Dispersal Sinks Research Articles

Long-term demographic changes of deer mouse (Peromyscus maniculatus) populations in a forest landscape with cumulative clearcutting

The deer mouse (Peromyscus maniculatus) is a generalist species and is widespread among the various early-successional forest habitats associated with post-clearcut landscapes. Deer mice seem to occupy clearcuts more than undisturbed late successional forest, at least for abundance, because harvested sites apparently act as dispersal sinks. We investigated the responses in abundance of P. maniculatus populations to cumulative clearcutting of coniferous forests on a landscape that had four independent clearcutting events (Periods 1 to 4) over a 42-year interval from 1979 to 2020 in south-central British Columbia, Canada. We tested three hypotheses (H) that populations of P. maniculatus would (H1) increase in abundance owing to the ongoing availability of early seral post-harvest habitats associated with cumulative clearcutting; (H2) have higher mean abundance, reproduction, and survival in clearcut than forest sites owing to greater food resources; and (H3) occupy clearcut sites as dispersal sinks thereby having lower adult body mass (a proximate measure of condition) and reduced reproductive output than in forest sites.Mean annual abundance of P. maniculatus was highest and similar in Periods 2 and 4 with comparable numbers in Period 1 for the first four post-harvest years of these clearcutting periods. This pattern did not support H1 that P. maniculatus would increase in abundance associated with cumulative clearcutting over the 42-year period. Deer mouse populations reached relatively high densities (range of 23 to 53 per ha) on clearcut sites and these irruptions were likely related to supply of lodgepole pine seed from cones in logging debris. Overall mean abundance of P. maniculatus ranged from 1.4 to 3.6 times higher in clearcut than forest sites, thereby supporting the abundance part of H2. Recruitment, number of successful pregnancies, and breeding young-of-the-year deer mice were all higher in clearcut than forest sites, thereby supporting the reproduction part of H2. However, equivocal measures of overall survival did not support this hypothesis. The general similarity in mean body mass of adult mice between treatment sites and higher reproductive output in clearcut than forest sites did not support H3 that clearcut sites would act as dispersal sinks for deer mice. This study is the first to measure the long-term demographic responses of deer mice to cumulative clearcutting over a coniferous forest landscape. Clearcut sites may provide high-quality habitat for deer mice in this forest landscape.

Trophic and spatial complementarity on seed dispersal services by birds, wild mammals, and cattle in a Mediterranean woodland pasture

Most earth surfaces have undergone intensive land-use changes, creating habitat mosaics. Seed dispersal by animals is a crucial process in such mosaics, but community-wide studies comparing the functional complementarity and response to man-imposed habitat heterogeneity are rare. Here, we investigate the trophic and spatial seed dispersal networks underpinning a strong, woody vegetation expansion over a pastureland inside the largest forest remnant in western Sicily, Italy. Over two fruiting seasons, we surveyed transects in three distinct biomes within our study area: forest, pastureland, and unpaved road. In total, we collected 659 feces and tested for differences in defecation patterns and seed rain density of birds, wild mammals, and cattle. We also tested the degree of trophic and spatial specialization and modularity using a network approach. Overall, birds dispersed 1208 seeds/ha of nine plant species, including six exclusive. Mammals dispersed 679 seeds/ha from four wild species, three of which also dispersed by birds, and 38 seeds/ha of three cultivated species. In turn, mammals dispersed exclusively the seeds of wild pear ( Pyrus amygdaliformis ), the most abundant tree in the woodland pasture. Cattle only dispersed wild pear, but accounting for 56% of the dispersed seeds. Seed rain densities were significantly higher in woodland pastures than in forests. However, almost of half the seeds dispersed by cattle and red fox were deposited on unpaved roads. While both trophic and spatial networks were more specialized than expected, we did detect distinct modules. Our study demonstrated the magnitude of the effects of man-made habitat heterogeneity on seed dispersal services, giving baseline information for restoration programs as well as high nature value pastureland management strategies. • Birds and mammals are complementary in their seed dispersal services and key for vegetation dynamics. • Rewilding with cattle can just partially compensate this services, but not replace wild mammals role. • Fruit-rich woodland pasture was more relevant than fruit poor managed forest for seed dispersal networks. • Soft linear developments may act both as seed dispersal corridors or as seed dispersal sinks. • Seed dispersal networks can be good indicators to define high nature value agroforestry systems.

