## Over time the populations of most species

## This film explores the foundational research in the serengeti

Consider a population of 1,000 people who were all born at the same time and in the same town. As time goes by, several people die, and each year there are less and fewer people left. But when do the majority of people die? Do most people live to be old, or do many people die at a young age? Survivorship curves are used by ecologists to show how the number of individuals in a population decreases over time. Ecologists define a generation, which is a group of individuals of the same species born at the same time in the same population, in order to quantify a population. When each person in a population dies, data is collected. Generations, populations, and even separate organisms can all be compared using survival curves. Survivorship curves are graphs that show how many people in a cohort have survived: Cohorts can be used to explain the survival of a population if they are identical over time. Since survivorship varies so much depending on the climate, this metric isn’t generally thought of as a species property. Aside from the general life history strategy of a species, biotic and abiotic factors such as competition and temperature may influence the shape of survivorship curves.

## The logistic equation and models for population – example 1

Understanding what determines the abundance of species within a population and why this abundance shifts over time is a major focus of modern ecological research. A population is a group of individuals of the same species that remain in the same place for a period of time. The way a species’ populations change over time is referred to as population dynamics. The aim of studying a species’ population dynamics is typically to find answers to questions like:

There are many mechanisms going on at the same time that can have an effect on population size and dynamics. First, the per capita population growth rate, or the rate at which the population size increases per person in the population, has an effect on population size. The birth, death, emigration, and migration rates in the population decide this growth rate. The population will experience exponential growth followed by exponential decline if the per capita growth rate remains constant. Charles Darwin was one of the first scientists to recognize that rapid population growth would result in major mortality events, which he linked to evolutionary changes in heritable traits or genes. The intrinsic rate of increase refers to a population’s overall per capita growth rate.

### Cbse class 12 biology || organisms and populations || full

To explain the rate of change in the size of a population over time, the two simplest models of population growth use deterministic equations (equations that do not account for random events). The first of these models, exponential growth, describes imaginary populations that rise in size without ever reaching a limit. The second model, logistic development, imposes reproductive growth constraints that become more severe as the population grows. Neither model accurately represents natural populations, but they do include comparisons.

The English clergyman Thomas Malthus inspired Charles Darwin in establishing his theory of natural selection. In 1798, Malthus published his book, claiming that populations with ample natural resources grow rapidly, but that further growth is limited by resource depletion. Exponential growth is the term for the early trend of growing population size.

Bacteria provide the best example of exponential growth in species. Bacteria are prokaryotes that depend on binary fission to replicate. For several bacterial species, this division takes about an hour. If 1000 bacteria are put in a large flask with an abundant supply of nutrients (so that the nutrients do not become exhausted quickly), the number of bacteria would have doubled from 1000 to 2000 in just one hour. Each of the 2000 bacteria will divide in an hour, yielding 4000 bacteria. After the third hour, the flask should contain 8000 bacteria. The main principle of exponential growth is that the rate of growth—the amount of species added with each reproductive generation—is increasing; in other words, the population size is expanding at an ever-increasing rate. The population would have risen from 1000 to more than 16 billion bacteria after 24 cycles. A J-shaped growth curve is created when the population size, N, is plotted over time ([Figure 1]a).

### 14. species and speciation

Changes in population size over time are referred to as population growth. Figure (PageIndex1) depicts one method of population growth: adding babies by births. Can you think of any other ways that a population’s size changes over time?

Populations evolve over time. They are constantly adding and subtracting people as a result of births and deaths. When people join or leave a community, migration may result in a substantial increase or decrease in population size. All of these variables work together to decide when and how rapidly a population increases. The rate of population growth will fluctuate over time. Exponential and logistic growth are two well-studied trends of population growth rate change.

Figure 2: (PageIndex2) Curve A depicts a population growth curve that is exponential. It’s worth noting that this population is increasingly increasing. Curve B depicts a population growth logistic sequence. The carrying ability is reached when the population size reaches a certain amount.