Ideas for Surviving Food Shortages

Food For Freedom

Water supplies around the US and the world are starting to dry up, and more and more people are being left without the water that they need to survive. How can you guarantee the safety of your family and friends and loved ones when you can't control the water yourself? You may not be able to control how much water your have available, but you CAN control what you do with the water that you have! This guide by expert survivalists can teach you all that you need to know about how to provide for your family during times of drought and bad seasons. You will learn how to build greenhouses for your family so that you can grow food with less water at a time! You will learn how to take control of the food that your family needs to survive and build systems that will make sure you are never without food! Read more...

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Types of Adaptations Relevant to Population Dynamics

Energy allocation to growth and reproduction may also closely respond to local environments, with anticipated harsh conditions (e.g., freezing lakes, food shortages, long-distance migration) often preceded by buildup of storage lipids in many taxa. This may remove energy from growth and reproduction in the short term but improve lifetime success. A species that occupies a large latitudinal range, for instance, may vary its 'energy budget' in response to temperatures. For example, northern Atlantic forms of silverside fishes allocate more energy to lipids than southern populations, and animals may channel energy to growth (when small and vulnerable as juveniles) and later allocate energy to fat reserves for periods of resource shortage and to reproduction.

Questions of behaviour and ecology of otters

The question of what limits numbers of otters in any one area can be approached from two different angles. From one direction, we study the effects of factors such as food by estimating availability and quantifying foraging effort, and we determine habitat utilization and possible restrictive mechanisms therein. From another angle, we study otter populations, patterns and causes of mortality (including disease, human-made hazards, food shortage and predation), changes in reproductive success, and deduce possible mechanisms in population dynamics from age structures. I have attempted to use both approaches, and to bring these together.

Measurements of eggs and chicks

Linear dimension relative to the average value from a fitted growth curve for the species gives a good indication of growth relative to the population mean. In species in which bone and feather growth are less affected by food shortage than the rate of increase in body weight, it may be possible to calculate an index of condition for chicks of unknown age by expressing weight relative to some function of linear dimensions. More details of condition indices can be found in Chapter 4.

The Costs and Benefits of Being Small

A simple, verbal model that explains the continental-scale variation in body size of the weasels as a group (see Figure 2.8) was worked out, independently of Sandell's calculations, by King (1989a). The small size and elongated shape of weasels give them a huge advantage when hunting rodents in their burrows, but the physiological consequences of smallness are nearly all unfavorable. In years when rodents are scarce, a weasel's constant need for well-insulated shelter is a handicap, because just at the very time when food is most hard to find and energy most precious, hunting expeditions must be long and ready-made dens are few. Weasels can store surplus food as caches, liable to theft and decay, but not as extensive body fat they are extremely vulnerable to temporary food shortages, yet hibernation and long-distance migration are impossible. The huge additional energy needed for breeding is hard to find, except when rodents are abundant.

Lessons from Coastal Bird IBMs and Future Developments

Which span multiple bird generations. These factors have simplified the development of coastal bird IBMs and their use in advising policy, and so the next step is to identify species that share some or all of these characteristics. Marine ducks, such as the common scoter and common eider Somateria mollissima, feed on similar, bottom-dwelling prey as do inter-tidally feeding shorebirds, and the extent of their feeding habitat changes through the tidal cycle as water depth changes. An IBM has recently been developed to predict the effect of habitat loss and disturbance from wind farms on common scoter wintering in the Irish Sea (Kaiser et al., 2005). Seed-feeding farmland birds have relatively simple diets and the abundance and food value of seeds can be quantified. These birds have considerably declined in recent decades, thought in several species to be associated with over-winter food shortages caused by change in agricultural practices (Robinson and Sutherland, 2002). IBMs can...

Studying Interactions

Long-term studies or field experiments designed to test mechanisms underlying the 10-year cycle have been reported only relatively recently. Vaughan and Keith (1981) conducted such an experiment in eight enclosures (about 3-6 ha in size) of natural habitat to measure the demographic response of hares to food shortages during the winter. They used two levels of hare density, averaging about four and 13 hares ha at the start of each experiment, and two levels of food availability (high and low). They found that food shortage greatly affected the reproductive characteristics of adult hares, including the onset and termination of breeding, pregnancy rates, and ovulation and implantation rates. These changes corresponded to shorter breeding seasons and a reduction in mean natality. Survival of juveniles was also markedly reduced, although survival of adults was unaffected. Vaughan and Keith concluded that the results of their field experiments were consistent with the view that cyclic...

