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What are the limiting factors of pathogen population size in human populations?

What are the limiting factors of pathogen population size in human populations?


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I understand that one limiting factor in non-human animal populations is that increased pathogen populations decrease animal populations from killing them, which decreases the density of the animals, which makes it harder for pathogens to infect other animals. However, pathogens, at least in developed nations, don't seem to kill a large enough number of people for this to be the main limiting factor. So what else are the limiting factors?


One of the limiting factors is how many uninfected hosts exist. As a pathogen becomes more widespread, the probability of someone infected with it infecting someone else decreases, as it's more likely that the person will spread the disease to someone who already has it instead of someone who doesn't have it, preventing the virus's population size from increasing. See this for more information.

Also the more common a pathogen is, the more likely it is that people will take actions against it, for example by being more sanitary or getting vaccinated against it.


Limiting Factors

A limiting factor is anything that constrains a population's size and slows or stops it from growing. Some examples of limiting factors are biotic, like food, mates, and competition with other organisms for resources. Others are abiotic, like space, temperature, altitude, and amount of sunlight available in an environment. Limiting factors are usually expressed as a lack of a particular resource. For example, if there are not enough prey animals in a forest to feed a large population of predators, then food becomes a limiting factor. Likewise, if there is not enough space in a pond for a large number of fish, then space becomes a limiting factor. There can be many different limiting factors at work in a single habitat, and the same limiting factors can affect the populations of both plant and animal species. Ultimately, limiting factors determine a habitat's carrying capacity, which is the maximum size of the population it can support.

Teach your students about limiting factors with this curated collection of resources.


Natality: increases population size as offspring are added to the population.

Immigration: increases population size as individuals have moved into the area from somewhere else and so this adds to the population.

Mortality: decreases the population as some individuals get eaten, die of old age or get sick.

Emigration: decreases the population as individuals have moved out of the area to go live somewhere else.


Example Nine

The number of pythons found throughout Everglades National Park has increased in recent years. These huge snakes are not native to Florida and are believed to have been released into the wild by pet owners. Wildlife biologists have initiated attempts to capture and remove these pythons. Which statement best explains the biologists' reasons for removing these pythons from the Everglades?

A. The pythons could upset the territorial boundaries of native organisms.

B. The pythons could adapt to overcome diseases common to native snakes.

C. The pythons could prey on native organisms and cause native population to decline.

D. The pythons could begin to interbreed with native snakes and produce a more successful species.


What is the host range of pathogens?

Some pathogens are limited to infecting a single host species, whereas others can infect a multitude of host species. Host ranges can feel highly idiosyncratic if not outright puzzling. For example, leprosy in humans is caused by two related intracellular bacteria Mycobacterium leprae and Mycobacterium lepromatosis, which are essentially restricted in the wild to humans, as well as armadillos in the Americas and red squirrels in Scotland [4].

Conversely, Yersinia pestis, another intracellular obligate bacterium and the agent of plague, has a natural life cycle involving alternating infections of rodents and fleas, but can infect essentially any mammalian host. An interesting twist in the case of plague is that Y. pestis is not well adapted to the human host. With the exception of uncommon occurrences of human-to-human transmissions, referred to as pneumonic plague, plague epidemics (bubonic plague) are caused by plague-infected fleas biting humans. Somewhat ironically for a pathogen that is possibly the biggest killer in human history, bubonic plague is a complete evolutionary disaster. The human host is at a very high risk of dying, the flea cannot reproduce on a meal of human blood and the bacterium is stuck in an evolutionary dead-end as it cannot transmit to another host.

There is no obvious predictor for the host range of different pathogens. Intuitively, it may be tempting to predict that pathogens with a more intimate relationship with their host are more closely adapted to their host, and thus have a more restricted host range. However, there is no obvious pattern suggesting that viruses (that rely on the host cells’ machinery for reproduction) have a narrower host range than bacteria. Also, intracellular bacteria do not seem to have a markedly narrower host range than extracellular ones, despite being more intimately tied to their host.

