Ecology  Community  Ecology  

Community Ecology

Populations are linked by interspecific interactions that impact the survival & reproduction

of the species involved

Community Structure •  Community−an assemblage of

populations living close enough together for potential interaction

•  Dominant Species−most abundant, highest biomass, powerful control over occurrence and distribution of other species… VA Sugar Maple

•  Keystone Species−NOT necessarily most abundant, exert strong control due to their ecological roles or niches… Sea Otters!!!

•  Richness number of species & abundance

•  Species diversity older = greater diversity larger areas = greater diversity climate = solar input & H2O available

Biodiversity  

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•  Communities with higher diversity are – More productive and more stable regarding

their productivity – Better able to withstand and recover from

environmental stresses – More resistant to invasive species, organisms

that become established outside their native range

Species  Diversity  

                                       Species  Richness    

                       (#  of  different  species)    

Species  Diversity      =            +                                            Rela9ve  abundance    

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(propor9on  each  different  species  represents  of  all  the  individuals  in  the  community)  

Species  Richness  

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Which  community  is  richer?  

A  

B  

Sample  Data  

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The data below represents the abundance of macro-invertebrates taken from three different river communities in Georgia. A variety of diversity indices may be used to calculate species diversity. Based on the data below, which community has the greatest diversity?

Observa9on  Of  Sea  O>er  Popula9ons    And  Their  Preda9on  

Food chain before killer whale involve- ment in chain

(a) Sea otter abundance

(b) Sea urchin biomass

(c) Total kelp density

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ber p

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1972 1985 1989 1993 1997 0 2 4 6 8

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200

300

400 G

ram

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2 O

tter n

umbe

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ax. c

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20

60

80

100

Year

Food chain after killer whales started preying on otters

Killer  Whales  vs.  Sea  O>ers  Predator-­‐Pray  Energe9cs  

The  daily  caloric  requirements  for  male  versus  female  killer  whales  (orcas)  is  shown  below:  

•  Male  killer  whale:  308,000  kcal/day  

•  Female  killer  whale:  187,000  kcal/day  

Calculate  the  average  caloric  value  of  a  sea  o>er  assuming  a  male  orca  consumes  five  sea  o>ers  each  day  to  meet  its  caloric  requirement.      

Killer  Whales  vs.  Sea  O>ers  Predator-­‐Pray  Energe9cs  

Calculate  the  average  caloric  value  of  a  sea  o>er  assuming  a  male  orca  consumes  five  sea  o>ers  each  day  to  meet  its  caloric  requirement.      

 Using  dimensional  analysis  or    simple  arithme9c:    

Killer  Whales  vs.  Sea  O>ers  Predator-­‐Pray  Energe9cs  

Assume  a  popula9on  of  4  male  orcas  feed  solely  on  sea  oSers.    How  many  oSers  are  lost  to  the  community  over  a  6-­‐year  period?    

Why  the  change?  –  Some  fish  popula9ons  have  declined  in  recent  decades  

–  Shortage  of  seals  and  sea  lions  resulted  in  killer  whales  preying  on  smaller  sea  oSers  

–  Shortage  of  certain  fish  caused  substan9al  declines  in  harbor  seals  and  sea  lions  

Interes9ngly,  The  Sea  O>er  Is  Not  Usually  The  Orca’s  Food  of  Choice  

Why  Should  We  Care  About    Declining  Numbers  of  Sea  O>ers?  

•  Sea  oSers  are  an  important  part  of  the    coastal  community  

•  The  loss  of  sea  oSers  affects  the  community  directly  and  indirectly    

A  keystone  species  is  one  that  has  a  strong  effect  on  the  composi9on  of  the  community  

–  Removal  of  keystone  species  causes  a  decrease  in  species  richness  

–  Sea  oSers  eat  sea  urchins  which  are  fierce  compe9tors  having  a  diet  of  kelp  

Indirect  Effect  on  the  Community  

Sea  Urchin  Popula9on  vs.  Kelp  Density  

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Early Hypotheses of Community Structure Individualistic

Individualis9c  Hypothesis  −  a  chance  group  of  species  is  linked  &  distributed  according  to  its  tolerance  ranges  for  abio9c  factors  

Early Hypotheses of Community Structure Individualistic

Interac9ve  hypothesis  -­‐  states  that  the  community  is  an  integrated  unit  comprised  of  closely-­‐related  species  associa9ng  with  each  other  due  to  bio9c  interac9ons.    