Measuring Ecological Efficiency

Organisms, there are typically periods during which energy income exceeds the possibilities for enhanced growth and reproduction and periods of food shortage when organisms can only rely on stored energy. Higher values for ash-free total calorie concentration in animals are found either when food is particularly abundant or when energy is being stored in advance of some process that will make higher energy demands than can be met immediately by feeding. Large seeds, yolky eggs, prehi-bernating mammals, and premigratory birds have high caloric density.

Weasels And Other Predators Compared

The populations of lynxes, which specialize on snowshoe hares. When hare populations begin to decrease because of food shortage, pressure by abundant predators on the remaining hares is intense. Hare populations are reduced and kept at low levels until predators starve. During that period, plants recover and can support high hare populations again. And so the cycle continues.

Heterotrophic succession See succession

Day length or food shortage may stimulate the hibernation mechanism, but in temperate and arctic animals the stimulus is cold. A 'hibernation hormone' has been suggested but not isolated. Temperature regulation is maintained but at a lower level. When hibernation ends, body temperature rises spontaneously, starting at the body core.

Migration and body size

The sex and age groups of a population often differ in body size. This may affect their ability to withstand cold and food shortage, their dominance status and many other features that could in turn influence movement patterns. In most bird species, males are bigger than females, but in raptors, owls, some shorebirds and others, females are bigger than males. The differences in body size between the sexes are usually slight, with some overlap between them, but in some species (notably most raptors and some shorebirds) the differences are substantial, with no overlap between sexes. In most bird species, juveniles are also slightly smaller than older birds of the same sex.

Irruptive migrations boreal seedeaters

Among the specialist seed- and fruit-eaters, most individuals stay in the north in years when food is plentiful there, wintering within, or just south of, their breeding areas, but moving further south in years when food is scarce. Their so-called invasions or irruptions, in which they appear in large numbers well beyond their usual range, follow periodic widespread crop failures. Irruptive migrations therefore occur in response to annual, as well as to seasonal, reductions in food supplies. The effect of food shortage is often accentuated because the birds themselves tend to be numerous at such times, as a result of good breeding and survival in previous years when food was plentiful (Lack 1954, Keith 1963, Berndt & Henss 1967, Koenig & Knops 2001). The greater the imbalance between the birds and their food, the greater the proportion of individuals that leaves, presumably as a result of competition (for Bohemian Waxwing Bombycilla garrulus, see Siivonen 1941, Cornwallis 1961,...

Coping with a boomandbust economy

By changing breeding and wintering areas between years, irruptive seed-eaters lessen the effects of the massive food shortages they would experience if they occupied the same areas every year. Nevertheless, they may still be exposed to a hugely fluctuating food supply, as reflected in their reproductive rates. For example, in an area of northern Sweden, Bramblings Fringilla montifringilla bred every year over a 19-year period, but in greatly varying numbers, depending on food supply (Lindstr m et al. 2005). Post-breeding juvenile-to-adult ratios varied more than 10-fold over this period, from 3.54 in good food years to 0.33 in poor ones. The Redpolls Carduelis flammea and Siskins Carduelis spinus mentioned earlier can breed for more than twice as long in good spruce years than in other years (giving time for 2-3 broods instead of 1-2), and could thereby double their production of young (Peiponen 1967, Shaw 1990). In good seed years, both species begin nesting as early as March, when...

Impacts of Acorns on Denning

Denning evolved as an accommodation to inclement weather and food shortage (Lindzey and Meslow 1976, Johnson and Pelton 1980, Rogers 1981). Even southern bears den, despite year-round mild weather and presence of food (Hamilton and Marchinton 1980, Novick et al. 1981, LeCount 1983). Thus, the onset of denning is likely determined by a circannual rhythm modified by weather and fall food supply (Johnson and Pelton 1980). In areas where bears are coincidental with oak forests, oak trees are related to denning behavior two ways (1) acorn production can influence the timing of denning, and (2) bears may select oak trees as den sites.

Owls And Other Predators

Owls and raptors that depend on cyclically fluctuating prey species often suffer food shortages in years when their prey numbers crash. In some northern species this leads to massive southward emigration. Two main prey systems underlie their behaviour (1) an approximately 3-5 year cycle of small (microtine) rodents in the northern tundras, open parts of boreal forests and temperate grasslands 1 and (2) an approximately 10-year cycle of Snowshoe Hares Lepus americanus in the boreal forests of North America (Elton 1942, Lack 1954, Keith 1963, Stenseth 1999). The numbers of certain grouse species also fluctuate cyclically, in some regions in parallel with the rodent cycle and in others with the longer hare cycle (Hornfeldt 1978, Keith & Rusch 1988, Newton 1998b). Any predators that specialise on such prey species, as well as more generalist predators that also eat other things, are affected by the fluctuations in these prey.

