Food limitation of a Delaware salt marsh population of the mummichog,Fundulus heteroclitus (L.)

Summary The relationship of the mummichog, Fundulus heteroclitus, to its food supply was examined using 8 m x 20 m enclosures under field conditions. Density effects were examined by stocking these enclosures with fish at densities ranging from 1/8 to four times the natural population density. Mean growth rate in each pen was estimated from measurements on individually marked fish. Fish enclosed at normal density grew at the same rate as fish from the natural population, indicating that caging had minimal effects on growth rate. When enclosed below natural density, mummichogs had a growth rate that was 2–3 times higher than that of the natural population. Fish enclosed at four times normal density had a negative mean growth rate, a higher mortality rate than fish enclosed at natural density, and no egg production.In further experiments, food was added to the same type of enclosure at fish densities ranging from one-half to four times natural density. Food additions had a positive effect on growth rate at all densities. Food additions also caused egg production of fish enclosed at the highest density to increase to a rate equivalent with the natural population. We conclude that the total food supply may regulate the maximum size of the Canary Creek mummichog population by affecting the individual growth rate, mortality rate, and fecundity.     

Evidence for competition between mudsnails (Hydrodiidae): a field experiment

The paper describes an experimental investigation of competition between Hydrobia ulvae and H. ventrosa using enclosed populations at a site at which the species coexist naturally. lntraspecific competition is more intense than interspecific competition and may have a regulatory influence on snail densities. Other experimental studies of competition between mudsnails are reviewed and found to infer strongly the occurrence of competition in natural field populations. However, features of the species' field distributions previously attributed to interspecific competition can be more convincingly explained by other processes.     

Experimental tests of predation and food hypotheses for population cycles of voles

Pronounced population cycles are characteristic of many herbivorous small mammals in northern latitudes. Although delayed density-dependent effects of predation and food shortage are often proposed as factors driving population cycles, firm evidence for causality is rare because sufficiently replicated, large-scale field experiments are lacking. We conducted two experiments on Microtus voles in four large predator-proof enclosures and four unfenced control areas in western Finland. Predator exclusion induced rapid population growth and increased the peak abundance of voles over 20-fold until the enclosed populations crashed during the second winter due to food shortage. Thereafter, voles introduced to enclosures which had suffered heavy grazing increased to higher densities than voles in previously ungrazed control areas which were exposed to predators. We concluded that predation inhibits an increase in vole populations until predation pressure declines, thus maintaining the low phase of the cycle, but also that population cycles in voles are not primarily driven by plant-herbivore interactions.     

Density-dependent regulation of natural and laboratory rotifer populations

Density-dependent regulation of abundance is fundamentally important in the dynamics of most animal populations. Density effects, however, have rarely been quantified in natural populations, so population models typically have a large uncertainty in their predictions. We used models generated from time series analysis to explore the form and strength of density-dependence in several natural rotifer populations. Population growth rate (r) decreased linearly or non-linearly with increased population density, depending on the rotifer species. Density effects in natural populations reduced r to 0 at densities of 1–10 l^–1 for 8 of the 9 rotifer species investigated. The sensitivities of these species to density effects appeared normally distributed, with a mean r=0 density of 2.3 l^–1 and a standard deviation of 1.9. Brachionus rotundiformis was the outlier with 10–100× higher density tolerance. Density effects in laboratory rotifer populations reduced r to 0 at population densities of 10–100 ml^–1, which is 10⁴ higher than densities in natural populations. Density effects in laboratory populations are due to food limitation, autotoxicity or to their combined effects. Experiments with B. rotundiformis demonstrated the absence of autotoxicity at densities as high as 865 ml^–1, a much higher density than observed in natural populations. It is, therefore, likely that food limitation rather than autotoxicity plays a major role in regulating natural rotifer populations.     