We know relatively little about the underlying genetic changes required for a pathogen to infect a new host, though, interestingly, only a few mutations can be required for a host jump. For example, avian influenza is only around five mutations away from being able to transmit in mammals [5], and a single amino acid change was sufficient for the human-adapted bacterium Staphylococcus aureus to become a pathogen of rabbits [6].


Demographic Transition – a dramatic change in birth and death rates

  • United States, Japan, and Europe population growth has stopped
  • ZERO POPULATION GROWTH
  • In order to achieve ZPG, each couple has only 2 children

Age Structure Diagrams can be used to evaluate populations

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45.3 Environmental Limits to Population Growth

By the end of this section, you will be able to do the following:

  • Explain the characteristics of and differences between exponential and logistic growth patterns
  • Give examples of exponential and logistic growth in natural populations
  • Describe how natural selection and environmental adaptation led to the evolution of particular life history patterns

Although life histories describe the way many characteristics of a population (such as their age structure) change over time in a general way, population ecologists make use of a variety of methods to model population dynamics mathematically. These more precise models can then be used to accurately describe changes occurring in a population and better predict future changes. Certain long-accepted models are now being modified or even abandoned due to their lack of predictive ability, and scholars strive to create effective new models.

Exponential Growth

Charles Darwin, in his theory of natural selection, was greatly influenced by the English clergyman Thomas Malthus. Malthus published a book in 1798 stating that populations with unlimited natural resources grow very rapidly, which represents an exponential growth , and then population growth decreases as resources become depleted, indicating a logistic growth.

The best example of exponential growth is seen in bacteria. Bacteria reproduce by prokaryotic fission. This division takes about an hour for many bacterial species. If 1000 bacteria are placed in a large flask with an unlimited supply of nutrients (so the nutrients will not become depleted), after an hour, there is one round of division and each organism divides, resulting in 2000 organisms—an increase of 1000. In another hour, each of the 2000 organisms will double, producing 4000, an increase of 2000 organisms. After the third hour, there should be 8000 bacteria in the flask, an increase of 4000 organisms. The important concept of exponential growth is the accelerating population growth rate —the number of organisms added in each reproductive generation—that is, it is increasing at a greater and greater rate. After 1 day and 24 of these cycles, the population would have increased from 1000 to more than 16 billion. When the population size, N, is plotted over time, a J-shaped growth curve is produced (Figure 45.9).

The bacteria example is not representative of the real world where resources are limited. Furthermore, some bacteria will die during the experiment and thus not reproduce, lowering the growth rate. Therefore, when calculating the growth rate of a population, the death rate (D) (number organisms that die during a particular time interval) is subtracted from the birth rate (B) (number organisms that are born during that interval). This is shown in the following formula:

The birth rate is usually expressed on a per capita (for each individual) basis. Thus, B (birth rate) = bN (the per capita birth rate “b” multiplied by the number of individuals “N”) and D (death rate) = dN (the per capita death rate “d” multiplied by the number of individuals “N”). Additionally, ecologists are interested in the population at a particular point in time, an infinitely small time interval. For this reason, the terminology of differential calculus is used to obtain the “instantaneous” growth rate, replacing the change in number and time with an instant-specific measurement of number and time.

Notice that the “d” associated with the first term refers to the derivative (as the term is used in calculus) and is different from the death rate, also called “d.” The difference between birth and death rates is further simplified by substituting the term “r” (intrinsic rate of increase) for the relationship between birth and death rates:

The value “r” can be positive, meaning the population is increasing in size or negative, meaning the population is decreasing in size or zero, where the population’s size is unchanging, a condition known as zero population growth . A further refinement of the formula recognizes that different species have inherent differences in their intrinsic rate of increase (often thought of as the potential for reproduction), even under ideal conditions. Obviously, a bacterium can reproduce more rapidly and have a higher intrinsic rate of growth than a human. The maximal growth rate for a species is its biotic potential, or rmax , thus changing the equation to:

Logistic Growth

Exponential growth is possible only when infinite natural resources are available this is not the case in the real world. Charles Darwin recognized this fact in his description of the “struggle for existence,” which states that individuals will compete (with members of their own or other species) for limited resources. The successful ones will survive to pass on their own characteristics and traits (which we know now are transferred by genes) to the next generation at a greater rate (natural selection). To model the reality of limited resources, population ecologists developed the logistic growth model.