World population and food supplies

The food situation differs greatly in different areas. In Western Europe, North America, Australia and New Zealand the consumption of food is, in general, fully adequate for daily needs, and health problems arise more from over-indulgence or improperly balanced diets than from any insufficiency of quantity. In Africa, South America and the Near East, however, food intake is precariously balanced with requirements, and often inadequate, while serious food shortages occur in parts of Asia.

Developing national capacity for disaster preparedness

There was a strong impression that preparedness at the community and local level was particularly important. This planning must deal with displaced persons, surveillance systems, managing and preventing psychological problems, food shortages, water purification, and methods to prevent damage to healthcare facilities.

Other Concepts Of Control In Ecology And Engineering

Density dependence in which the severity of mortality factors is correlated with population density (such as for mortality caused by disease, predators or parasites, and food shortage). Another group led by Andrewartha and Birch (1954) believed in density independence in which the severity of mortality is the same at all population densities (such as for mortality caused by extreme weather events). This is a critically important distinction because density dependence allows for a self-regulation mechanism within a population. Cole (1957) reviewed the subject in terms of the search for a governor or controlling influence on population size. He showed the governor as a term added to the population growth equation, which converts uncontrolled, exponential growth (Figure 7.9a) into controlled, logistic or limited growth (Figure 7.9b)

Absorption and Excretion

PCBs are readily absorbed through many tissue membranes, including by microorganisms, the gills of fish, the skin of many animals, and the gastrointestinal tract. In plants PCBs may enter the cellular structure, but are more likely to be contained within the cuticular layers. PCBs accumulate in lipid-rich tissues, and in animals it is usual to express concentrations on a lipid weight basis. Lipid-rich tissues are the main storage sites in animals, and the liver (being the first organ chemicals absorbed into the blood from the gastrointestinal tract encounter, highly blood-perfused, and the main metabolic site) is generally relatively enriched in PCBs on a lipid-weight basis. The release ofPCBs occurs when fat is redistributed due to food shortage, poor health, hibernation, lactation, incubation, and migration. However, PCBs are not necessarily released pro rata, and adipose PCB concentrations tend to increase at these times. PCB concentrations found in tissues of a dairy cow in the UK...

Civil War and Disease as Public Bads

The economist Paul Collier notes that collapsed states, often as the result of civil war, act as the epicenters of disease and regional turbulence.87 In this sense, state failure can be also seen as a public bad, which then generates further externalities such as pathogens, which may then function to afflict a specific region, or indeed come to burden the entire world. Toole et al. comment that the civil war in Bosnia during the early 1990s revealed extensive disruption to basic health services, displacement of more than 1 million Bosnians, severe food shortages in Muslim enclaves in eastern Bosnia, and widespread destruction of public water and sanitation systems. War-related violence remains the most important public health risk civilians on all sides of the conflict have been intentional targets of physical and sexual violence.88 Moreover, the extensive empirical analysis of Ghobarah confirms that contiguous civil wars are significantly associated with the spread of HIV in...

Food availability starvation and population limitation

The effects of lack of food as a cause of mortality are complicated because starvation acts together with other environmental stresses. It is rare to find any animal dying just from starvation, as usually some other, more immediate, cause can also be identified and labelled as the culprit. In England and Germany, starvation was deemed the sole cause of death in none (Simpson 1997) and only 1.3 (Hauer et al. 2002a) of inspected otter carcasses. However, the role of starvation may show in different ways. For instance, if food shortage were involved as a major factor causing mortality, one might expect more otters to die (from whatever proximate cause) at

Orientation to stimuli

For protozoa, this results in an environment of alternating feasts and famines, with patches of high bacterial concentrations dispersed among large areas of low concentrations. This makes it important for protozoa to be capable of chemotactic orientation to such patches so as to utilize the available food resources. In addition,

Mate choice and promiscuity

This is because a recent study on the mealworm beetle (Tenebrio molitor) has shown that, in both sexes, mating reduces the activity levels of phenoloxidase, a key enzyme in the insect immune system (Rolff and Siva-Jothy, 2002) (see below). This mating-induced downregulation of immunity lasts for at least 24 h and appears to be mediated by juvenile hormone. Thus, by mating frequently, individuals may be compromising their immune systems. Similar conclusions were reached in an earlier study by McKean and Nunney (2001) using the fruitfly Drosophila melanogaster (Fig. 10.2). They found that as the number of females housed with each male was increased (from 0 to 4), so there was a highly significant decrease in the male's ability to immunologically clear an experimental injection of E. coli bacteria. This reduced immunocompetence was not due to food shortage, since it was prevalent also when food was in excess. Nor was it simply due to crowding effects, since males housed with four males...

