2. The eastern screech owl (Megascops asio) roosts in tree cavities. As long as there are enough tree cavities available for all the owls in the population, the number of

tree cavities has no effect on per capita birth or death rates. If there are more owls than the number of tree cavities, the per capita death rate increases with increased population size (because of increased rates of predation or death by exposure). How does this scenario differ from the traditional idea of density-dependent population regulation, as described by Nicholson (1956) or Turchin (1995)? Does it make more or less sense, and why? Turchin states : Density dependence is a dependence of per capita population growth rate on present and-or past population densities. Density dependence is considered to be direct if population growth rate is negatively affected by density, and inverse if it is positively affected. It is also well known that the presence of density dependence is not a sufficient condition of regulation. For regulation to occur, three general conditions must hold: (1) density dependence must be of the right sign(i.e., there has to be a return tendency), (2) return tendency needs to be strong enough to counteract the disruptive effects of density-independent factors, and (3) the lag with which return tendency operates must not be too long otherwise it may cause diverging oscillations. To summarize, regulation is not equivalent to return tendency, since return tendency is a necessary but not sufficient condition of regulation.Generally accepted that population regulation cannot occur in the absence of density dependence.

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