Local Population Growth Rates Research Articles

The Evolution of Patch Selection in Stochastic Environments

A null model for habitat patch selection in spatially heterogeneous environments is the ideal free distribution (IFD), which assumes individuals have complete knowledge about the environment and can freely disperse. Under equilibrium conditions, the IFD predicts that local population growth rates are zero in all occupied patches, sink patches are unoccupied, and the fraction of the population selecting a patch is proportional to the patch's carrying capacity. Individuals, however, often experience stochastic fluctuations in environmental conditions and cannot respond to these fluctuations instantaneously. An evolutionary stability analysis for fixed patch-selection strategies reveals that environmental uncertainty disrupts the classical IFD predictions: individuals playing the evolutionarily stable strategy may occupy sink patches, local growth rates are negative and typically unequal in all patches, and individuals prefer higher-quality patches less than predicted by their carrying capacities. Spatial correlations in environmental fluctuations can enhance or marginalize these trends. The analysis predicts that continually increasing environmental variation first selects for range expansion, then selects for persisting coupled sink populations, and ultimately leads to regional extinction. In contrast, continually increasing habitat degradation first selects for range contraction and may select for persisting coupled sink populations before regional extinction. These results highlight the combined roles of spatial and temporal heterogeneity on the evolution of habitat selection.

Abundance–occupancy relationships in metapopulations: examples of rock pool Daphnia

Intraspecific positive relationships between abundance and occupancy are observed for many species, suggesting that the same processes drive local and regional species dynamics. Two main groups of mechanisms explain this relationship: spatiotemporal variation in local population growth rates due to variation in habitat quality, or dispersal effects that increase occupancy of a species when locally abundant. Several studies show that spatiotemporal variation in population growth rates causes positive abundance-occupancy relationships, but few have shown dispersal effects. It is believed that such effects should be more evident for species whose dispersal is limited, e.g. metapopulations, but those studies are limited. This study investigates abundance-occupancy relationships in three Daphnia metapopulations in rock pools and the degree to which dispersal or habitat quality affect their local abundances and occurrence. Daphnia longispina and Daphnia magna showed positive abundance-occupancy relationships, but not Daphnia pulex. No single ecological factor could explain the abundance-occupancy relationships of any given species. Instead, dispersal processes and rock pool quality (mainly salinity and depth) seem to act together to shape the abundance-occupancy relationships. Such a conclusion is also supported by an immigration experiment in natural rock pools. This study suggests that although positive abundance-occupancy relationships may be commonly found for metapopulations, both dispersal processes and variation in habitat quality can be factors determining the abundance-occupancy relationship of metapopulations experiencing habitat heterogeneity.

Interactions between habitat quality and connectivity affect immigration but not abundance or population growth of the butterfly,Parnassius smintheus

Habitat geometry has been a primary focus in studies of spatially structured systems. Recent studies have indicated that a more comprehensive approach including habitat quality may be needed, however most previous studies have neglected potential interactions between quality and geometry. We investigated the effects of habitat quality for the butterflyParnassius smintheusamong a series of 17 sub‐populations. Specifically, we examined how habitat connectivity and local nectar flower density affect dispersal, and local population abundance and growth. We first determined which flower species were potentially important by examining nectar flower electivity and then quantified nectar flower density in meadows over a five year period (2003–2007). These data along with meadow connectivity were compared to local population statistics derived from mark–recapture over the same time period. The number of immigrants to a meadow increased as meadow connectivity increased, but showed no direct relationship with nectar flower density; however, there was a significant interaction between meadow connectivity and nectar flower density such that meadows with high connectivity and a high density of nectar flowers received the greatest number of immigrants. The number of emigrants from a meadow increased with increasing habitat quality and connectivity, but showed no interactive effect.The abundance of butterflies increased with meadow connectivity, but showed no relationship with habitat quality or any interactive effect. Separate experiments showed that access to nectar flowers significantly increased female reproductive output, but not lifespan. Despite the effects on immigration and reproductive output, local population growth rates also showed no relationship to nectar flower density. Our results indicate that habitat quality can be important for immigration in spatially structured populations; however, effects of habitat quality may not necessarily translate into higher abundance or population growth. Additionally, habitat quality should not be considered independently from habitat isolation, particularly if it directly affects dispersal. Preserving or augmenting habitat quality will do little to bolster immigration or colonization without adequate connectivity.