Jonathan Cohen and Joseph J Amon

The relationship between the health status of a population and the behavior of the government under which that population lives has been previously explored in war or civil crisis settings, and in the context of chronic health issues such as nutrition, famine, and child mortality (Sen, 1999 Dreze and Sen, 2002). These studies have found that government behavior (sometimes captured by the term governance), as measured by indicators such as accountability, stability, rule of law, respect for human rights, and the existence of an independent civil society, plays a significant role in health outcomes - a role independent of, and perhaps even superior to, host genetics, insect vectors, or individual behaviors. Famines stem not solely from bad weather or genetics, but also from the failure of governments to protect their populations from civil strife, or to equitably distribute food aid. Occupational illnesses, such as pneumoconiosis (or black lung), can be understood in terms of the risk...

Carnivore Scavenger Interactions

Grizzly bears are important scavengers throughout most of their geographic range. In most cases, however, they hibernate during the winter months as a means to survive periods of chronic food shortages. However, Grizzly bears are known to forego hibernation in conditions when the supply of carrion is high. This may often happen in winters with high snow depth because moose and elk species that comprise the prey base for wolves are encumbered by deep snow and thus are especially vulnerable to being captured. Under such conditions, wolves frequently abandon partially eaten carcasses in favor of capturing new prey, leaving a continuous and plentiful supply of left-over meat, bone and hide to be scavenged.

From winterlimited to summerlimited

Over several years, the same population might change from one state to another, as its status with respect to available resources changed. Several species of geese increased during the latter half of the twentieth century in response to reduced hunting pressure in their wintering areas, but then came up against food shortage

Nongovernmental organizations

In some regions of the world where health services are limited, faith-based clinics and hospitals work with local NGOs as the main health providers. During disasters and complex humanitarian emergencies (CHEs), these local and international NGOs are usually the front-line emergency responders for refugees and displaced persons in isolated and insecure areas. The CDC defines CHEs as situations affecting large civilian populations which usually involve a combination of factors including war or civil strife, food shortages, and population displacement, resulting in significant excess mortality (Connolly, 2000 Toole et al., 2005). Much like the fragmentation already described on multi-lateral and bi-lateral levels, the services these NGOs provide frequently are not well coordinated either among themselves or with the host government. Particularly during emergencies, NGOs can have diverging priorities and approaches, and adhere to different standards of care. (Toole et al., 2005 Telford et...

Concluding Remarks

Competitive interactions and food shortage At stopover sites, territorial and other interactions between individuals can operate to adjust densities to local food supplies at the time, causing hungry birds to move elsewhere, to places where they may survive or die of starvation or something else. Secondly, food shortage at stopover sites may reduce population size through lowering breeding rates (as in some geese), not necessarily entailing the starvation of full-grown birds. Without special study, this type of effect may be hard to detect because of the time lag between the food shortage and the resulting decline in breeding numbers. In some long-lived species, individuals do not normally breed until they are several years old, so it may take several years before the effects of poor breeding are reflected in decline in the numbers of adult breeders. As a further complication, the effects of food shortage on any population may be accentuated by the presence of people or natural...

Mass mortality of migrants

While the previous chapter alluded to the ongoing mortality resulting from food shortage, predation and other factors operating at stopover sites, this chapter reviews some additional major mortality incidents described in the ornithological literature. They establish the importance of such additional mortality as a frequent hazard for migratory birds, even though their effects on populations are hard to estimate. Almost all such incidents occurred during adverse weather, either during the journey itself, soon after arrival in breeding areas in spring, or just before departure from breeding areas in late summer or autumn. Typically, those incidents that occurred in breeding areas were not matched by concurrent mortality among local resident species, and could have been avoided if the migrants had arrived some days later or left some days earlier. They are therefore classed here as migration-related.

Unseasonable Cold Before Departure From Breeding Areas

Similarly, many large-scale mortality incidents have been recorded from time to time in seabirds (so-called wrecks), when large numbers are blown ashore, sometimes far inland (for review see Newton 1998b). It is usually unknown whether these birds were on migration at the time, or in their wintering areas. However, because some seabird species seem to remain on the move for most of the time between leaving their breeding places in one year and returning there the next, migration periods cannot always be separated from other parts of the annual cycle. The same could be said for some landbird species, which seem able to move long distances at almost any time during the course of a winter to avoid food shortages (Chapter 16).