Carrying Capacity and the Logistic Model

In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals gets large enough, resources will be depleted, slowing the growth rate. Eventually, the growth rate will plateau or level off (Figure 45.9). This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or K .

The formula we use to calculate logistic growth adds the carrying capacity as a moderating force in the growth rate. The expression “KN” indicates how many individuals may be added to a population at a given stage, and “KN” divided by “K” is the fraction of the carrying capacity available for further growth. Thus, the exponential growth model is restricted by this factor to generate the logistic growth equation:

Notice that when N is very small, (K-N)/K becomes close to K/K or 1, and the right side of the equation reduces to rmaxN, which means the population is growing exponentially and is not influenced by carrying capacity. On the other hand, when N is large, (K-N)/K comes close to zero, which means that population growth will be slowed greatly or even stopped. Thus, population growth is greatly slowed in large populations by the carrying capacity K. This model also allows for the population of a negative population growth, or a population decline. This occurs when the number of individuals in the population exceeds the carrying capacity (because the value of (K-N)/K is negative).

A graph of this equation yields an S-shaped curve (Figure 45.9), and it is a more realistic model of population growth than exponential growth. There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time.

Role of Intraspecific Competition

The logistic model assumes that every individual within a population will have equal access to resources and, thus, an equal chance for survival. For plants, the amount of water, sunlight, nutrients, and the space to grow are the important resources, whereas in animals, important resources include food, water, shelter, nesting space, and mates.

In the real world, phenotypic variation among individuals within a population means that some individuals will be better adapted to their environment than others. The resulting competition between population members of the same species for resources is termed intraspecific competition (intra- = “within” -specific = “species”). Intraspecific competition for resources may not affect populations that are well below their carrying capacity—resources are plentiful and all individuals can obtain what they need. However, as population size increases, this competition intensifies. In addition, the accumulation of waste products can reduce an environment’s carrying capacity.

Examples of Logistic Growth

Yeast, a microscopic fungus used to make bread and alcoholic beverages, exhibits the classical S-shaped curve when grown in a test tube (Figure 45.10a). Its growth levels off as the population depletes the nutrients. In the real world, however, there are variations to this idealized curve. Examples in wild populations include sheep and harbor seals (Figure 45.10b). In both examples, the population size exceeds the carrying capacity for short periods of time and then falls below the carrying capacity afterwards. This fluctuation in population size continues to occur as the population oscillates around its carrying capacity. Still, even with this oscillation, the logistic model is confirmed.

Visual Connection

If the major food source of the seals declines due to pollution or overfishing, which of the following would likely occur?


What are the limiting factors of pathogen population size in human populations? - Biology

5.3.2 Populations and Sustainability

a) explain the significance of limiting factors in determining the final size of a population

  • A habitat cannot support a population ledger because of factors that limit poplulation size.
  • Factors:
    • food
    • water
    • light
    • oxygen
    • space
    • shelter
    • parasites/predators
    • competition

    b) explain the meaning of the term carrying capacity

    c) describe predator–prey relationships and their possible effects on the population sizes of both the predator and the prey

    • Predation can act is a limiting factor on the population of prey which can then be a limiting factor on the population of the predator.
    • Predators population increases hence more prey are eaten
    • Prey population gets smaller hence less food for predators
    • Less food hence fewer predators survive
    • Fewer predators hence fewer prey eaten hence prey population increases
    • More prey hence more food hence higher predator population

    d) explain, with examples, the terms interspecific and intraspecific competition

    • Intraspecific
      • Competition between members of the same species.
      • Survival of the fittest
        • The best adapted will survive
        • Population size decreases hence competition decreases hence population increases
        • Population size increases hence competition increases hence population decreases
        • Competition between different species hence can effect the population size of a species and the distribution
        • Example:
          • Two species of Paramecium - Paramecium aurelia and Paramecium caudatum
          • Both occupied the same niche however Paramecium aurelia was better adapted
          • Paramecium caudatum died out
          • This is know as competitive exclusion principle

          e) distinguish between the terms conservation and preservation (HSW6a, 6b)