Intrinsic and extrinsic determinants of mountain pine beetle population growth

Abstract1 Mountain pine beetleDendroctonus ponderosaepopulations have large, economically significant outbreaks. Density dependence and environmental variability are expected to have important effects on their dynamics. We analysed time series data from an outbreak in the 1930s to determine the relative importance of population density and environmental variability on local population growth rates.2 Resource depletion occurred rapidly at the scale of 0.4 ha and population growth rates were strongly density dependent. Annual environmental changes did not have detectable effects on population growth rates, leading to the conclusion that intrinsic processes influenced local population density more than extrinsic factors during this outbreak.3 Our calculated value ofrmax(1.16) does not suggest intrinsically cyclic population dynamics. Our estimate ofrmaxand density dependence will be useful in developing applied models of mountain pine beetle outbreaks, and the subsequent evaluation of management strategies.

Fitness differences associated withPgiSNP genotypes in the Glanville fritillary butterfly (Melitaea cinxia)

Allozyme variation at the phosphoglucose isomerase (PGI) locus in the Glanville fritillary butterfly (Melitaea cinxia) is associated with variation in flight metabolic rate, dispersal rate, fecundity and local population growth rate. To map allozyme to DNA variation and to survey putative functional variation in genomic DNA, we cloned the coding sequence of Pgi and identified nonsynonymous variable sites that determine the most common allozyme alleles. We show that these single-nucleotide polymorphisms (SNPs) exhibit significant excess of heterozygotes in field-collected population samples as well as in laboratory crosses. This is in contrast to previous results for the same species in which other allozymes and SNPs were in Hardy-Weinberg equilibrium or exhibited an excess of homozygotes. Our results suggest that viability selection favours Pgi heterozygotes. Although this is consistent with direct overdominance at Pgi, we cannot exclude the possibility that heterozygote advantage is caused by the presence of one or more deleterious alleles at linked loci.

Approximating the ideal free distribution via reaction–diffusion–advection equations

We consider reaction–diffusion–advection models for spatially distributed populations that have a tendency to disperse up the gradient of fitness, where fitness is defined as a logistic local population growth rate. We show that in temporally constant but spatially varying environments such populations have equilibrium distributions that can approximate those that would be predicted by a version of the ideal free distribution incorporating population dynamics. The modeling approach shows that a dispersal mechanism based on local information about the environment and population density can approximate the ideal free distribution. The analysis suggests that such a dispersal mechanism may sometimes be advantageous because it allows populations to approximately track resource availability. The models are quasilinear parabolic equations with nonlinear boundary conditions.

TROUT PREDATORS AND DEMOGRAPHIC SOURCES AND SINKS IN A MAYFLY METAPOPULATION

Habitat patches often differ in environmental conditions, and the degree of environmental heterogeneity among patches is thought to influence population dynamics at the regional (metapopulation) scale. In a previous study, I found evidence of a source– sink dynamic in a metapopulation of the mayfly Callibaetis ferrugineus hageni occupying beaver ponds with or without brook trout predators, where adult females frequently dispersed among ponds, and frequently oviposited in ponds with trout. Here, I tested the hypothesis that trout create habitat sinks by manipulating trout densities and estimating local population growth rates (λ) of Callibaetis. In addition, Callibaetis emergence rates were estimated in the mayfly generation before the manipulation. Among ponds, λ ranged from 0.062 to 2.631, and nine Monte Carlo simulations scenarios revealed that four ponds had λ significantly below replacement (sinks) while the presence of source ponds was less conclusive. The manipulation of trout did not result in a detectable treatment effect, perhaps because of low statistical power resulting from high within-treatment variation in trout densities. However, point estimates of λ and emergence rates were negatively and exponentially related to trout density, such that nearly all ponds with more than 1 trout/100 m2 were sinks. Further, multiple regression analyses were consistent with the hypothesis that trout reduced λ and emergence rates, while other biotic and abiotic factors did not provide explanatory power. The results support the hypothesis that source–sink population dynamics may be common in freshwater habitats, and that prey metapopulation dynamics may be predicted from the distribution of predators, prey movement and habitat selection behaviors, and factors influencing the outcome of predator–prey interactions.