Flea species richnessBody mass and complexity of burrow

Performance of larvae of the two species in terms of their developmental success in mixed-species and single-species treatments under different air temperatures, relative humidities, substrate textures and food abundance. It was found that the developmental success (evaluated as the number of individuals that survived until emergence) of X. conformis depended on the presence of competing species, being, in general, lower in mixed-species compared with single-species treatments (Fig. 16.8a). The decrease in developmental success of X. conformis in mixed-species treatments was found mainly during food shortage. In contrast, presence of the competitor did not affect developmental success of X. ramesis (Fig. 16.8b).

Evolution Of Alloheimy

Whatever the pattern of geographical segregation among wintering migrants, if such differences are genetically influenced, through migratory behaviour, it is not hard to envisage how they might have come about (Figure 23.9). Imagine that populations which breed in separate areas come together on common wintering grounds. If they were limited by food in the wintering area, and individuals of one population were better adapted to that area, they would in time be expected to eliminate individuals of the other population completely, or force them by selection to winter elsewhere. As a second scenario, imagine that, under food shortage in their joint wintering area, they competed on equal terms, with the same proportion of each population surviving the winter. If individuals of one population had, on average, a consistently higher reproductive rate than the other, then in time they would be expected to replace individuals of the other population completely in their shared wintering area,...

Exploring incidence functions

In contrast, were absent from many islands in the 2-10 ha range. Temporary food shortages, or 'energy crises' might be more problematic for the smaller species than for the larger S. araneus, thus potentially explaining their lower frequency of occurrence as a function of environmental stochas-ticity. Arguably, interspecific competition might also have a role to play, but Peltonen and Hanski (1991) subsequently reported no evidence for a significant effect of competition on extinction rates. Both studies conclude that the body size of species has a bearing on their incidence. As the larger species do not appear to be affected by interactions with the smaller species within the guild, they predict that they should occur consistently on islands greater than a minimum size set by demographic stochasticity. Small species should have a more erratic distribution, because of their increased susceptibility to environmental stochasticity. This interpretation combines elements of the incidence...

The Lotka Volterra model

Putting the two isoclines together (Figure 10.2c) shows the behavior ofjoint populations. Predators increase in abundance when there are large numbers of prey, but this leads to an increased predation pressure on the prey, and thus to a decrease in prey abundance. This then leads to a food shortage for predators and a decrease in predator abundance, which leads to a relaxation of

The maintenance and ecological effects of flight

The obvious difference between these two groups of organisms is that the former (insects and birds) retained a functional distinction between the flight apparatus and the legs they can both walk or run without using their wings. In pterosaurs and bats this is not the case and both groups would be relatively ineffective on the ground, hindering the transition to a terrestrial lifestyle again. In birds and insects, loss of flight could mean a reallocation of energy away from the flight apparatus and increases in reproductive expenditure (Roff 1990, 1994). They lost much of course, and in birds loss of flight is only viable under special circumstances. It has happened mostly in a few taxonomic groups (rails notably) and under special ecological circumstances (notably on islands, see Figure 13.3) (Roff 1994). There are three likely reasons for the latter first, an absence of land predators that makes escape (and especially nesting off the ground) less important. Second, on...

Facultative movements in relation to weather

Many hard weather movers usually return soon after conditions improve again, sometimes less than a week later. One probable reason for their return over several hundred kilometres is the avoidance of competition for food, which is likely to be more intense in the overcrowded hard weather refuges than in the areas previously left. It is not that birds are necessarily driven back by competition, but this could be the ultimate factor involved. The proximate stimulus may be a favourable change in temperature or wind direction, for it would be advantageous for the birds to leave before they were weakened by food shortage. In Europe, winds from the south and west bring warmer temperatures and suitable tailwinds. It is almost as though such birds shuttle back and forth along part of their migration route, on average getting further from their breeding areas as winter advances. It seems that some facultative migrants have some sense of where they ought to be - that is, as near to the breeding...

Lifehistory Patterns and Plasticity

Non-adaptive effects of poor environments. In the speckled wood butterfly Pararge aegeria, for instance, levels of drought and food shortage have a strong impact on adult longevity (Gotthard et al., 2000b) (see also next section for an example from P. c-album involving host-plant quality). Perhaps more surprisingly, a strong genetic component to variation in ageing and lifespan can also often be demonstrated, a fact that has led to considerable discussion (Stearns, 1992). One approach to explaining such variation is in terms of future reproductive success. From an optimality perspective, it does not pay to invest in a very durable soma if there is little chance of reproduction in late life. In a previous section we saw a result of this equation, with the light form of P. c-album investing more in current reproduction and less in a durable thorax and wings than the long-lived dark form a demonstration that plastic variation in lifespan can sometimes be adaptive.

