An Origin for Some Natural Conventions

There are several published hypotheses that consider spacing behavior to be a significant factor causing the multiannual density fluctuations characteristic of some microtine rodent populations. Recent modeling efforts have concluded, however, that spacing behavior should have a stabilizing rather than a destabilizing effect on population dynamics. Why doesn't spacing behavior stabilize these cyclic populations? We argue that while spacing behavior does have a stabilizing influence on population dynamics by limiting the number of breeding individuals, reproduction continues and population size is not limited in an asymptotic manner. Rather, microtine social organization produces demographic changes within a population that allow density cycles to occur under certain conditions. Using a simulation model, we demonstrate that in a strongly seasonal environment populations with low density dependence in reproduction will cycle whereas populations with high density dependence in reproduction will have relatively stable densities. Given such complicating factors as the "annual species" nature of microtine rodents, occasionally intense predation, and the tendency for territoriality to break down during the non-breeding season, individuals with low density dependence in reproduction will always be able to invade and eventually dominate populations with high density dependence in reproduction, regardless of the resulting destabilization of population dynamics.

Replenishment of many marine populations occurs through the entry of juveniles to adult populations following a pelagic larval stage. Because mortality during the pelagic stage is thought to be high and density independent, larval abundance and traits of individual larvae should have strong effects on overall population dynamics in marine organisms. Surprisingly, few experiments have tested how localized interactions among breeding adults affect the quantity and phenotypic traits of larvae they produce. Here I experimentally test for the influence of food competition, mate limitation, and population density on somatic growth, fecundity, and offspring provisioning (larval length and energy reserves) in a planktivorous, territorial coral reef damselfish, Stegastes partitus. I manipulated food supply and adult S. partitus density on isolated patch reefs in the Bahamas and also made behavioral observations of S. partitus occurring on nearby natural reefs at a range of population densities. On the experimental reefs, females experienced density-dependent growth and fecundity; male reproductive success was density dependent, but male growth was not. Density-dependent growth and reproduction were not moderated by food supplementation, and density-dependent reproduction was not influenced by mate availability. On natural reefs, the frequency of aggressive interactions, particularly involving females, increased with population density, implicating aggression-related energetic costs as the source of both forms of density dependence in the experiments. Food supplementation increased female somatic growth and larval energy reserves, suggesting that females allocated surplus energy to future reproductive potential and enhanced offspring quality. Neither experimental treatment affected larval length. By altering patterns of reproduction, the interplay between spatial variation in food availability and population density may drive population dynamics in a broad range of benthic marine organisms.

We report on a series of experiments performed on a population of free—living eastern chipmunks, Tamias striatus, inhabiting a forest in northwestern Pennsylvania. The experiments were designed to examine, via pertubations of food supply and/or population density, the relationship between home range size, food availability, and population density. When food levels were increased within an area, a simultaneous effect was noted: mean home range size decreased and population density increased through recruitment from neighboring habitats. Whether the cause of the decreased patterns of movement was the increased food supply or the elevated population density was determined in subsequent experiments. When population density was held constant and food supplies were experimentally increased, there was a significant reduction in mean home range sizes when compared to the normal (control) situation. When food supplies were undistributed, put population density was greatly reduced, there was no change in the mean home range size over that determined at higher mean home range size, whereas population density, at least at the levels we examined experimentally, has no effect on movement patterns. These results are in accord with those theories relating movement patterns to resource abundance, but are not in accord with hypotheses suggesting that the home range sized and population density and inversely associated.

