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Community Composition, Interactions, and Productivity


Understanding the patterns of and controls on distribution of ... animal taxa (fish, amphibians, crayfish, mussels) are used for distinguishing ecoregions. ... – PowerPoint PPT presentation

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Title: Community Composition, Interactions, and Productivity

Community Composition, Interactions, and
  • Biodiversity
  • Population Interactions
  • Productivity Controls
  • Understanding the patterns of and controls on
    distribution of organisms in aquatic habitats is
    essential to the study of ecology, particularly
    in the fields of conservation biology and
    fisheries management.
  • Species over-exploitation, habitat destruction,
    and introduction of exotic (alien) species by
    human activities has lead to dramatic community
    alterations and species extinction (locally and

  • Measures of biological diversity help define
    patterns and infer controls on community
    structure over various scales
  • spatially (globally to between and within
  • temporally (evolutionary to seasonal)
  • These measures permit monitoring of ecosystem
    stability and/or impacts from outside disturbance
    (e.g. human activities).
  • Species Richness (S)
  • Total number of species in an area.
  • Evenness (or equitability E)
  • Degree of equal representation for each species.
  • Shannon-Weaver Index (H)
  • Incorporates information on both S and E.
  • H increases when either S or E increases.

Where p is the proportion of species j to the
total of all individuals ( Nj / N)
Where lnS is the maximum diversity or maximum
evenness for S species.
Biodiversity over Spatial Scales Within-Habitat
(a diversity) versus
Between-Habitat (ß-diversity)
  • Consider the two sets of four ponds A-D and
  • Overall diversity of each set is similar.
  • Set A-D has lower a diversity one species per
    habitat dominated community.
  • Set E-H has lower ß diversity little difference
    in community between habitats.

Global Scale
Ecoregions classification of large geographic
areas based on their distinct assemblages of
natural communities. Information on organisms and
abiotic characteristics are considered. Presently,
only particular animal taxa (fish, amphibians,
crayfish, mussels) are used for distinguishing
ecoregions. North America has been divided into
76 ecoregions.
Evolution as the Source of Biodiversity
  • Uninterrupted time and reproductive isolation are
    key to evolution of new species.
  • Few freshwater ecosystems have fulfilled this
    criteria (contrast marine ecosystems) due to
    climate variation (e.g., glaciations).
  • Most freshwater ecosystems have cosmopolitan
    species (wide spread geographically), and few
    have many endemic species (unique to a
    particular habitat).

  • Tectonic lakes (deep and old) have a much greater
    proportion of endemic species as compared to
    glacier lake.
  • Compare Lake Baikal (high endemic crustacean
    diversity) and the African Rift Lakes (high
    endemic teleost diversity).
  • Both show examples of adaptive radiation (many
    species from a single founder).

Baikal Gammarids (amphipods)
Tanganyika Cichlidea family
Short-term Variation in Diversity
  • 1) Habitat diversity (many types in a single
  • 2) Size of habitat (positive relationship with
  • 3) Connectivity of habitats (ecotones
    colonization conduits).

  • 4) Sources of recruitment (dormancy and
  • 5) Species interactions (specialize to avoid
    competition niche).
  • 6) Productivity (timing and location coincident
    with recruitment).

Species space spawning activity to limit
Phytoplankton Diversity
  • Phytoplankton require light, CO2 (inorganic
    carbon) and nutrients (P, N, etc.) to grow
    through photosynthesis most aquatic environments
    are nutrient limited.
  • Many species competing for the same nutrient
    resources in the same areas should lead to
    competition and ultimately competitive exclusion.
  • Instead, MANY different species of plankton
    co-exist at once. This has been termed The
    Paradox of the Plankton.

