Density-dependent Immigration Research Articles

Immigration Rates during Population Density Reduction in a Coral Reef Fish.

Although the importance of density-dependent dispersal has been recognized in theory, few empirical studies have examined how immigration changes over a wide range of densities. In a replicated experiment using a novel approach allowing within-site comparison, we examined changes in immigration rate following the gradual removal of territorial damselfish from a limited area within a much larger patch of continuous habitat. In all sites, immigration occurred at intermediate densities but did not occur before the start of removals and only rarely as density approached zero. In the combined data and in 5 of 7 sites, the number of immigrants was a hump-shaped function of density. This is the first experimental evidence for hump-shaped, density-dependent immigration. This pattern may be more widespread than previously recognized because studies over more limited density ranges have identified positive density dependence at low densities and negative density dependence at high densities. Positive density dependence at low density can arise from limits to the number of potential immigrants and from behavioral preferences for settling near conspecifics. Negative density dependence at high density can arise from competition for resources, especially high quality territories. The potential for non-linear effects of local density on immigration needs to be recognized for robust predictions of conservation reserve function, harvest impacts, pest control, and the dynamics of fragmented populations.

Modeling ecological traps for the control of feral pigs.

Ecological traps are habitat sinks that are preferred by dispersing animals but have higher mortality or reduced fecundity compared to source habitats. Theory suggests that if mortality rates are sufficiently high, then ecological traps can result in extinction. An ecological trap may be created when pest animals are controlled in one area, but not in another area of equal habitat quality, and when there is density-dependent immigration from the high-density uncontrolled area to the low-density controlled area. We used a logistic population model to explore how varying the proportion of habitat controlled, control mortality rate, and strength of density-dependent immigration for feral pigs could affect the long-term population abundance and time to extinction. Increasing control mortality, the proportion of habitat controlled and the strength of density-dependent immigration decreased abundance both within and outside the area controlled. At higher levels of these parameters, extinction was achieved for feral pigs. We extended the analysis with a more complex stochastic, interactive model of feral pig dynamics in the Australian rangelands to examine how the same variables as the logistic model affected long-term abundance in the controlled and uncontrolled area and time to extinction. Compared to the logistic model of feral pig dynamics, the stochastic interactive model predicted lower abundances and extinction at lower control mortalities and proportions of habitat controlled. To improve the realism of the stochastic interactive model, we substituted fixed mortality rates with a density-dependent control mortality function, empirically derived from helicopter shooting exercises in Australia. Compared to the stochastic interactive model with fixed mortality rates, the model with the density-dependent control mortality function did not predict as substantial decline in abundance in controlled or uncontrolled areas or extinction for any combination of variables. These models demonstrate that pest eradication is theoretically possible without the pest being controlled throughout its range because of density-dependent immigration into the area controlled. The stronger the density-dependent immigration, the better the overall control in controlled and uncontrolled habitat combined. However, the stronger the density-dependent immigration, the poorer the control in the area controlled. For feral pigs, incorporating environmental stochasticity improves the prospects for eradication, but adding a realistic density-dependent control function eliminates these prospects.

Identifying hidden sinks in growing populations from individual fates and movements: The feral horses of Sable Island

Identifying the existence of population sinks is critical for conservation and management. However, because of density-dependent dispersal, sinks can sometimes be masked by immigration events, especially during phases of population growth. We present a large-scale, empirical demonstration of within- population source-sink dynamics using the feral horses (Equus ferus caballus) of Sable Island National Park Reserve, Nova Scotia, Canada, as a model. We tracked the fates and movements of 98.7% of the female population (n ¼190-237) across 3 demographic clusters (subunits) during a period of rapid population growth (2008-2010; 24.7% increase in density). All subunits experienced increases in population size each year (l>1.0). Our individual-based analysis showed that western Sable Island, where water availability was greatest, behaved as a source and would have grown with or without immigration in all years. However, the central (andfastest growing subunit) would havedeclinedfrom 2008-2009(l ¼0.951)without immigration. Further, the eastern subunit would have declined in 2 intervals (l ¼0.932, 0.999) without immigration. Our study demonstrates that the propensity of habitat to act as a sink can be masked during a period of population growth because of density-dependent immigration from adjacent habitats. These findings present a caution to managers charged with conserving wide-ranging species with long population cycles for which effects of immigration on local population growth rate can be difficult to isolate using standard methods of enumeration. 2013 The Wildlife Society.

