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Logistic growth describes birds populations well

Logistic growth describes birds populations well. d N/ dt = r N (1-N/K) r = intrinsic rate increase (exponential growth) K = carrying capacity. Exponential growth of populations of introduced birds: Cattle Egrets and House Finches (r = 0.2).

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Logistic growth describes birds populations well

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  1. Logistic growth describes birds populations well • dN/dt = rN (1-N/K) • r = intrinsic rate increase (exponential growth) • K = carrying capacity

  2. Exponential growth of populations of introduced birds: Cattle Egrets and House Finches (r = 0.2)

  3. Ring-necked Pheasants on Protection Island (r=3.75)

  4. Density-dependent Regulation • Growth slows to 0 as population approaches K • K = habitat • Reproduction, survival decline as population size increases

  5. Density-dependence in Great Tit • Reproduction declines as density increases • Survival of adults, juveniles during winter declines as density increases • Effect stronger in juveniles • Beechnut crop is the key • Interaction between food supply, density determines survival

  6. Food supply in the non-breeding season generally is important in the regulation of bird populations • Annual fluctuations in the food supply produce fluctuations in bird populations • Food shortages, bad weather, high nest predation and other events knock populations back below K • Populations often on erratic track back to K

  7. Other factors produce variation in survival, reproduction at a given population density

  8. Predators of adults and juveniles generally do NOT have an important role in regulation of bird populations Habitat (K), food supply and population density are much more important in regulation of bird populations

  9. Population Regulation of Migrants • K may be set by non-breeding or breeding habitat, but not both (usually unclear which) • K set by one area, numbers will be below what the habitat can hold in the other • Populations can be knocked back below K by events on the non-breeding area, the breeding area or during migration

  10. Population Regulation in Cavity Nesters • Primary cavity nesters (like woodpeckers) are like other birds, but secondary cavity nesters are limited by availability of cavities • K = number of cavities available • Populations can be increased by providing nest boxes

  11. Density-dependent effects are often mediated by territorial behavior in birds Territorial behavior affects how individuals experience impacts of density on reproduction and survival

  12. Ideal free regulation: impacts are experienced equally by all individuals

  13. Ideal despotic regulation: some individuals (those forced into suboptimal habitat or without a territory) are impacted more than others (those with a territory in good habitat)

  14. Ideal Despotic Regulation • Territory holders can control space, prevent others from without space from taking it • Poor success of those in suboptimal habitat pulls down average reproduction, survival of population • These individuals feel the effects of density the most, there are more of them as population size increases

  15. Site dependent regulation:extreme despotism, the most realistic scenario for birds

  16. Site Dependent Regulation • Individuals in best territories can hold them • Additional individuals acquire territories in suboptimal habitat, become floaters when these run out • Severe impacts of density on the lowest ranking individuals, none on the highest ranking individuals • Average reproduction, survival decline as individuals added at low end territory quality

  17. Great Tit example: suboptimal (secondary) habitat may be a different type (pine forest) than primary habitat (oak forest)

  18. Site Dependent Regulation • Probably widespread in birds • Individuals attempt to move up to better territories when vacancies occur

  19. Territory vacancies filled rapidly in many bird species • Some replacement by birds from worse territories in site dependent systems • Replacement by floaters in site dependent systems and other systems • Red-cockaded Woodpeckers: floaters account for 9-15% of adult population, most stay in a particular area • Rufous-collared Sparrow: floater hierarchy

  20. Rufous-collared Sparrow Floaters • Hierarchy of 1-3 secretive floater females in each territory • Floater males have positions in hierarchies on several territories • Nearly 50% of adults are floaters

  21. Dispersal Affects Population Dynamics • High rates of dispersal between populations in some species (poorly known) • Dispersal may be density dependent (an alternative to being a floater) • Immigration may keep population closer to K • Regulation may occur at the level of the regional population if dispersal is common

  22. Ptarmigan example: local recruits come from regional population

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