Does movement toward better environments always benefit a population?

We study the effects of advection along environmental gradients on logistic reaction–diffusion models for population growth. The local population growth rate is assumed to be spatially inhomogeneous, and the advection is taken to be a multiple of the gradient of the local population growth rate. It is also assumed that the boundary acts as a reflecting barrier to the population. We show that the effects of such advection depend crucially on the shape of the habitat of the population: if the habitat is convex, the movement in the direction of the gradient of the growth rate is always beneficial to the population, while such advection could be harmful for certain non-convex habitats.

REMOVING BROWN-HEADED COWBIRDS INCREASES SEASONAL FECUNDITY AND POPULATION GROWTH IN SONG SPARROWS

Parasitic Brown-headed Cowbirds may reduce the reproductive success of their songbird hosts and thus threaten rare and frequently parasitized host populations with extinction. The demographic impacts of cowbirds on hosts, however, have seldom been studied experimentally. We removed cowbirds to estimate how much they reduce the sea- sonal fecundity and population growth rates of a non-endangered cowbird host, the Song Sparrow. Over five control years, 61 % of 515 sparrow nests were parasitized at three riparian study sites. When we removed female cowbirds from two of these sites, parasitism fell to 43% of 65 nests at Westham in 1996, to 30% of 117 nests at Westham in 1997, and 18% of 78 nests at Delta in 1998. The mean seasonal fecundity of sparrows increased from 1.67 fledglings per territory in controls to 3.40 fledglings per territory at Westham and from 1.23 to 3.16 fledglings per territory at Delta. At the third control site, Deas, seasonal fecundity averaged 2.07 fledglings per territory. The mean sizes of sparrow broods at fledging were increased in removal years from 0.53 to 0.95 young at Westham and from 0.47 to 1.15 young at Delta. Daily nest failure rates were reduced from 0.046 in control years to 0.030 during cowbird removals at Westham and from 0.049 to 0.035 at Delta. Survival over a 25-d nest cycle rose from an average of 0.32 in eight controls to 0.45 in three removals. Cowbird removals increased local population growth rates of song sparrows from well below replacement levels (X = 0.70-0.87) to a stable condition (X = 0.93-1.06). If cowbirds have similar or stronger effects on rarer and more vulnerable hosts, cowbird removal could improve the demographic performance of such hosts substantially.

Evolutionary Dynamics of Prey Exploitation in a Metapopulation of Predators

In well-mixed populations of predators and prey, natural selection favors predators with high rates of prey consumption and population growth. When spatial structure prevents the populations from being well mixed, such predators may have a selective disadvantage because they do not make full use of the prey's growth capacity and hence produce fewer propagules. The best strategy then depends on the degree to which predators can monopolize the exploitation of local prey populations, which in turn depends on the spatial structure, the number of migrants, and, in particular, the stochastic nature of the colonization process. To analyze the evolutionary dynamics of predators in a spatially structured predator-prey system, we performed simulations with a metapopulation model that has explicit local dynamics of nonpersistent populations, keeps track of the number of emigrants entering the migration pool, assumes individuals within local populations as well as within the migration pool to be well mixed, and takes stochastic colonization into account. We investigated which of the predator's exploitation strategies are evolutionarily stable and whether these strategies minimize the overall density of prey, as is the case in Lotka-Volterra-type models of competitive exclusion. This was analyzed by pairwise invasibility plots based on short-term simulations and tested by long-term simulation experiments of competition between resident and mutant predator-types that differed in one of the following parameters: the prey-to-predator conversion efficiency, the per capita prey consumption rate, or the per capita emigration rate from local populations. In addition, we asked which of these three strategies are most likely to evolve. Our simulations showed that under selection for conversion efficiency the predator-prey system always goes globally extinct yet persists under selection for consumption or emigration rates and that the evolutionarily stable (ES) exploitation strategies do not maximize local population growth rates. The most successful exploitation strategy minimizes the overall density of prey but does not make it settle exactly at the minimum. The system did not settle at the point where the mean time to co-invasion (i.e., immigration of a second predator in a local prey population) equals the mean local interaction time (an idea borne out from studies on host exploitation strategies in host-pathogen systems) but rather where the mean time to co-invasion was larger. The ES exploitation strategies represent more prudent strategies than the ones that minimize prey density. Finally, we show that-compared to consumption-emigration is a more likely target for selection to achieve prudent exploitation and that prudent exploitation strategies can evolve only provided the prey-to-predator conversion efficiency is subject to constraints.