Mortality and morbidity patterns in modern conflicts

In addition to causing the displacement of millions of people, conflict also takes an important toll on both agrarian and industrial economies. Although there is some dispute as to the magnitude of the impact of conflict on national economies, some estimate that a 15-year civil war would reduce gross domestic product by as much as 30 percent (Collier, 1999 Imai and Weinstein, 2000). Local economies may be even more devastated, with more immediate consequences for the health of the population - particularly through food shortages resulting in a high prevalence of under-nutrition. In rural areas, where a relatively high proportion of food is derived from subsistence farming, farmers may be physically unable to plant as much as they might in the absence of conflict. Where even small-scale commercial food production is a way of life, farmers may be obstructed from bringing their produce to market. Resulting scarcities can be responsible for higher food prices, which, combined with the...

Immediate Causes of Death

Many immediate causes of death can be associated positively or negatively with anthropogenic effects. For example, humans reduce the number of wolves and foxes in urbanized areas which reduces the predation rate in deer and rabbit populations. Humans release pathogens, parasites, and predators for biological control of agricultural and urban pests. Agricultural ecosystems provide excessive food supply for many herbivorous insects and reduce their mortality due to food shortage. Forest clear-cuts increase the amount of light that reaches the ground, which causes mortality in plants that cannot tolerate direct sun light, and reduces mortality in other plant species which survive better in open conditions. Also forest cuts remove the food source for herbivores that feed on trees and shelters for many forest animals. Irrigation affects humidity and water supply for many animals and plants which changes their mortality patterns. River dams create barriers for fish movement and causes their...


While many birds alleviate seasonal food shortages by migrating elsewhere, many other animals cope with seasonally difficult periods by hibernating, remaining dormant for up to several months at a time. They survive at much reduced metabolic rate on body reserves, and emerge when conditions improve. At one time, the disappearance of most birds from high latitudes for the winter was attributed to hibernation rather than migration. In fact, at least one species of bird does hibernate in winter. This was discovered in 1946, when a Common Poorwill Phalaenoptilus nuttallii (a sort of nightjar) was found in a torpid state in a rock crevice in a California desert (Jaeger 1949). The bird was inert, its respiration and heart rate were barely detectable, and its body temperature was 18-20 C, about half the usual level for birds. The individual was ringed, and in subsequent winters it was found hibernating again in the same crevice. Since then other poorwills have been found in similar sites in...

Demographic Pressure

Population-resource imbalance caused by demographic pressure is one of several univariate explanations for the origins of agriculture (Cohen 1977 Smith and Young 1972 Smith and Young 1983). In the best known formulation of this idea, Mark Cohen (Cohen 1977) argued that worldwide population growth explained why hunter-gatherers living in different locations independently turned to agriculture at the end of the Pleistocene. The argument was based on the premise that the adoption of agriculture resulted in a net increase in workload and a decrease in food diversity and sufficiency, and therefore an overall reduction in the quality of life, a situation that any rationally minded hunter-gatherer would not enter into freely. Cohen argued that as hunter-gatherers exceeded environmental carrying capacity, food shortages pushed them to experiment with plants and animals and, ultimately, with agriculture. Hunter-gatherers over-filled salubrious habitats worldwide and were compelled to augment...

The grazing rate

Although the interactions between plant and animal populations are difficult to elucidate, the grazing rate of the herbivorous zooplankton is certainly one of the factors which regulates the size of the standing stock of phytoplankton, and therefore influences the production rate. The quantity of epipelagic zooplankton generally correlates more closely with the quantity of plant nutrients in the surface layer than with the size of stock of phytoplankton, indicating how greatly grazing reduces the number of plants in fertile water. In the long term, the primary productivity of an area must determine the size of the animal population it supports, but in the short term there are often wide, and sometimes rapid, changes in both numbers and composition of populations due to a variety of causes. Interactions between species often involve a time lag, and there is consequently a tendency for numbers to fluctuate about mean levels. Although some natural populations show homoeostatic mechanisms...

Onegeneration cycles

Predator-prey generation cycles occur essentially when the generation length of the consumer is roughly half that of its host - as it often is. Any small, chance peak in host abundance tends to generate a further peak in host abundance one host generation later. But any associated peak in consumer abundance occurs half a host generation length later, creating a trough in host abundance between the twin peaks. And this host trough creates a further host trough one generation later, but a consumer trough coinciding with the next host peak. Thus, the consumers have alternate 'feasts' and 'famines' that accentuate the originally small peaks and troughs in host abundance, and hence promote one-generation cycles (Figure 10.4).

