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Food Webs
Chapter 17
Arctic Food Web
Bear Island in High Arctic.
feeding activities of a few species may have a
dominant influence on community structure.
Keystone Species
• Keystone species – species that, despite low
biomass, exert strong effects on the structure
of the community they inhabit
• http://www.prairiedogs.org/keystone.html
• If keystone species reduce likelihood of
competitive exclusion, their activities would
increase the number of species that could
coexist in communities.
Figure 53.15 Sea otters as keystone predators in the North Pacific
Keystone Species
Food Web Structure and Species
Diversity
• Paine found as number of species in intertidal
food webs increased, proportion of the
foodweb represented by predators also
increased.
– higher proportion of predators produces higher
predation pressure on prey populations, in turn
promoting higher diversity.
• Removal of starfish (top predator) caused decline in
diversity from 15 to 8 species.
Fish as River Keystone Species
Effects of Predation by Birds on
Herbivory
• Atlegrim studied influence of birds on
herbivorous insects and insect-induced plant
damage.
– Insectivorous birds may act as keystone species
via effects on herbivorous insects.
• Larval insect densities peak when many insectivorous
birds are feeding their young.
Keystone Species: Summation
• Power : Keystone species exert strong effects
on their community structure, despite low
biomass.
Exotic Predators
• Exotic species have dramatic impacts on
communities because they were outside the
evolutionary experience of local prey
populations.
– Nile Perch (Lates nilotica) exotic fish predator in Lake
Victoria.
• Fish fauna dramatically reduced.
Exotic Predators
Exotic Predators
Exotic Predators
• Kaufman pointed out changes in Lake Victoria
fish community coincide with other ecosystem
changes.
– Dissolved oxygen concentrations significantly
decreased.
– Cultural eutrophication.
Primary Production and Energy Flow
Chapter 18
NEVER FORGET!!!
ENERGY FLOWS:
solar (or nuclear) input
heat output
NUTRIENTS RECYCLE:
mass conservation
inorganic ↔ organic
Focus Areas:
• Trophic Levels
• Terrestrial Production Controls:
– Actual evapotranspiration (AET)
– Precipitation
– Soil fertility
– Consumers
• Aquatic Production Controls:
– Nutrients
– Light
– Consumers
Introduction
• Trophic Level: Position in a food web
determined by number of energy transfers
from primary producers to current level:
– Primary producers occupy first level.
– Primary consumers occupy second level.
– Secondary consumers occupy third level.
– Tertiary consumers occupy fourth level.
• Primary production: Fixation of energy by
autotrophs in an ecosystem.
– Rate of primary production: Amount of energy
fixed over a given period of time.
• Gross primary production: Total amount of energy fixed
by autotrophs.
• Net primary production: Amount of energy leftover
after autotrophs have met their metabolic needs.
Primary Production Controls
• Bottom-Up Controls
– Influences of physical and chemical factors of
an ecosystem.
• Top-Down Controls
– Influences of consumers.
Soil Fertility and Terrestrial
Primary Production
• Significant variation in terrestrial primary
production can be explained by differences in
soil fertility.
– Shaver and Chapin found arctic net primary
production was twice as high on fertilized plots as
unfertilized plots.
– Bowman suggested N is main nutrient limiting net
primary production in a dry tundra meadow, and N
and P jointly limit production in a wet meadow.
18_04.jpg
Primary Production on the Serengeti
• McNaughton estimated
Serengeti grazers consume
an average of 66% of
annual primary production.
– Rate of primary production
in the Serengeti is positively
correlated with rainfall
quantity.
Primary Production in the Serengeti
• Found grazers can increase primary
production.
– Increased growth rate.
• Compensatory Growth
– Lower respiration rate due to lower biomass.
– Reduced self-shading.
– Improved water balance due to reduced leaf area.
18_15.jpg
Patterns of Aquatic Primary Production
• Several studies have found quantitative
relationship between phosphorus and
freshwater phytoplankton biomass.
• Several studies support generalization that
nutrient availability controls rate of primary
production in freshwater ecosystems.
18_06.jpg
Global Patterns of Marine Primary
Production
Biogeochemical cycles
Concepts of Ecology
Carbon cycle
• Is strongly tied to the energy flow
• Carbon in the form of CO2 (and Carbonates in
aquatic eco systems) is fixed into organic
compounds in the process of photosysnthesis,
passses through the food chain, and returned
to the atmosphere through the process of
respiration
• the Carbon cycle exhibits both annual and
diurnal fluctuations
Nitrogen cycle
• Is characterized by the fixation of atmospheric
nitrogen by mutualistic nitrogen fixing
bacteria associated with roots of many plants,
largely legumes and cyanobactria
• Other processes are
– Ammonification, the breakdown of amino acids by
decomposer organisms to produce ammonia;
– Nitrification, the bacterial oxidation of ammonia
to nitrates and nitrites
– Denitrification, the reduction of nitrates to
gaseous nitrogen
Phosphorous cycle
• Is wholly sedimentary, with reserves coming
largely from phosphate rocks
• Once in the soil, inorganic P is taken up by
plants and incorporated into organic
compounds
• During the process of decomposition of living
matter, it becomes again available through
mineralization
• In aquatic eco systems, P exists in three major
states;
– Particulate Organic Phosphorous (POP)
– Dissolved Organic Phosphorous (DOP)
– Dissolved Inorganic Phosphorous (DIP)
Transfer among theses states is controlled by the
processes of uptake by primary producers,
decomposition, and excretion of DIP by
zooplankton.