This paper reports the effects of food supply, predation and the interaction between them on the population dynamics of root voles, Microtus oeconomus , by adopting factorial experiments in field enclosures. This two‐factor experiment proved the general hypothesis that food supply and predation had independent and additive effects on population dynamics of root voles. The experimental results proved the following predictions: (1) predation reduced population density and recruitment significantly; (2) food supply increased population density; (3) predation and food supply influenced spacing behavior of root voles separately and additively: Exposure to predation reduced long movements of root voles between trapping sessions; additional food supply reduced aggression level and home range size of root voles. Less movement of individuals that exposed to predators possibly reduced their opportunity of obtaining food and lessened population survival rate, which led population density to decrease. Smaller home range and lower aggression level could make higher population density tolerable. The interactive effect of predation and food on home range size was highly significant (P=0.0082<0.01). The interactive effect of food and predation on dispersal rate was significant (P<0.01). From the experimental results, we conclude that the external factors (predation, food supply) were more effective than internal factors (spacing behavior) in determining population density of root voles – under the most favorable external conditions (−P, +F treatment), the mean density and mean recruitment of root vole population was the highest; under the most unfavorable external conditions (+P, −F treatment), the mean density and mean recruitment of root vole population was the lowest.

Temporal variation in survival, fecundity, and dispersal rates is associated with density-dependent and density-independent processes. Stable natural populations are expected to be regulated by density-dependent factors. However, detecting this by investigating natural variation in density is difficult because density-dependent and independent factors affecting population dynamics may covary. Therefore, experiments are needed to assess the density dependence of demographic rates. In this study, we investigate the effect of density on demographic rates of the Seychelles Warbler (Acrocephalus sechellensis). This species, endemic to a few islands in the Indian Ocean, went through a severe population bottleneck in the middle of the last century, with only approximately 30 individuals left on one small island, but has since recovered. Our monitoring shows that since reaching the island's carrying capacity, population density has remained stable. However, we detected neither density-dependent reproduction nor survival on the basis of natural density variation during this stable period. For conservation reasons, new populations have been established by transferring birds to nearby suitable islands. Using the change of numbers during the process of saturation as a natural experiment, we investigated whether we can detect regulation of numbers via density-dependent survival and reproduction within these new populations. We found that populations were mainly regulated by density-dependent reproduction, and not survival. Variation in density between islands can be explained by food abundance, measured as insect density. Islands with the highest insect densities also had the highest bird densities and the largest breeding groups. Consequently, we suggest that the density-dependent effect on reproduction is caused by competition for food.

The term “furbearer” is loosely applied to mammals that are, or have been, harvested primarily for their pelts. Furbearers typically include species ranging from 1–20 kg mass in the orders Marsupialia, Rodentia, and Carnivora. Studies of demography and regulation of populations have important implications to furbearer management and conservation, as well as to population ecology generally. Reproductive potential varies considerably among species, although most species attain sexual maturity within 1 year and have high fertility rates. There is solid evidence for inverse density-dependence in fertility and/or recruitment in muskrats. In longer-lived omnivores and carnivores, variation in reproduction is most often related to changes in pregnancy rates, and/or survival of young. Generalization about density-dependent reproduction among these groups is not warranted from the evidence. For many North American furbearers, the largest fraction of mortality is human-related, primarily from legal harvest, but also vehicle collisions and accidents in some areas. Shorter-lived species respond to increased harvest mortality in a compensatory fashion. However there is evidence that increased harvest may be more additive than compensatory among longer-lived carnivores. Disease may periodically affect annual losses, but evidence that harvest reduces disease outbreaks is equivocal. Dispersal is a prominent behavior among most species and, along with other social behaviors, may influence population regulation. Density-dependent effects in reproduction, mortality, and dispersal often limit effectiveness of controlling population levels of many furbearers. Where lightly-harvested populations are dependent on a single prey species, food supply may regulate carnivore populations — a situation especially apparent in northern landscapes. Simulation has been used effectively to study disease, exploitation, and other aspects of population dynamics. We need a quantitative, theoretical framework to more completely understand how competing sources of mortality interact, particularly the effects of time-lagged responses. Managers need efficient methods to assess population density and rates of change. Population genetics of species of special concern are poorly known and potentially significant to conservation efforts. Although we recognize that habitat heterogeneity and fragmentation influence population dynamics, we are only beginning to quantify the consequences at the metapopulation level. There is need for manipulative, replicated, long-term field experiments to study all aspects of population dynamics.