ISLAND POPULATIONS OF RODENTS: THEIR ORGANIZATION AND FUNCTIONING

1. Data from 14 confined populations living under natural conditions (mainly on small islands) and from a few laboratory populations have been used. 2. Several differences between a typical, confined rodent population (i.e. living under natural conditions in a well isolated, homogeneous and relatively small area) and an open one have been determined. 3. Island populations are characterized by: (a) Attainment and maintenance of high densities not observed at all, or recorded only periodically, in open populations of comparable species. (b) Stability of population numbers, expressed either by very slight fluctuations or by a very regular cycle with only small differences between peaks of consecutive years. (c) Lack of emigration resulting from any or all of the barriers surrounding the confined population, from homogeneity of the habitat enclosed by these barriers, or from a lower tendency to dispersal of individuals forming such populations. (d) Decreased reproduction. (e) Low losses of independent individuals. (f) Mortality of sucklings changing with the population density. (g) An age structure that is probably more differentiated and more regularly changing in an annual and long-term cycle. (h) Different spatial organization based on smaller home ranges, that are differently arranged in relation to one another from those in open populations; the arrangement of home ranges allows the population to squeeze in a greater number of individuals with possibly the lowest number of interactions between individuals. 4. In confined populations there must be different mechanisms for the regulation of numbers from those in open populations: (I) in the latter the regulation occurs by the outflow of the surplus of independent individuals, i.e. through the dispersal and high mortality of migrants; (2) in confined populations the inflow of independent individuals is regulated by controlled reproduction and early mortality of offspring. Both mechanisms are induced by social pressure but in the second case (regulation of inflow) a much stronger social organization is indispensable Only those species that are capable of the formation of such an organization can survive as confined populations. 5. Knowledge of phenomena occurring in confined populations is useful for predicting the fate of populations of different species which become isolated as a result of human activity in transforming and subdividing the natural environment.     

Zooplankton of the lagoons of the Laccadives: diel patterns and emergence

Zooplankton populations in the water column increased substantiallyat night in four lagoons of the Laccadives. Epibenthic formsemerging into the water column during night, as well as non-epibenthicforms such as calanoid and cyclopoid copepods, were responsiblefor this. Unsealed traps were found subject to contaminationand hence less reliable in estimating densities of emergingfauna compared with sealed traps. Direct sampling of the substratumduring daytime showed that the actual densities of epibenthicforms are much higher than those observed in traps. Densitiesof emerging plankton varied geographically. Most of the zooplanktonorganisms which contribute to the lagoon fauna are residentin the lagoons and rarely occurred in the surrounding sea     

Street fighters: Bite force,injury rates,and density of urban Australian water dragons (Intellagama lesueurii)

In an increasingly urbanized world it is imperative that we understand how wildlife responds to this novel anthropogenic landscape, both at the individual‐ and population‐level. Urbanization generally reduces biodiversity, but can also favour particular species and increase their abundance relative to wild populations. When population density increases, so too does the frequency and cost of social interactions. We studied Australian water dragons (Intellagama lesueurii), a species common in urban, semi‐natural and natural areas, to firstly test the prediction that urban populations occur at higher densities, and then determine the consequences of urbanization for combat rates (quantified using wounding) and bite force. We established that urban populations are denser than ones from semi‐natural and natural habitats. We also recorded significantly more wounds in females from urban populations than females from both natural and semi‐natural populations. Urban males also had significantly higher incidence of wounding than males from natural populations. We did not find a difference in male or female bite force among any populations across the urban‐natural gradient. Overall, we found evidence that urbanization results in a higher population density of water dragons and more frequent conspecific combat, but this was not associated with an increase in bite force. These finding suggests that there may be a physiological cost to living in urban habitats related to increased contest rates and wounding.     

