Inversely density-dependent natal dispersal in brown bears Ursus arctos

There is considerable controversy in the literature about the presence of density dependence in dispersal. In this study, we exploit a data series from a long-term study (>18 years) on radio-marked brown bears (Ursus arctos L.) in two study areas in Scandinavia to investigate how individual-based densities influence the probability of natal dispersal and natal dispersal distances. Cumulatively, 32% and 46% of the females and 81% and 92% of the males dispersed before reaching 5 years of age in the northern and southern study area, respectively. Density had a negative effect on both the probability of dispersal and dispersal distances for the dispersing animals, when controlling for study area, sex and age, making this the first study to show that natal dispersal probability and distances are inversely density dependent in a large carnivore. We suggest that female–female competition for space caused females in higher density areas to settle closer to their natal area. For males, however, merging of demes, resulting in decreased relatedness and increased heterozygosity in an expanding population, might be the reason for shorter dispersal distances in males living at higher densities. This has been hypothesised for small mammals. The high proportion of dispersing female brown bears in Scandinavian compared with North American studies might be due to lower densities in Scandinavia and recent population expansion, with unoccupied areas available at the edges of the population. The longer dispersal distances in female Scandinavian brown bears suggest less social constraints on movements than for North American females. The longer dispersal distances by Scandinavian males may be due to increased searching for potential mates in peripheral areas with lower densities of females. These results, in addition to results of other brown bear studies, suggest that brown bears might be more territorial than previously thought, and that density is regulated by social interactions.     

Marine metapopulations: a useful concept?

We discuss the potential and limitations of the metapopulation concept in marine ecology. The usefulness of the concept in terrestrial ecology is neither based on its simplicity or generality nor on overwhelming empirical evidence. The usefulness is in the questions which are asked when the metapopulation concept is applied. These questions address spatial phenomena and processes on different spatial scales. They help in acknowledging that every population, be it terrestrial or marine, has a spatial organization. Understanding this spatial organization is also important for tackling specific applied problems, i.e. to avoid overexploitation of living marine resources or for configuring marine reserves. The 'openness' of coastal populations, whose larvae enter larval pools or which are holoplanktonic, is no reason for not asking the questions implied by the metapopulation concept. For marine ecology, the real problem is to delineate populations, which then may possibly correspond to the 'local populations' of metapopulations. Thus, the answer to the question in the title of this paper, whether 'marine metapopulation' is a useful concept, is 'yes', if the concept is considered a working hypotheses, if the concept is explicitly defined, and if the questions linked to the concept are clearly stated. Even if it eventually transpires that only very few marine metapopulations actually exist, marine ecology would still have gained some important new insights. Electronic Publication     

Synchronous dynamics and rates of extinction in spatially structured populations

We explore extinction rates using a spatially arranged set of subpopulations obeying Ricker dynamics. The population system is subjected to dispersal of individuals among the subpopulations as well as to local and global disturbances. We observe a tight positive correlation between global extinction rate and the level of synchrony in dynamics among thesubpopulations. Global disturbances and to a lesser extent, migration, are capable of synchronizing the temporal dynamics of the subpopulations over a rather wide span of the population growth rate r. Local noise decreases synchrony, as does increasing distance among the subpopulations. Synchrony also levels off with increasing r: in the chaotic region, subpopulations almost invariably behave asynchronously. We conclude that it is asynchrony that reduces the probability of global extinctions, not chaos as such: chaos is a special case only. The relationship between global extinction rate, synchronous dynamics and population growth rate is robust to changes in dispersal rates and ranges.     

