Organisms and Population Test - 9

Natality and Immigration add to the population density. Mortality and emigration decrease the population density.

Triangular age pyramid has high proportion of reproductive individuals, moderate number of reproductive individuals and fewer post-reproductive individuals. It represents young or rapidly growing population.

In urn-shaped age pyramid, the number of reproductive individuals is higher than the number of pre-reproductive individuals. It represents declining or diminishing population with negative growth

In bell-shaped age pyramid, the number of pre-reproductive and reproductive individuals is almost equal. Post reproductive individuals are comparatively fewer. It represents stable or stationary population where growth rate is nearly zero.

A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of increase and decrease and finally the population density reaches the carrying capacity. A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst- Pearl Logistic Growth as explained by the following equation:

Where N = Population density at a time t;
r = Intrinsic rate of natural increase and;
K = Carrying capacity.

Plant-animal interactions involve co-evolution of mutualists. This means, the evolution of the plant (e.g., flower) and the animal (e.g. pollinator species) are closely linked with one another.

Mutualism is an association between individuals of two species, both of which are benefitted. The species participating in mutualism are called mutualists. Sea anemone, Adamsia is habitually found fastened by its characteristically lobed pedal disc to the snail's shell used as a portable home by the hermit crab (Eupagurus prideauxi). The anemone protects the crab from the enemies with its nematocysts. In return, the anemone receives pieces of food dropped by the crab and is carried to new places by it. Other examples of mutualism are mycorrhiza living on roots of higher plants and wasp pollinating fig inflorescence.

Competition is defined as the active demand by two or more individuals of the same species or members of two or more species at same trophic levels for a common resource. Intraspectific competition is competition amongst members of the same species for a common resource such as for food, space and mate. interspecific competition is rivalry amongst members of different species. The severity of competition depends upon similarity in the requirement of food and shelter.

Total number of plants (in the beginning ) = 30
New plants added by reproduction = 25
Birth rate (of a population)

=0.83 individuals per lotus per year

Number of plants died =8

Sometimes, for certain ecological investigations, there is no need to know the absolute population densities. Relative densities serve the purpose equally well. In this case, population size is indirectly estimated without actually counting them. For example, the number of fishes caught per trap is good enough measure of its total population density in the lake. The tiger census in our National parks and tiger reserves is often based on pug marks (animal's foot print) and faecal pellets.

Population density is the number of individuals present per unit area or volume at a given time. If the total number of individuals is represented by letter N and the number of units of space (area for land organisms and volume for water organisms) by letter S, then the population density D can be represented as D =N/S. For instance, number of animals per square kilometer, number of trees per acre in a forest, etc.

Age distribution is percentage of individuals of a given age or age group. A population at any given time is composed of individuals of different ages. If age distribution is plotted for the population, the resulting structure is called as age pyramid.

Population ecology is the subfield of ecology that deals with the dynamics of species populations and how these populations interact with the environment. It is an important area because it links the ecology to population genetics and is responsible for evolution.

Number of deaths is more than the number of births, showing a declining population.

The age structure of a population is the percentage of individuals of different ages such as young, adult and old. 
Age structure is shown by organisms in which individuals of  more than one generation time are important related to it. 

Different age groups have different reproductive capabilities. Pre-reproductive individuals are the young individuals which will enter the reproductive age after some time. They are the potential source of increas in population. Reproductive individuals are the ones which are actually adding neq members to the population. Post-reproductive individuals are older individuals which no longer take part in reproduction.

migration is permanent outward movement of individuals from a population for settlement into a new area. If decreases the local population.

A and B are natality and immigration. They add to population density. C and D are mortality and emigration. They decrease population density.

Immigration is the permanent inward movement of some individuals coming from outside into an existing population. This increase population density and would not limit its growth.

Immigration is not necessarily equal to emigration in a population.

When food and space for a population are unlimited, each species has the ability to realise fully its inherited potential to grow. Then the population grows in an exponential or geometric ratio.