Synchrony, extinction, and dynamics of spatially segregated, heterogeneous populations

Most theoretical assessments of synchrony assume that populations are identical in their ability to produce and absorb dispersing individuals. Relaxing this assumption, I assessed how the local population growth rate, the proportion of individuals dispersing between populations, and different patterns in the immigration rate and carrying capacity with habitat patch size affect synchrony in local population dynamics and metapopulation extinction. All factors influenced both synchrony and extinction, and there were strong interactions between the factors. Local population growth rates had the largest effect on synchrony, followed by the pattern of carrying capacity with patch size and the proportion dispersing. The pattern of immigration with patch size had the smallest effect on synchrony. Metapopulation extinction was largely affected by the population growth rate, with the other factors having smaller but similar effects. Highly synchronous dynamics tended to be associated with greater metapopulation extinction within regions of chaotic local population growth; however, across the range of population growth investigated, high amounts of synchrony did not necessitate extinction. Differences among populations in the ability to absorb individuals mediated by interactions between growth rate, dispersal, patch size, and carrying capacity can substantially affect synchrony and extinction. Not considering how these factors interact may lead to erroneous conclusions regarding synchrony and extinction. Under certain situations, heterogeneity among local populations can promote metapopulation stability despite highly chaotic local dynamics.

How Do Any Species Persist? Lessons for Conservation Biology

Abstract: Recent reformulation of the birth‐and‐death process model to include the effects of environmental variation on stochastic demography presents a pessimistic view of the prospects for persistence of small populations. The models indicate that modest increases in population size, or in the mean (positive) growth rate of the population, do not dramatically increase the mean persistence time. The key to long‐term persistence seems to lie in reducing the variance in the population growth rate.Natural populations, of those species that are not numerous, achieve their persistence through a variety of variance‐reduction mechanisms. These notably include: I) risk‐averaging by wide geographic distribution over areas that experience mutually independent environmental variation, and 2) reliance on restricted “hot spots” of especially favorable habitat where the local population growth rate is almost invariably strongly positive when the population is not crowded.These natural mechanisms depend on a spatial scale, and on species‐specific richness of habitat, which is not readily attained for many species on biological reserves with a “hamis‐off” management policy. However, these mechanisms can be mimicked by management intervention strategies‐particularly by reintroduction programs in an “archipelago” of geographically dispersed reserves that are otherwise mutually isolated and by resource augmentation, predator and disease control, and other survival‐and‐reproduction enhancement programs implemented specifcally at times when the population is at a low ebb.

A comparative analysis of factors influencing the socioeconomic impacts of electric generating facilities

In recent years electric generating facility developments have become the foci of extensive controversy and resulting project delays, due in part to uncertainties and unknowns pertaining to a broad range of socioeconomic impacts which may occur. Unfortunately, socioeconomic impact research has not resolved many of these unknowns. Case study research findings have generally provided little new knowledge as to socioeeonomic impact dimensions and their causes. This paper attempts to avoid some shortcomings of prior research via a comparative analysis of some socioeconomic impact dimensions as they occurred in a sample of U.S. electric generating facilities. The paper focuses on an investigation of factors which may influence the extensiveness of developmentinduced local impacts. Both facility characteristics and characteristics of the locale which hosts facility development are suggested as possible impact determinants. Using correlational and regression procedures, expectations regarding the effects of facility size, construction time, distance to a metropolitan center, local population density and growth rates, and local labor force characteristics on overall “community service” impacts are examined. Results indicate both consistency and inconsistency with prior research conclusions, and suggest that some characteristics of the development and the development area are important in projecting the likely levels of some facility-induced socioeconomic impacts.