Early Hypotheses of Community Structure Individualistic

Community  composi9on  seems  to  change  con9nuously,  with  each  species  independently  distributed  

Factors that Impact Communities 1.  Disease  2.  Interspecific  Interac9ons:  

•  Compe99on  

•  Preda9on  •  Symbiosis  

! Mutualism  −  mycorrhizae  ! Commensalism    

Defense Mechanisms Mullerian-Two or more unpalatable, aposematically colored species resemble each other

Cryptic-camouflage  Aposematic-warning

Batesian-palatable/ harmless species mimics an unpalatable/ harmful model  

Ecological Niches An organism’s niche is the specific role it plays in its environment…its job!

•  All of its uses of biotic and abiotic resources in its environment

•  Ex: oak tree in a deciduous forest !  Provides oxygen to plants,

animals !  Provides a home for

squirrels !  Provides a nesting ground

for blue jays !  Removes water from the

soil

The Niche

•  Ecological niche is the total of an organism’s use of biotic and abiotic resources in its environment

Ex: Barnacle species on the coast of Scotland

Competition Between Organisms Of Different Species Can Be Direct Or Indirect

•  Interference−Directly  figh9ng  over  resources  

•  Exploita9ve−  Indirectly  compe9ng  by  consuming  a  common  limi9ng  resource  (space)  

•  Apparent-­‐  Indirectly  between  2  species  both  preyed  upon  by  the  same  predator.  

Example:    Species  A  and  species  B  are  both  prey  of  predator  C.  The  increase  of  species  A  will  cause  the  decrease  of  species  B  because  the  increase  of  As  would  increase  the  number  of  predator  Cs  which  in  turn  will  hunt  more  of  species  B.  

Competitive Exclusion Principle

Some9mes  referred  to  as  Gause's  law  of  compe99ve  exclusion  states  that  two  species  compe9ng  for  the  same  resources  cannot  coexist  if  other  ecological  factors  are  constant.    •  The  compe9ng  species  that  has  even  the  slightest  advantage  will  dominate  in  the  long  term  and  emerge  the  victor.  

•  The  loser  will  either  relocate  or  become  ex9nct.        •  The  principle  has  been  paraphrased  as  "complete  compe9tors  cannot  coexist".  

Competition Between Organisms Of Different Species

Solutions to Competitive Exclusion

•  Resource partitioning− sympatric species consume slightly different foods or use resources in different ways

Ex: Anolis lizard sp. perching sites in the Dominican Republic

Solutions to Competitive Exclusion

Character displacement− sympatric species tend to diverge in the characteristics that overlap

Ex: Darwin’s finch beak size on the Galapagos Islands

Succession •  Ecological  succession−  

transi9on  in  species  composi9on  over  ecological  9me  

•  Pioneer  organisms  =  bacteria,  lichen,  algae  

•  Climax  community  =  stable  •  Primary−  begun  in  lifeless  

area;  no  soil,  perhaps  volcanic  ac9vity  or  retrea9ng  glacier.      

•  Secondary  an  exis9ng  community  has  been  cleared  by  some  disturbance  that  leaves  the  soil  intact  

Human  Impact  on  Ecosystems

•  Humans are the most widespread agents of disturbance –  Reduces diversity –  Prevent some naturally

occurring disturbances

Human  Impact  on  Ecosystems  

•  Combus9on  of  Fossil  Fuels  

– Leads  to  acid  precipita9on  

– Changes  the  pH  of  aqua9c  ecosystems  and  affects  the  soil  chemistry  of  terrestrial  ecosystems  

Increasing  Carbon  Dioxide  Concentra9on  in  the  Atmosphere  

Created  by:  

Susan  Ramsey  VASS  Notable  contribu9ons  by  S.Meister