population| types| function| definition| structure| curve| commensalism| mutualism| Amensalism|

Population

It can be defined as the total number of individuals of a species in a specific geographical area, which can interbreed under natural conditions to produce fertile offspring and function as a unit of biotic community. 

Population Ecology 

It is an important area of ecology as it links ecology to the population genetics and evolution.

Population Attributes 

These are certain characteristics of a population. Some of them are as follows:

Population Size or Density 

It is the number of individuals of a species per unit area or volume at a given time.

Population Density: -

Death Rate or Mortality 

• It is expressed as the number of deaths per 1000 individuals of a population per year, e.g. if 4 individuals in a population of 40 fruit flies died during a specified time interval (say a week). 
• Death rate= 4/40 = 0.1 (individual per fruit fly per week)

Sex Ratio 

• It is expressed as the number of females and males per 1000 individuals of a population in a given time, e.g., 60% females and 40% males in population. 

Age Pyramid 

1. When the age distribution (per cent individuals of a given age or age group) is plotted for the population in which. pre-reproductive age is at base, reproductive is in middle and post-reproductive is at top, we get a pyramid called age pyramid.
2. The shape of the pyramids reflects the growth status of the population that whether it is expanding (triangular-shaped), stable (bell-shaped) or declining (Urn-shaped).

   

   Fig: - Representation of age pyramids for human population
   
   
   

Population Growth: -

• The size of a population for any species is   not a static parameter. 
• It keeps changing with time, depending on various factors including food availability, predation, pressure and adverse weather conditions.
• The density of a population in a given habitat during a given period fluctuates due to changes in four basic processes:

(a) Natality is the number of births during a given period in the population that are added to the initial density.

(b) Mortality is the number of deaths in the population during a given period.

(c) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. 

(d) Emigration is the number of individuals of the population who left the habitat and  moved elsewhere during a given time Period. 

Figure: -Factors influencing population density

Note: - Natality and immigration contribute to an increase in population density and mortality and emigration contributes to a decrease.

• Population density can be given by the following equation 

          N1+1 = N1 + [(B + I) - (D + E)] this is density at time t + 1.

Where, N = population density, t = Time,

       B = Birth rate,I = Immigration, D = Death rate, E = Emmigration, N1 = Population in beginning.

The equation demonstrates that the population density will increase, if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E), otherwise it will decrease. 

• Although total number is the most appropriate measure of population density. In some cases, it is difficult to determine. So, in these cases, the per cent cover or biomass is a more appropriate measure of the population size, e.g. 200 Parthenium plants and a single huge banyan tree. In this case, the banyan tree produces much more biomass. 
• Total number is not an appropriate measure in case of huge population size. In such case, relative densities are used. For example, the number of fishes caught per trap is a good measure of its total population density in the lake. It is an indirect method to estimate population size.
• In some cases, population size is estimated indirectly. Without actually counting or seeing them, e.g. the tiger census is based on pug marks and faecal pellets.
• Growth curves of population are of the following types: -

(a) Exponential growth (A-curve) occurs normally when resources, i.e., food and space are unlimited. 

• Equation for exponential growth can be represented as follows: -

dN/dt = (b-d) N

let (b-d) = r, then 

dN/dt = rN

Where,

N = Population size,

Nt = Population density after time t

N.= Population density at time zero, 

r = Intrinsic rate of natural increase

e = Base of natural logarithms (2.71828), 

b = Birth rate and 

d = Death rate

Fig: - Population growth curve, A- when responses are limiting the growth, plot is exponential,
B- when responses are limiting the growth, plot is logistic, K- is carrying capacity.

• r is an important parameter assessing impacts of any biotic or abiotic factor on population growth.
• In exponential growth, when N in relation to time (t) is plotted on graph, the curve becomes J-shaped as shown in graph. 

(b) Logistic growth (B-curve) occurs when resources become limited at certain point of time, so no population can grow exponentially. 

• Due to competition between individuals for limited resources, the fittest individual will survive and reproduce. 
• In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This is called carrying capacity (K) of a habitat.
• A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity. 
• The logistic growth shows sigmoid curve and this is also called Verhulst-Pearl logistic growth. 
• The logistic growth model is considered more realistic since, resources for groWth of most animal populations are finite and become limited sooner or later.

