ICA 8 COMMUNITY STRUCTURE + DEVELOPMENT
SPATIAL STRUCTURE OF COMMINITIES
1. What is a community? Assemblage of interacting species that co-occur.
Define spatially species co-occurring in same area
Define functionally .association of interacting species
2. Are there boundaries to a community? depends; some are abrupt; others blur
Do boundaries differ depending on mobility of organism? Some organisms
are ‘community-specific’ and very sensitive to boundaries. More mobile and less
specialized species move across boundaries and function in multiple communities.
3. What is an ecotone? a prominent boundary separating communities 18.4, 18.5
What causes them to arise? When physical conditions change abruptly, e.g.
as geology and soil conditions change; or as land changes to water
How does species composition change from one side to other? Some species
are restricted to one side or other of the ecotone; others have broader tolerance
and cross ecotones.
Are there clear ecotones between all communities? No
Is soil tolerance an adequate explanation for a species’ presence? No{What else
can determine its distribution? competition 18.6
4. Contrast a closed: All species are roughly same tolerance of environmental conditions
and hence occur together consistently across some gradient.
vs. open community. Species differ in breadth and position along a gradient and
hence don’t co-occur consistently. 18.3, 18.7
Which is more likely to have ecotones? Closed; ecotones separate groups of species.
Contrast holistic = closed; all included together on a gradient and have dependencies.
vs. individualistic view of community. Each species is independent of others.
What is gradient analysis and how is it used to study the nature of the community?
Field study to determine species distributions along an environmental gradient.
Use to test whether communities are open or closed. Also whether the species
fit a closed or continuum concept. If species replace one another continuously
along physical gradients the community is open. 18.7, 18.9
5. How test whether identical communities develop in identical environments?
Build sterile experimental areas, leave alone, and determine species number and
composition after exposure for colonization.
What is result from experiment? Communities differ in number of species
and species composition. Some species are found in all; others are very
unpredictable in their occurrence in a community.
What is the predictability of species composition of a community? Not high,
especially for more rare species
What role does chance play? A great deal, at least for a subset of species.
FUNCTIONALITY OF COMMUNITIES
6. How strong are the connections among species in a community?
Contrast the rivet: tight linkage among species; obligate assocation or
exclusion of species
vs. the redundancy model: loose linkage; species indifferent to each other;
one species or process can be accomplished by another species; species
are redundant and can be lost with no consequence
7. What organizes community webs? Feeding relationship between species 18.11
Does species richness drive the structure of a food web? not entirely
What else contributes? # guilds; # feeding links (per species + total); # levels
What changes: # species; # guilds; # links; # trophic levels
vs. doesn’t change with increased food web complexity? # links/species 18.12
8. What are 3 different ways of organizing food webs? 18.14
Connectedness web shows feeding relationships among species.
Energy flow web shows connections quantified as energy flux.
Functional web emphasizes influence of species on growth rates of other species.
9. What is an example of the effect of food web structure on species diversity?
E.g. Number of species on one trophic level may depend on consumption by
species at higher trophic levels. (e.g starfish removal experiment).
10. What is a keystone species? non-redundant, key species in maintaining community
stability and diversity 18.13
What is an example for a plant? Palm species provide fruit throughout the year
and carry over fruit eaters when rest of plant community has low fruit.
an herbivore? beetle feeds on dominant plant and large numbers of subdominant
plants flourish
a predator? starfish eating dominant herbivore and large number of subdominant
herbivores flourish
How would you test whether an herbivore is acting as a keystone species?
If a keystone herbivore species controls community diversity,
then species diversity will be greater with than without keystone species.
present.
11. What are two hypotheses about what influences abundance of each trophic level?
1. top-down control from predators to lower trophic levels.
2. bottom-up control from abiotic resources and plants to higher levels. 18.19
What is a trophic cascade? indirect effects of trophic level interactions that
extend through additional trophic levels 18.23, 24, 25
How do trophic sizes change going up the trophic structure with bottom up control?
