Dispersal Data Research Articles

Estimating Population Density Per Unit Area from Mark, Release, Recapture Data

In this paper, we describe a method for estimating animal population density per unit area from mark, release, recapture (MRR) data. Standard procedures are available for estimating population size from MRR data, but not density per unit area, which is often of more practical value. If data on dispersal movements are available in addition to capture/recapture records, they can be used to estimate the area over which the study population ranges. These area estimates can then be combined with MRR abundances to yield estimates of population density. As a concrete example of how these methods can be applied, we calculated population density estimates for seven MRR data sets for the screwworm, Cochliomyia hominivorax (Diptera: Calliphoridae), from Costa Rica. As a standard of comparison, we also used a different method to calculate estimates of screwworm density for five sets of USDA APHIS Screwworm Eradication Program data from Mexico. Mean densities for both sets of analyses were similar, ranging from 10 to 120 adult flies/km2. General agreement between the Mexican and Costa Rican estimates lends support to the dispersal approach we propose here. Although we have applied these methods to the screwworm as a specific example, they are entirely general and can be applied to virtually any organism for which both MRR records and dispersal data are available.

Dispersal Data and the Spread of Invading Organisms

Models that describe the spread of invading organisms often assume that the dispersal distances of propagules are normally distributed. In contrast, measured dispersal curves are typically leptokurtic, not normal. In this paper, we consider a class of models, integrodifference equations, that directly incorporate detailed dispersal data as well as population growth dynamics. We provide explicit formulas for the speed of invasion for compensatory growth and for different choices of the propagule redistribution kernel and apply these formulas to the spread of D. pseudoobscura. We observe that: (1) the speed of invasion of a spreading population is extremely sensitive to the precise shape of the redistribution kernel and, in particular, to the tail of the distribution; (2) fat—tailed kernels can generate accelerating invasions rather than constant—speed travelling waves; (3) normal redistribution kernels (and by inference, many reaction—diffusion models) may grossly underestimate rates of spread of invading populations in comparison with models that incorporate more realistic leptokurtic distributions; and (4) the relative superiority of different redistribution kernels depends, in general, on the precise magnitude of the net reproductive rate. The addition of an Allee effect to an integrodifference equation may decrease the overall rate of spread. An Allee effect may also introduce a critical range; the population must surpass this spatial threshold in order to invade successfully. Fat—tailed kernels and Allee effects provide alternative explanations for the accelerating rates of spread observed for many invasions.

Reconsideration of pollen dispersal data from field trials of transgenic potatoes

During the initial field evaluation of transgenic plants, it is usual to isolate them genetically from other plants of the same species. Several field experiments on potatoes, using transgenes as markers, have shown that transgene dispersal by pollen to other potato plants is limited and very unlikely at distances over 10 m. In a recent study in Sweden, a frequency of transgene-containing progeny of over 30% is reported from non-transgenic potato plants grown at distances of 10-1000 m from transgenic plants containing nptII and gus marker genes. Data from the Swedish study is discussed along with other relevant observations, and it is concluded that the high frequency of gene dispersal in that study results from a high frequency of false positives during PCR analysis of the nptII gene. From the data available in potato, it is concluded that a distance of 20 m is generally adequate for the initial field evaluation of transgenic potatoes containing novel gene constructs.

Reconsideration of pollen dispersal data from field trials of transgenic potatoes

Reconsideration of pollen dispersal data from field trials of transgenic potatoes

Interpatch movement of the red milkweed beetle, Tetraopes tetraophthalmus: individual responses to patch size and isolation.

Individual movement patterns and the effects of host plant patch size and isolation on patch occupancy were examined for red milkweed beetles, Tetraopes tetraophthalmus, residing in a heterogeneous landscape. Male beetles were found to move both more often and farther between host plant patches than female beetles, and this difference affected the patterns of patch occupancy observed. Overall, unoccupied milkweed patches were smaller and more isolated than patches occupied by beetles. Patches uninhabited by females tended to be more isolated, but not necessarily smaller, than patches with female beetles, indicating that females may be affected more by patch isolation than patch size. Presence of male beetles on patches showed a stronger response to patch size than to patch isolation. Differences in movement between males and females illustrate the need for demographically based dispersal data. Comparisons of Tetraopes interpatch movement patterns between landscapes composed of patches of different size revealed that landscapes with overall smaller patches may have greater rates of interpatch movement.