Metapopulation structure in the lagoon cockle <Emphasis Type="Italic">Cerastoderma lamarcki</Emphasis> in the northern Wadden Sea

Benthic invertebrates in discontinuous inshore habitats and with short or no pelagic larval dispersal are likely to exhibit regional metapopulation dynamics with partially isolated local populations. Near the island of Sylt, the bivalve Cerastoderma (Cardium) lamarcki (Reeve, 1844, syn. in part with C. glaucum Bruguiére, 1789) was widespread in intertidal seagrass beds, coexisting with the sibling species C. edule (Linné, 1758). However, the last C. lamarcki in this habitat was found in 1980. At present the lagoon cockle is restricted to disjunct ditches, creeks and ponds within island salt marshes. There it differs in year-class structure between localities. Successful recruitment events did not coincide. At one locality, a period with regular recruitment was followed by 5 years of recruitment failure, resulting in an overaged population probably at the rim of extinction. In a nearby brackish pond, extinction was followed by recolonization 3 years later. Other lagoonal habitats which seem to be suitable are without cockles. It is speculated that small and isolated habitats occasionally receive colonizers by eggs and juveniles adhering to avian vectors. Electronic Publication     

Relative movement patterns of a tephritid fly and its parasitoid wasps

The extent of within-patch dispersal by a tephritid fly and its four major parasitoids was examined over three field seasons. Hosts and parasitoids were marked using acrylic paint and observed as they oviposited into the flowerheads of marsh thistle, Cirsium palustre. The average recapture rate pooled across all species was 22%. The four parasitoids showed consistently greater rates of movement than the host in all three years. In nearly all comparisons, male dispersal was less than female dispersal. There was no evidence that parasitoids moved longer distances after visiting low quality rather than high quality patches. In the one season it was studied, no correlations between movement and insect size were observed. The relevance of these observations to host-parasitoid population dynamics is discussed.     

Temporal diversification and no geographic population structure in the most abundant European ectomycorrhizal fungus Russula ochroleuca

Although Russula ochroleuca is a common ectomycorrhizal (ECM) species in European forests, its reproduction mode, dispersal range, and population structure remains unknown. Altogether, 462 fruit bodies from three spruce forests (two 15 km apart, one 230 km distant from the first two) were analysed using AFLP markers. A mixed mode of reproduction, with several large genets accompanied by a majority of single fruit body genets, was found. Low level of inbreeding (Fis = 0.0021–0.0072), low level of genetic differentiation among subpopulations (PhiPT = 0.037–0.190) and negligible isolation by distance (r = 0.033, P = 0.005) indicates effective spore dispersal and a high level of outcrossing across the sampled area. Finally, we discovered temporal diversification within this species, as evidenced by genetic differentiation between early (August) and late (September) fruiting genotypes.     

The effect of the rate of successful dispersal of a Phytoseiid mite,Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite, Phytoseiulus persimilisAthias-Henriot in response to the density of its prey, Tetranychus kanzawaiKishida . And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows:

1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas.
2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other.
3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day.
4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one.
5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.

     

Patchy patterns due to group dispersal

The presence of multiple foci in population patterns may be due to various processes arising in the population dynamics. Group dispersal, which has been lightly investigated for airborne species, is one of these processes.We built a stochastic model generating the dispersal of groups of particles. This model may be viewed as an extension of classical dispersal models based on parametric kernels. It has a hierarchical structure: at the first stage group centers are drawn under a classical dispersal kernel; at the second stage the particles are diffused around their group centers. Analytic and simulation results show that group dispersal is a sufficient condition to generate patterns with multiple foci, i.e. patchy patterns, even if the population can remain particularly concentrated.     

The effect of the rate of successful dispersal of a phytoseiid mite,Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite, Phytoseiulus persimilisAthias-Henriot in response to the density of its prey, Tetranychus kanzawaiKishida . And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows:

1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas.
2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other.
3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day.
4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one.
5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.

     

Closing the larval loop: linking larval ecology to the population dynamics of marine benthic invertebrates

The majority of marine benthic invertebrates exhibit a complex life cycle that includes separate planktonic larval, and bottom-dwelling juvenile and adult phases. To understand and predict changes in the spatial and temporal distributions, abundances, population growth rate, and population structure of a species with such a complex life cycle, it is necessary to understand the relative importance of the physical, chemical and biological properties and processes that affect individuals within both the planktonic and benthic phases. To accomplish this goal, it is necessary to study both phases within a common, quantitative framework defined in terms of some common currency. This can be done efficiently through construction and evaluation of a population dynamics model that describes the complete life cycle.