Population dynamics of the heather vole (Phenacomys intermedius) in commercial forest landscapes of south-central British Columbia, Canada

AbstractLong-term population dynamics are essentially unknown for the western heather vole (Phenacomys intermedius), a relatively rare microtine species. Our objectives were to provide a description of demographic changes in populations of P. intermedius and determine possible causes of these changes. We analyzed four detailed 12- to 22-year data sets that recorded changes in abundance of P. intermedius in a variety of managed forests in south-central British Columbia, Canada from 1993 to 2018. We evaluated three hypotheses (H) that may help explain population changes: heather voles will (H1) have higher populations in sites with (i) abundant dwarf shrubs or (ii) cover of woody debris; (H2) colonize new clear-cuts as dispersal sinks for young voles; and (H3) be at lower abundance when other Microtus species are present at equal or higher numbers. Populations of P. intermedius occurred commonly at ≤ 1 animal/ha (as a density index). However, in favorable habitats such as some new clear-cuts and in “open” pine stands, P. intermedius occurred at mean annual peak numbers of 5–6/ha (as a density index). There were two relatively high populations of P. intermedius in 1994–1995 and 1998, and possibly again in 2002 in the open pine stands. Mean numbers of lactating females and recruits per year tended to follow the pattern of changes in abundance. High populations of P. intermedius occurred in stands with abundant dwarf shrubs in open pine stands but not those with a cover of woody debris structures in clear-cuts, H1, therefore, was only partially supported. A relatively higher proportion of adult than juvenile heather voles on clear-cuts did not support H2: that these harvested sites would be dispersal sinks for young voles. Mean annual numbers of P. intermedius were positively correlated with those of M. longicaudus, at least on clear-cut sites, hence H3 was not supported. Phenacomys intermedius was essentially absent from our closed-canopy mature/old-growth forests but occurred in many managed forest habitats at low abundance (density index of ≤ 1/ha). In early successional habitats after clear-cutting and in open young pine stands this species occurred at moderate abundance and generally appeared stable in numbers across these forest landscapes.

Dispersal sinks and handling effects: interpreting the role of immigration in common brushtail possum populations

Summary An evaluation of the potentially adverse effects of measurement must be made before concluding that one is dealing with a ‘dispersal sink’. We conducted a spatially and temporally replicated removal experiment on common brushtail possums (Trichosurusvulpecula) in uniformly suitable old‐growth eucalypt forest in south‐eastern Australia, that was designed to address the question: does immigration ‘rescue’ populations from extinction? Despite taking precautions to minimize potential harm, analyses indicated some evidence of an adverse effect of handling on the survival of pouch‐young and strong evidence of effects on adult survival. In addition, symptoms of stress associated with handling observed at our site, corresponded to symptoms reported in connection with the long‐term (15 + years) trapping study on possums conducted by Efford et al. in the Orongorongo Valley (OV) of New Zealand. Initial projections from a demographic model indicated that the resident population at our site was not replacing itself (births < deaths), suggesting that the site was a dispersal sink. This was inconsistent with the fact that the site was in prime habitat. Moreover, the measured rate of true immigration in response to experimental removals was not sufficient to maintain the population density. Using data from Efford (1998), our model confirmed his suggestion that the OV site also appears to be a dispersal sink for possums, despite being in prime habitat. When otherwise undiagnosable deaths among adults were assumed to be due to handling and ‘right‐censored’ (excluded), the projection was that the resident population at our site was stable (r ≅ 0), and therefore not in need of ‘rescue’ by immigration. Similarly, when survival estimates for the OV site were corrected by the same amounts, the projection was that the population at that site was also stable. Most vacancies created by our experimental removals were filled by neighbouring residents that expanded their ranges into the removal areas. We suggest that the artificial ‘removal’ of residents as a consequence of deaths due to handling, may often induce an influx of such apparent immigrants, thereby giving the impression that immigration is ‘rescuing’ populations from extinction.

A Response to Solving the Enigma of Population Cycles with a Multifactorial Perspective