小慈姑的开花状态、传粉机制与交配系统

在自然种群中对沼生植物小慈姑（Sagittaria potamogetifolia Merr.）的开花状态和传粉过程作了观测,并用同工酶遗传标记法定量估测了其3个种群中5个样本的异交率。该种为中心媒传粉为主的虫／风 媒兼性传粉机制,访花昆虫的飞行距离多在0-2m范围内,并与花序密度呈不显著的正相关。异交率（－／^／t）为50.0％－92.8％,表明该种为异交为主的混合交配系统（ixed mating system）。对雌雄同株的小慈姑而言,异交率与各种群花期植株密度无关,异交水平取决于种群中开花花序的两个或两个以上的植株所占比例。     

Contrasting effects of natural habitat loss on generalist and specialist aphid natural enemies

The greater susceptibility of higher trophic levels to habitat loss has been demonstrated to disrupt important trophic interactions such as consumer control of prey populations. This pattern is predicted to break down for generalist species that can use matrix habitats, yet empirical studies comparing generalist and specialist enemy pressure in response to natural habitat loss are lacking. Here we examined the effects of landscape simplification resulting from habitat conversion to agriculture on nettles, Urtica dioica , their specialized aphid herbivore, Microlophium carnosum , and associated natural enemies that varied broadly in their degree of specialization. Both nettles and their specialized aphid herbivore were significantly more abundant in complex than simple landscapes. Different enemy groups showed contrasting responses. Aphid specialists (parasitic wasps and cecidomyiid midges) reached higher densities in complex than simple landscapes, and this effect was primarily related to shifts in local resource abundance (i.e. nettle aphid densities). In contrast, densities of generalists (coccinellid beetles and spiders) were significantly higher in simple landscapes, presumably due to spillover of generalists from surrounding cropland habitats. Natural enemy-prey ratios did not differ significantly across landscape types for specialist groups but were significantly higher in simple than complex landscapes for generalist groups, suggesting that enemy pressure on nettle aphids likely increases with landscape simplification. This was supported by our finding that aphid population growth rates were lower in simple than complex landscapes, and declined significantly with increasing coccinellid densities. Thus, in marked contrast to previous work, our results suggest that natural habitat loss may augment rather than disrupt consumer–prey interactions, and this will depend greatly on the degree of specialization of functionally dominant natural enemies.     

The contribution of white clover to a mixed upland sward

Summary The density and effectiveness ofRhizobium trifolii in six experimental areas were investigated. The densities of the nodule organism ranged from 100 cells per g soil at pH 3.5 to 1.8×10⁵ at pH 5.7. There was a high correlation between density and soil pH. In the open mountain areas the densities and overall levels of symbiotic effectiveness were lower than in the enclosed areas at a lower elevation. The results are given of the densities and effectiveness of nodule isolates on the plots of an inoculation trial two years after sowing. The effect of the inoculation and the liming treatments were still apparent. These results of Rhizobium strain distribution are discussed in relation to its possible effect on the contribution of white clover to a sown sward.     

Living in the city: urban environments shape the evolution of a native annual plant

Urban environments are warmer, have higher levels of atmospheric CO₂ and have altered patterns of disturbance and precipitation than nearby rural areas. These differences can be important for plant growth and are likely to create distinct selective environments. We planted a common garden experiment with seeds collected from natural populations of the native annual plant Lepidium virginicum, growing in five urban and nearby rural areas in the northern United States to determine whether and how urban populations differ from those from surrounding rural areas. When grown in a common environment, plants grown from seeds collected from urban areas bolted sooner, grew larger, had fewer leaves, had an extended time between bolting and flowering, and produced more seeds than plants grown from seeds collected from rural areas. Interestingly, the rural populations exhibited larger phenotypic differences from one another than urban populations. Surprisingly, genomic data revealed that the majority of individuals in each of the urban populations were more closely related to individuals from other urban populations than they were to geographically proximate rural areas – the one exception being urban and rural populations from New York which were nearly identical. Taken together, our results suggest that selection in urban environments favors different traits than selection in rural environments and that these differences can drive adaptation and shape population structure.     