Statistical Resampling Methods in Biology: A Case Study of Beaver Dispersal Patterns

SYNOPTIC ABSTRACTThe performances of the bootstrap, generalized bootstrap and jackknife resampling methods are compared using beaver (Castor canadensis) dispersal distance data. The results show that female beavers disperse slightly farther from their natal families than males, but the difference is not significant. Compared with results from simple statistics, the jackknife gives unbiased estimates for the mean and standard error. The performances of the bootstrap and generalized bootstrap are similar for estimation of the mean, standard error, and 95% confidence interval. The bootstrap requires a slightly smaller sample size than the generalized bootstrap to achieve the same desired results. However, the generalized bootstrap appears to be more consistent in parameter estimation than is the bootstrap. For similar studies, we recommend 500 as the minimum number of bootstrap replications.

Localization of a Contact Zone between Two Highly Divergent Mitochondrial DNA Lineages of the Brown Bear Ursus arctos in Scandinavia.

In Europe the brown bear (Ursus arctos) is represented by two different mitochondrial DNA (mtDNA) lineages, which probably diverged about 0.85 million years ago. Scandinavia has been colonized by representatives of both lineages, from the north (eastern lineage) and from the south (western lineage), and now bears occur primarily in four main regions called female concentration areas. For management purposes the localization of the contact zone between these two genotypes is important. Using hairs as a source of DNA, 127 individual brown bears from throughout the Scandinavian populations were assayed for lineage assignment. A part of the mtDNA control region was amplified via the polymearase chain reaction, and the product was either sequenced (14 individuals) or digested with two diagnostic restriction endonucleases (113 individuals). Fifty-six and 71 bears were assigned to the western and eastern lineages, respectively. The geographic distribution of the two genotypes allowed precise localization of the contact zone. Only two males from each lineage had crossed the border between the two lineages. We used dispersal data from bears radiomarked as yearlings to determine whether potential mtDNA introgressions agreed with the dispersal behavior of bears. The males in the "wrong" areas were all within the 95th-percentile dispersal distance from the "correct" area. Females were more philopatric than males, and none were found in the wrong areas. The two female concentration areas flanking the contact zone were 134 km apart. Thus, radiotelemetry results on dispersal distances could explain the occurrence of the males in the wrong genetic area. In the absence of information concerning possible male-mediated gene flow, a conservative management approach would be to consider the southern and the three northern female concentration areas as two distinct conservation units. Localización de una zona de contacto entre dos linajes de ADN mitocondrial muy divergentes del oso pardo Ursus arctos en Escandinavia.

Effective Population Size and Demography of the Rare Flightless Galapagos Cormorant

The population structure and effective population size of the Galapagos Cormorant (Compsohalieus [Nannopterum] harrisi), a rare seabird endemic to the Galapogos Islands, Ecuador, were investigated using standard demographic techniques and data collected over nine years (1970‐1973, 1977, 1978, and 1989‐1992). The study population was located around Punta Espinosa, Fernandina Island, where cormorants bred at seven traditional places scattered along 9 km of bare rocky coastline. This population, averaging 96 ± 13 adults (mean ± 1 SD) (47 ± 8 males; 50 ± 7 females), is an estimated 10% of the total number (≈980) distributed along 350 km of coastline around Fernandina and the western coast of Isabela Island. The Galapagos Cormorant's restricted distribution and small population size probably result from its adaptation to rich upwelling systems where food is highly reliable and abundant. This adaptation apparently is also shaping the population's general breeding ecology. The population has high potential fecundity and is probably regulated through food competition by density‐dependent processes which affect reproductive success and juvenile recruitment more than adult survival. Demographic analyses and direct counts along the species' entire range showed that the population is stable both in its demographic structure (e.g., sex ratio and age structure) and number. The population is characterized by a high and relatively constant annual adult survival, long life‐span, and a generally low and variable reproductive success and annual productivity. The serial polyandrous mating system apparently has little effect on these demographic traits. Dispersal data indicated that gene flow is extensive (≈28 km per generation) and that the whole population is distributed in at least 10 relatively discrete demes or neighborhoods. The demographic analysis yielded an expected among‐deme genetic variance component (Fst) of 0.049 (4.9%), which implies that the population is not genetically substructured to a large degree. Thus, for analyses of the effect of genetic drift, the whole population can be considered as a single panmictic unit. However, the exceptionally small total population size, and its even smaller effective population number (Ne ≈650), predicts a rate of loss of neutral genetic variation (ΔF = 1/2 Ne) of 0.0008 (0.08%) per generation, equivalent to 0.006% per year. The population is therefore expected to be highly homozygous (G = 0.997). These population genetic traits should be taken into account for conservation purposes. Although after a crash (e.g., in 1982) the population may rapidly recover in number and otherwise remain relatively stable, its genetically effective size barely reaches the minimum number judged to be required for long‐term persistence in the face of genetic and environmental stochasticity. Moreover, the recurrent human disturbance of the habitat where cormorants find their prey, mainly through alteration of the sea bottom for the capture of sea cucumbers, may severely affect the survival of this endemic species, one of the world's rarest seabirds.