  • One mechanism proposed to explain this paradox is
    the fact that lake conditions are not in a state
    of equilibrium for more than 1 month before the
    system is disturbed it would take longer than
    this for 1 species to become dominant.
  • Disturbances can be difficult to characterize
    (vary in magnitude from slight shifts from
    equilibrium to punctuated events.
  • Lakes, groundwaters less prone to major
    disturbance events but experience seasonal
  • Streams, rivers, wetlands experience regular
    disturbance (flooding, drying, etc.)
  • Systems prone to disturbance are less likely to
    achieve a classic equilibrium state (climax
    community) rather dynamic equilibrium is more

  • Succession is the sequence of species colonizing
    newly available habitat and niches.
  • The sere (sequence of specific organisms) is
    based on an organisms characteristics for
    colonization (recruitment), growth rate, resource
    competition, predator avoidance, physicochemical
    tolerances, disease resistance, and relative
    community scale.
  • Over time, the habitat may become modified so to
    favor the next organisms in the sere (e.g.
    nutrient depletion shifts competition).
  • Stages of Succession
  • Early invaders rapid reproducers and colonizers
    (r selection)
  • Mid- to late-succession Better long-term
    competitors (K selection)
  • Maximum diversity occurs during mid-succession
    stages, as both early-stage and late-stage
    species are present and competing for resources.
  • Disturbance and succession within a larger
    ecosystem will favor an increase in diversity up
    to some limit.

Intermediate-Disturbance Hypothesis
competition (K)
recruitment/ colonization (r)
Long-Term Lake SuccessionLake Aging
  • Over thousands of years, a newly formed lake will
    eventually fill with sediments and return to a
    more terrestrial state, regardless of trophic
    state. (30m lake at 1 mm/y will take 30,000 y to
  • Although many exceptions exist hypothetically
    lake succession proceeds from oligotrophic ?
    mesotrophic ? eutrophic ? senescence (marsh) ?
  • Over decadal scale a subclimax may be observed.
  • Mean depth, lake size and watershed size and
    fertility are major facts on controlling the
    timing of lake succession.
  • Catastrophic change in watershed, climate, or
    nutrient loads can rapidly shift subclimax state.
  • Some manmade impacts on trophic state have been
    demonstrated to be reversible when appropriately
    mitigated (i.e. rejuvenation).

Population Interactions
  • Competition for Resources
  • Exploitative competition Both organisms
    competing for the same resource(s).
  • Interference competition (amensalism) Organism
    exert direct, negative effects on another
    (allelochemical and allelopathy)
  • Competitive interactions can get interesting when
    two species compete for more than one resource
    with differing capabilities.
  • Predation (mortality)
  • Prey population declines when growth rates slows
    below predation rate (and other mortality terms)
  • Predator Avoidance
  • Mechanical defenses spines, filaments,
    gelatinous aggregates.
  • Chemical defenses allelochemical and allelopathy
    (taste nasty)
  • Life history defenses growth rate / reproduction
  • Behavioral defenses diel vertical migrations
    (e.g. zooplankton)
  • Predator-Prey (Functional Response) Models.

Two species competing for Si and P resources.
Curves represent growth rate under given nutrient
concentrations. Note that the two species differ
in their abilities to compete for different
resources. Species 1 needs higher Si to survive
competition. Species 2 needs higher P to
survive competition.
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Predator-Prey Models
  • Type I e.g. Lotka-Volterra. For a given
    predator density, prey consumption increases
    linearly with prey density.
  • Type II e.g. Holling disc equation. Includes
    search and handling time of prey, following
    structure of Michaelis-Menton equation. (e.g.
    microbes, zooplankton)
  • Type III Introduces concept of learning and
    increase in predator efficiency with increase in
    prey density. (e.g. fish)

Trophic Cascades
  • Interactions at higher levels of the food chain
    have a cascading influence down through lower
  • Bottom-up control Primary production is
    controlled by limitations of abiotic factors
    (light, nutrients, etc.)
  • Top-down control Primary production is
    controlled by predation on herbivores.
  • Trophic cascades in aquatic systems e.g.
    piscivores and phytoplankton biomass.
  • With piscivore, larger population of zooplankton
    crustaceans, graze down phytoplankton.
  • Removal shift dominance to planktivorous fish and
    loss of large zooplankton and shitch to rotifers
    phytoplankton bloom that are resistant to rotifer

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Controls of 1º Productivity
  • Tolerance to temperature, pH and other physical
    chemical conditions.
  • 2) Light
  • Decreases with depth.
  • Decreases faster with turbid water.
  • Compensation depth
  • Depth when cell photosynthesis respiration.
  • More turbidity causes a shallower (lower)
    compensation depth.
  • At a depth where only 1 photosynthetic light
    remains Euphotic Zone

