Biology Test - 9

The correct answer is Diapause.

Key Points

• Diapause
  • Under unfavorable conditions, many zooplankton species in lakes and ponds enter diapause. Hence, Option 2 is correct.
  • It is considered to be a physiological state of dormancy with very specific initiating and inhibiting conditions, resulting in a delay in development.
  • Diapause can be defined as the physiological state of dormancy or developmental arrest where most life processes are shut down.
  • It is initiated during unfavorable conditions and is most commonly observed in insects, especially in arthropods.
  • The diapause stage can occur at any stage during an insect’s life cycle – from its pupae stage to the active, adult stage.
  • When this phenomenon occurs during the egg or the pupil stage, the development slows down to a halt. In the adult stage, feeding habits or reproductive behavior is halted or slowed down.

Additional Information

The correct answer is 1, 2, and 3.

• There are some plants that grow in extremely dry conditions throughout the year.
• Structural adaptations are important to plant survival in the desert.
• For this purpose, they undergo some morphological adaptations in the xerophyte stem. These are as follows - 
  • The stems of woody xerophytes are comparatively stunted, hard, and rigid. They may be covered with thick bark. e.g. Acacia arabica.
  • Wax coatings on leaves prevent water loss through evaporation because the loss of water from both the surface and the inside of leaves occurs in the hot desert.
  • The stem is covered by hairs or waxy coating.
  • Hence, statement 1 is correct.
  • The main stem and branches may occur thick, fleshy, flattened, and have a green modified structure called phylloclade.
  • Hence, statement 2 is correct.
  • Leaves are also smaller on desert plants, further reducing the possibility of water loss.
  • Phylloclades are found in succulent xerophytes. They do photosynthesis. e.g. Opuntia, Cacti etc.
  • Usually, in phylloclade bearing xerophytes, leaves are modified into spines that are known to check the rate of transpiration.
  • They have hard, thick coatings and some are covered in prickly spines to prevent water loss.
  • Hence, statement 3 is correct.
  • It is to protect them from animals who might try to chew through them to get to their moisture.
  • The stem may occur as an underground modified structure to store food and takes part in reproduction. e.g. Saccharum.

Concept:

• Growth models help us understand if the growth of any population over time follows a particular pattern or not.
• This also helps us to understand several other attributes of the population.
• Growth models explain 2 types of growth curves:
  • Exponential - This takes place when the resources are unlimited and the population keeps growing.
  • Logistic - This is a growth model that takes the limitations of resources into account.

Important Points

Logistic Growth:

• Resources are not available to any species population in unlimited amount.
• Thus, the species compete for the available resources to survive.
• This competition restricts the exponential or unlimited growth of any population.
• Any given habitat can only provide resources to support a maximum possible number, beyond which it further growth of population is not possible.
• This is known as the carrying capacity (K) for a particular species in a habitat.
• Such growth pattern is also known as the Verhulst-Pearl Logistic Growth.
• It can be denoted by the equation:

\({dN \over dt} = rN ({K - N \over K})\)

where, N = Population Density,

K = Carrying Capacity,

\(dN \over dt\) = Rate of change of population density.

• Here, 'r' represents the intrinsic rate of natural increase, which is the difference between the per capita birth and death (b-d).
• The environmental resistance is represented in the equation as \(K- N \over K\).

Explanation:

• When the population density (N) is plotted against time (t), it produces a sigmoid curve (S-shaped curve).

• Logistic growth shows 3 different phases:
  • Lag phase - This represents the initial lag time that a population shows before it starts growing.
  • Log phase - This is the phase of accelerated growth, which is itself an exponential growth.
  • Asymptote - This is the stationary phase where the population density reaches the carrying capacity.
• Hence, asymptote is obtained when K = N.

The correct answer is  Commensalism.

Key Points

Species interaction or population interaction

• In an ecosystem living organism continuously interact and influence the other organism and with their environment directly or indirectly, in order to carry out their vital processes.
• This is called species interaction or population interaction.
• Sometimes interaction between species or population can be - useful, harmful or neutral to one of the species or both.
• Types of interaction between organisms can be broadly classified as follows :

1. Positive interaction: In this type, interacting organisms or populations benefit one another. The benefit may be in respect of - food, shelter, substratum, or transportation.
2. Negative interaction: In this type, one of the interacting organisms is benefited and another is affected (harmed). One organism or population may eat other organisms, compete for food or excrete harmful waste.

