{"id":9562,"date":"2026-06-01T21:33:48","date_gmt":"2026-06-01T21:33:48","guid":{"rendered":"https:\/\/kapdec.com\/help\/?p=9562"},"modified":"2026-06-01T21:33:48","modified_gmt":"2026-06-01T21:33:48","slug":"population-ecology-and-effect-of-density","status":"publish","type":"post","link":"https:\/\/kapdec.com\/help\/population-ecology-and-effect-of-density\/","title":{"rendered":"Population Ecology And Effect Of Density"},"content":{"rendered":"

Unit : Ecology<\/strong><\/h2>\n

Chapter: Population ecology and effect of density on populations<\/strong><\/h3>\n

Reference:<\/em><\/strong> Population attributes, growth models, life history variations, Community interactions<\/em>                                          <\/em><\/strong><\/p>\n

Learning objectives<\/strong><\/p>\n

    \n
  • To understand population attributes, growth models<\/li>\n
  • To learn life history variation and population interactions<\/li>\n<\/ul>\n

    Population attributes<\/strong><\/p>\n

      \n
    • Population is total number of interbreeding individuals of species found in a geographical area, sharing, and competing for similar resources.<\/li>\n
    • Population density- the number of individuals per square unit area (in terrestrial organisms) or per cubic unit area (in aquatic or aerial organisms). Sometimes the population density is not required and only Relative density serves the purpose. For example, the fish caught per trap in a lake can be used as population density.<\/li>\n
    • A set of local populations connected by dispersing individuals is called a metapopulation.<\/strong><\/li>\n
    • Age distribution Pyramids<\/strong> – For constructing age pyramids, 3-age groups are taken into consideration.<\/li>\n<\/ul>\n

      1. pre-reproductive age (0-14 years)<\/p>\n

      2. Reproductive age (15-60 years).<\/p>\n

      3. Post reproductive age (> 60 years).<\/p>\n

      There are 3-types of pyramids-<\/p>\n

      1. Triangular<\/strong>-It indicates expanding population with high growth rate.<\/p>\n

      2. Bell shaped-<\/strong> It indicates stable population with Zero growth rate<\/p>\n

      3. Urn-shaped-<\/strong> It indicates declining population with growth rate in minus.<\/p>\n

      \"\"<\/p>\n

        \n
      • The gross value of population growth is calculated as the difference of birth rate and death rate.<\/li>\n
      • The population density is not a static parameter and keeps changing in times. The population size depends on the availability of food, weather conditions, predation pressure etc. The population size or density in each area or habitat, during a given period depends upon the 4- basic processes- natality, mortality, emigration, and immigration.<\/li>\n
      • If N is the population density at time t, then density at time (t + 1) will be N (t + 1) = N(t) + [ (B + I) – (D + E)] Until and unless a new habitat is being colonized, the birth and death are the most important factors for influencing the population density.<\/li>\n<\/ul>\n

        \"\"<\/p>\n

          \n
        • Population growth = (Birth rate + Immigrants) – (Death rate + Emigrants)<\/strong><\/li>\n
        • Birth rate or natality rate-<\/strong> It is the number of births per thousand of a population per year. It can also be represented in percentage, or per capita, i.e., per individual.<\/li>\n
        • Death rate or mortality rate-<\/strong> It is the number of deaths occurring in a population of one thousand per year. This can also be represented in percentage or per capita.<\/li>\n
        • Immigration- <\/strong>The movement of individuals into a population.<\/li>\n
        • Emigration-<\/strong> movement, out of the population.<\/li>\n
        • Zero Population Growth or demographic transition–<\/strong> When birth rate equals death rate, and the growth of the population is Zero, i.e., the size of the population remains constant. Considering migration, at zero population growth- Birth rate + immigrants = Death rate + emigrants.<\/li>\n
        • Growth models-<\/strong><\/li>\n<\/ul>\n
            \n
          1. Exponential growth model-<\/strong> When resources like food and space etc. in a habitat are unlimited, each species realizes its full reproductive or biotic potential and grows in an exponential or geometric fashion. Ex. Algal bloom, insects during rainy season. If the population size is ‘N’ and birth rate and death rate per capita are respectively ‘b’ and ‘d,’ then the change in population size, i.e., increase or decrease during a unit time period ‘t’, will be-  dN\/dT<\/u>=(b-d)<\/strong><\/li>\n<\/ol>\n

