximately one month after
the controlled burn, virtually all of the biomass
in this prairie is living.
• The large-scale fire in Yellowstone National Park
in 1988
– Demonstrated that communities can often respond
very rapidly to a massive disturbance
(a) Soon after fire. As this photo taken soon after the fire shows, the
burn left a patchy landscape. Note the unburned trees in the
distance.
(b) One year after fire. This photo of the same general area taken the
following year indicates how rapidly the community began to recover.
A variety of herbaceous plants, different from those in the former
forest, cover the ground.
The End