1. Small population dynamics depend importantly on the strength and shape of density dependence. Unfortunately, the lack of reliable life-history data often prevents to make accurate demographic predictions for populations regulated by density dependence. 2. We created a gradient from low to high densities in small experimental populations of common lizards (Zootoca vivipara) and investigated the shape and strength of the density dependence of life-history traits during a yearly cycle. We then analysed stochastic population dynamics using one-sex and two-sex age-structured matrix models. 3. Body growth and reproductive performances decreased with density, yearling and adult survival and body size at birth were density-independent, and juvenile survival increased with density. The density dependence of reproduction was partly explained by positive effects of body size on age at first reproduction and clutch size. 4. Parturition date decreased with density in sparse populations and then increased, providing one of the first empirical evidence of a component Allee effect in the phenology of reproduction. 5. Population growth rate (λ) was most affected by variations in juvenile and yearling survival. However, density at equilibrium was most affected by juvenile access to reproduction and yearling clutch size. 6. Stochastic simulations revealed that negative density dependence buffers the effects of initial density on extinction probability, has positive effects on the persistence of sparse populations and interacts with sex ratio fluctuations to shape extinction dynamics. 7. This study demonstrates that negative density dependence modifies the dynamics of small populations and should be investigated together with Allee effects to predict extinction risks.

Effects of population density of mule deer Odocoileus hemionus on forage selection were investigated by comparing diet characteristics of two subpopulations of deer in southern California, USA, that differed in population density during winter. Quality of diet for deer, as indexed by faecal crude protein, was higher at the low‐density site than at the high‐density site in winter, when deer densities were different. Quality of diet was similar in summer when both areas had comparable densities of deer. Both outcomes are consistent with predictions from density‐dependent selection of diets by deer. Dietary niche breadth, however, differed in a manner opposite to predictions of niche theory based on diet selection under an ideal‐free distribution. During winter, when differences in density between the two study sites were pronounced, niche breadth along the dietary axis in the low‐density area was twice that of the high‐density site. Generalist herbivores feeding primarily on low‐quality browse at high population density in winter would be expected to increase their dietary breadth by feeding on additional species of plants as they depleted their food supply. Mule deer in our study, however, decreased the breadth of their dietary niche as population density increased. We hypothesize that by rapidly eliminating high‐quality forages from an area by heavy grazing, deer at higher population densities narrowed their dietary niche. Theoretical models for changes in niche dimensions, including the ideal‐free distribution, need to consider such empirical outcomes.

This research analyzes the influence of bioenergy use on food supply in the 28 European Union (EU) countries under the governance quality system in 1990–2018. We used the pooled ordinary least squares technique and found that bioenergy growth (under the quality governance system), fossil fuel, and gross domestic product appears to increase food supply more in 13 European Union (EU13) underdeveloped countries. On the other hands, governance quality and population density have greatly affected food supply in 15 European Union developed countries (EU15) compared with EU13 European countries. These findings imply that the number of bioenergy resources which could be used directly in agricultural food chain end-use sectors will effectively increase food supply and thus contribute to the global warming objective. The results were verified by the pooled least squares (Pooled OLS) and fixed effect methods. The study recommends that the Member States of the European Union increase bioenergy in the energy mix to improve food supply, as systematic output does not compete with food-producing resources. The authorities should illustrate organized food supply and bioenergy policy by developing alternative strategies for reducing fossil fuel power and related CO2 emissions, according to the unique characteristics of both developed and developing countries in the EU.