  • 2. Turbulence (mixing)
  • Low when stratified.
  • Population stays in the light and grows.
  • High when stormy.
  • Population mixed too deep will die/declines.
  • Critical depth.
  • Population alive above
  • Population death below

  • 3) Nutrients (P, N, Si)
  • Deep winter mixing replenishes surface nutrients
  • Stratification minimizes supply from deep waters
  • 4. Grazing
  • Refers to the process of primary production being
    eaten by herbivores (e.g. cow).
  • Crustaceans like copepods and krill.
  • Grazing zooplankton populations typically
    increase after phytoplankton increase.

Seasonal Phytoplankton Succession
An annual cycle of species dominance in
response to abiotic and biotic factors associated
with seasonal changes in a temperate /
cold-temperate lake.
  • Eight stage model
  • Mid-winter
  • Late winter
  • Spring circulation
  • Initial summer stratification
  • Summer clearwater phase
  • Late summer stratification
  • Fall circulation
  • Late autumn decline

1. Midwinter
  • Low temperature
  • stable water column (inverse thermal
  • high light reflectance due to snow cover (low
  • moderate to high nutrient availability
  • Phytoplankton community dominated by small,
    motile, low-light adapted phytoplankton
  • Though not common, in some cases rates of primary
    production under ice cover can be constitute a
    significant portion of annual production when
    there is no snow.

2. Late winter
  • Low temperature
  • Stable water column
  • Moderate to high nutrient availability
  • Increasing light availability due to longer days,
    ice melt
  • Rapid increase in motile species, particularly
  • In lakes that do not ice over (e.g. temperate
    monomictic lakes), phytoplankton biomass remains
    low due to deep mixing and decreased light levels.

3. Spring Circulation
  • Low but increasing temperature
  • Mixing water column with low stability
  • Low (but variable and increasing) light
  • high nutrient availability (why?)
  • Rapid growth and increases in phytoplankton
    biomass, particularly diatoms. Often represents
    period of highest annual biomass.
  • Increasing light is dominant contributing factor
    zooplankton grazing remains low for now.

4. Initial Summer Stratification
  • Rapidly increasing temperature
  • Water column stabilizes
  • Light availability increasing rapidly to maximum
  • Declining nutrient availability (why and what
  • Phytoplankton biomass declines rapidly due to
    sedimentation of diatoms, compensated by rapid
    growth of small flagellates
  • Grazing by zooplankton increases rapidly during
    this period, due to hatching and response to prey

5. Summer Clearwater Phase
  • High temperatures
  • High water column stability
  • High light availability
  • Sharply reduced nutrient availability (why?)
  • Precipitous decline in phytoplankton populations
    due to nutrient limitations and high zooplankton
    grazing (clearance rate exceeds reproductive
  • Zooplankton biomass high due to timing of
    hatching, high production in response to spring
    bloom silica limitation common due to
    sedimentation of diatoms

6. Late Summer Stratification
  • High temperature
  • Stable but decreasing water column stability and
    deepening of metalimnion
  • High but decreasing light availability
  • Low but increasing nutrient availability
  • Increasingly diverse phytoplankton community,
    especially cyanobacteria and green algae (diatoms
    still silica-limited)

7. Fall Circulation
  • Rapidly declining temperatures
  • Rapid vertical mixing, no water column stability
  • Decreasing light availability
  • High nutrient availability (why and what
    nutrients in particular?)
  • Phytoplankton dominated by large algae,
    particularly diatoms
  • Zooplankton populations in decline, grazing
    pressure is reduced.

8. Late Autumn Decline
  • Low temperature
  • Decreased mixing of water column
  • Light availability rapidly declining to annual
  • Rapidly decreasing nutrient availability (why?)
  • Rapid decline in phytoplankton biomass due to
    reduction in light and nutrient levels.
  • Grazing rates decreasing to annual minimum

Seasonal community structure beyond phytoplankton
Fig. 20.5
Seasonal community structure in stream systems