ORGANISMS AND POPULATIONS�

POPULATION:

• Population attributes:
• Population: a group of individual living in a well defined geographical area, share or compete for similar resources, potentially interbreed.
• Birth rate and death rate refers to per capita births and deaths respectively.
• Another attribute is sex ratio. The ratio between male female in a population.
• If the age distribution is plotted for a population the resulting structure is called age pyramid.
• The shape of the pyramids reflects the growth status of the population like growing, stable or declining.
• The population size is more technically called as population density.
• Methods for measurement of population density:
• Counting the number
• Percent cover
• Biomass.
• Pug marks and fecal pellets for tiger census

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Population growth:

• The size of the population changes depending on food availability, predation pressure and reduce weather.
• Population size fluctuated due to changes in four basic processes, two of which (Natality and immigration) contribute an increase in population density and two (mortality and emigration) to a decrease.
• Natality: number of birth in given period in the population.
• Mortality: number of deaths in the population in a given period of time.
• Immigration: is the number of individuals of same species that have come into the habitat from elsewhere during a given period of time.
• Emigration: number of individuals of the population who left the habitat and gone elsewhere during a given time period.
•   If  ‘N’  is  the  population  density  at  time  ‘t’,  then  its  density  at  time  t  +  1  is  :

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Exponential growth

• The Exponential growth equation is  N_t = N₀e^rt
• Nt  =  Population  density  after  time  t
• N0  =  Population  density  at  time  zero
• r  =  intrinsic  rate  of  natural  increase
• e  =  the  base  of  natural  logarithms  (2.71828)

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Exponential growth (‘J’ shape curve is obtained).

�When resources are not limiting the growth.

�Any  species  growth  exponentially  under  unlimited  resources  conditions  can  reach enormous  population  densities  in  a  short  time.

�  Growth is not so realistic.�

• Logistic growth model

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VerhulstPearl  Logistic  Growth  is  described  by  the  following  equations 

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dN/dt  =  rN  (K–N  /  N)

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• Where  N  =  Population  density  at  time  t
• r  =  Intrinsic  rate  of  natural  increase
• K  =  Carrying  capacity

Logistic Growth (Sigmoid curve is obtained)

• When responses are limiting the Growth.
• Resources  for  growth  for  most  animal  populations  are  finite  and  become  limiting.
• The logistic growth model is a more realistic one.

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POPULATION INTERACTIONS:

• Predation:
• Organism of higher trophic level (predator) feeds on organism of lower trophic level (prey) is called the predation.
• Even the herbivores are not very different from predator.
• Predator acts as a passage for transfer of energy across trophic level.
• Predators keep prey populations under control.
• Exotic species have no natural predator hence they grow very rapidly. (prickly pear cactus introduced in Australia created problem)
• Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. (Pisaster starfish field experiment)

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Defense developed by prey against predators:

• Animals:
• Insects and frogs are cryptically coloured (camouflaged) to avoid being detected by the predator.
• Some are poisonous and therefore avoided by the predators.
• Monarch butterfly is highly distasteful to its predator (bird) due to presence of special chemical it its body. The chemical acquired by feeding a poisonous weed during caterpillar stage.
• Plants:
• Thorns in Acacia, Cactus are morphological means of defense.
• Many plants produce and store some chemical which make the herbivore sick if eaten, inhibit feeding, digestion disrupt reproduction, even kill the predators.
• Calotropis produces poisonous cardiac glycosides against herbivores.
•  Nicotine, caffeine, quinine, strychnine, opium etc. are produced by plant actually as defenses against the grazers and browsers

Competition

• Interspecific competition is a potent force in organic evolution.
• Competition generally occurs when closely related species compete for the same resources that are limiting, but this not entirely true:
• Firstly: totally unrelated species could also compete for the same resources.
• American lakes visiting flamingoes and resident fishes have their common food, zooplanktons.
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• Secondly: resources need not be limiting for competition to occur.
• Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, due to greater browsing ability.

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• Competitive release: A species, whose distribution is restricted to a small geographical area because of the presence of a competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally removed.
• Connell’s elegant field experiment showed that superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.
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• Gause’s ‘competitive Exclusion Principle’: two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior will be eliminated eventually.
• Resource partitioning: If two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging pattern.
• MacArthur showed five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioral differences in their foraging activities.

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Parasitism

• Parasitic mode of life ensures free lodging and meals.
• Some parasites are host-specific (one parasite has a single host) in such a way that both host and parasite tend to co-evolve.
• Parasitic adaptation
• Loss of unnecessary sense organs.
• Presence of adhesive organs or suckers to cling on to the host.
• Loss of digestive system.
• High reproductive capacity
• Parasites having one or more intermediate host or vectors to facilitate parasitisation of its primary host.
• Liver fluke has two intermediate hosts (snail and a fish) to complete its live cycle.

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•  Effects on the host:
• Parasite always harms the host.
• They reduce the survival, growth and reproduction of the host.
• Reduce its population density.
• They make the host more vulnerable to the predators, by making it physically weak.

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•  Ectoparasite: feeds on the external surface of the host.
• Lice on human
• Ticks on dog
• Marine fish infested with copepods
• Cuscutaa parasitic plant grow on hedge plants.

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• Endoparasites: are those that live inside the host body at different sites.
• Life cycle is more complex.
• Morphological and anatomical features are greatly simplified.
• Highly developed reproductive system.

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• Brood parasitism:
• Special type of parasitism found in birds.
• The parasitic birds lay its eggs in the nest of its host and let the host incubate them.
• The egg of the host is very similar with the egg of the host.
• Cuckoo lays eggs in the nest of the crow.

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Commensalism

• This is the interaction in which one species benefits and the other is neither benefited nor harmed.
• Orchids growing as an epiphyte on a mango branch.
• Clown fish living among tentacles of sea anemone.
• Barnacles on back of whales.
• Cattle Egret and grazing cattle.

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Mutualism

• interaction between two living organism, both are equally benefited, no one is harmed.
• Lichen: a mycobiont and a Phycobiont.
• Mycorrhiza: relationship between fungi and root of higher plant.
• Pollinating insects and flowering plants.
• Fig trees and its pollinating agent wasp.

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Sexual deceit

• Mediterranean orchid Ophrys employs ‘sexual deceit’.
• Petal of the flower resembles the female bee.
• The male bee attracted to what it perceives as a female, ‘pseudocopulates’ with the flower but does not get any benefits.

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