Two forms that such a model might assume are reviewed: a stage-based, population matrix model, and a model that specifies discrete stages of the population, on the bottom and in the water column, in terms of simultaneous differential equations that may be solved in both space and time. Terms to be incorporated in each type of model can be formulated to describe the critical properties and processes that can affect populations within each stage of the life cycle. For both types of model it is shown how this might be accomplished using an idealized balanomorph barnacle as an example species. The critical properties and processes that affect the planktonic and benthic phases are reviewed. For larvae, these include benthic adult fecundity and fertilization success, growth and larval stage duration, mortality, larval behavior, dispersal by currents and turbulence, and larval settlement. It is possible to predict or estimate empirically all of the key terms that should be built into the larval and benthic components of the model. Thus, the challenge of formulating and evaluating a full life cycle model is achievable. Development and evaluation of such a model will be challenging because of the diverse processes which must be considered, and because of the disparities in the spatial and temporal scales appropriate to the benthic and planktonic larval phases. In evaluating model predictions it is critical that sampling schemes be matched to the spatial and temporal scales of model resolution.     

Global phylogeography of the dolphinfish (Coryphaena hippurus): the influence of large effective population size and recent dispersal on the divergence of a marine pelagic cosmopolitan species

Pelagic fish that are distributed circumtropically are characterised by a low population structure level as a result of a high capacity for dispersion and large population sizes. Nevertheless, historical and contemporary processes, including past demographic and/or range expansions, secondary contact, dispersal, gene flow, and the achievement of large effective population sizes, may play a part in the detection of divergence signals, especially in the case of tropical pelagic species, whose distribution range depends strongly on the sea surface temperature. The connectivity and historical demography of Atlantic, Indian, Pacific and Mediterranean populations of dolphinfish (Coryphaena hippurus) was studied using partial sequences of the mitochondrial DNA NADH dehydrogenase subunit 1 (ND1). AMOVA analyses revealed significant inter-oceanic divergence with three phylogroups located in the Indo-Pacific, Eastern Atlantic, and Mediterranean Sea, the last one being the most divergent. However, it was not possible to clearly observe any genetic differentiation between the Indo-Pacific and Atlantic populations, as has been reported for most tropical pelagic species of tuna and billfishes. This supports the assumption of recent dispersal among basins facilitated by the actual continuous distribution of dolphinfish populations. Moreover, the lack of a divergence signal for populations separated by the Panamanian Isthmus reveals that genetic drift does not exert a strong influence on tropical pelagic species with large effective population sizes.     

Seed and microsite limitation of recruitment in plant populations

Summary Availability of seed and microsites, respectively, are two factors that potentially may limit recruitment in plant populations. Microsites are small-scale sites suitable for germination and survival of seedlings. We discuss this dichotomy of recruitment limitation both from a theoretical and empirical point of view. Investigations of recruitment in 14 woodland species showed that 3 species were seed limited, 6 species were limited by a combination of seed and microsite availability, and 5 species were found not to be seed limited, but the limiting factor was not identified. A combination of seed and microsite limitation implies that recruitment is promoted by increasing both seed and microsite availability. We suggest that the importance of seed limitation in plant populations has been underestimated, and that the operating limiting factors may be dependent on spatial and temporal scale. We expect that many species, if adequately studied, will turn out to be both seed and microsite limited. Experimental field studies that incorporate a range of seed and microsite densities in various spatial and temporal scales are needed to examine the extent to which plant populations are seed and microsite limited.     

Genetic structure of black abalone (Haliotis cracherodii) populations in the California islands and central California coast: Impacts of larval dispersal and decimation from withering syndrome

The genetic structure of black abalone populations in the southern California Islands and central California coast was investigated by protein electrophoresis. Detailed sampling of San Nicolas Island (SN) permitted investigation of microgeographic genetic differentiation among local geomorphological features. In addition, temporal genetic differentiation was assessed by comparing juveniles and adults at three islands, San Miguel (SM), Santa Cruz (SC) and San Nicolas (SN). Mainland and island locations were genetically differentiated based on allele frequency differences and the presence of private alleles in some island populations. Although microgeographic genetic structure among sites on SN was weak and not statistically significant, heterozygosity varied among sites, with diversity decreasing from west to east. In addition, investigation among cohorts showed that adults were genetically differentiated among island locations, whereas no differences among juveniles were detected. Genetic differentiation between adult and juvenile abalones was detected at SC but not SM or SN. These data are generally consistent with local recruitment augmented by relatively more gene flow among island populations than among island and mainland populations, and possible selection acting on immigrant recruits.     

