POPULATION CONSEQUENCES OF WINTER HABITAT LOSS IN A MIGRATORY SHOREBIRD .2. MODEL PREDICTIONS

1. This paper models empirically how habitat loss in winter might affect the size of the European population of oystercatchers Haematopus ostralegus ostralegus. It explores how a density-dependent mortality rate in winter interacts with a density-dependent production rate in summer to determine the total, year-round population size following a loss of winter habitat which itself leads to intensified competition for food and hence increased winter mortality rates. 2. Simulations over a range of probable parameter values show that the density at which winter mortality becomes density-dependent, cW, simply determines the point at which population size is affected as habitat is gradually removed. The population is affected sooner in the more widely fluctuating Continental subpopulations than in the less variable Atlantic subpopulations. 3. Once winter density reaches cW, the consequences depend on the slope, bW, of the density-dependent winter mortality function. In all subpopulations, the reduction in population size increases sharply as bW increases, but only at low values; above a certain level, further increases in bW make less difference. Because of their higher reproductive rate, inland subpopulations are initially less affected by winter habitat loss than coastal subpopulations. These conclusions are robust over a range of assumptions about competition for territories in summer and age difference in mortality in winter. 4. Adding density-dependent hedging success to the basic model reduces the effect of winter habitat loss on population size, but only when low proportions of the habitat are removed. A higher mortality rate in females, whether only in postfledging young birds or in birds of all ages, makes little additional difference to the population consequences of habitat loss. 5. Field studies on winter habitat loss in migratory bird populations should first test whether density has already reached the critical level, cW; i.e. whether some birds already die of food competition. The parameter bW should then be estimated to determine whether its probable value lies in the range within which predictions are sensitive or insensitive to its precise value. Whether the summer density-dependent functions are linear or curvilinear needs also to be explored, as does the effect of interactions between subpopulations which have different fledgling production rates but share the same winter habitat.