Density dependence in reproduction plays an important role in stabilizing population dynamics via immediate negative feedback from population density to reproductive output. Although previous studies have shown that negative density‐dependent reproduction is associated with strong spacing behavior and social interaction between individuals, the proximal mechanism for generating negative density‐dependent reproduction remains unclear. In this study, we investigated the effects of density‐induced stress on reproduction in root voles. Enclosed founder populations were established by introducing 6 (low density) and 30 (high density) adults per sex into per enclosure (four enclosures per density in total) during the breeding season from April to July 2012 and from May to August 2015. Fecal corticosterone metabolite (FCM) levels, reproductive traits (recruitment rate and the proportion of reproductively active individuals), and founder population numbers were measured following repeated live trapping in both years. The number of founders was negatively associated with recruitment rates and the proportion of reproductively active individuals, displaying a negative density‐dependent reproduction. FCM level was positively associated with the number of founders. The number of founder females directly affected the proportion of reproductive females, and directly and indirectly through their FCM levels affected the recruitment rate; the effect of the number of male founders on the proportion of reproductive males was mediated by their FCM level. Our results showed that density‐induced stress negatively affected reproductive traits and that density‐induced stress is one ecological factor generating negative density‐dependent reproduction.

Context Feral pigs (Sus scrofa) are highly fecund, and populations can increase rapidly under favourable conditions. Population size can also fluctuate widely, driven largely by changes in juvenile mortality in response to food availability, but these relationships have only been explored on a limited number of sites and over short periods. Aims The present study aimed to investigate and quantify the numerical response of feral pig populations to changes in their food supply in north-eastern Australia. Methods Pig population densities were determined from aerial surveys conducted over a 21-year period on 10 regional blocks (~2000–6000 km2) throughout the Queensland rangelands. Densities were used to calculate annual exponential rates of increase (r), which were then corrected for anthropogenic mortality (baiting and commercial harvesting). Six proxy measures of annual food supply, including rainfall, pasture biomass and pasture growth (using the AussieGRASS model), were calculated for each survey block, and assessed as predictors of corrected r. The rates of increase predicted from the first half of the data series were then applied to initial population densities to estimate successive pig densities during the second period in each bioregion. Key results The most parsimonious model of the numerical response had parameters common to three bioregions, with rainfall in the 12 months between surveys being the best predictor variable. Modelled densities for each bioregion were a good fit to actual, observed densities. Relationships between r and each measure of food supply at the individual block level were inconsistent. Conclusions Using rainfall as a measure of food supply, the numerical response relationship provides a method for predicting the dynamics of feral pig populations at the bioregional scale. Predicting population dynamics at any one site using this relationship is less precise, suggesting that differences in landscape composition affect utilisation of resources supporting population growth. Implications The results from the present study could be used to predict feral pig population changes at the bioregional level, supplementing or reducing the need for more frequent, expensive population surveys. This improved ability to predict fluctuations in regional feral pig populations can help guide future management actions.

1. Male and female European robins, Erithacus rubecula, defended separate territories from August to January. Pairs formed between January and March. 2. The benefits of early pairing were twofold: first, as 20% of males failed to pair there was strong competition for females; second, although pairing was initially costly in terms of individual foraging success, bachelors invested more time advertising in the long term. 3. Birds on individual territories encountered large food items more frequently than those on pair territories. This decrease in foraging success within pairs was identified as a cost of territory sharing. 4. To test the hypothesis that food availability underlies the defence of individual territories in winter, and the timing of switches to pair territoriality, I manipulated food supply and recorded individual behaviour. 5. Compared to controls, males provided with extra food were forced to repel intruders more frequently, yet advertised for mates earlier and paired earlier. 6. Pair members whose supplementary food supply was temporarily withdrawn spent less time consorting than controls. This suggests that an elevated food supply helped individuals to afford the costs of sharing their food resources, and thus to pair and lay clutches early. 7. The influence of food supply and territoriality on population density and social behaviour is discussed.

