Predator or Prey: Who’s in
Control?
Many real populations
look like this. Why?
What factors control population
growth?
• Competition for:
– space
– food
– mates
• Disease
• Catastrophic events
• Predation – this is what we’ll focus on
today
Question
Does the number of predators control the
number of prey?
Or
Does the number of prey control the number
of predators?
Isle Royale: Wolves and Moose
• Today we’ll look at
long-term data for the
population of wolves
and moose on Isle
Royale, Michigan.
• Populations of these
two species have
fluctuated for the last
several decades.
Top-down or bottom-up?
• There’s 2 competing hypotheses:
Top-down: Wolf predation on moose keeps
the moose population down
Bottom-up: Plant growth is limited by
climate, which limits the amount of plants
available to moose, which in turn limits the
number of moose available for wolves
Tree-rings
• Can provide records of
temporal changes in climate
and nutrient availability
• Ring widths larger on salmon
spawning streams in Alaska
(Helfield and Naiman 2001;
Reimchen et al. 2002; Drake et al.
2003)
(photo © H.D. Grissino-Mayer)
Isle Royale (McLaren and Peterson
1994)
•
Wolf→Moose→Balsam fir
•
Three decade correlation study
•
Wolves common when old moose (>9 yrs
old) were abundant
•
High wolf numbers led to low calf survival
and release of balsam fir
•
Over 30 years, balsam fir trees displayed
cyclic intervals of ring growth suppression
that accompanied elevated moose
densities.
•
Only when moose densities were low did
tree ring growth correlate with climate