Diversity is not only a characteristic of living organisms but

11.1 Populations

Ecology is the branch of biology that studies the interactions among organisms and between organisms and their physical (abiotic) environment. It is concerned with four levels of biological organization: organisms, populations, communities, and biomes. This section focuses on populations, which are groups of individuals of the same species inhabiting a defined geographical area, sharing or competing for similar resources, and potentially interbreeding. Although interbreeding implies sexual reproduction, populations resulting from asexual reproduction are also considered for ecological studies. Examples include all cormorants in a wetland, rats in an abandoned building, teakwood trees in a forest, bacteria in a culture plate, and lotus plants in a pond. Population ecology is significant because evolutionary changes through natural selection occur at the population level. While an individual organism experiences environmental changes, natural selection acts on populations to evolve desired traits. Populations have attributes that individuals do not, such as birth rates, death rates, sex ratio, and age distribution. Birth and death rates in populations are expressed as per capita rates, indicating the number of births or deaths per individual per unit time. For example, if 8 new lotus plants are added to a population of 20 in a year, the birth rate is 8/20 = 0.4 offspring per plant per year. Similarly, if 4 fruit flies die in a population of 40 over a week, the death rate is 4/40 = 0.1 individuals per fruit fly per week. Sex ratio is another important attribute, representing the proportion of males and females in a population, such as 60% females and 40% males. Age distribution refers to the proportion of individuals of different ages in a population. When plotted, this distribution forms an age pyramid, which can indicate whether a population is growing, stable, or declining. For humans, age pyramids typically show the distribution of males and females in different age groups. Population size, or population density (denoted as N), is a key indicator of population status in a habitat. It can range from very small numbers, such as fewer than 10 Siberian cranes in Bharatpur wetlands, to millions, such as Chlamydomonas algae in a pond. Population density is usually measured by total number but can also be expressed as percentage cover or biomass, especially when comparing species of vastly different sizes or ecological roles, like carrot grass and a banyan tree. In some cases, absolute counts are impractical; relative density measures, such as the number of fish caught per trap, are used instead. Indirect methods like counting pug marks or fecal pellets are employed for elusive species like tigers. Understanding populations and their attributes is foundational to studying population dynamics, growth, interactions, and ecological roles in subsequent sections.

• Population: group of individuals of the same species in a defined area sharing resources and potentially interbreeding
• Population attributes include birth rate, death rate, sex ratio, and age distribution
• Birth and death rates are expressed as per capita rates (per individual per unit time)
• Sex ratio indicates proportion of males and females in a population
• Age distribution plotted as age pyramid reflects population growth status
• Population size (density) can be measured by number, biomass, or percent cover depending on species
• 📌 Population: group of individuals of the same species in a defined geographical area
• 📌 Birth rate: number of births per individual per unit time in a population
• 📌 Death rate: number of deaths per individual per unit time in a population

11.1.2 Population Growth

Population size is dynamic and changes over time due to four basic processes: natality (births), mortality (deaths), immigration (arrival of new individuals), and emigration (departure of individuals). Natality and immigration increase population density, while mortality and emigration decrease it. Mathematically, if N is the population density at time t, then the population density at time t + 1 is given by: N_(t+1) = N_t + [(B + I) - (D + E)] where B = number of births, I = number of immigrants, D = number of deaths, and E = number of emigrants during the time interval. Under normal conditions, births and deaths are the primary factors influencing population density; immigration and emigration become significant mainly during colonization or migration events. Population growth patterns can be modeled to understand how populations change over time: (i) Exponential Growth: When resources such as food and space are unlimited, populations grow exponentially. The per capita birth rate is b, and the per capita death rate is d. The rate of change of population size N with respect to time t is: dN/dt = (b - d) × N Let r = (b - d), called the intrinsic rate of natural increase, then: dN/dt = rN This results in a J-shaped growth curve where population size increases rapidly without constraints. The integral form is: N_t = N_0 × e^(rt) where N_0 is the initial population size, e is the base of natural logarithms (~2.71828), and t is time. Examples of r values include 0.015 for Norway rat, 0.12 for flour beetle, and 0.0205 for human population in India in 1981. (ii) Logistic Growth: In natural habitats, resources are limited, leading to competition and limiting population growth. The maximum population size that a habitat can sustain is called the carrying capacity (K). Logistic growth initially shows a lag phase, followed by acceleration, deceleration, and finally stabilizes at K, forming an S-shaped or sigmoid curve. The logistic growth equation is: dN/dt = rN × ((K - N) / K) where N is population size at time t, r is intrinsic rate of increase, and K is carrying capacity. This model is more realistic for natural populations where resources become limiting. Governments use knowledge of population growth to implement measures to control human population expansion. Understanding these growth models helps in managing wildlife, conserving endangered species, and controlling pests.

• Population density changes due to natality, mortality, immigration, and emigration
• Population growth can be exponential (unlimited resources) or logistic (limited resources)
• Intrinsic rate of natural increase (r) = per capita birth rate - per capita death rate
• Exponential growth produces J-shaped curve; logistic growth produces S-shaped (sigmoid) curve
• Carrying capacity (K) is the maximum population size a habitat can support
• Logistic growth accounts for resource limitation and competition
• 📌 Natality: number of births in a population during a given time
• 📌 Mortality: number of deaths in a population during a given time
• 📌 Immigration: arrival of individuals into a population