The distribution and density of every species of animal is at least loosely controlled by the distribution and abundance of its food. Particular attention has been paid recently to the distribution of salmonids as affected by selection of the site of the fish's feeding territory (Chapman & Bjornn 1969) and to their numbers as affected by varying abundance of food in different streams (Egglishaw 1967). Evidence that food supply does more than permit or prevent a population of salmonids from inhabiting a stream is as yet equivocal. Mason & Chapman (1965) found that the biomass and numbers ofjuvenile coho salmon (Oncorhynchus kisutch Walbaum) remaining in two stream channels from which they could emigrate were greater in the channel which had a greater food supply. However, since there was no replicate in which the varying rations were switched between channels, the possibility remained that the channel with the greater food supply retained more fish because of a greater number of hiding places, better cover, etc. In comparing two streams, one of which had three times as much food as the other, Egglishaw (1967) found that there were fewer trout in his food-poor stream, but more salmon, so that the total density of salmonids differed little (about 0 45 fish/m2 in the poor compared to 0 55 fish/m2 in the rich stream). Biomass differed even less. The streams were different in other respects besides food, however, which, in this case, may have masked the effect of food supply on density. Le Cren (1965) suggests that while a population of fish may be limited by food in the long run, in some instances other regulatory mechanisms usually reduce the importance of food supply as a regulatory factor. Effects offood supply on behaviour, in contrast to those on population density, have been reasonably well defined. In laboratory experiments with young salmon (Salmo salar L.), provision of food in two daily feedings was followed shortly afterwards by an increase in aggression which subsided gradually 30 min later (Keenleyside & Yamamoto 1962). Symons (1968) found when aggression was measured after immediate effects of feeding had waned, that fish deprived of food for 18-66 h were more aggressive than those receiving an abundance of food. Together these results indicate that, in those natural habitats having a continuous abundant supply of food, young salmon are probably less aggressive than in habitats where they are constantly hungry and meals are sporadic. Chapman (1962) has postulated that aggression of some territorial fish may cause the emigration of others with inferior fighting ability. Presumably an increase in aggression would speed this emigration process. Symons (1968) also suggested that the increase in aggression associated with food-deprivation might, in a natural environment, result in an

livelihoods of the population. Different population densities, land fertility, and differences in income levels initiate the use of different approaches to providing the world’s population with food. The impact of sanctions against Russia on food supply and food security in the world, in particular considering the “grain deal,” remains poorly understood. The purpose of the work is to analyze the processes of food supply and achieving food security in the countries of the world, considering the introduction of sanctions by Western countries against Russia. The objectives of the study are as follows: to analyze the current situation in food supply, to identify the main threats to food security in the world as a whole, to propose solutions to reduce the negative impact of sanctions imposed against some states on third countries. The results of the study provide substantiation of proposals aimed at resolving issues of ensuring food security in the world under sanctions pressure on exporters of agricultural products.

Coupling of two Lotka–Volterra type competition systems with density‐dependent migration was surveyed. We assumed that species x and y are each exclusively superior in subhabitat 1 and subhabitat 2, respectively, and that population densities that exert intra‐and interspecific competitive effects also impose pressures for migration of individuals from a subhabitat. If the two species are, respectively, abundant in the subhabitats in which either species is competitively superior, and the migration has a mixing effect, then, it would be intuitively expected that, as potential migration rates increase, the two species are mixed well and coexist in the whole habitat. An analysis of this competitive situation using our model under the assumption of linear diffusion predicted that, even though weak mixing maintains coexistence in the whole habitat, strong mixing collapses coexistence and leads to the exclusion of one species. The assumption that migrations occur due to self‐ and cross‐population pressures provides different predictions: (i) weak dominance and strong mixing destabilize the coexistence state and lead to a monopolizing equilibrium of either species (bi‐stability of monopolizing equiliblia); (ii) conspicuous weakness of the inferior species makes the mixing equilibrium stable, regardless of the potential migration rate; and (iii) tri‐stability exists in between situations (i) and (ii). In the third case, the attainable state is the mixing equilibrium or either of the monopolizing equilibria, depending on the initial state. Migration mechanisms with self‐ and cross‐population pressures tends to mediate spatial segregation and makes coexistence possible, even with strong mixing.