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Community  Ecology  BIOL  221  

Ch.  54  

Overview:  A  Sense  of  Community  

•  biological  community    

•  an  assemblage  of  popula9ons  of  various  species    

•  living  close  enough  for  poten9al  interac9on  

•  interspecific  interac<ons    

•  rela9onships  between  species  in  a  community  

•  compe99on,  preda9on,  herbivory  

•  and  symbiosis    

•  parasi9sm,  mutualism,  and  commensalism  

•  Can  affect  the  survival  and  reproduc9on  of  each  species  

•  effects  can  be  summarized  as  posi9ve  (+),  nega9ve  (–),  or  no  effect  (0)  

Community  Interac<ons  

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Interspecific  Compe<<on  •  compe<<ve  exclusion    

•  –/–  interac9on    

•  occurs  when  species  compete  for  a  resource  in  short  supply  

•  Strong  compe99on  leading  to  local  elimina9on  of  a  compe9ng  species  

•  Compe99ve  exclusion  principle    

•  two  species  compe9ng  for  the  same  limi9ng  resources    

•  cannot  coexist  in  the  same  place  

Ecological  Niches  •  ecological  niche    

•  The  total  of  a  species’  use  of  bio9c  and  abio9c  resources  

•  Can  also  be  thought  of  as  an  organism’s  ecological  role  

•  Ecologically  similar  species  can  coexist  in  a  community    

•  if  there  are  one  or  more  significant  differences  in  their  niches  

•  Resource  par<<oning    

•  differen9a9on  of  ecological  niches,  enabling  similar  species  to  coexist  in  a  community  

Ecological  Niches  

A. ricordii

A. insolitus usually perches on shady branches.

A. distichus perches on fence posts and other sunny surfaces.

A. aliniger A. distichus

A. insolitus

A. christophei

A. cybotes A. etheridgei

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•  As  a  result  of  compe99on  

•  a  species’  fundamental  niche  may  differ  from  its  realized  niche  

Realized  Niches  

Ocean

Chthamalus

Balanus

EXPERIMENT

RESULTS

High tide

Low tide

Chthamalus realized niche

Balanus realized niche

High tide

Chthamalus fundamental niche

Low tide Ocean

Character  Displacement  •  Character  displacement    

•  a  tendency  for  characteris9cs  to  be  more  divergent    

•  in  sympatric  popula9ons  of  

two  species    

•  than  in  allopatric  popula9ons  of  the  same  

two  species  

•  varia9on  in  beak  size    

•  between  popula9ons  of  two  

species  of  Galápagos  finches    

Los Hermanos

G. fuliginosa G. fortis

Beak depth

Daphne

G. fuliginosa, allopatric

G. fortis, allopatric

Sympatric populations

Santa María, San Cristóbal

Beak depth (mm)

Perc

enta

ges

of in

divi

dual

s in

eac

h si

ze c

lass

60 40 20

0

60 40 20

0

60 40 20

0 8 10 12 14 16

Preda<on  •  Preda<on    

•  +/–  interac9on    

•  one  species  (predator)  kills  and  eats  the  other  (prey)  

•  adapta9ons  of  predators  

•  claws,  teeth,  fangs,  s9ngers,  and  poison  

•  Prey  defensive  adapta9ons  

•  Behavioral  defenses  

•  hiding,  fleeing,  forming  herds  or  schools,  self-­‐defense,  and  alarm  calls  

•  Morphological  and  physiological  defenses  

•  Cryp<c  colora<on  (camouflage)  makes  prey  difficult  to  spot  

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Aposema<c  Colora<on  •  Aposema<c  colora<on    

•  Animals  with  effec9ve  

chemical  defense    

•  oYen  exhibit  bright  warning  colora9on,  

called  

•  Predators  are  par9cularly  cau9ous    

•  in  dealing  with  prey  that  display  such  

colora9on  

•  In  some  cases,  a  prey  

species  may  gain  

significant  protec9on    

•  by  mimicking  the  

appearance  of  

another  species  

•  Batesian  mimicry  

•  palatable  or  harmless  

species  mimics  an  

unpalatable  or  

harmful  model  

Batesian  Mimicry  

Hawkmoth larva

Green parrot snake

•  Müllerian  mimicry  

•  two  or  more  

unpalatable  species  

resemble  each  

other  

Mullerian  Mimicry  

Cuckoo bee

Yellow jacket

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Herbivory  •  Herbivory    

•  +/–  interac9on    

•  refers  to  an  interac9on  in  which  an  herbivore  eats  

parts  of  a  plant  or  alga  

•  has  led  to  evolu9on  of  plant  mechanical  and  

chemical  defenses    

•  and  adapta9ons  by  herbivores  

Symbiosis  

•  Symbiosis    

•  A  mutualism  where  

two  or  more  species  

live  in  direct  and  

in9mate  contact  with  

one  another  

Parasi<sm  

•  parasi<sm    

•  +/–  interac9on  

•  Parasite  

•  derives  nourishment  from  another  organism  

•  Host  

•  which  is  harmed  in  the  process  

•  endoparasites    

•  live  within  the  body  of  their  host  

•  Ectoparasites  

•  live  on  the  external  surface  of  a  host  

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Mutualism  •  mutualism    

•  +/+  interac9on  

•  interspecific  interac9on  that  

benefits  both  species  

•  Obligate  

•  where  one  species  cannot  survive  

without  the  other  

•  Faculta9ve  

•  where  both  species  can  survive  alone  

Acacia tree and ants (genus Pseudomyrmex)

Area cleared by ants at the base of an acacia tree

Commensalism  

•  Commensalism    

•  +/0  interac9on  

•  one  species  benefits    

•  and  the  other  is  apparently  unaffected  

•  hard  to  document  in  nature    

•  because  any  close  associa9on    

•  likely  affects  both  species  

Species  Diversity  •  Species  diversity  

•  The  variety  of  organisms  that  make  up  the  community  

•  Two  components  

•  species  richness  and  rela9ve  abundance  

•  Species  richness  

•  Total  number  of  different  species  in  the  community  

•  Rela<ve  abundance    

•  Propor9on  each  species  represents  of  the  total  individuals  in  the  community  

Community 1 A: 25% B: 25% C: 25% D: 25%

Community 2 A: 80% B: 5% C: 5% D: 10%

A B C D

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Trophic  Structure  

•  Trophic  structure    

•  feeding  rela9onships  between  organisms  in  a  community  

•  key  factor  in  community  

dynamics  

•  Food  chains    

•  link  trophic  levels  from  

producers  to  top  carnivores  

Carnivore

Carnivore

Carnivore

Herbivore

Plant

A terrestrial food chain

Quaternary consumers

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

A marine food chain

Phytoplankton

Zooplankton

Carnivore

Carnivore

Carnivore

Food  Webs  

•  Food  web    

•  Branching  food  chain  with  complex  trophic  

interac9ons  

•  More  realis9c  than  food  

chain    

Humans

Smaller toothed whales

Baleen whales

Sperm whales

Elephant seals

Leopard seals

Crab-eater seals

Birds Fishes Squids

Carnivorous plankton

Copepods Euphausids (krill)

Phyto- plankton

Dominant  Species  •  Dominant  species    

•  Those  that  are  most  abundant    

•  or  have  the  highest  biomass  

•  Biomass  is  the  total  mass  

of  all  individuals  in  a  

popula9on  

•  Exert  powerful  control  over  the  

occurrence  and  distribu9on  of  

other  species