Subplinian eruption mechanisms inferred from volatile and clast dispersal data

Abstract Subplinian tephras erupted during the North Mono eruption of 1350 A.D. contain abundant obsidian clasts. The weight percent of water in the clasts from each tephra bed can be used to infer the depths from which the clasts originated within the magmatic system. A comparison of the depths of origin with inferred eruption dynamics and local bedrock stratigraphy suggests that the clasts were eroded from conduit walls during repeated withdrawal of magma from a replenished shallow magmatic system. Furthermore, the obsidian was preferentially formed at and eroded from sections in the conduit where increased cooling and magma fragmentation took place due to proximity to groundwater or to the surface. Finally, the data are compatible with magma withdrawal during separate explosive phases resulting from the movement of a disruption surface downward into magma columns that became larger in cross-sectional area with each explosive phase.

Accelerating Forest Succession in a Fragmented Landscape: The Role of Birds and Perches

Previous research suggests that in highly fragmented forest landscapes ecological succession can be arrested by lack of seeds, but that seed deposition abundance and diversity of bird‐dispersed plants can be enhanced by bird‐attracting structures such as snags. Consequently, bird perches remain a potential tool for accelerating ecological succession and reforesting disturbed land. Consequently, in order to determine the effectiveness of bird perches in reclaiming forested landscapes, seed dispersal, seedbank storage, and recruitment of bird‐dispersed plants was studied on a central Florida mined site with clay‐rich soil undergoing primary succession over a seven‐year period. Data collection included 20 continuous months of seed dispersal data, an analysis of the total and germinable seedbanks, and plant recruitment at one and two years after a fire destroyed perches and burnt vegetation. Seed dispersal to perches reached a peak seedfall by weight in August, which was attributable to nonmigratory birds. Myrica cerifera, the most abundant species dispersed to the sites, was the only species dispersed during the winter and spring months, and it may be a keystone species for the frugivorous bird guild in central Florida. Seedfall beneath perches had a higher diversity of seed genera, and seed numbers (340 seeds m−2 yr−1) were 150 times greater than in sites without perches. Seeds of bird‐dispersed plants in the seedbank under perches numbered 77 ± 33 (m−2) in total and 17 ± 5 for the viable seedbank. The population density of bird‐dispersed plants was 1.4 and 2.0 plants m−2 at one and two years afler the fire. Less than 0.06% of the dispersed seeds survived to become seedlings. Species composition shifted from seedfall to seedlings, with small‐seeded, early‐successional (r‐selected) shrubs and herbs becoming relatively more common than the desired large‐seeded, late‐successional (K‐selected) tree species. Perches attracted birds and associated seeds, but the physically harsh conditions created by primary succession and/or high predation on seeds appeared to reduce the success of the desired late‐successional plant species. Nonetheless, there was a higher abundance and diversity of bird‐dispersed plants under perches, suggesting that perch structures have a limited ability to enhance plant diversity under conditions of primary succession.

DIRECT AND INDIRECT ESTIMATES OF NEIGHBORHOOD AND EFFECTIVE POPULATION SIZE IN A TROPICAL PALM, ASTROCARYUM MEXICANUM.

To estimate the relative importance of genetic drift, the effective population size ∗∗∗(Ne ) can be used. Here we present estimates of the effective population size and related measures in Astrocaryum mexicanum, a tropical palm from Los Tuxtlas rain forest, Veracruz, Mexico. Seed and pollen dispersal were measured. Seeds are primarily dispersed by gravity and secondarily dispersed by small mammals. Mean primary and secondary dispersal distances for seeds were found to be small (0.78 m and 2.35 m, respectively). A. mexicanum is beetle pollinated and pollen movements were measured by different methods: a) using fluorescent dyes, b) as the minimum distance between active female and male inflorescences, and c) using rare allozyme alleles as genetic markers. All three estimates of pollen dispersal were similar, with a mean of approximately 20 m. Using the seed and pollen dispersal data, the genetic neighborhood area (A) was estimated to be 2,551 m2 . To obtain the effective population size, three different overlapping generation methods were used to estimate an effective density with demographic data from six permanent plots. The effective density ranged from 0.040 to 0.351 individuals per m2 . The product of effective density and neighborhood area yields a direct estimate of the neighborhood effective population size (Nb ). Nb ranged from 102 to 895 individuals. Indirect estimates of population size and migration rate (Nm) were obtained using Fst for five different allozymic loci for both adults and seeds. We obtained a range of Nm from 1.2 to 19.7 in adults and a range of Nm from 4.0 to 82.6 for seeds. We discuss possible causes of the smaller indirect estimates of Nm relative to the direct and compare our estimates with values from other plant populations. Gene dispersal distances, neighborhood size, and effective population size in A. mexicanum are relatively high, suggesting that natural selection, rather than genetic drift, may play a dominant role in patterning the genetic variation in this tropical palm.