Explanation:

Commensalism (Neutral type of interaction)

• In this type, one organism is benefited while the other organism is neither benefited nor harmed.
• Example 1 - Cattle and Egret Birds. As the cattle move, they stir up the small insects hiding in the grass and birds feed on it. So, here birds are getting food however cattle are neither harmed nor benefited.
• Example 2 - Remora fish and Shark. Remora attaches itself to the fins of Sharks or Whales and feeds on leftover food and benefit for transportation and protection from predators. In this interaction host organism i.e. shark is neither harmed nor benefited.

Additional Information

1. Mutualism (Positive type of interaction)

• In this type of interaction, both organisms are benefited.
• Example. Association between hermit crab and Sea anemone. Crab carries the Sea anemone to a fresh feeding site and Sea anemone provides protection to crab from enemies (predators).

2. Parasitism (Negative type of interaction)

• In this type of interaction, one species harms the other by making its direct or indirect use for food or shelter.
• A parasite is an organism that lives on or inside the body of another organism and derives its food from it. Harmed organism is called a host and benefitted one is called a parasite.
• Example: Cuscuta, a parasitic plant that is commonly found growing on hedge plants, does not have chlorophyll and leaves for photosynthesis. Hence it derives its nutrition from the host plant which it parasite over other plants.

3. Predation (Negative type of interaction)

• In this type of interaction, one organism kills and feeds on another.
• Predator is the organism that kills and feeds the other and prey is the one who dies or becomes a feed.
• However, predation is an important interaction because it helps to maintain constancy in a number of prey populations and thereby maintaining a balance within an ecosystem.
• Ex. Snake eats a Frog. Here snake is a predator and the frog is a prey. The frog is killed and eaten by a snake and thereby snake is benefitted and the frog is affected.

Key Points

• A population at any given point of time comprises of individuals of different age groups.
• The percentage of individuals of a given age or age group is known as age distribution, which is an important population attribute.
• Age pyramid is a structure that results from the plotting of the age distribution of a population.
• For a human population, 3 age groups are considered:
  • Pre-reproductive
  • Reproductive
  • Post-reproductive
• The pre-reproductive age group occupies the lowermost layer of the pyramid.
• Thus, a narrow base would indicate low number of young individuals.

Important Points

• The age pyramids usually include both male and female individuals.
• The shape of the pyramid shows the growth status of a population.
• They can be of 3 basic types:
  • Expanding - Pre-reproductive group is maximum and post-reproductive group is minimum.
  • Stable - The pre-reproductive and reproductive groups are almost equal, while the post-reproductive is less.
  • Declining - Pre-reproductive group is lesser than the other age groups.

Concept-

• The food chains and webs that exist in nature.
• Starting from the plants (or producers) food chains or rather webs are formed such that an animal feeds on a plant or on another animal and in turn, is food for another.
• The chain or web is formed because of this interdependency.
• No energy that is trapped in an organism remains in it forever.
• The energy trapped by the producer, hence, is either passed on to a consumer or the organism dies.
• The death of an organism is the beginning of the detritus food chain/web.

Explanation-

Herbivorous

• All animals depend on plants (directly or indirectly) for their food needs.
• They are hence called consumers and also heterotrophs.
• If they feed on the producers, the plants, they are called primary consumers.
• Obviously, the primary consumers will be herbivores.
• Some common herbivores are insects, birds, and mammals in the terrestrial ecosystems and molluscs in the aquatic ecosystem.

Carnivorous

• The consumers that feed on these herbivores are carnivores, or more correctly primary carnivores (though secondary consumers).
• Those animals that depend on the primary carnivores for food are labelled secondary carnivores.
• Carnivorous form the third tropical level.

Producers

• The green plant in the ecosystem is called producers.
• Producers prepare food from solar energy(sun).
• In a terrestrial ecosystem, major producers are herbaceous and woody plants.
• Likewise, producers in an aquatic ecosystem are various species like phytoplankton, algae and higher plants.

Decomposer

• The detritus food chain (DFC) begins with dead organic matter.
• It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.
• They meet their energy and nutrient requirements by degrading dead organic matter or detritus.
• These are also known as saprotrophs (sapro: to decompose).
• Decomposers secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.