          • Conservation
            • Active management and reclamation of land
            • Protecting land and leaving it in it's untouched form e.g. National Parks

            f) explain how the management of an ecosystem can provide resources in a sustainable way, with reference to timber production in a temperate country

            • Sustainable Management
              • Sustainable management means maintaining biodiversity whilst also financially securing timber production companies.
              • Coppicing
                • Cutting a deciduous tree close to the ground to encourage shoots to grow
                • These shoots can be cut and used for fencing, firewood or furniture
                • Once cut new shoots grow and the cycle continues
                • Same as coppicing but higher up
                • This is to keep them out of reach of herbivores like deers
                • Dividing a woodland into sections and cutting down different sections at a time to allow the other sections to regrow
                • Some trees are left to produce larger timber these are called standards
                • Very good for biodiversity as unmanaged woodlands end up going through secondary succession thus blocking out light to the woodland floor
                • Clear-felling
                  • All the trees in an area are cut down
                  • This reduces mineral levels and leaves soil susceptible to erosion
                  • Leaving each section of the woodland for 50-100 years before felling is economically unviable
                  • Modern Sustainable Practices:
                    • Any tree harvested is replaced by another tree
                    • The ecological function of a forrest is not disturbed by the extraction of timber
                    • Local people derive benefit
                    • Cut down only the most valuable trees hence the biodiversity in maintained and the habitat unaffected
                    • control pests and pathogens
                    • only plant tree species they know will grow well
                    • position trees optimal distances apart

                    g) explain that conservation is a dynamic process involving management and reclamation

                    • Conservation required careful management to maintain a stable community
                    • Strategies to manage conservation:
                      • Increasing carrying capacity by providing more food
                      • Move individuals to enlarge populations or help with the natural dispersion
                      • Restrict dispersal of individuals by fencing
                      • Control predators and poachers
                      • Vaccination against diseases
                      • Preservation of habitats

                      h) discuss the economic, social and ethical reasons for conservation of biological resources (HSW6b, 7c)

                      • Many species are a valuable food source
                      • Genetic diversity of wild strains may provide useful characteristics in the future
                      • Provide access to drugs that we may use in the future
                      • Natural predators of pests can act as biological control agents
                      • Wild insects help pollinate plants
                      • Reduction in biodiversity leads to reduced climate stability

                      i) outline, with examples, the effects of human activities on the animal and plant populations in the Galapagos Islands (HSW6b).

                      • Habitat Disturbance
                        • Dramatic increase in population has placed demands on water, energy and sanitation services
                        • Increases pollution, expansion of agricultural land and building have destroyed the habitat
                        • Seal and whale hunters killed large populations of the animals faster than they could replenish
                        • Tortoises need little food and could be stored on ships for long time as a source of food
                        • Demand for exotic marine life such as the sea cucumber and shark fins have devastated populations
                        • Humans on purpose brought some species on to the islands such as goats, cats, fruits and vegetables and not on purpose brought over other species such as rats and insects.
                        • These new species out competed the locals, destroyed native habitats and just out right ate the locals.
                        • Combated this by:
                          • Adding a new quarantine system to prevent the introduction of non native species by tourists
                          • Natural predators exploited to kill pests
                          • Culling of feral goats and pigs

                          “A man who dares to waste one hour of time has not discovered the value of life.” -Charles Darwin (Discovered Theory of Evolution) />


                          Limiting Factors

                          A rabbit can raise up to seven litters a year. So why are we not overrun with rabbits? In nature, limiting factors act on populations to keep them in check.

                          Rabbits in the Field

                          Female cottontail rabbits (Sylvilagus floridanus) are especially fertile, able to give birth to seven litters a year. While this would suggest areas with cottontail rabbits would be overrun by them, but this isn't the case. Rabbit populations are restricted by traits like food availability and predation.

