Ecosystems

1
Chapter 54

• Ecosystems

2
Outline

• Energy flow
• Trophic structure
• Biogeochemical Cycles
• nutrient cycling
• water cycle
• N cycle
• C cycle
• Global warming and human impacts

3
Intro

• Ecosystems all organisms living in area and
  abiotic components (nutrients, air, water,
  sunlight, soil, temp.)
• composed of multiple communities along w/ abiotic
  environment
• S and nutrient flows link biotic and abiotic
  environments
• ecosystem ecology study of how S flows among
  components of an ecosystem how C, Nn, and other
  elements cycle through organisms, sediments,
  oceans, and atmosphere and how humans affect
  abiotic environment
• Many global environmental problems (global
  warming, acid rain, ozone hole, anaerobic dead
  zones, algal blooms)
• humans changed chemical/physical characteristics
  of environment

4
Energy Flow and Trophic Structure

• All ecosystems consist of 4 components linked by
  flow of S
• 1 producers autotrophs use solar or chemical
  S to manufacture their own food
• form basis of ecosystems by transforming S in
  sunlight or reduced inorganic compounds into
  chemical S (sugar)
• chemical S for maintenance, R, growth,
  reproduction
• S invested in new tissue
  net
  primary productivity (NPP)
• available for others to eat
• consumers eat other organisms
• herbivores eat plants
• carnivores eat animals
• decomposers detritivores
  feed on
  dead remains and waste

5
Global Patterns in Productivity

• NPP varies among ecosystems by region and biome

6
Global Patterns in Productivity

• Terrestrial NPP lowest in deserts and arctic
  regions
• limited by combination of temp., water and
  sunlight
• highest productivity wet tropics
• 16 of surface but 43 of NPP
• deserts and tundra lowest productivity

7
Global Patterns in Productivity

• Except for major deserts, terrestrial
  productivity declines from equator toward poles
• Marine productivity high along coast/upwells, low
  in open ocean
• limited by nutrient availability
• coastal areas receive input from terrestrial
  systems
• upwelling returns nutrients to surface

8
0100 100200 200400 400600 600800 gt800
Terrestrial productivity
Productivity ranges (g/m2/yr)
lt35 3555 5590 gt90
Productivity ranges (g/m2/yr)
Marine productivity
9
Energy Flow and Trophic Structure

• S flow through temperate forest ecosystem in
  Hubbard Brook
• begins when plants capture S via Ps
• 1,254,000 kCal of solar radiation per m2
  reached forest per year
• gross Ps total Ps per area per year 10,400
  kCal/m2
• gross photosynthetic efficiency efficiency that
  plants use total S available to them
• ratio of gross Ps to solar radiation 0.8
  (tiny fraction of total radiation received)
• 1 producers use S captured by Ps in 2 ways
• 45 for synthesis of new tissue (NPP) (S
  available to herbivores)
• 55 for maintenance or respiratory costs

10
(No Transcript)
11
Energy Flow and Trophic Structure

• Grazing food web network of herbivores (1
  consumers) organisms that eat herbivores (2
  consumers)
• amnt of S entering grazing food web varies btwn
  1 to 40 NPP
• 1 consumers harvested 31 kCal/m2 each year
• 82 assimilated and 18 excreted
• 1.6 used in production of new tissue
• production of new tissue by 1 consumers 2
  production
• 2 production higher in ectotherms than
  endotherms
• tiny fraction of available solar radiation
  involved in 2 production

12
(No Transcript)
13
Energy Flow through Ecosystems

• Plant tissue not consumed by herbivores
  eventually dies and enters decomposer food web of
  sp. that eat dead remains
• detritus dead animals and plant tissue,
  consumed by 1 decomposers (bacteria, fungi,
  worms, millipedes)
• 1 decomposers consumed by carnivores (spiders,
  shrews)
• 75 NPP enters
  decomposer food web
• not all detritus consumed
• OM in soil
• leave community

14
Energy Flow and Trophic Structure

• Large amounts of S leave ecosystem as detritus
  (washes into streams)
• major source of S for aquatic organisms
• represents flow of S out of ecosystem
• Ps by aquatic plants 10 kCal/m2/yr of energy
• in contrast, 6039 kCal/m2/yr washed from
  surrounding forest
• most of S inputs to aquatic system come from
  terrestrial plants