围栏条件下母体密度应激对根田鼠 F1代性器官指数的影响

为探讨自然条件下母体密度应激对根田鼠F1代性器官指数的影响,通过在围栏内建立不同密度的母体种群获得相应的F1代个体,以此建立了不同密度的子代种群,并测定了母体种群建群者的血浆皮质酮含量,将建立子代种群后剩余的F1代个体带回实验室饲养.实验结束后,测定了全部F1代个体的性器官指数及皮质酮含量.结果表明,高密度母体种群建群者的皮质酮含量显著高于低密度母体种群建群者;出生于高密度母体种群的F1代个体处于高密度子代种群,其性器官指数显著低于出生于低密度母体种群且处于低密度子代种群的F1代个体,而皮质酮含量显著高于后者;出生于高密度母体种群的F1代个体处于低密度子代种群,其性器官指数及皮质酮含量与出生于低密度母体种群且处于低密度子代种群的F1代个体间无显著差异.此外,实验室饲养条件下,出生于高密度和低密度母体种群的F1代个体间的睾丸指数无显著差异.本研究结果说明,在根田鼠种群中,单一的母体密度应激对子代的性器官指数无影响,但生前应激子代在性成熟后,当再次遭遇密度应激时,其性器官指数显著降低,母体密度应激和当前环境对种群的未来繁殖能力具有耦合效应.     

Colonization of vineyards by Kampimodromus aberrans (Oudemans) (Acari: Phytoseiidae): dispersal from surrounding plants as indicated by random amplified polymorphism DNA typing

Abstract

• 1 Kampimodromus aberrans (Oudemans) is the most important predatory mite found in vineyards of southern France. This mite also occurs in surrounding uncultivated areas from where it disperses to colonize adjacent vineyards
• 2 To determine accurately origins of immigrant mites and to study their establishment in vineyards, a study using RAPD markers (Random Amplified Polymorphism DNA) was performed. Females of K. aberrans were sampled on leaves collected in an experimental vineyard, and from several adjacent areas including neighbouring vine plots and natural plants, both of which harboured high densities of the mite. Samples were taken in May and July before and after major dispersal of K. aberrans into the experimental plot occurred.
• 3 For both dates, genetic distances within population were lower than between populations and three groupings of mites were observed. Strong relationships were observed between (1) females from different parts of the same experimental vineyard (variety Cabernet‐Sauvignon), (2) females from several plants in the woody margin neighbouring this experimental plot, and (3) females from two neighbouring vineyards (variety Carignan). Populations seemed to be structured and no correlation between genetic and geographical distances was observed. Hence, definitive conclusions about origins of migrants were not possible.
• 4 Once in a vineyard, mites are probably exposed to selection pressures (i.e. pesticide applications or vine variety characteristics) that largely determine differentiation of populations. Thus, despite many immigrants moving into vineyards, our study indicated that there was limited survival or reproduction of immigrants. Further studies of within vineyard selection factors and impacts on immigrant mites are needed to determine the influence of natural colonization on grape pest management.

     

Genetic structure of the Danish red deer (Cervus elaphus)

The red deer (Cervus elaphus) population in Denmark became almost extinct in recent historical times due to over‐hunting. The species has subsequently recovered within remote areas, but non‐Danish individuals have been introduced at several localities. To assess genetic structure, past demographic history, and the possibility of a still existing original stock, we analysed 349 specimens from 11 geographically separate areas and from three enclosed areas, genotyping 11 microsatellite loci. Moreover, an 826‐bp fragment of the control region of the mitochondrial DNA was sequenced for 116 recent specimens and seven museum specimens. There was a significant difference in mean expected heterozygosity (H_E) between the three enclosed areas and the 11 unenclosed areas. Significant departures from Hardy–Weinberg equilibrium were observed in the three enclosed areas and in nine of the unenclosed areas. The overall degree of genetic differentiation among all 14 areas was significant (F_ST = 0.09, P < 0.01), primarily because the mean pairwise F_ST for the three enclosed areas was significantly higher than that for the 11 unenclosed areas. A Bayesian clustering procedure detected three genetically distinct populations and indicated reduced gene flow between the enclosed and unenclosed areas. The individuals in the unenclosed areas show genotypic mixture, presumably as a result of gene flow among them. Markov Chain Monte Carlo simulations, based on the genealogical history of the microsatellite alleles, suggest a drastic decline in the effective population size of the enclosed areas some 188–474 years ago. Mitochondrial DNA analysis of the recent specimens showed seven haplotypes. Individuals from the enclosed Jægersborg Dyrehave contain haplotypes that occur all over Denmark and also are found in Western Europe. A close relationship between Scandinavian and Western European red deer is most likely. Only individuals from the unenclosed Lindenborg Estate and the enclosed Tofte Skov did not group with any other Danish individuals. As six of seven museum specimens had haplotypes also found in modern Danish samples, the current population of red deer in Denmark is genetically close to the original Danish red deer. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 688–701.     