Body Size

The influence of environmental factors is size dependent. Small animals have relatively more surface area and relatively less metabolically active tissue with which to generate heat. Therefore, we would expect a small animal, when challenged with cold, to lose heat more rapidly than a larger animal. If such an animal attempted to generate heat to offset the heat loss and maintain Tb, it would require a relatively higher rate of energy utilization. Small endothermic homeotherms, by virtue of their higher metabolic requirements, are therefore dependent on a reliable supply of energy-yielding foods. Exposure to extreme cold and or a food shortage would place a small endothermic homeotherm under profound metabolic stress. Obvious energetic savings are to be made under such conditions by lowering the thermal set-point for Tb regulation and depressing metabolism (i.e., torpor, Figure 8). Examples of torpor are found in all three infraclasses of mammals and some groups of birds.

Clutch size

Although the Lack hypothesis is generally supported over the long run, there are many cases where clutch size is modified through brood reduction after egg laying. Studies of red-winged blackbirds (Agelaius phoeniceus) (Forbes et al. 1997), for example, confirm the insurance hypothesis, which states that hatching asynchrony creates marginal offspring that serve as replacements for failed earlier-hatched core offspring. Forbes et al. showed that marginal offspring had very high mortality rates in control nests, but survived well if broods were experimentally or naturally reduced. In asynchronous hatching the older siblings beg more vigorously for food and or the older siblings may simply kill the weaker sibling. If food shortages arise, the late-hatched young usually die of starvation. In such cases, the parents, who cannot predict food resources, can use brood reduction to adjust reproduction to actual food availability.

Defenses of the prey

Such specialized behavior is especially well suited to avoid predators, since it need only be employed when predators are actually present. One must assume that there are costs associated with the avoidance of predators (e.g., the prey has less time to spend searching for food). The costs of predator avoidance will be mentioned in several of the subsequent chapters. It has frequently been demonstrated that organisms foraging under predation risk suffer sublethal consequences of restricted time budgets, suboptimal habitats, and excess energy allocated to defenses. The avoidance of predatory stonefly nymphs, for example, results in lower growth and fecundity in mayflies Baetis, similar to those caused by food shortage (Peckarsky et al. 1993).


Few empirical data are available to support the generalities just mentioned, but exceptions exist. Berger (1990) addressed the issue of MVP by asking how long different-sized populations persist. He presented demographic and weather data spanning up to 70 years for 122 bighorn sheep (Ovis canadensis) populations in southwestern North America. His analyses revealed that 100 percent of the populations with fewer than 50 individuals went extinct within 50 years, populations with more than 100 individuals persisted for up to 70 years, and the rapid loss of populations was not likely to be caused by food shortages, severe weather, predation, or interspecific competition. Thus, 50 individuals, even in the short term of 50 years, are not a minimum viable population size for bighorn sheep. However, Krausman et al. (1993) questioned this result because they know of populations of 50 or less in Arizona that have persisted for more than 50 years.

Hares and hareeaters

Migrating Great Horned Owls fly by night, and because they move into more southern regions already populated by resident Great Horned Owls, their irruptions have been less well documented. However, all those that I could find recorded coincided with Goshawk invasions, and hence with low hare numbers, again providing indications that food shortage stimulated large-scale emigration in this mainly resident species (Table 19.5). In a study at Kluan, Yukon Territory, annual emigration rates of radio-marked Great Horned Owls increased over a period of years from 0 to 33 for territory holders (N 2-54 in different years) and from 0 to 40 for non-territorial floaters (N 2 18 in different years), as hares declined (Rohner 1996). To judge from ring recoveries, migration of Great Horned Owls from Saskatchewan is mainly to the southeast, but over greater distances in years with low hare numbers, with some individuals travelling more than 1000 km from their breeding sites (Houston & Francis...

Evolutionary aspects

Migration might be expected to occur wherever individuals benefit more, in terms of survival or reproduction, if they move seasonally between different areas than if they remain in the same area year-round (Lack 1954). The usual reason why breeding areas become unsuitable during part of the year is lack of food. Such food shortages occur for many birds because plant growth stops for part of the year, and many kinds of invertebrates die or hibernate or become inaccessible


Although analyses of the relationship between conflict and disease are novel in the domain of political science, the public health, medical, and historical communities have documented such issues for centuries. In the domain of public health, the epidemiologist Steve Morse has characterized war as a significant historical factor associated with the spread of infectious disease.6 The epidemiologist William Foege concurs in his analysis of war's proximate and distal effects on population health. Foege argues that organized violence, as seen in small or large conflicts, has direct and indirect effects on health as it leads to famines, epidemics, social dislocations, and the disruption of public health programs in general.7 The medical historians Richard Garfield and Alfred Neugut agree, holding that social disruptions associated with the practices of war correlate with increases in the mortality rate among civilian populations. Such practices include war-induced material deprivations...