Therefore the correct option is 2 (a - ii, b - i, c - iv, d - iii).

The correct answer is Both 1 and 2.

Key Points

• Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients and the process is called decomposition.
• Dead plant remains such as leaves, bark, flowers, and dead remains of animals, including faecal matter, constitute detritus, which is the raw material for decomposition.
• The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification, and mineralization.
• Detritivores (e.g. earthworms) break down detritus into smaller particles. This process is called fragmentation.
• By the process of leaching, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. Hence, Statement 1 is correct.
• Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called catabolism.
• It is important to note that all the above steps in decomposition operate simultaneously on the detritus.
• Humification and mineralization occur during decomposition in the soil.
• Humification leads to the accumulation of a dark-coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Hence, Statement 2 is correct.
  • Being colloidal in nature it serves as a reservoir of nutrients.
  • The hummus is further degraded by some microbes and the release of inorganic nutrients occurs by the process known as mineralization.

Concept:

• Organisms need a constant supply of nutrients for various metabolic activities.
• The nutrient amount varies in different ecosystems and also due to seasonal variations.
• The amount of nutrients present in the soil at any given point of time is known as the standing state.
• Though variable, but nutrients are never lost from the ecosystem.
• This is due to the process of nutrient cycling, which refers to the movement of nutrient elements through the different components of the ecosystem.
• They are also known as biogeochemical cycles.
• The rate of release of nutrients into the atmosphere is controlled by environmental factors like soil, moisture, pH and temperature.

Important Points

• Biogeochemical cycles are of 2 types:

1. Gaseous -
   • These include the nitrogen and carbon cycles.
   • The main reservoir for these cycles is in the atmosphere.
2. Sedimentary -
   • These include the sulphur and phosphorus cycles.
   • Their reservoir is in the Earth's crust.

• The function of these reservoirs is to meet the deficit that might be caused due to imbalance in the rate of influx and efflux.

Additional InformationPhosphorus Cycle:

• Phosphates in the rocks are the main reservoir for this biogeochemical cycle.
• Here gaseous exchange of phosphorus between organisms and the environment is negligible.

Concept:

• The sequential, gradual, and predictable changes in the species composition in an area are called succession or ecological succession.
• The entire sequence of communities that successively changes in a given area is called sere(s).
• The individual transitional communities are termed as seral stages or seral communities.
• The community that is in near equilibrium with the environment is called a climax community.
• The species that invade a bare area are called pioneer species.​

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Explanation:

Succession is of two types - Primary succession & Secondary succession.

Primary Succession-

• Primary Succession is a process that starts in an area where no living organisms are there.
• Examples of areas where primary succession occurs are newly cooled lava, bare rock, newly created pond or reservoir.
• The establishment of a new biotic community is generally slow.
• Before a biotic community of diverse organisms can become established, there must be soil.
• Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.

Secondary Succession- 

• Secondary succession begins in areas where natural biotic communities have been destroyed such as in abandoned farmlands, burned or cut forests, lands that have been flooded. 
• Since some soil or sediment is present, succession is faster than primary succession.

Concept:

• An ecosystem is the basic ecological unit in which living organisms interact among themselves and with their surrounding physical environment.
• In an ecosystem, an organism occupies a specific place in the food chain called trophic level like Producers, Consumers, Decomposers.
• Each trophic level has a certain mass of living material at a particular time called the standing crop, this standing crop is measured as the biomass of living organisms, as the number in a unit area.
• An ecological pyramid is a pictorial representation of the relationship between different organisms in an ecosystem.
• It is of three types: Pyramid of the number, Pyramid of biomass, and Pyramid of energy.

Explanation:

• When the relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of biomass, it is called a pyramid of biomass.
• It is of two types:

1. Upright - The amount of biomass in each trophic level decreases as we move from producers to decomposers. Eg. Grassland ecosystem
2. Inverted - The amount of biomass in each trophic level increases as we move from producers to decomposers. Eg. In a Marine ecosystem like a pond producers (phytoplankton) have lesser biomass compared to consumers (Fishes) 

Additional Information

• Tundra, Forest, and grassland ecosystems show an upright pyramid of biomass.

Important Points

• The pyramid of energy is always upright