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