Intercontinental tests of the effects of habitat patch type on the distribution of chitons within and among patches in intertidal boulder field landscapes

Patchy distributions within landscapes may be caused by migration in response to different types of habitat patches. Intertidal boulder fields are landscapes in which boulders are discrete habitat patches, often with chitons attached to their under-surfaces. Chiton densities and associations with patch edges differed between boulders overlying coarse- versus fine-sediment types, with greater densities occurring over fine sediments. We tested whether adult migration caused between-boulder distributions by measuring immigration and emigration following experimental replacement of coarse sediments with fine sediment under boulders. We also assessed whether the manipulations altered chiton positions relative to patch edges, and large-scale generality was tested by including data from two continents. The manipulations did not influence the association of chitons with edges or amounts of emigration, but chitons did display positive density-dependent immigration that mirrored their distributional patterns, indicating the importance of immigration. Strikingly, all results were consistent between continents despite involving different species and even genera of chitons. By using boulder fields as a small-scale, easily manipulated landscape, we show that, even in sedentary organisms, patchy distributions within landscapes can be caused by migration alone, without the need to invoke mortality or larval recruitment.

CONTROL OF A METAPOPULATION HARVESTING MODEL FOR BLACK BEARS

ABSTRACT. We develop a metapopulation harvesting model that includes density-dependent immigration and emigration and apply Pontryagin's maximum principle to derive an optimal harvesting and reserve design strategy. The model is designed to mimic the black bear population of eastern Tennessee and western North Carolina. Model results suggest that a forest region's population can be maintained despite high harvest levels due to emigration from a connected, un-harvested park region. The amount of shared border between the park and forest region is important in determining the optimal harvesting strategy. This technique offers new insight on the spatial control of protected populations.

The effect of emigration and immigration on the dynamics of a discrete-generation population

The recent paper of Sinha and Parthasarathy investigated the effect of modifying the Ricker and logistic population models to simulate the effects of immigration to, and emigration from, the population. Immigration of a fixed number of individuals was shown to reduce the probability of observing chaos in the Ricker model but not the logistic one. Here, isocline analysis is used to investigate why these effects occur. The stabilization effect for the Ricker equation occurs over a wide range of values of the immigration parameter. There are no values of the parameter, however, which increase the stability of the logistic equation substantially. In contrast density-dependent immigration is found to destabilize both the Ricker and logistic models. Density-dependent emigration serves to reduce the propensity of both models to exhibit chaos.

Population dynamics of Mytilus edulis along environmental gradients:density-dependent changes in adult mussel numbers

The densities of each age class of adult (>l yr old) mussels Mytilus edulis were measured in autumn and spring over 8 yr on each of 12 intertidal mussel beds in the Exe estuary, southwest England. The possibility that density-dependent changes in adult numbers in each of 9 yearly age classes occurred over the summer and winter periods was explored directly on individual beds and indirectly by combining data from all beds. There was direct evidence of density-dependent changes in both seasons in some age classes on some beds. The indirect tests suggested, however, that density-dependence was much more widespread over beds, age classes and seasons than had been suggested by the direct tests. The critical density above which density-dependent losses occurred, and to which the density-dependence tended to return numbers irrespective of the abundance of the mussels at the start of summer or winter, varied considerably among beds in several age classes. These critical densities were correlated most closely with the time for which a bed was exposed over low tide but were also influenced by the distance of a bed from the estuary mouth and the softness of its substrate. Between them, these 3 variables defined the environmental gradient of intertidal mussels in the Exe estuary. The residual, density-independent variations in the changes in numbers of several age classes of adult mussels about the density-dependent relationships were correlated with weather variables (temperature, gales and rainfall) in several cases and in both seasons. Surprisingly, there was evidence of widespread density-dependent immigration of adult mussels, especially in summer and in the younger age classes. While the passive movement by adult mussels in storms is well established, it is not known whether active movements occur in these older mussels. Although much of the density-dependence was due to mortality, immigration was therefore often an important component of the densitydependent relationships detected on individual beds. However, immigration on some beds countered losses on others. Perhaps because of this, density-dependence was often masked, and thus not detected, at the estuary scale in all but the youngest mussels whose estuary-wide abundance was dominated by 2 large mussel beds, where their numbers tended to CO-vary. Thus, strong densitydependence at one scale may not be detected at a higher scale.