Life History Variation 

(i) Darwinian fitness (high r value) states that the Populations evolve to maximize their reproductive fitness in the habitat in which they live. 

(ii) Under a particular set of selection pressures, organisn1s evolve towards the most efficient reproductive strategy.

(iii) The rate of breeding varies from species to species, e.g. Pacific salmon fish and bamboo breed only once in their lifetime, while most birds and mammals breed many times in their life.

(iv) Some organisms produce a large number of small-sized offspring (oysters, pelagic fishes), while birds and mammals produce a small number of large-sized offspring.

(v) Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live.

Population Interactions

Population interactions explain that in nature, animals, plants and microbes cannot live in an isolation but interact in various ways to form a biological community.

(i) Interspecific interactions arise from the interaction of populations of two different species. 

(ii) This interaction may be 

(a) Beneficial/positive (+) 

(b) Detrimental/harmful (-) 

(c) Neutral (neither harm nor benefit) to one of the species or both (O) 

Population Interactions

Some of the population interactions are as follows: -

Predation: -

It is an interspecific interaction, where an animal called predator kills and consumes the other weaker animal called prey. This is a biological control method.

(i) It is the nature's way of transferring energy to higher trophic levels, which is fixed by plants, e.g. tiger eating a deer, sparrow eating seeds, etc. 

(ii) Important roles of predators are: - 

• They keep prey population under control. 
• They help in maintaining species diversity in a community by reducing the intensity of competition among prey species. For example, in American Pacific coast, the star fish Pisaster is an important predator. When all the fishes were removed from an area more than 10 species of invertebrates became extinct within a year due to in Ter specific competition.
• In the absence of predators, prey species could achieve very high population densities and cause instability. So, besides acting as 'conduits' for energy transfer across trophic levels, predators play very important role to provide population stability.

(iii)When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have natural predators, e.g. prickly pear cactus introduced in Australia (1920s) caused havoc by spreading rapidly over a large area. It was controlled only after a cactus feeding predator (a moth) was brought from its natural habitat. 

(iv) If a predator is too efficient and over exploits its prey, then the prey might become extinct. Following it, the predator will also become extinct because of the lack of food.

(v) Prey species have evolved various defense mechanisms to lesser the impact of predation. Some of them are: -

• Some species of insects and frogs are cryptically colored (camouflaged) to avoid being detected easily by the predator. Some are poisonous and therefore, avoided by the predator.
• Phytophagous insects like Monarch butterfly is highly distasteful to its. predators (birds) because of a special chemical present in its body. The . butterfly acquires this chemical dunng its caterpillar stage by f ceding on a poisonous plant (weed). 
• Some plants have thorns or spines fo r defence mechanism, e.g. Acacia, cactus. 
• Some plants produce highly poisonous chemicals like cardiac glycosides, nicotine, caffeine, quinine, strychnine, opium, etc. These are actually defence mechanisms against grazers and browsers. e.g. Calotropis. These chemicals make the herbivore sick when th ey are eaten, inhibit digestion, disrupt reproduction or even kill them.

Competition: -

It is a type of interaction, where both the species suffer. It occurs when species (closely related or even unrelated), compete for the same resources that are limited, e.g. in some shallow South American lakes visiting flamingoes and resident fishes compete for their common food, i.e. Zooplanktons. In other words, it can be defined as a process in which the fitness of one species (measured in terms of its 'r' the intrinsic rate of increase) is lower in the presence of another species. 

(i) It may be 

• Intraspecific (within same species). 
• Interspecific (between different species).

(ii) In interference/interspecific competition the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of other species, although the resources (food and space) are plenty. 

(iii) Competitive release is a phenomenon. in which a species whose distribution is restricted to a small geographical area because of the presence of competitively superior species, is found to expand its distributional range dramatically when the competing species is experimentally · removed. 

(iv) Connels' elegant field experiments showed that on the rocky seacoasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chatha malus from that zone. 

(v) Gause's competitive exclusion principle states that two closely related species competing for the Sarne resources cannot coexist indefinitely and the competitively inferior one will be eliminated eventually (true if resources are limiting), e.g. when goats were introduced in Galapagos Islands, the Abingdon tortoise became extinct within a decade due to greater browsing efficiency of the goats.