We believe that Lidicker's (1988) paper on the enigma of microtine cycles, based on a plenary address presented at the 1987 annual meeting of the American Society of Mammalogists, needs response because it contains several unclear points and conclusions of uncertain validity. Lidicker (1988) argued that the enigma of microtine cycles will not be solved by a single-factor approach. He reiterated his earlier claim that a multi-factorial perspective (MFP) is absolutely essential if we are going to understand microtine demography (Lidicker, 1988:226). In contrast, we believe that the multifactorial perspective in its present form is difficult to comprehend; our objective is to address in detail difficulties with it. If these difficulties can be overcome, the multifactorial perspective may be acceptable to more researchers in the field. Three major aspects in Lidicker's (1988) presentation of the multifactorial perspective that could lead to confusion are imprecise terminology, misinterpretation of empirical results, and philosophy of science. Imprecise terminology.-One of the greatest potential sources of confusion in science is imprecise terminology. Presaturation dispersal and dispersal sinks are two terms used by Lidicker (1988) that need to be clarified. The best way to eliminate confusion is to define these terms operationally. Lidicker (1988) asserted that presaturation dispersal is one of eight major components in the multifactorial perspective having a role in the density-regulation machinery of California voles (Microtus californicus). Unfortunately, Lidicker (1975, 1978, 1988) has not defined presaturation dispersal operationally. Certainly, the general characteristics of these animals could not be relied upon because Lidicker (1975:106) defined them so broadly as ... in relatively good condition, and may include any sex and age group including pregnant females. Another approach would be to use the density of the resident population as an indicator of presaturation dispersal. Animals emigrating from the resident population at carrying capacity would be saturation dispersers, whereas animals emigrating before the resident population reaches carrying capacity would be presaturation dispersers. However, numbers of voles and abundances of resources vary over time, thus, it is impossible to measure the carrying capacity. If the operationality of presaturation dispersal is dependent on the operationality of carrying capacity, it behooves Lidicker to suggest how carrying capacity in fluctuating populations of voles is measured. On the positive side, Lidicker's (1975) concept of presaturation dispersal brought attention to the fact that animals may leave a population before food, space, and mates become limiting. Interestingly, Krebs et al. (1973) recognized the effect of dispersal during phases of increasing density on the population dynamics of voles in fenced enclosures (fence effect), but their observations were not widely appreciated at the time. A solution to the problem is to use the rate of population instead of carrying capacity in distinguishing between the two types of dispersal. Presaturation dispersal would occur when population-growth rates are positive, whereas saturation dispersal occurs when population-growth rates are zero or negative. At least the rates of the population can be measured. Lidicker (1988:229) stated the growth rate of a local population is affected strongly by the availability of a dispersal sink (Lidicker, 1975, 1980). Peak densities occur when sinks are filled and dispersal becomes frustrated. These statements are confusing because of Lidicker's ambiguous definition of dispersal sink. Lidicker (1975:117) defined dispersal sink as some empty or unfilled suitable habitat, or perhaps marginal or even unsuitable habitat in which at least short-term survival is possible. If all spatially accessible unsaturated habitats are dispersal sinks, it may be impossible to find a habitat that is not a dispersal sink. Furthermore, if dispersal sinks are unsuitable or marginal habitats where dispersers presumedly suffer high mortality rates, it is difficult to understand how the sinks will become saturated and frustrate dispersal in

Are small-scale landscape features important factors for field studies of small mammal dispersal sinks?

Interest in the influence of landscape features on animal movement has been widespread; however, few field studies of the emigration of small mammals from patches of habitat directly consider the effects of the small-scale landscape features. The simulation models of Stamps et al. (1987a, b) and Buechner (1987a, b) suggest that the size of a dispersal sink relative to the size of the source patch, the average distance traveled by dispersers in the sink, the ease with which dispersers cross the edge between the sink and a source patch, and source patch perimeter:area ratio may all be important influences on emigration rates. A review of field studies of small mammal dispersal into sinks suggests that in a substantial fraction of such studies the values of these factors fall within the ranges that the simulation models indicate have the greatest potential effect on emigration rates. New field studies of dispersal sinks that include a consideration of these factors are necessary in order to evaluate the magnitude of the impact of these factors on natural populations.

Response of the white-footed mouse (Peromyscus leucopus) to the transition between disturbed and undisturbed habitats

The relationship between social behavior, demography, and habitat selection in the white-footed mouse (Peromyscus leucopus) was studied at the transition zone between disturbed (clear-cutting and strip-mine) and undisturbed (woodland) habitats. Demographic characteristics included a tendency toward overrepresentation of young mice (juvenile and subadults) in disturbed habitats and higher disappearance rates for young woodland males than for females. Home range data indicated that movements between habitats were minimal and mostly restricted to the ecotone. Behavioral interaction studies revealed that woodland mice were more likely to initiate contact and that mice from the clear-cutting responded by retreating or fleeing. During a reciprocal removal experiment, 43% of the resident population of the clear-cutting moved to woodland habitat after the original woodland population had been removed; no residents of woodland habitat moved to a disturbed site following removal. The suggestion that disturbed habitats serve as dispersal sinks for behaviorally subordinate individuals is supported by these data.