Seed,dispersal, microsite,habitat and recruitment limitation: identification of terms and concepts in studies of limitations

Recently, there is an increase in number of studies concerned with the effect of various types of limitations on species local population size and distribution pattern at the landscape scale. The terminology used to describe these limitations is, however, very inconsistent. Since the different terms often appear in conclusions of the papers, the inconsistency in their use obscures the message of these papers. In this study, we review the current uses of these terms, identify the basic concepts involved in the discussion of a limitation and link the concepts with the currently used terms. Finally, we discuss the experimental approaches used to assess the different types of limitations. We differentiated four basic concepts resulting from the combination of limitation by environment versus ability to grow and spread, and two spatial scales (local and regional scale). The two concepts at each spatial scale are expected to form a gradient of all possible combinations of the two respective types of limitations. In the considerations of various experimental approaches used to assess these limitations, we conclude that sowing experiments, i.e. seed addition into existing populations or seed introduction into unoccupied habitats, are the only reliable types of evidence for the different types of limitations.     

Isolating the components of activity-density for the carabid beetle Pterostichus melanarius in farmland

The activity and density of the carabid beetle, Pterostichus melanarius, were studied over 10 weeks in a continuous mark-recapture experiment using a grid of pitfall traps spanning a hedgerow and extending approximately 30 m into two cereal fields; 1777 beetles were individually marked. The recapture rate was approximately 60% and 40% for males and females, respectively. Activity-density rose and fell four times between early June and mid August. Jolly-Seber estimates of density showed population density increasing to a single peak in late July. The mean population density in late July and August was relatively stable at approximately 0.26 m⁻². Activity varied over the 10 weeks and was significantly higher during August than in June or July. The daily displacement distance frequencies, calculated from 750 male and 485 female recaptures of individually marked beetles, were distributed exponentially. Mean displacement distances were approximately 2.6 m day⁻¹ during June and July, and 5.3 m day⁻¹ during August. A diffusion model provided diffusion coefficients of 23.7 and 27.9 m² day⁻¹ for female and male beetles, respectively. The spatial distribution of P. melanarius was aggregated in patches. Spatial analysis by distance indices showed the spatial distribution of counts between successive periods of activity-density to be significantly associated. Approximately 5.75% of recaptures were from releases on the opposite side of the hedgerow. Approximately 20% of recaptures were from releases in opposite halves of the grid within the same field. The hedgerow acted as a significant barrier to dispersal between fields, with implications for the metapopulation structure of the species. Received: 26 January 1998 / Accepted: 1 April 1998     

灭鼠干扰后高原鼢鼠的种群动态与扩散

本文采用去除取样法对高原鼢鼠的种群动态及扩散行为进行了研究。结果表明,高原鼢鼠的种群密度因栖息地不同而表现出一定的差异。土壤疏松、食物丰富的栖息地内鼠密度较高。去除前的自然种群内和去除后扩散区内的鼢鼠中,其种群密度基本相同,种群结构相对稳定,成体鼢鼠的数量均占种群的７３％左右,雌雄性比为１１;去除区内鼢鼠的密度较低,雄性幼体的数量相对增加,成体的数量相对减少,其性比亦为１１。相邻动物之间的距离受密度因素的影响,在同一密度条件下,其距离无性别间的差异,动物呈镶嵌分布型。去除区内鼢鼠的平均扩散率为２６．５％,平均扩散距离为６６．９ｍ,扩散距离受土壤因素的影响。去除区内鼢鼠的成体体重小于扩散区内鼢鼠的成体体重,而在幼体间无差异。引起高原鼢鼠扩散的主要原因是种群密度,当种群密度超过平衡密度后,由于种内的相互攻击和对食物资源的相互竞争加剧使部分个体扩散出去     