            If (b – d) = r, then dN\/dT =rN <\/strong><\/p>\n

            Here ‘r’ is called ‘Intrinsic rate of natural increase’ or Biotic potential (maximum capacity of reproduction), Which indicates the impact of biotic and abiotic factors in population growth.<\/p>\n

            \"\"<\/p>\n

            Fig.3. Population growth curve in J-shape.<\/strong><\/p>\n

              \n
            1. Logistic growth curve- <\/strong>In nature no species has unlimited resources at its disposal to permit exponential growth. There starts competition between individuals for limited resources. The population growing in a habitat initially shows ‘lag phase,’ followed by phases of acceleration or deceleration (log phase) and finally the stage of asymptote, when population density reaches carrying capacity (plateau phase or stationary phase). If the population density ‘N’ is plotted against time ‘t,’ the result is ‘S’ shaped growth pattern\/ curve (Sigmoid curve). This type of growth pattern is also called as ‘Verhulst-Pearl logistic growth’ and can be represented as following equation.<\/li>\n<\/ol>\n

              dN\/dT=rN(K -N\/K)<\/p>\n

              K = Nature is carrying capacity in that habitat (K – N)\/ K or 1– N\/ K = environmental resistance.<\/p>\n

              \"\"<\/p>\n

              Fig.4. Population growth curve in sigmoid or S-shape<\/strong>.<\/p>\n

              Carrying capacity (K) –<\/strong> It can be defi ned as the maximum number of individuals which the environment can support or sustain. Hence, it is the capacity of environment and not of individuals.<\/p>\n

              Environment resistance–<\/strong> The factors like predators, diseases, calamities, shortage of food etc. which impose a check on population, form environmental resistance.<\/p>\n

              \"\"<\/p>\n

              Life History Variation<\/strong><\/p>\n

              The populations evolve to maximize their reproductive fitness with high ‘r’ value, in the given habitat. This, organisms do by developing the most efficient reproductive strategy under the given selection pressure.<\/p>\n

              There are two type of population growth-<\/p>\n

                \n
              1. R-selection-<\/strong> abundance of resources, natality is high, size of individual is small and population seldom reaches carrying capacity due to high degree of predation.<\/li>\n<\/ol>\n

                Ex. Population of rabbit<\/p>\n

                  \n
                1. K-selection-<\/strong> natality is low, individual size is large, longevity is so high that population exists at carrying capacity. Ex. Population of wolf.<\/li>\n<\/ol>\n

                  \"\"<\/p>\n

                  \"\"<\/p>\n

                  1. Parasitism-<\/strong> one species (Parasite) is benefitted and the other (Host) is harmed (+, –). Ex. Cuscuta, Petromyzon<\/em>, hagfishes, koel or cuckoo is a brood parasite, liver fluke. The parasites have following types of special adaptations for their survival<\/p>\n

                    \n
                  • Loss of unnecessary sense organs<\/li>\n
                  • Presence of suckers or adhesive organs to attach to the host body<\/li>\n
                  • Loss of digestive<\/li>\n
                  • High reproductive capacity<\/li>\n
                  • Complex life cycle with one or two hosts<\/li>\n<\/ul>\n

                    2. Commensalism-<\/strong>one species is benefitted and other is neutral i.e., neither benefitted nor harmed (+ ,0). Ex. egret and the cattle, orchid growing as an epiphyte on mango tree, clown fish hiding in the colony of sea anemone, barnacle growing on the back of whale, orchid growing as an epiphyte on mango tree.<\/p>\n

                    3.<\/strong> Mutualism-<\/strong> interaction both the species are benefitted (+, +). Ex. Lichens, Mycorrhizae, female wasp pollinates the fig inflorescence while searching for suitable site for egg laying. The Mediterranean orchid Ophrys employs ‘sexual deceit’ to get pollination done by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour, and markings. The male bee is attracted to what it perceives as a female, ‘pseudo copulates’ with the flower, and during that process is dusted with pollen from the flower. When this same bee ‘pseudo copulates’ with another flower, it transfers pollen to it and thus, pollinates the flower<\/p>\n