Less is more: density influences the development of behavioural life skills in trout

Theory suggests that habitat structure and population density profoundly influence the phenotypic development of animals. Here, we predicted that reduced rearing density and increased structural complexity promote food search ability, anti-predator response and the ability to forage on novel prey, all behavioural skills important for surviving in the wild. Brown trout were reared at three densities (conventional hatchery density, a fourth of conventional hatchery density and natural density) in tanks with or without structure. Treatment effects on behaviour were studied on trout fry and parr, whereupon 20 trout from each of the six treatment groups were released in an enclosed natural stream and recaptured after 36 days. Fry reared at natural density were faster to find prey in a maze. Moreover, parr reared at natural density were faster to eat novel prey, and showed more efficient anti-predator behaviour than fish reared at higher densities. Furthermore, parr reared at reduced densities were twice as likely to survive in the stream as trout reared at high density. In contrast, we found no clear treatment effects of structure. These novel results suggest that reduced rearing densities can facilitate the development of behavioural life skills in captive animals, thereby increasing their contribution to natural production.     

Ecologically heterogeneous populations of the invasive ant Wasmannia auropunctata within its native and introduced ranges

Abstract 1. The biology of most invasive species in their native geographical areas remains largely unknown. Such studies are, however, crucial in shedding light on the ecological and evolutionary processes underlying biological invasions. 2. The present study focuses on the little fire ant Wasmannia auropunctata, a species native to Central and South America that has been widely introduced and which has become invasive throughout the tropics. We characterise and compare several ecological traits of native populations in French Guiana with those in one of its introduced ranges, New Caledonia. 3. We found ecologically heterogeneous populations of W. auropunctata coexisting in the species’ native geographical area. First, we found populations restricted to naturally perturbed areas (particularly floodplains) within the primary forest, and absent from the surrounding forest areas. These populations were characterised by low nest and worker densities. Second, we found dominant populations in recent anthropogenic areas (e.g. secondary forest or forest edge along road) characterised by high nest and worker densities, and associated with low ant species richness. The local dominance of W. auropunctata in such areas can be due to the displacement of other species (cause) or the filling‐up of empty habitats unsuitable to other ants (effect). With respect to their demographic features and ant species richness, the populations of native anthropogenic habitats were to a large extent similar to the invasive populations introduced into remote areas. 4. The results point to the need for greater research efforts to better understand the ecological and demographic features of invasive species within their native ranges.     

Survival and fidelity of an enclosed white-tailed deer population using capture–recapture-reporting data

The number of animal populations enclosed by impermeable fences has increased, which poses issues related to the behavior of individuals and populations. Despite the increased number of fenced enclosures, there is a paucity of survival and fidelity data on white-tailed deer (Odocoileus virginianus) from fenced enclosures. Therefore, we examined marked deer recaptures and resightings over 13 years for an enclosed population of white-tailed deer in Oklahoma, USA, to estimate survival and fidelity parameters. We found that a step model was the best model of survival for both sexes. Survival of females and males was greater after hunting was suspended. Average female survival was 77% before hunting was suspended but increased to 98% after. Male survival was also greater after (99%) hunting was suspended compared to before (58%). Females exhibited greater site fidelity (84–94%) than males for all age groups except old individuals, which showed similar site fidelities for males and females. Fidelity was highest for old males (85%), followed by adult (74%), fawn (61%), and yearling males (56%). Our high-tensile electric fence allowed management goals to be achieved through increased survival while potentially maintaining genetic diversity through the exchange of limited numbers of individuals from surrounding areas. However, if the goal is to confine deer to limit disease spread or protect sensitive areas (e.g., airports) from deer encroachment, then other fence designs may be necessary, because our fence was not completely effective at controlling deer movements for these purposes.     