(vi) Resource partitioning states that if two species compete for the same resource, they could avoid competition by choosing, for instance, different lines for feeding or different foraging patterns. In this relation, McArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and coexist due to behavioral differences in their foraging activities.

Parasitism: -

It is the mode of interaction between two species in which one species (parasite)depends on the other species (host) for food and shelter and in this process the parasite damages the host. Hence, one organism is benefitted (parasite), while the other is being harmed (host).

(i) Adaptions of a parasite are: -

• Parasite is host specific in a way that both and parasite tend to coevolve. 
• Loss of unnecessary sense organs.
• The presence of adhesive organs or suckers.
• Loss of digestive system 
• High reproductive capacity.

(ii)  The life cycles of parasites are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host, e.g.

• Human liver fluke (a trematode parasite) depends on two intermediate hosts ( a snail and a fish) to complete its life cycle.
• Malarial parasite (Plasmodium) needs a vector (mosquito) to spread disease to other hosts. 

(iii) Majority of parasites harm the host. They reduce the survival, growth and reproduction ability of the host and reduce its population density, by making it physically weak. They render the host more vulnerable to predation.

(iv) Parasites can be of the following two types 

• Ectoparasites live on the external surface of the host organism for food and shelter. Examples are the lice on humans, ticks on dogs, copepods, Cuscula, etc.
• Endoparasite's live inside the hosts body at different sites like liver, kidney, lungs, etc., for food and shelter, e.g., tapeworm, liver fluke, Plasmodium, etc. The life cycles of endoparasites are more complex because of their extreme specialization.

(v)female mosquito is not considered as parasite because it does not complete its life cycle in human body for reproduction.

(vi) Brood parasitism is a phenomenon in which one organism (parasite) lays its eggs in the nest of another organism, e.g., eggs of cuckoo (Koel) and the crow resemble in the size and colour. To reduce the chances of the crow (host) detecting the foreign eggs (cuckoo's) and ejecting them out from the nest, cuckoo lays its egg in crow's nest and let the host incubate them. 

Commensalism: -

It is the interaction between two species, where one species is benefitted and the other is neither harmed nor benefitted.

Some examples of commensalism are: -

• An orchid growing as an epiphyte on a mango tree. Orchid gets shelter and support from mango tree, while the mango tree is neither benefitted nor harmed. 
• Barnacles growing on the back of whale. Barnacles are benefitted to move to different locations for food as well as shelter, while the whales are neither benefitted nor harmed. 
• Egrets always forage close to where the cattle are grazing. Because, the cattle move, they stir up the bushes and insects are flushed out from the vegetation to be catched by cattle egrets.
• Sea anemone has stinging tentacles and the clown fish lives among them. The fish gets protection from predators, which stay away from the stinging tentacles.
• The anemone does not appear to derive any benefit by hosting the clown fish. 

Mutualism: -

It is an interaction that confers benefits to both the interacting species. 

Some examples of mutualism are: -

• Lichens represent an intimate mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria. Here, the fungus helps in the absorption of nutrients and provides protection, while algae prepare the food.
• Mycorrhizae are close mutual association between fungi and the roots of higher plants. Fungi help the plant in absorption of nutrients, while the plant provides food for the fungus.
• Plants and animals also show mutual relationship. Plants need help from animals for pollination and dispersal of seeds. In return, plants provide nectar, pollens and fruits to the pollinators, e.g. the female wasp uses the fig fruit not only as an oviposition (egg-laying) site but uses the developing seeds within the fruit for nourishing its larvae. The wasp pollinates the fig's inflorescence, while searching for suitable egg-laying sites. In return, fig provides the wasp some seeds as food for the developing wasp larvae. 
• Not all orchids offer rewards, e.g. Mediterranean orchid Ophrys employs 'sexual deceit' to get pollinated 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 and pseudo copulates with the flower.
• During that process, pollen is dusted from the flower. When the same bee pseudo copulates with another flower. It transfers pollen to it and hence, pollinates the flower. This case indicates the process of coevolution. If the female bee's colour pattern changes even slightly during evolution, pollination success will be reduced unless the orchid flower coevolve to maintain the resemblance of its petal to female bee.

Amensalism: -

It is an interaction between different species, in which one species is harmed and the other is neither benefitted nor harmed, e.g. Penicillium, a Mould secretes penicillin, which kills bacteria but the Mould itself remains unaffected.