Nutrient Cycling and Retention
Chapter 19
www.sws.uiuc.edu/ nitro/biggraph.asp
Focus Areas
• Nutrient Cycles
– Phosphorus
– Nitrogen
– Carbon
• Rates of Decomposition
– Terrestrial
– Aquatic
• Organisms and Nutrients
• Disturbance and Nutrients
Phosphorus Cycle
• Global phosphorus cycle does not include
substantial atmospheric pool.
– Largest quantities found in mineral deposits and
marine sediments.
• Much of this in forms not directly available to plants.
– Slowly released in terrestrial and aquatic
ecosystems via weathering of rocks.
Phosphorus Cycle
http://arnica.csustan.edu/carosella/Biol4050W03/figures/phosphorus_cycle.htm
Nitrogen Cycle
• Includes major atmospheric pool - N2.
– Nitrogen fixers can use atmospheric supply directly
(only prokaryotes).
• Energy-demanding process; reduces to N2 to
ammonia (NH3).
– Industrial N2- fixation for fertilizers equals the
biological process annually.
– Denitrifying bacteria release N2 in anaerobic
respiration (they “breath” on nitrate).
– Decomposer and consumers release waste N in form
of urea or ammonia.
– Ammonia is nitrified by bacteria to nitrate.
Nitrogen Cycle
http://muextension.missouri.edu/xplor/envqual/wq0252.htm
Carbon Cycle
• Moves between organisms and atmosphere as
a consequence of photosynthesis and
respiration.
– In aquatic ecosystems, CO2 must first dissolve into
water before being used by primary producers.
– Although some C cycles rapidly, some remains
sequestered in unavailable forms for long periods
of time.
Carbon Cycle
http://www.ucar.edu/learn/images/carboncy.gif
Rates of Decomposition
• Rate at which nutrients are made available to
primary producers is determined largely by
rate of mineralization.
– Occurs primarily during decomposition.
• Rate in terrestrial systems is significantly influenced by
temperature, moisture, and organic chemical
compositions (labile versus refractory).
Decomposition in Temperate Forest
Ecosystems
• Melillo et.al. used litter bags to study
decomposition in temperate forests.
– Found leaves with higher lignin:nitrogen ratios lost
less mass.
• Suggested higher N availability in soil might have
contributed to higher decomposition rates.
– Higher environmental temperatures may have also played a
role.
Decomposition in Aquatic Ecosystems
• Gessner and Chauvet found leaves with a
higher lignin content decomposed at a slower
rate.
– Higher lignin inhibits fungi colonization of leaves.
• Suberkropp and Chauvet found leaves
degraded faster in streams with higher nitrate
concentrations.
Decomposition in Aquatic Ecosystems
Nutrient Spiraling in Streams
Animals and Nutrient Cycling in
Terrestrial Ecosystems
• MacNaughton found a positive relationship
between grazing intensity and rate of turnover in
plant biomass in Serengeti Plain.
– Without grazing, nutrient cycling occurs more slowly
through decomposition and feeding of small
herbivores.
• Huntley and Inouye found pocket gophers altered
N cycle by bringing N-poor subsoil to the surface.
Animals and Nutrient Cycling in
Terrestrial Ecosystems
Human influences on nutrient cycling
• Loss of nutrients via deforestation
• Addition of nutrients
– Fertilizers
– Nitrogen in atmosphere from coal burning
Consequences
– Reduced biodiversity
– Reduction of mycorrhizal fungi
– Eutrophication of lakes
– http://www.epa.gov/maia/html/eutroph.html
(Peierls et al., 1991)
Succession and Stability
Chapter 20
SUCCESSION
• Succession: Gradual change in plant and animal
communities in an area following disturbance.
– Primary succession on newly exposed geological
substrates.
– Secondary succession following disturbance that does
not destroy soil.
• Climax Community: Late successional community
that remains stable until disrupted by
disturbance.
Primary Succession at Glacier Bay
Secondary Succession in Temperate
Forests
• Oosting found number of woody plant species
increased during secondary succession at
Piedmont Plateau.
– Johnston and Odum found increase in bird
diversity across successional sequence closely
paralleled increase in woody plant diversity
observed by Oosting.
Succession in Stream Communities
• Fisher studied rapid succession in Sycamore
Creek, AZ.
– Evaporation nearly equals precipitation - flows
generally low and intermittent.
