Transcript Community Composition and Productivity

Community Composition,
Interactions, and Productivity
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 globally).
Biodiversity
• Measures of biological diversity help define patterns and infer controls
on community structure over various scales:
– spatially (globally to between and within habitats).
– 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.
S
H '   p j ln p j
j 1
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 (α diversity) versus
Between-Habitat (β-diversity)
• Consider the two sets of
four ponds A-D and E-H.
• Overall diversity of each
set is similar.
• Set A-D has lower α diversity; one
species per habitat dominated
community.
• Set E-H has lower β 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.
(1999)
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 ecosystem).
2) Size of habitat (positive relationship with diversity).
3) Connectivity of habitats (ecotones; colonization conduits).
4) Sources of recruitment (dormancy and dispersal).
5) Species interactions (specialize to avoid competition; niche).
6) Productivity (timing and location coincident with recruitment).
Species space
spawning activity to
limit competition.
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.”
Disturbance
• 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 changes.
– 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 normal.
Succession
• Succession is the sequence of species colonizing newly available
habitat and niches.
• The sere (sequence of specific organisms) is based on an
organism’s 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 earlystage 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
Theoretical Relationship Between Diversity and
"Disturbance"
Biotic Diversity
competition
(K)
Frequency of Disturbance
Intensity of Disturbance
recruitment/
colonization
(r)
Long-Term Lake Succession
“Lake 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 fill)
• Although many exceptions exist; hypothetically lake succession
proceeds from oligotrophic → mesotrophic → eutrophic →
senescence (marsh) → terrestrial.
• 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 tradeoff
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.
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 levels.
– 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
grazing.
Controls of 1º Productivity
1) 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:
1)
2)
3)
4)
5)
6)
7)
8)
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 stratification)
• high light reflectance due to snow cover (low penetration)
• moderate to high nutrient availability
Phytoplankton community dominated by small, motile, lowlight 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 dinoflagellates
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 availability
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 nutrients?)
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 density.
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 rates).
• 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 silicalimited)
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 minimum
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…