They steadily decrease; although amount of decrease depends on size of
resources and plants. 18.19
How to they change in a four-layer: 4 reduces 3 increases 2 reduces 1;
vs. three-layer trophic structure with top down control? 3 reduces 2 increases 1
12 Does food web complexity lead to increased community stability? Debatable 18.15
Constancy = ability of community to resist change from outside influences (resistance)
Pro: Complexity brings alternative resources so less dependency on single resource;
Species are redundant so removal of species has little effect
Disrupt one energy pathway is no problem because of multiple routes possible
Con: So much dependency that great time lags result that destabilize population
dynamics
13. Resilience = ability of community to return to some reference state after disturbance.
Which are more stable (resistant) when disturbed: Species-rich or species-poor
communities? In this natural experiment in plots exposed to drought, biomass
loss decreases as number of species increases. 18.17
14. What are alternative stable states? A system can exist in more than one stable
reference state. 18.18
What causes the switch? An extreme external perturbation, such as removal of
keystone consumer and new keystone responses differently to key important factor.
Chap 18 Summary: 1-5,7-12
DEVELOPMENT OF COMMUNITES AFTER DISTURBANCE (SUCCESSION)
1. Are communities stable over time? Depends on time scale; over short term
(decades) perhaps. But over longer periods, environmental conditions change
and so does species composition of a community.
Contrast equilibrium stable
vs. non-equilibrium transient/instable views of the community.
2. What is succession? Sequence of community change after exposure of new
substrate or disturbance 19.2
What initiates succession? New substrate is exposed or old habitat disturbed
What is the final end point of succession? Climax community pg. 398-9; 19.13
What must be true for no further change to occur? No further change in
community structure and species composition. Species able to replace themselves.
3. What are examples of disturbance? Earthquake, flood, fire, treefall, lava flow; plow;
cut down forest
How do disturbances differ? size; frequency (predictability); intensity; level of
environmental heterogeneity created; seed source availability; soil availability
4. What is primary succession? community change on bare substrate with no soil
or local seed source pg. 392-3; 19.4, 19.5, 19.11
What are examples of disturbance that lead to primary succession? flood; glacial
retreat; lava flow; sand dunes
What steps must occur for the land to be colonized? seed arrival from outside
sources; establishment under extremes of water and nutrient limitations.
What are seed sources? Wind, water, and later animals from distant habitats
Which type of dispersal mode arrives first? abiotic modes
What factors affect survival and the rate of primary succession? How quickly
soil develops (with water-holding capacity and an available nutrient pool.
Arrival of nitrogen-fixing organims.
5. What is secondary succession? regeneration following disturbance that
removed vegetation but NOT soil or seed source in soil. 19.3
What are examples of disturbance that lead to secondary succession? windstorm,
flood, fire, plow, cut forest
Which is faster: primary or secondary? secondary Why? Have a local source
of seeds in seed bank in soil; have soil with water and nutrients. Primary has neither
and it takes a very long time to develop soil – and then for seeds to arrive.
6. Using examples, how does a species’ life history affect when a species
enters into the succssional sere? 19.10 Species have different lifespans and different
conditions that stimulate germination; annuals may come in early but not stay
long; perennials may come in early or late but have longer pre-reproductive period.
What traits are common in early successional (r-selected = many small seeds,
abiotic mode of dispersal; seed dormancy in soil; rapid growth; small mature size;
low stress tolerance and low competitive ability.
vs. late successional organisms? (opposite of above) Table 19.2, 19.12
7. Describe three mechanisms proposed that govern the course of succession?
1. facilitation: species in earlier change environmental conditions so they
can’t survive but new species can. 19.9
2. inhibition: existing species suppressing colonization by later species.
Succession becomes ‘arrested’.
3. tolerance: existing species do not affect establishment of others.
Species may persist “under-performing’ for long time before it finally flourishes.
Why does species replacement stop? Environmental conditions reach point in which
existing species can flourish and replace themselves; no other species enter.
8. Is there a successional sequence of species for animals? Yes
Do animals respond to succession? In part do they affect plant succession? They
can by being seed predators/ herbivores of plants, thus affecting their relative
abundance and even presence.
Summary Chap 19: 1-8