Population growth
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What is population growth in biology?
Population growth in biology refers to changes in the number of individuals within a plant or animal species in a specific location over a defined time. Several factors influence this growth, including fertility and mortality rates. In animals, migration (immigration and emigration) also plays a significant role.
The population growth rate indicates whether a population is increasing (positive rate) or decreasing (negative rate). Environments have a carrying capacity, which is the maximum population size an environment can sustain based on available resources.
Notably, human activities can significantly impact population dynamics, often leading to biodiversity loss through habitat destruction, pollution, and climate change.
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How do scientists measure population growth?
Scientists calculate population growth by looking at a few key factors. For a basic understanding, in a closed population where people don't move in or out, the population at the end of a time period is the population at the beginning, plus births, minus deaths.
However, most populations are open, meaning people migrate in and out. So, the equation becomes: population at the end equals population at the beginning, plus births, minus deaths, plus immigrants, minus emigrants. Fertility, mortality, and migration are crucial in understanding population change, influencing population size, growth rates, and composition. Population growth rates are calculated by subtracting population loss (mortality and emigration) from population gain (fertility and immigration).
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What factors can cause a population to increase or decrease?
Population size is governed by births, deaths, and migration. A population increases through births and in-migration and decreases through deaths and out-migration. The rate of natural increase is the difference between the birth rate and the death rate.
Environmental factors also play a crucial role; limited resources, competition, and environmental stress factors influence fertility, mortality, and migration. Additionally, predator-prey relationships can affect population dynamics, causing oscillations in population size.
Life history strategies, such as the age of first reproduction and the allocation of resources to offspring, also affect population dynamics.
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How do birth rate and death rate affect population growth?
Birth and death rates are key factors in population growth. A closed population increases with births and decreases with deaths. The rate of natural increase is the birth rate minus the death rate. High birth rates and low death rates result in rapid population growth, while low birth rates and high death rates lead to population decline. Migration can also affect population size.
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What is carrying capacity and how does it limit population growth?
Carrying capacity is the average population size or density of a species that an environment can sustain without deterioration.
Geometric or exponential population growth slows due to ecological factors like food availability, competition for resources, predation, or disease. As competition increases, population growth slows, forming an S-shaped curve known as the logistic curve. When a population reaches its carrying capacity K, growth nears zero.
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How do human activities impact population growth in other species?
Human activities significantly influence population growth in other species through several key factors. Habitat loss and degradation, driven by agriculture and urbanization, reduce living spaces and food resources, leading to declines or extinctions, especially for species unable to migrate. The introduction of invasive species can outcompete native species, disrupting ecosystems and causing population declines.
Overexploitation, such as overfishing or hunting, depletes populations faster than they can recover, potentially leading to extinction and trophic cascades. Pollution introduces harmful substances into the environment, causing health and reproductive problems that threaten species survival. Climate change from burning fossil fuels alters temperature and precipitation patterns, challenging species' adaptive capacities and potentially causing significant extinction risks. These drivers, intensified by human population growth and resource consumption, often interact, exacerbating biodiversity loss.
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Population growth, in population ecology, a change in the number of members of a certain plant or animal species in a particular location during a particular time period. Factors affecting population growth include fertility, mortality, and, in animals, migrationâi.e., immigration to or emigration from a particular location. The average change in a population over time is referred to as the population growth rate. A positive growth rate indicates a population increase, and a negative growth rate indicates a population decrease. The upper limit of a population in a given environment, referred to as the environmentâs carrying capacity, is determined by the amount and availability of resources that are life-sustaining for that population.
Calculating population growth
Population growth rates in a given location and time period can be calculated by subtracting population loss, or the combined rates of mortality and emigration, from population gain, or the combined rates of fertility and immigration. Fertility is the number of offspring produced on average by an individual species member under certain environmental conditions over a period of time. (Fertility is not to be confused with fecundityâthe theoretical maximum number of offspring that can be produced by a species member in a given time period. The sex ratio and age structure of a population affect fertility rates, because they constrain the number of individuals capable of reproducing.
Population dynamics
The natural environment plays a crucial role in population growth through its carrying capacity. The life-sustaining resources in an environment are limited and can be reduced for individual species members by greater population density and competition from other species, among other factors. Moreover, fertility, mortality, and migration are all affected by food availability, mate availability, and environmental stress factors such as pollution and natural disasters.
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Population growth dynamics may be graphically depicted in an S-shaped curve, known as a logistic curve, as depicted in Figure 1. The logistic curve (right) represents an initial lag in growth, a burst of exponential growth, and finally a decline in population growth. When population density is high, mortality tends to increase because of competition for resources, predation, or increased disease transmission, resulting in the plateau in growth at the end of the curve. Population growth rates may also fluctuate in correlation with seasonal variations in the environment. For example, in deep lakes, a spring thaw causes colder, deeper waters to rise to the lakeâs surface, releasing nutrients that then cause bursts in the growth of plankton, including algae, bacteria, and protozoans (see water bloom).
The population growth of different species may also be determined by predator-prey relationshipsâfor example, following an increase in the population of zooplankton (minute aquatic animals), the population of their algae prey may decrease or grow at a slower rate. This results in an oscillating population pattern, such as the cyclical fluctuation of the snowshoe hare and lynx populations, as depicted in Figure 2.
Human population growth
The worldâs human population experienced exponential growth from the 18th century, but growth rates have been declining since the second half of the 20th century. Although population growth rates vary significantly between countries and some regions continue to experience increasing growth rates, the overall rate of growth is decreasing.
The worldâs human population reached 8 billion in November 2022 and is predicted to peak at 10.4 billion by 2080 and to remain at that level until the end of the century. Although human mortality rates have been decreasing on average, the main reason for the decreasing population growth rate is lower fertility. Fertility varies based on reproductive behaviour patterns, which in turn depend upon factors such as cultural traditions, socioeconomic conditions, access to contraception, and ecological variables, such as population density.
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Decreasing mortality among humans in recent decades has changed the age structure of the population. It has been predicted by the Population Division of the United Nations Department of Economic and Social Affairs that by 2050 the worldâs population of people age 65 or older will be twice as large as the population of children under age 5. That means that proportionately fewer people will be of reproductive age in the coming decades, which will contribute to a decline in population growth.