Adaptive Timing

Some food shortages are predictable because they occur at about the same time every year, in response to the changing seasons. This enables birds, through evolutionary mechanisms, to anticipate periods of shortage, and to get out while they are still able to accumulate fat reserves for the journey. Similarly, birds can leave their wintering areas at a time that allows them to reach their breeding areas at an appropriate date in spring, when breeding again becomes possible. It is in response to these predictable seasonal changes in food supplies that most migration is presumed to have evolved. Many obligate migrants react to proximate environmental cues, such as daylength, in conjunction with an endogenous rhythm, in such a way that they can prepare for migration ahead of time (Chapters 11 and 12). On current understanding, this is the situation in complete migrants, in which the whole population leaves the breeding area each autumn in anticipation of a predictable loss of winter food...


Other environmental effects on adult otter mortality, either direct, or indirect by reducing fish populations, may be caused by pollution, which affects aquatic habitats more than most others. Effects of pollution have to be considered in a population context, jointly with other mortality factors such as food shortage, and this has been attempted to some extent in the Shetland studies. One possible scenario in Shetland was that otter numbers were limited by food, and that older animals, with high levels of mercury in their bodies, would suffer the highest mortality rate. Diseases may also affect survival.

Delayed Implantation

Small rodents comprise ideally convenient packages of meat to feed to all young weasels they are easy to kill, light to carry, and wrapped in ready-made waterproof packets. Populations of common and least weasels have developed their opportunistic way of life to such a fine art that they can survive the most dangerous conditions, such as in the fierce cold and regular famines of the far north. Local extinction is common, but there are always a few patches of habitat with enough rodents to support at least a few individual weasels, which in turn can survive and disperse to recolonize other areas vacated when times were bad.


There can be no doubt, therefore, that many birds die on migration, and that in some weather-induced incidents, the numbers can be very large. In some of the examples mentioned above, the daily mortality during migration, or soon after arrival in breeding areas, was almost certainly much greater than the mean daily rate at other times of year. Estimates from the Black-throated Blue Warbler Dendroica caerulescens, Barnacle Goose Branta leucopsis, Brent Goose B. bernicla and Greater Snow Goose Chen caerulescens in the previous chapter provide striking examples, and in other species (such as Northern Lapwing Vanellus vanellus and King Eider Somateria spectabilis), the losses immediately after arrival in breeding areas in unusually cold springs exceeded the normal total annual mortality expected in these species. Following post-arrival losses in spring, local breeding populations of various species fell by 25-90 , and following heavy pre-departure losses in autumn, subsequent breeding...

Sexual Dimorphism

Male weasels of all species are substantially larger than females (see Table 4.1 Figure 14.1). The reason for this difference was at first considered to have something to do with food (Brown & Lasiewski 1972). Because the two sexes are so different in size, they tend to eat different things so, the argument ran, the difference must have arisen so that each could avoid trespassing on the other's food supplies. In times of food shortage, this trick might be valuable to both. Indeed, the overlap in the diets of males and females is substantial at all times, especially when food is short (Chapter 5). The fluctuations of populations of voles and lemmings over a 3 or 4-year period create alternate feasts and famines for weasels. Across these fluctuations, the chances of a female weasel producing young that survive to breed in their turn range from practically nil to very high. The population densities of the weasels themselves vary over the same period, during which the chances of a male...

Effects of crowding

More generally, predators have mutual interference been assumed in the models discussed thus far to consume prey at a rate that depends only on prey abundance (in Equation 10.2, for example, the consumption rate per predator is simply aN). In reality, consumption rate will also often depend on the abundance of the predators themselves. Most obviously, food shortage - the abundance of prey per predator - will commonly result in a reduction in the consumption rate per individual as predator density increases. However, even when food is not limited, the consumption rate can be reduced by a number of processes known collectively as mutual interference (Hassell, 1978). For example, many consumers interact behaviorally with other members of their population, leaving less time for feeding and therefore depressing the overall feeding rate. For instance, humming-birds actively and aggressively defend rich sources of nectar. Alternatively, an increase in consumer density may lead to an...

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