Intrinsic rate of natural increase (r)

= Birth rate − death rate

=0.15−0.08=0.07

In nature, a given habitat has resources to support a certain number of individuals of a population, beyond which no further growth is possible. This limit is called as nature's carrying capacity (K) for that species in that habitat.

S-shaped growth curve is also called Verhulst-Pearl logistic curve and is represented by the following equation :

where dN/dt= rate of change in population size, r= intrinsic rate of natural increase, N= population density, K= carrying capacity and 

Since resources of growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered as more realistic.

A population with large number of older individuals than younger ones is likely to decline since older individuals do not take part in reproduction.

Predation is an interaction between members of two species in which members of one species capture, kill and eat up members of other species. The former are called predators and the latter are called preys.

Parasitism is a relationship between two living organisms of different species in which one organism called parasite obtains its food directly from another living organism called host. The parasite is usually smaller as compared to its host. It spends a part or whole of its life on or in the body of the host. Obligate parasites obtain their nutrition from living organisms only.

The evolution of complementary adaptations in two species caused by the selection pressures that each exerts on the other is called co-evolution. It is common in symbiotic associations e.g., many insect-pollinated plants have evolved flowers whose shapes, colours, etc., make them attractive to particular insects, at the same time the pollinating insects have evolved sense organs and mouth parts specialised for quickly locating and extracting nectar from particular species of plants.

According to the mechanism of resource partitioning (supported by Mac Arthur), if two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging patterns. Two species evolve to become different too reduce competition, so that species can co-exist. 

In mutualism, both species are benefitted. Gut bacteria are dependent on us for food. They release certain enzymes which help us in digestion.

If mitochondria are still considered to be separate organisms, they would form mutualistic relationship with us. This is because they would get the resources from us and in turn produce energy for various activities of the body.

In competition, both species are harmed. In predation and parasitism, one species derives benefit and the other one is harmed. Commensalism is an interaction in which one species is benefitted and other one remains unaffected.

Brood parasitism in birds is an example of parasitism in which the parasitics bird lays its eggs in the nest of its host and the host incubates them. This way, the bird can save the energy that it would have spent in incubation and can use that energy if foraging and other activities. During the course of evolution, the eggs of the parasite bird have evolved to resemble the host's eggd in size and colour to reduce the chances of the host bird detecing the foreign eggs and removing them from the nest. Laying eggs by cuckoo (koel) in crow's nest is brood parasitism. Cowbirds also show brood parasitism.

Coevolution is caused by the selection pressures that each of the two species exerts on the other. Since there has been no coevolved herbivore with prickly pear cactus, it became abundant in Australia.

Prey species have evolved various defenses to lessen the impact of predation. For plant, herbivores are the predators. Plants have evolved an astonishing variety of morphological and chemical defences against herbivores. Calotropis produces highly poisonous cardiac glycosides and that is why we never see any cattle or goat browsing on this plant. A wide variety of chemical substance that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.) are produced by them actually as defences against grazers and browsers.

Two or more species with closely similar niche requirements cannot exist indefinitely in the same area as sooner or later they come into competition for possession of it. This is called as Gause's competitive exclusion principle, which states that an ecological niche cannot be simultaneously and completely occupied by established populations of more than one species. Two species can live in same habitat but not in the same niche. More similar the two niches are, severe the competition is

When species 1 was kept with species 2, its number of individuals fell drastically while when species 2 was kept with species 1, its number of individuals was not much affected. Hence, both species are affected by inter-specific competition but species 2 is less affected.

Growth rate = Birth rate - death rate 
For country P, it is 10/1000. For country Q, It is 15/1000
For country R, it is 17/1000. For country S, It is 7/1000
Hence, country S has the least population growth rate 

A can be either predation or parasitism since one species is harmed and the other is benefitted. B is commensalism since one species is benefitted and other has no effect on it. C can be amensalism since one species is harmed and the other has no effect on it.

Bacteria decompose the organic matter which is not really living. Hence, it cannot be considered as a prey predator relationship.