                          Photograph by Thai Yuan Lim/EyeEm

                          A female cottontail rabbit (Sylvilagus floridanus) can give birth as often as seven times a year. A female American toad (Anaxyrus americanus) can lay thousands of eggs every spring. So why are the meadows and forests of the eastern United States not literally hopping with rabbits and toads? In nature, the size of a population and the rate of population growth are influenced by what ecologists call &ldquolimiting factors.&rdquo

                          Take It to the Limit

                          Think about all the different resources that two common animals need to stay alive. Cottontail rabbits need food to eat (grasses and other plants), water to drink, and a safe place to raise their young. American toads eat insects and, though they often live in forest habitat, need ponds or puddles to lay their eggs. Both toads and rabbits have to watch out for predators. But even if they avoid a hungry hawk or snake, they face other potentially deadly dangers, including diseases, forest fires, or drought.

                          Any of these factors&mdashfood, shelter, breeding sites, predators, and more&mdashmay serve to limit the growth of a rabbit or toad population. Often, the population is affected by several limiting factors that act together.

                          Density Matters&mdashUnless It Does Not

                          Limiting factors fall into two broad categories: density-dependent factors and density-independent factors. These names mean just what they say: Density-independent factors have an impact on the population, whether the population is large or small, growing or shrinking. For example, a wildfire that sweeps through a dense forest in the Everglades has a big impact on every population in the community, regardless of the density of any one population.

                          Wildfire is abiotic (nonliving), and most density-independent limiting factors fall in this category. Other density-independent factors include hurricanes, pollutants, and seasonal climate extremes.

                          Density-dependent limiting factors tend to be biotic&mdashhaving to do with living organisms. Competition and predation are two important examples of density-dependent factors.

                          Mountain chickadees (Parus gambeli) compete for a special kind of nest site&mdashtree holes. These little cavities are excavated and then abandoned by woodpeckers. Scientists who added new nest sites in one expanse of forest saw the chickadee nesting population increase significantly, suggesting that nest sites are a density-dependent limiting factor.

                          A small furry rodent found in eastern Greenland called the collared lemming (Dicrostonyx groenlandicus) is a good example of how predation can be a density-dependent limiting factor. The population goes through a boom-and-bust cycle every four years. The lemming population grows to as much as 1,000 times its initial size, then crashes.

                          The cause is stoats (Mustela erminea), a type of weasel that hunts and eats lemmings almost exclusively. Stoats do not reproduce as fast as lemmings, so after a crash, when both stoat and lemming numbers are low, stoats do not have much impact on the lemming population. But by the fourth year, after the stoat population has had time to grow to greater numbers, the stoats&mdashtogether with other predators&mdashcause another lemming crash, and the cycle continues.

                          If a population is small and resources are plentiful, a population may grow quickly. But over time, because of limiting factors, population growth tends to slow and then stop. The population has reached the &ldquocarrying capacity&rdquo of the ecosystem.


                          Density-independent factor

                          Our editors will review what you’ve submitted and determine whether to revise the article.

                          Density-independent factor, also called limiting factor, in ecology, any force that affects the size of a population of living things regardless of the density of the population (the number of individuals per unit area). Density-independent factors often arise from physical and chemical (rather than biological) phenomena.

                          Such factors stemming from weather and climate—as well as flooding, wildfires, landslides, and other disasters—affect a population of living things whether individuals are clustered close together or spaced far apart. For example, for most organisms that breathe oxygen, oxygen availability is a density-independent factor if oxygen concentrations decline or breathable oxygen is suddenly made unavailable, such as when oxygen-using plants are covered by rising floodwaters, those organisms perish and populations of the various affected plant species decline.

                          The dynamics of most populations of living things are influenced by a combination of density-independent factors and density-dependent factors (that is, those factors that emerge when the concentrations of individuals in a population rise above a certain level). The relative importance of these factors varies among species and populations.

                          The Editors of Encyclopaedia Britannica This article was most recently revised and updated by John P. Rafferty, Editor.


                          Watch the video: Limiting Factors in an Ecosystem (May 2022).


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