DISPERSAL IN THE WOOD FROG (RANA SYLVATICA): IMPLICATIONS FOR GENETIC POPULATION STRUCTURE.

Recapture of marked juvenile and adult wood frogs in five Appalachian Mountain ponds showed adults to be 100% faithful to the ponds in which they first bred, but approximately 18% of the juveniles dispersed to breed in ponds other than the one of origin. Effective population sizes were generally smaller than the population censuses and genetic neighborhoods had an average radius of 1,126 meters. Values of standardized genetic variance based on effective population size and mating success were relatively small. Genetic population structure estimated from the dispersal data suggested that ponds within about a 1,000 meter radius should show little genetic differentiation; ponds separated by a distance greater than 1,000 meters should experience little gene flow and show higher genetic differentiation. Wood frogs in these ponds do not show a meta-population structure as suggested for newts.

Pollen analysis for the generation of environmental hypothesis

Abstract Q-mode factor analysis combined with R- and Q-mode cluster analysis were used as hypothesis-generating tools on modern pollen dispersal data for Cerro Domuyo, Neuquén (Argentina). Q-mode cluster analysis partitioned the data matrix into three groups of samples that were consistent with plant geographic units (Subantartic forest, bogs, patagonian and andean xeric communities). Q-mode factor analysis produced three principal components that accounted fot 87% of the total variance and were assigned respectively to temperature, topographic gradient and precipitation. Three particular combinations of these components corresponded to the group of samples generated by Q-mode cluster analysis. When R-mode cluster analysed, each group of samples reflected a peculiar correlation of plant taxa. A model was postulated stating that the ‘causal’ structure of the system is the particular combination of those three physical components of the environmental complex and that the peculiar correlations of plant taxa are surface structures generated by biological ‘responses’ to the gradients of the physical parameters. The procedure is recommended for constructing explanatory modern analogs for Quaternary paleoecological research and as a fast and low cost survey aimed to planification of research work in plant ecology.

Cyclic Drought, Dispersal, and the Conservation of the Snail Kite in Florida: Lessons in Critical Habitat

Abstract:Studies of the dispersal of the Florida Snail Kite, an endangered hawk that inhabits flooded wetlands and feeds almost solely on apple snails, illustrate the problems associated with protecting habitats for migratoty and wandering species with disjunct habitat requirements. Before a drought in 1985, most kites 2oere sighted in the large tracts of Everglades that were used for nesting. During the drought however, 60 percent of the sightings of kites reported to our sighting hotline were from smaller wetlands and other drought‐related habitats, mostly in Palm B e d and surrounding counties Although these small wetlands are important emergency habitats used by kites to suruive during drought, they are not designated as “critical habitat” and are being developed at a rapid rate We discuss the problems of uncertainty, cumulative effects, and compromise in theprocess of evaluating development proposals and protecting small; isolated habitats. We identify key sites used by kites during drought and suggest ways to conserve this species in Florida by mitigating drought‐induced population crashes The usefulness of sighting hotlines for assessing dispersal data is also evaluated.

Modeling Vertebrate Dispersal Distances: Alternatives to the Geometric Distribution

The geometric probability distribution is a common way by which to model frequency distributions of vertebrate dispersal distances. This model has two weaknesses: (1) it requires that the probability of stopping, p, is constant across the range of habitat units, and (2) it provides no mechanism for modeling the maximum dispersal distance. The weaknesses of this model are discussed and a family of alternative models, which allows the stopping probability to change and which takes into account the maximum dispersal distance (dispersal scale) is presented. The new models in their simplest form reduce to the goemetric probability distribution model. The new models are applied to five sets of published field dispersal data to determine the general properties of their fit as compared to the geometric and to assess their potential as better models of dispersal patterns.