15
Energy Flow and Trophic Structure

• Key points about S flow through ecosystems
• S enters ecosystems as sunlight via Ps by 1
  producers
• plants use tiny fraction of total radiation
  available
• tiny fraction of fixed S actually available to
  consumers
• most NPP enters decomposer food web
• only small fraction used for 2 production by
  herbivores/carnivores
• most S fixed during Ps used for R, not syn. of
  new tissues

16
Trophic Levels

• Organisms that use same type of S source occupy
  same trophic level (position in
  food chain)
• organisms at top trophic level not eaten by other
  organisms
• productivity highest at lowest trophic level
• productivity declines from one level to next

17
Food Chains and Food Webs

• Food chain connects trophic levels in
  particular ecosystem
• describes how S moves btwn trophic levels
• typically embedded in more complex food webs
• maximum links in food chain 1 to 6
• only 2 to 7 trophic levels

18
Energy Loss between Trophic Levels

• Pyramid of productivity productivity greatest
  at first (bottom) trophic level and declines at
  higher levels
• S lost in keeping organism alive

19
Energy Flow and Trophic Structure

• 3 hypotheses to explain why ecosystems dont have
  8 trophic levels
• S transfer hypothesis food-chain length is
  limited by productivity
• as S transferred up food chain, large fraction of
  S is lost
• by time S reaches top trophic level, not enough
  left to support additional group of consumers
• should be more trophic levels in ecosystems w/
  higher productivity or higher S -transfer
  efficiency

20
Energy Flow and Trophic Structure

• stability hypothesis long food chains are more
  fragile (Pimm)
• easily disrupted by environmental disturbances
  thus eliminated
• long food chains unlikely to persist in variable
  environment and likely to take longer to return
  to previous state following a disturbance
• should be longer in more stable environments
• environmental complexity hypothesis food-chain
  length function of ecosystems physical structure
  
• 3-dimensional ecosystems should have longer food
  chains than 2-dimensional ones

21
Biogeochemical Cycles

• Nutrients also transferred when one organism eats
  another
• biogeochemical cycle path an element takes as
  it moves from abiotic systems through living
  organisms and back again
• Humans disrupting biogeochemical cycles on global
  scale
• Researchers study 3 basic aspects of
  biogeochemical cycling
• nature and size of reservoirs of elements
• rate of movement btwn reservoirs and factors
  influencing these rates
• how biogeochemical cycles interact

22
Biogeochemical Cycles

• Nutrients cycle from organism to organism via
  assimilation, consumption, and decomposition
• nutrients exported from by migration of organisms
  out of area or water
• Nutrients are recycled/reused

23
Biogeochemical Cycles

• Summary of process
• nutrients taken up from soil and incorporated
  into plant tissue
• if plant is eaten, nutrients pass to animals of
  ecosystem
• nutrients excreted in fecal matter or urine,
  taken up by parasite or predator, or enter dead
  biomass pool
• if plant dies, nutrients and plant biomass become
  litter
• decomposers break down nutrients in plant litter,
  animal excretions, and dead animal bodies
• nutrients then part of OM of soil
• nutrients in OM converted to inorganic form
• nutrients available for uptake by plants

24
Biogeochemical Cycles

• Factors controlling rate of nutrient cycling in
  ecosystems
• decomposition of detritus
• influenced by temperature and precipitation
• quality of detritus as nutrient source for
  decomposers
• presence of large compounds difficult to digest
• growth of decomposers also inhibited if low N
• rate of nutrient loss 10x as high in devegetated
  site vs. control site
• devegetation has huge impact on nutrient export
• devegetated areas much less productive than
  intact, vegetated sites

25
The Water Cycle
26
The Carbon Cycle
27
(No Transcript)
28
The Nitrogen Cycle
29
Human Impacts on Ecosystems

• 2 factors responsible for human impacts on
  ecosystems
• rapid increase in human population
• rapid increase in human resource use
• people in industrialized countries have
  disproportionately large impact b/c consumption