Populations

POPULATIONS A collection of individuals from the same species dwelling in a specific geographic area make up a population. Local populations or demes, which are subgroups of populations, refer to the organisms living in a particular area. For example, Homo sapiens inhabiting hills or plains. Sister populations are distinct groups of the same species found in different locations. Hill ↓ Plains ↓ Local population → Local population ↓ Sister population Meta population - A set of local population which are interconnected by dispersing individuals. Population Attributes (A) Birth rate and Death rate: Populations exhibit distinct characteristics that differ from individual organisms. While individuals. experience birth and death, populations express birth rate (natality) and death rate (mortality). Within a population, these rates are referred to as per capita birth and death rates. For instance, if a pond had 20 lotus plants last year and 8 new plants have been added, the current population is 28. Therefore, the birth rate of the population is calculated as 0.4. If, out of the 20 lotus plants, 4 have died, then the death rate of the population is 0.2. (B) Sex ratio: An individual is either male or female, but a population has sex ratio. i.e., ratio of male and female like 60% of population are female and 40% are male. (C) Age pyramid: At any particular moment, a population consists of individuals across various age groups. When the percentage of individuals of different ages or age groups within a population is represented graphically, it forms an Age Pyramid. In the case of the human population, the age pyramid typically displays the age distribution of both males and females in a single diagram. Age pyramid reflects the growth status of the population. i.e., (A) Growing (B) Stable (C) Declining The reproductive status of a population is influenced by its composition across different age groups. Ecologically, populations are often categorized into three age stages: Prereproductive, Reproductive, and Post-reproductive. The distribution of age groups within a population significantly impacts its growth patterns. Three types of age pyramids have been recognized - Triangular, Bell shaped, Urn shaped age pyramids. Class 12 NEET Biology (a) Triangular age pyramid In this specific type of age pyramid, the population is characterized by a significant number of pre-reproductive individuals compared to both reproductive and post-reproductive individuals. This imbalance suggests a potential for considerable population growth in the future. Postreproductive Reproductive Prereproductive Expanding Population (i) Triangular Stable Population (ii) Bell shaped Declining Population (iii) Urn shaped Fig: Human population - Representation of age pyramids (b) Bell shaped age pyramid Pre-reproductive and reproductive individuals are almost equal in number and the individuals of post-reproductive age are fewer. Thus population is stable. (c) Urn shaped age pyramids Population of Reproductive age group is more than prereproductive age group. Number of postreproductive individuals is also sizeable. Thus population shows negative growth. Population Density or Population size: Population density refers to the measure of how crowded a population is within a specific area or volume. It is calculated by dividing the total number of individuals by the area or volume they occupy at a given moment. For instance, there might be 50 individuals of a tree species per hectare or 70 individuals of a grass species cynodon per square meter. The density of plants is determined through the study of a predetermined area size. Human population census, or counting, is conducted every decade. Species density fluctuates over time and across different geographical regions. Typically, there is a higher density of plants during the rainy season compared to the dry season.. The size of a population is influenced by the availability of resources such as nutrients and water at any given time. (E) Biotic potential and Environmental resistance: Biotic potential The inherent maximum capacity of an organism to reproduce or increase in number is termed as biotic potential, represented by the symbol 'r'. Biotic potential can only be fully realized when environmental conditions are extremely favorable, leading to the highest possible birth rate and the lowest possible death rate. Under such conditions, the population size increases at its maximum rate. Environmental Resistance The environment regulates the size of populations by affecting their biotic potential. As the population grows, nature imposes greater resistance, which means it becomes harder for the population to keep expanding. Environmental resistance includes factors like the availability of water and space, as well as competition for food. These factors prevent organisms from reaching their maximum potential for growth, thus keeping the population size relatively lower. The equation dN/dt = rN demonstrates a population's geometric or exponential growth, which is visualized as a J-shaped curve. However, population decline can swiftly occur due to limited resources or environmental obstacles. The mentioned equation illustrates the exponential or geometric growth of a population, leading to a J-shaped curve when plotted against time. If you possess a basic understanding of calculus, you can derive the integral form of the exponential growth equation. where N= No ert NPopulation density after time t No Population density at time zero r = intrinsic rate of natural increase e the base of natural logarithms (2.71828) (2) Logistic growth: In nature, a habitat can only sustain a specific number of individuals from a population due to its limited resources. This maximum limit is referred to as the carrying capacity (K). When resources are limited, a growing population experiences a delay in growth initially, followed by phases of increase and decrease, until it stabilizes at the carrying capacity. This pattern is known as Verhulst-Pearl logistic growth. dN/dtr N K-N K NPopulation density at a time time t r Intrinsic rate of natural increase K= Carrying capacity It is also called S or sigmoid growth form. Thus logistic growth model is more realistic. Life History Variation Populations adapt over time to enhance their ability to reproduce, known as Darwinian fitness (measured by a high 'r' value), within their specific habitat. In response to various selection pressures, organisms evolve towards the most effective reproductive strategies. Some organisms, like Pacific salmon fish and bamboo, breed only once in their lifetime, while others, such as most birds and mammals, breed multiple times. Additionally, some species produce numerous small offspring (like oysters and pelagic fishes), while others produce fewer, larger offspring (like birds and mammals). Determining which strategy is most advantageous for maximizing fitness depends on the ecological context. Ecologists suggest that the life history traits of organisms evolve in response to the limitations imposed by the abiotic and biotic factors of their habitat. The study of how life history traits evolve in different species is currently a significant area of research among ecologists. Community/Biological Community/Biotic Community A biotic community refers to the arrangement of populations from various species within a habitat, where they rely on each other and interact. Nature hosts numerous biotic communities due to two main reasons: 1. Diverse habitats exist with unique environmental conditions. 2. Various species coexist whose tolerance ranges overlap with the environmental conditions of that habitat. When comparable conditions occur elsewhere, the same biotic community is established there as well. Each biotic community possesses ecological traits that set it apart from others.