                    4.Predation- <\/strong>interspecific interaction also one species is benefitted and the other is harmed (+, –). Ex. tiger and deer, cat and mouse, and lion and zebra, introduction of prickly pear (cactus) into Australia; Nile perch into Lake Victoria (East Africa) and Eichhornia into water bodies in India. The other examples of exotic species that spread rapidly due to absence of predators are Parthenium, Lantana and Clarias etc.<\/p>\n

                    5.Competition- <\/strong>interspecific process in which the fitness of one species (measured in terms of ‘r’) is significantly lower in the presence of another species. It is (–, –) relationship. Ex. the Flamingos (birds), visiting S. American lakes, compete with the resident fishes for the common food, zooplanktons.<\/p>\n

                    Interference Competition-<\/strong> feeding efficiency of one species may be reduced due to interfering presence of other species. The species facing competition may sometimes evolve such mechanisms that promote co-existence rather than exclusion. One such mechanism is ‘Resource partitioning,’<\/strong> i.e., avoiding competition by either choosing different timings of feeding or behavioural differences in feeding pattern.<\/p>\n

                    Competitive release-<\/strong> It has also been found in nature that when a competitively superior species is experimentally removed, a species whose distribution is restricted to small geographical area expands its distributional range dramatically and expands into a larger area.<\/p>\n

                    6.Ammensalism- <\/strong>association one species is harmed but the other remains neutral (–, 0). Ex. Penicillium notatum growing close to bacteria.<\/p>\n

                    Density dependent limiting factor<\/strong><\/p>\n

                    Density-dependent factors include disease, competition, and predation. Density-dependant factors can have either a positive or a negative correlation to population size. With a positive relationship, these limiting factors increase with the size of the population and limit growth as population size increases.<\/p>\n

                    Solved examples<\/strong><\/p>\n

                    Example 1<\/strong>. These types of organisms which multiply rapidly in numbers<\/p>\n

                      \n
                    1. R-strategists   b) K-strategists c) predators   d) mutualist<\/li>\n<\/ol>\n

                      Solution 1:<\/strong> a. R-strategists multiply rapidly in numbers.<\/p>\n

                      Example 2<\/strong>. If the number of births in a population is the same as the number of deaths in a population, what will happen to the growth of the population?<\/p>\n

                      a) It will increase.  b) It will decrease.<\/p>\n

                      c) It will stay the same.  d) It will fluctuate.<\/p>\n

                      Solution 2:<\/strong> If the number of births in a population is the same as the number of deaths in a population, the growth of the population will increase.<\/p>\n

                      Summary <\/strong><\/p>\n

                        \n
                      • Ecological factors regulate population size, proved from population number, population biomass, etc. and vary in species to species.<\/li>\n
                      • Population growth depends upon the resources available, which rise exponentially when resources are in abundance and show logistic growth when they are limited.<\/li>\n
                      • Also, birth and migration add up population while death and emigration decrease population.<\/li>\n
                      • Species often interact with each other in many ways such as mutualism, commensalism, predation, ammensalism, competition and parasitism.<\/li>\n<\/ul>\n

                         <\/p>\n

                         <\/p>\n","protected":false},"excerpt":{"rendered":"

                        Unit : Ecology Chapter: Population ecology and effect of density on populations Reference: Population attributes, growth models, life history variations, Community interactions                                           Learning objectives To understand population attributes, growth models To learn life history variation […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[629],"tags":[],"class_list":["post-9562","post","type-post","status-publish","format-standard","hentry","category-ap-biology"],"_links":{"self":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts\/9562","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/comments?post=9562"}],"version-history":[{"count":0,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/posts\/9562\/revisions"}],"wp:attachment":[{"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/media?parent=9562"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/categories?post=9562"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/kapdec.com\/help\/wp-json\/wp\/v2\/tags?post=9562"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}