Variation in Sex Ratios of Banner-Tailed Kangaroo Rats in Relation to Population Density

Over a 5-year period, sex ratios of juvenile Dipodomys spectabilis varied significantly at one study site, whereas there was a recurrent and significant male bias at a second site. Population density was variable at the first site, but at the second site it was consistently at high levels. The proportion of males was correlated positively with population density. Demographic and dispersal data revealed no variation in competition for local resources or in differential mortality that ultimately could account for the variation in sex ratio. However, male weights relative to female weights at the end of parental care were lower where sex ratios were male-biased, suggesting a correspondence between male-biased sex ratios and relatively cheaper males. The adjustment to male-biased sex ratios apparently occurred in utero or during lactation. A hypothesis that might explain the variation in sex ratio is that it is an adaptive response either to variation in the relative costs of producing sons and daughters or to variation in the life-history expectations of sons and daughters that occurs with variable population density. Another hypothesis is that competition among siblings results in higher mortality in one sex, and this effect is more pronounced at higher population density.

Estimating buffer zone widths for pesticide applications

AbstractA technique for estimating the width of buffer zones required around sensitive areas during pesticide applications has been devised and tested. The technique has been used to estimate the buffer width required around water bodies during ground‐based permethrin applications in Canadian forests to prevent significant impact on fish and their food populations. A worst‐ case scenario was developed for environmental impact in water bodies result ing from ground‐based permethrin applications, and a spray application was made under these worst‐case conditions. Permethrin deposit on ground sheets was measured downwind of overlaid crosswind swaths. From these measurements the deposit at various downwind distances from a single crosswind swath was calculated, and a curvilinear regression line fitted to these values. Permethrin deposit downwind of multiple‐swath applications was computed by adding the contributions from individual swaths. Mortality resulting from various permethrin concentrations was measured for Aedes aegypti larvae. Although these larvae are not an important food species for the fish species of interest, salmon and trout, they are more sensitive to permethrin than most aquatic invertebrates. Predicted mortality in populations of this species and Salmo gairdneri, rainbow trout, at various downwind distances from the permethrin application was calculated from the toxicological and spray‐cloud dispersal data. Buffer width was estimated by choosing an acceptably low mortality, and determining the downwind distance at which this value was obtained. For example, a 20 m swath width was found to be adequate to limit mortality in A. aegypti andS. gairdneri populations to 10 and 0.1% during ground‐based permethrin applications.

Sedimentology of an ancient erg margin: the Lower Jurassic Aztec Sandstone, southern Nevada and southern California

ABSTRACTThe Lower Jurassic Aztec Sandstone is an aeolian‐deposited quartzose sandstone that represents the western margin of the southerly‐migrating Navajo‐Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments.In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue‐shaped aeolian sand bodies interstratified with fluvially‐deposited coarse sandstone and mudstone. Above, evaporite‐rich fine sandstone and mudstone are overlain by thick, cross‐stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west.Thin aeolian sequences are interbedded with volcanic flow rocks, ash‐flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian‐deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc‐derived sediment.A two‐part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross‐strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore‐erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian‐deposited sediment encased in fluvial and sabkha deposits. Continued sand‐sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central‐erg sediments buried the fore‐erg deposits. The trailing, upwind margin of the erg is represented by back‐erg deposits in northern Utah and Wyoming.

Dispersal and gene flow in a butterfly with home range behavior: Heliconius erato (Lepidoptera: Nymphalidae).

Heliconius butterflies have been found to have low rates of dispersal in previous mark-recapture studies, and this lack of movement is due home-range behavior. An experiment on Heliconius erato was designed to investigate movement from the site of pupal eclosion. It was found that most of the movement occurs before the first capture of an individual in a mark-recapture study. After incorporating this early movement, the dispersal parameter, σ, is estimated to be at least 296 m (±30 m jackknifed standard error), and the "neighborhood population size", N, is about 50-150 individuals. These estimates of σ and N are more than 2 and 5 times larger, respectively, than estimates based on standard mark-recapture data, though they are small compared with estimates from other butterfly species. Severe limitations of using dispersal experiments to estimate gene flow and neighborhood size are discussed. Genetic data from color pattern loci in hybrid zones and from electrophoresis suggest that, if anything, the estimates of σ and N that I have obtained are still too low. Genetic and dispersal data together show that kin selection is an unlikely mechanism for the evolution of warning color and other supposed altruisms in Heliconius, unless occasional genetic drift is also involved.

Diffusion Model for Insect Dispersal

The physical model for the self-diffusion of a gaseous material is applied to the analysis of insect dispersal data. Because this model is derived from well-understood mathematical principles, the model provides biological information not available from other models. In particular, through application of the model in analyzing dispersal data, one can obtain an independent estimate of mortality in the field. The model also provides some behavioral data. Examples of the application of the model to data sets from the literature are given.