Local dynamics and dispersal in a structured population of the whirligig beetle Deneutus assimilis

The study illustrates the ecological determinants and evolutionary consequences of dispersal in the pond-living water beetle Dineutus assimilis (Coleoptera: Gyrinidae). Over 2 years, local populatiopn dynamics were studied in 51 ponds within a 60-km² study area. In most of the 31 occupied ponds, and even in large populations, abundances changed dramatically from one year to the next. Nine extinction and nine colonisation events were observed. These temporal patterns show no sign of spatial autocorrelation. Such a habitat distribution should favour high dispersal rates. Indeed, D. assimilis was found to be a very effective coloniser of newly available sites (mean propagule size: 23). A mark-recapture study showed that most dispersal occurred after diapause and over distances ranging from 100 m to at least 20 km. Yet despite frequent movement, the local variability in environmental conditions maintiins a large variance in average reproductive success per pond. Furthermore, immigration rates vary widely within a season. The apparent lack of correlation between these two sources of variation should greatly strengthen the role of drift in this system. A companion paper (Nürnberger and Harrison 1995) documents a non-random distribution of mitochondrial haplotypes due to recent population bottlenecks.     

The influences of habitat, landscape structure and climate on local distribution patterns of the nuthatch (Sitta europaea L.)

The nuthatch, Sitta europaea L., is a small (23 g), cavity-nesting woodland bird which, since the 1970s, has been expanding its range in Britain. However, within this range, the species is notably scarce in an area of eastern England. This gap in the species distribution could arise for several reasons including habitat quality, local landscape structure, regional landscape structure and climate. Field surveys and logistic models of breeding nuthatch presence/absence were used to investigate the relative influences of habitat quality, landscape structure and climate on the prevalence of nuthatches in eastern England. Field surveys of woods in the study area indicated that habitat quality was sufficient to support a nuthatch population. A model of habitat occupancy in relation to local landscape structure, developed in the Netherlands, was applied to the study area. The number of breeding pairs predicted for the study area by the model was lower than expected from habitat area alone, suggesting an additional effect of isolation. However, observed numbers were even lower than those predicted by the model. To evaluate the possible roles of climate and large-scale landscape structure on distribution, presence/absence data of breeding nuthatches at the 10-km grid square scale were related to variables describing climate and the amount and dispersion of broadleaved woodland. While climate in the study area appeared suitable, models including landscape variables suggested that the study area as a whole was unlikely to support nuthatches. Although suitable habitat was available, woodland in the study area appeared to be too isolated from surrounding nuthatch populations for colonisation to be successful. This situation may change if current increases in both national and regional populations continue, thus increasing the number of potential colonists reaching the study area. Received: 3 November 1997 / 22 January 1998     

Invasions with density-dependent ecological parameters

The speed and the minimum carrying capacity needed for a successful population expansion into new territory are addressed using a reaction-diffusion model. The model is able to encapsulate a rich collection of ecological behaviours, including the Allee effect, resource depletion due to consumption, dispersal adaptation due to population pressure, biological control agents, and a range of breeding suppression mechanisms such as embryonic diapause, delayed development and sperm storage. It is shown how many of these phenomena can be characterised as density-dependence in a few fundamental ecological parameters. With the help of a powerful mathematical technique recently developed by Balasuriya and Gottwald (J. Math. Biol. 61, pp. 377-399, 2010), explicit formulae for the effect on the speed and minimum carrying capacity are obtained.     

Friends and strangers: a test of the Charnov-Finerty Hypothesis

Summary We tested the hypothesis that populations composed of unrelated animals should perform worse than those composed of related animals by setting up two moderatedly dense field populations in adjacent enclosures: one was composed of related females and one of unrelated females; both had unrelated males. The survival and reproductive success of a number of litters located by spooling were determined. Final population size, pregnancy success, number of young recruited per pregnancy, and survival were similar in both populations. Thus, differences in relatedness produced no differences in demography. We conclude that the Charnov-Finerty Hypothesis in unlikely to be an explanation for microtine population fluctuations.