• Subject to flash floods.
– Observed rapid changes in diversity and
composition of algae and invertebrates.
• Invertebrates found refuge because many adults in
aerial stage.
– Re-colonized after flooding.
Hypothetical Periphyton Community Succession
Mirror Lake, Yosemite National Park
El Capitan Meadow, Yosemite National Park © Dan Baumbach 2002
Mechanisms of Succession
• Clements, 1916
– Facilitation
• Connell and Slayter, 1977
– Facilitation
– Tolerance
– Inhibition
Facilitation
• Proposes many species may attempt to
colonize newly available space.
– Only certain species will establish.
• Colonizers “Pioneer Species” modify environment so it
becomes less suitable for themselves and more suitable
for species of later successional stages.
Tolerance
• Initial stages of colonization are not limited to
pioneer species.
– Early successional species do not facilitate later
successional species, but do change the
environment.
– Species remain, leave, or establish based on
tolerance to environment.
– Long lived species are most broadly tolerant to a
range of environmental change.
Inhibition
• Early occupants of an area modify the
environment in a way that makes it less
suitable for both early and late successional
species.
– Early arrivals inhibit colonization by later arrivals.
– Assures late successional species dominate an
area because they live a long time and resist
damage by physical and biological factors.
Mechanisms of Succession
20_25a.jpg
20_25b.jpg
20_25c.jpg
Why Alder?
20_25d.jpg
Successional Mechanisms in
Rocky Intertidal Zone
• Sousa investigated mechanisms behind
succession of algae and barnacles in intertidal
boulder fields.
– If the inhibition model is in effect, early successional
species should be more vulnerable to mortality.
• Results showed early successional species had lowest
survivorship and were more vulnerable to herbivores.
Community and Ecosystem Stability
• Stability: Absence of change.
– Ability to resist change from original state and/or
return to original state once disturbed
Community and Ecosystem Stability
• Resistance: Ability to maintain structure and
function in face of potential disturbance.
Community and Ecosystem Stability
• Resilience: Ability to recover from
disturbance.
X
X
– Large Scale Ecology and Global Change
Where have we been and where are we going?
Introduction to Landscape Ecology
Landscape =
Landscape Ecology =
Landscape
element =
What do landscape ecologists study?
Landscape structure – scale and configuration
Landscape processes – between patches
Ho do these landscapes differ structurally?
Quantifying landscape structure
Patch density
Quantifying landscape structure
Patch shape – area/perimeter
patch perimeter/circle perimeter
Concept of “the Edge” – why are edges important?
Landscape Processes
Metapopulation dynamics
-habitat patches
and dispersal
occur within the
context of a
landscape
Patch size and metapopulations
Patch size and population density
Landscape position
Position of lakes affects
proportion of groundwater and
surface water inputs
Ecosystem Interactions – e.g. riparian zones
Hillslope
Floodplain
Stream
RoSS
RoSS
Interface
High Water
Table
Permanent Groundwater
Bedrock
Unconstrained
Baseflow
Water Table
Constrained
Baker et al. 2000
Landscapes and Change
How does landscape structure change?
Geological processes and climate
Different aged soils have different properties (which affects
the kinds of plants that can live there).
END
17_07.jpg
Consumers’ Effects on Local Diversity
• Lubchenko studied influence of intertidal snail
(Littorina littorea) on structure of an algal
community.
– Snails fed on green (Enteromorpha spp.) and red
(Chondrus crispus) algae.
• Under normal conditions, Enteromorpha out-competes
Chondrus in tide pools, and Littornia prefers
Enteromorpha.
– In the absence of snails, Chondrus is competitively displaced.
Consumers’ Effects on Local Diversity
Consumers’ Effects on Local Diversity
• When snails are present in high densities,
Littorina grazes down Enteromorpha, releasing
Chondrus from competition.
– Green crabs (Carcinus maenus) prey on young
snails, preventing juveniles from colonizing tide
pools.
– Populations of Carcinus are controlled by seagulls.
Consumers’ Effects on Local Diversity
– Low snail density - Enteromorpha dominates tide
pool.
– Medium snail density - Competitive exclusion
eliminated, and algal diversity increased.
– High snail density - Feeding requirements are high
enough that snails eat preferred algae and lesspreferred algae.
• Algal diversity decreased.
Strong Interactions and Food Web
Structure
• Tscharntke studied food webs associated with
wetland reeds (Phragmites australis).
– Attacked by fly Giraudiella inclusa.
• Attacked by 14 species of parasitoid wasps.
– Predator specialization
– Distinguished weak and strong interactions.
• Determination of keystone species.
Fish as River Keystone Species
• California roach Hsperoleucas symmetricus
and steelhead trout Oncorhhyncus mykiss
significantly influence food web structure.
– Predatory fish decrease algal densities.
• Low predator density increased midge production.
– Increased feeding pressure on algal populations.
» Thus, fish act as Keystone Species.