Transcript Community Composition, Interactions, and Productivity

Patterns of Ecosystem Metabolism
in Streams and Rivers:
Lessons from Studies in 33 Systems
Dr. Thomas L. Bott
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Stroud Water Research Center
Avondale, PA 19311
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Tuesday, April 3rd, at 4:30 – 5:30 p.m.
Ruhl Student Center, Community Room
Biodiversity
Biodiversity Concept
Evolution (long-term change)
Factors of short-term change
• 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).
Four Levels of Biodiversity
• Genetic diversity within a species.
• Diversity of populations within a species
geographic range.
• Diversity of species within communities.
• Diversity of natural communities and
ecosystems throughout the world.
Biodiversity
• Measures of species biodiversity within communities can help define
patterns and infer controls on community structure over various scales:
– spatially (globally to between and within habitats).
– temporally (evolutionary time-scales 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.
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 stagger
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 selective)
– Mid- to late-succession: Better long-term competitors (K selective)
– 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
factors 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).