Eemb 148: Community Structure

Front 1

What is a community vs. a population?

Back 1

Community: 2 or more populations in a defined geographic region

Population: 2 or more individuals of the same species in a defined geographic region

Front 2

Are stream communities composed of strongly interacting species or loose assemblages adapted to particular environments?

What are the relative effects?

Back 2

01: Biotic Factors: communities at equilibrium -- stable conditions

02: Abiotic Factors: communities determined by environmental conditions

Front 3

How does the interaction between abiotic and biotic forces influence ecological communities?

Back 3

3 Hypotheses:

01: The harsh-benign hypothesis
02: Niche Based Models
03: Patch Dynamics

Front 4

Harsh Benign Hypothesis

Back 4

- Local environments vary from harsh (temperate:winter) to benign (late spring, early summer)
-Shift in the relative importance of biotic and abiotic forces with environmental condition
- Structuring forces change depending on state of stream
- Biotic processes do not produce community structure in unpredictable environmental conditions
- Stable streams: slower, warmer, less fluctuation in discharge: biotically controlled
- Biotic controls: competition/predation and herbivory
- Abiotic structuring force in flashy rivers with fast flow
- Emphasis on survival
- Abiotic: physical/chemical -- current velocity, flow, suspended sediment load, abundance and distribution
- High diversity in streams because disturbance resets conditions

Front 5

Harsh Benign Hypothesis Problems

Back 5

Difficult to measure/define environmental harshness

But, is useful:
01: for individual seasonal comparisons of one stream
02: localities that differ in environmental variability, can compare forces that change abundance and distribution

Front 6

Niche Based Models Hypothesis

Back 6

Biological Interactions

01: In constant environment, strong biological forces permit only a few superior species to maintain populations
02: Competition, predation, herbivory maintain community structure
03: Moderate levels of disturbance prevent biological dominance and allow other species to co-exist (intermediate disturbance)

--Communities persist with little change
--For environment to be constant need:
01: high density
02: limited resources
03: stability
-- Biology structures community
--Reduces diversity over time, except that intermediate disturbance resets and restores diversity

Front 7

Problems With Niche Based Models Hypothesis

Back 7

(-) Rivers are usually flashy (not stable)
--> discharge high, lots of resources, lower density more common

(-) Stability is short term in annual cycle
--> not always the case, late spring and early summer in temperate areas

Predictions:
01: Good for short periods of environmental stability
-- ex. late spring and early summer (N. hemisphere)
-- also another window: good late fall and early winter before the rain is too heavy or gets too dry

Front 8

Patch Dynamics Hypothesis

Back 8

01: Dispersal ability of organisms within a shifting mosaic of environmental conditions
02: Colonization and reproduction in variable environment determines community structure
03: Variable environment allows more species to co-occur
04: Overall community regularity because environmental conditions are predictable as a whole though locally unpredictable

ex. dicosmoecus blackfly and caddisfly: only comes around when flow is low because it is a poor colonizer but a good colonizer

- in stable conditions, higher density of better predators/competitors
- in flashy conditions, higher density of better colonizers

Front 9

How does the Patch Dynamics Hypothesis Work?

Back 9

01: Disturbance removes organisms and opens up space
-- Dispersal ability is critical because stability shifts frequently

02: New space is colonized
-- Bed is made of patches that result in different reactions based on substrate type.
-- Move to different undisturbed patch or risk extinction
-- Finding a new patch and reproducing = variable environment
-- Determines what the environment looks like, maintains lots of species variability and allows for more species diversity // heterogeneity

03: Different community trajectories are possible

Front 10

What are the different trajectories of the Patch Dynamic Hypothesis?

Back 10

Trajectory #1: Competitive ability inversely correlated to colonization ability

- succession: good colonizers to good competitors
- greater diversity with intermediate disturbance
- low disturbance: greater density of dominant predators
- high disturbance: greater density of better colonizers

Trajectory #2: Lottery Competition
- all species are equally good colonizers and competitors
- chance colonization determines community composition
---> sheer chance: whoever gets there first is best and will be seen over time
- rare in rivers (may only work for alga - first few days)
---> w/i 5 days is outcompeted by other forms, but first few, sheet chance alone
- good in theory, but hard to prove
- equal competitive strengths

Trajectory #3: High dispersal rates (geared to rapid dispersal)
- environmental fluctuations interrupt biotic interactions
- communities exist in permanent state of non-equilibrium
- biotic interactions rarely important
-- creates patchwork of species presence and absence
- organisms take advantage of resources patch to patch till have enough biomass to reproduce
- never stable, constantly reset
- disequilibrium: getting in and getting out
(-) overplays dispersal

Front 11

Patch dynamics: why does the overall community exhibit regularity in species composition?

Back 11

- Environmental conditions are predictable as a whole
- Even though they are unpredictable in any given local place or time (patch -- small spatial time scales)

-- seasonal predictions are general
-- summer: good competitors
-- winter: invertebrates, good colonizers
-- patches constantly change but can average the substrate type to make large scale general predictions

Front 12

Problems of the Patch Dynamics hypothesis?

Back 12

01: Underestimates the importance of biological interactions
-- spring and summer

Predictions: a good template for flashy streams, which most streams are
(+) up/down: unpredictable
-- abiotic factors important
-- orgs are good dispersers

Front 13

Which conceptual model is correct?

Back 13

- All and none: no single model is adequate
- All overlap: predictions are dependent on environmental conditions
---> All incorporate conditions and biological comparisons

01: Extreme environments do prevent biotic interactions (governed by abiotic)
02: Stable environments: biotic interactions can structure communities
03: Variable environments: dispersal and life history characteristics regulate community structure