Conservation in insular parks: Simulation models of factors affecting the movement of animals across park boundaries

Many parks represent insular areas of habitat and the movement of animals across park boundaries can be critical to park wildlife populations. I use computer simulation models in order to analyse factors affecting the movement and density of animals in patches of habitat (Buechner, 1987b; Stamps et al., 1987; in press). The results suggest that the perimeter: area ratios and edge permeability of parks, the appearance and stopping patterns and habitat preferences of moving animals, and the relative size of dispersal sinks and source pools can all potentially influence the direction and magnitude of the movement of animals across the park boundaries. The results suggest that scientists interested in park and reserve design n to consider other factors than just the size of potential sites, particularly park perimeter: area ratios. The simulations also suggests a strategy for solving some types of conservation and control problems by manipulating sinks or pools: (1) identify which factors can be practically altered in a given situation, (2) evaluate those factors to estimate their current levels, and (3) focus on the factor which is nearest the lower end of its range of values.

Demographic Substructure in a California Vole Population Inhabiting a Patchy Environment

We employed a live-trapping grid encompassing several discrete vegetation patches to analyze spatial differences in demographic structure of California voles ( Microtus californicus ). Densely populated patches had female-biased sex ratios, longer individual persistence of females, higher per individual productivity and emigration rates, and lower immigration rates than did sparsely populated patches. Male density was relatively constant among patches, while female density varied by a factor of three. The high degree of stability this population exhibited over the four years in which density was estimated may be a result of the presence of perpetually unfilled dispersal sinks. A laboratory study showed that demographic performance in a given patch is strongly associated with the dietary quality of the vegetation present in that patch. Vegetative cover may also be important in contributing to demographic differences among patches, but is less important than food. Because the quality of diet and, presumably, protective cover, had the same effect on both males and females, it appears that sex differences in response to habitat heterogeneity are a result of differences in spacing behavior.

Meadow vole cycles within fences

To test the idea that dispersal is necessary for cycling and to develop an enclosure that would permit vole population cycling, we partially and fully fenced grids that included woodland areas (dispersal sinks) that were continually trapped out. We studied three populations of the meadow vole, Microtus pennsylvanicus, in Massachusetts from 1978 to 1982. The control population and both fenced populations exhibited virtually identical demographic trends, which encompassed a vole cycle. Analysis of demographic parameters suggested that the major effect of the fence was to increase survival within. We attributed this increased survival to reduced dispersal because a dispersal sink attracted only 98 dispersers throughout the study, We suggest that there is a continuum of individual thresholds to disperse with only a very few animals motivated enough to enter a woodland dispersal sink.

Demography of southern red-backed voles (Clethrionomys gapperi) and deer mice (Peromyscus maniculatus) after logging in north-central Ontario

Populations of Clethrionomys gapperi and Peromyscus maniculatus were investigated on newly harvested upland black spruce (Picea mariana) clear-cuttings and in control areas (uncut, selective cut, older clear-cuttings) near Manitouwadge, Ontario. On control areas Clethrionomys had increased breeding intensity, better summer survival rates, and a relatively greater number of young in 1978 relative to 1976 and 1977. Those conditions appeared to be unrelated to summer weather and occurred after a cold, wet spring with a delayed snowmelt. Numbers of Clethrionomys on new clear-cuttings declined to nil over the first two summers following timber harvest. Demography on new clear-cuttings was similar to that on control areas except for poorer juvenile survival rates and fewer young. The disappearance of Clethrionomys from recent clear-cuttings was likely due to direct mortality. Demography of Peromyscus on control areas was similar each year except in 1978 when there was an increase in the breeding intensity of males and a decrease in the proportion of young. Numbers of Peromyscus on new clear-cuttings increased over the 3 years. Demography was similar on clear-cuttings and control areas in 1978, but in 1976 and 1977 the clear-cutting population had an excess of males, greater reproductive intensity of both males and females, more wounding of males, and less wounding of females. A highly mobile, young, male-dominated population with a high reproductive rate and a good summer survival rate was present on the 1- and 2-year-old cuts. Recent clear-cuttings in upland black spruce act as dispersal sinks for Peromyscus and provide new habitat for colonization by presaturation dispersers.

An Experimental Examination of an Artificial Dispersal Sink

Studies of artificial dispersal sinks (removal experiments) assume that they measure the dispersal pattern that would occur in an unmanipulated population. i conducted a removal experiment on the California ground squirrel, Spermophilus beecheyi , to test this assumption. The sex ratio of young immigrants removed from the artificial dispersal sink (0.72 males/female) was lower than that of young immigrants to an adjacent reference area (1.64 males/female), young immigrants to reference areas 1 km away (4.13 males/female), and young emigrants in an earlier study of the species (2.55 males/female). Similar results occurred in a study of the Arctic ground squirrel, S. parryii , conducted by J. E. Green. I conclude that removal data cannot be assumed to represent an accurate measure of the dispersal, or the characteristics of dispersers, occurring in unmanipulated populations of ground squirrels.