A factor that has a greater effect on population as the density increases or decreases.

Factors are typically biotic (living)

Ex:  The  common  loon,  a  bird  found  in  Ontario,  nest      on  a  lakeshore,  but  rarely  shares  a  lake  with    another  pair  of  loons.    

•  Density-­‐dependent:    the  population  is  limited  by  the                                                                        number  of  nesting  lakes  available  

• Ex:  The  population  of  pet  mice  in  a  cage  stops        increasing  even  though  enough  food  is  present    to  support  a  larger  population.  

• Density-­‐dependent:    the  size  of  the  population  has                              stopped  growth  due  to  the  number  of  

 organisms  being  limited  by  nesting  sites                                                                  (or  similar)  

"   Mangrove  trees  are  often  seen  in  the  Florida  everglades,  their  roots  alternately  exposed  to  the  air,  or  submerged  in  water.  Few  tree  species  can  survive  such  varied  exposure.  Mangroves  can  grow  outside  of  the  swamp,  but  rarely  do.  What  are  the  ecological  reasons  for  the  unusual  habitat  of  the  mangrove?      

"   One  animal  kills  and  eats  another  

"   Humans  raise  cattle  for  food  

"   Predation  is  beneficial  to  one  species,  but  usually  lethal  to  the  other    

"   Controls  the  number  of  a  population  of  prey  

"   Population  too  small  and  cannot  survive  due  to    difficulty  finding  a    mate  (population  density  is  too  low)  

"   Neanderthals  may  have  died  out  when  their  birth  rate  became  too  low  to  offset  their  death  rate.    

"   The  minimum  viable  pop.  size  consists  of  enough  individuals  so  that  the  population  can  cope  with  variations  in  natality  and  mortality  as  well  as  environmental  change  or  disasters.  

"   A  population  is  considered  at  risk  of  becoming  extinct  when  its  number  falls  below  the  minimum  viable  population  size    

Note:    

When  a  population  is  small  enough  to  be  far  below  its  carrying  capacity,  density  dependent  factors  have  little  effect  and  the  population  growth  is  rapid.    

Eventually,  the  population  reaches  a  density  at  which  these  factors  start  to  have  an  effect;  after  this  point  the  population  growth  slows,  and  it  eventually  stops  when  the  carrying  capacity  is  reached.    

"   Factors  that  influence  population  regulation  regardless  of  population  density  

"   Factors  tend  to  be  abiotic  (non-­‐living  factors)  

2.  Insecticide  application  –  biomagnification  (concentrates  up  the  food  chain)  

3.  Floods,  Droughts  

4.  Forest  fires,  hurricanes,  tornadoes  

"   An  essential  resource  that  is  in  shortest  supply  or  unavailable  

"   Limiting  factors  prevent  populations  from  achieving  their  biotic  potential  

"  è  Determines  the  carrying  capacity  "   ex.  light,  space,  water  or  nutrients  

Water  is  the              è  limiting  factor    

Practice  Questions:  pg  675  Q’s1-­‐4