Download Harvest Management: Population Size, Growth, and Sustainable Yield in Wildlife - Prof. Eri and more Study notes Zoology in PDF only on Docsity! 1 Harvest Management Page 1 Harvest Management Page 2 In the past, unregulated harvest has caused wildlife extinctions & near extinctions • Hunting by primitive humans • Unregulated harvest by Euro- Americans (especially market hunting) Harvest Management Page 3 • Even under modern regulations, commercial harvest has endangered certain populations – E.g., fisheries like cod and anchovies • However regulated recreational hunting/fishing/trapping has never caused a harvested population to become extinct or endangered Why Do People Hunt or Trap? • 13 million licensed U.S. hunters in 2001 – Declining percentage of population • Subsistence • Non- subsistence food • Recreation • Income (trapping) • Population control • Major source of $$ for research & habitat • State: licenses, stamps, permits; Pittman-Robertson $$ • Federal: federal “duck” stamps & user fees fund national wildlife refuges Page 4 Is Harvest Necessary? • Population control – Some species become overpopulated • Few large predators • Human activities improve habitat quality – Some species don’t • Unhunted populations of mourning doves not overabundant Page 5 Harvest Management Page 6 • Basic Premise: – Harvest reduces population size but increases population growth rate – Therefore, in good habitat wildlife populations can produce a sustainable yield 2 Harvest Levels • Who determines the “appropriate” level? – Migratory species : federal agencies – Nonmigratory species : state agencies • What is the “appropriate” level? – Ideally, harvest should equal what the population growth rate would be in absence of harvest Page 7 Factors Modifying Harvest Levels • Harvest can be higher if it is a compensatory mortality factor – biased toward individuals that would otherwise die of other causes – reduces competition among survivors • Harvest must be lower if it causes additive mortality – biased toward individuals that would otherwise survive • Harvest can be higher if it is biased toward sex/age classes with low reproductive value – very young or very old females – males (most species) Page 8 Yield -vs- population size Page 9 r0 = 0.4 K = 1,000 Yield is maximum at N = K/2 0 20 40 60 80 100 120 0 200 400 600 800 1000 N dN/dt = Yield Yield -vs- population size Page 10 0 20 40 60 80 100 120 0 200 400 600 800 1000 N dN/dt = Yield r0 = 0.4 K = 1,000 Yield is maximum at N = K/2 Maximum Sustained Yield Why NOT harvest at MSY? Page 11 • Maximum sustained yield should NOT be a goal because: – assumes the following are known exactly • Population size • Population growth rate • Relationship between size & growth • Harvest level – MSY produces an unstable equilibrium – If population size starts to decline, it will continue to decline unless yield is changed Yield -vs- population size Page 12 0 20 40 60 80 100 120 0 200 400 600 800 1000 N dN/dt = Yield r0 = 0.4 K = 1,000 Maximum Sustained Yield If N < K/2, then MSY > ∆N/∆t population will decline