Directed seed dispersal towards areas with low conspecific tree density by a scatter‐hoarding rodent

Scatter‐hoarding animals spread out cached seeds to reduce density‐dependent theft of their food reserves. This behaviour could lead to directed dispersal into areas with lower densities of conspecific trees, where seed and seedling survival are higher, and could profoundly affect the spatial structure of plant communities. We tested this hypothesis with Central American agoutis and Astrocaryum standleyanum palm seeds on Barro Colorado Island, Panama. We radio‐tracked seeds as they were cached and re‐cached by agoutis, calculated the density of adult Astrocaryum trees surrounding each cache, and tested whether the observed number of trees around seed caches declined more than expected under random dispersal. Seedling establishment success was negatively dependent on seed density, and agoutis carried seeds towards locations with lower conspecific tree densities, thus facilitating the escape of seeds from natural enemies. This behaviour may be a widespread mechanism leading to highly effective seed dispersal by scatter‐hoarding animals.     

The population and evolutionary dynamics of Vibrio cholerae and its bacteriophage: conditions for maintaining phage-limited communities

Although bacteriophage have been reported to be the most abundant organisms on earth, little is known about their contribution to the ecology of natural communities of their host bacteria. Most importantly, what role do these viral parasitoids play in regulating the densities of bacterial populations? To address this question, we use experimental communities of Vibrio cholerae and its phage in continuous culture, and we use mathematical models to explore the population dynamic and evolutionary conditions under which phage, rather than resources, will limit the densities of these bacteria. The results of our experiments indicate that single species of bacterial viruses cannot maintain the density of V. cholerae populations at levels much lower than that anticipated on the basis of resources alone. On the other hand, as few as two species of phage can maintain these bacteria at densities more than two orders of magnitude lower than the densities of the corresponding phage-free controls for extensive periods. Using mathematical models and short-term experiments, we explore the population dynamic processes responsible for these results. We discuss the implications of this experimental and theoretical study for the population and evolutionary dynamics of natural populations of bacteria and phage.     

Recruitment variability of resident brown trout in peripheral populations from southern Europe

1. Population regulation was studied for seven consecutive years (1992–98) in five rivers at the periphery of the distribution of Salmo trutta, where the fish were living under environmental constraints quite different from those of the main distribution area. 2. Recruitment is naturally highly variable and the populations had been earlier classified as overexploited. Thus we expected that densities of young trout in most populations would be too low for density‐dependent mortality to operate. We tested this by fitting the abundance of recruits to egg densities over seven consecutive years (stock–recruitment relationship), and used the results to judge whether exploitation should be restricted in the interests of conserving the populations. 3. The density of 0+ trout in early September, as well as the initial density of eggs and parents, varied greatly among localities and years. The data for all populations fitted the Ricker stock–recruitment model. The proportion of variance explained by the population curves varied between 32% and 51%. However, in most cases the observations were in the density‐independent part of the stock–recruitment curve, where densities of the recruits increased proportionally with egg densities. 4. Our findings suggest that recruitment densities in most rivers and years were below the carrying capacity of the habitats. Although density‐dependent mechanisms seemed to regulate fish abundance in some cases, environmental factors and harvesting appeared generally to preclude populations from reaching densities high enough for negative feedbacks to operate. The findings thus lend support to Haldane’s (1956) second hypothesis that changes in population density are primarily due to density‐independent factors in unfavourable areas and areas with low density due to exploitation. Exploitation should be reduced to allow natural selection to operate more effectively.