A harmful organism, that is harmful for one/more populations/species, can be made extinct; the food chain should not be affected. For example, the virus causing small pox has been controlled world over.

Since plants cannot move away from their predators (herbivores) and harsh environmental conditions, many of them have become extinct. But animals can move away from the harmful/unfavourable environments or their predators and evolution of favourable characters has thus taken place in them.

Control of foods and droughts:
1. Forests control flood by trapping and absorbing the rain water and slowly releasing it later.
2. The roots of plants hold the soil particles firmly and prevent soil erosion.
3. The forest cover prevents water directly falling on open soil.

Sacred Groves are the tracts of forests where all the trees and wildlife within were venerated and given total protection.
1. These are the last refuges for a large number of rare and threatened plants.

(i) Habitat loss and fragmentation
(ii) Over-exploitation by man for natural resources
(iii) Invasion of alien species, which complete with the native species and cause their extinction.
(iv) Co-extinction of related plants and animals.

Slope of regression represents the changes in the species richness with the area. Species richness decreases with the decrease in area. The value of 'Z' (regression coefficient) lies in between 0.1-0.2, regardless of the taxonomic groups in the region. For very larger areas like continents, the slope of the line becomes much steep with the Z values in the range of 0.6-1.

Genetic diversity, species diversity, ecosystem diversity.

In situ conservation involves the protection of species in their natural habitats. It helps in the conservation of threatened/endangered species via following means
(i) Biodiversity hotspots are regions with high levels of species richness and high degree of endemism (i.e. species confined to that region are not found anywhere else). These hotspots can reduce the ongoing mass extinctions by almost 30%.
(ii) Protected areas are ecologically unique and biodiversityrich regions.These are legallyprotected as biospherereserves.nationalparks and sanctuaries. Hence, can help in conservation of endangered species. 
(iii) Sacred groves are forest areas set aside,all the trees and wildlife within it are venerated and given total protection. These are religious and cultural places, which are protected for conservation of endangered species.

(i) Site Y has the lowest content of dissolved oxygen due to increased bacterial population. It is caused by sewage run-off from a pig farm.
(ii) There are very few fisher at size Z due to low oxygen Concentration and choked lake (by algae and weeds)
(iii) Algae and weeds will thrive after site Z.

The human activities like deforestation, over-cultivation, unrestricted grazing, poor irrigation practices, result in arid patches of land (deforestation). When these large barren patches extend and remain unattended for long, a desert is created. Because the formation of fertile top layer of soil takes millions of years, desertification easily takes over.

Biomagnification or biological magnification.

(a) Government, through a new auto fuel policy, has laid out a road map to cut down vehicular pollution in Indian cities including Delhi.
(i) Switching over the entire public transport (i.e., buses autorickshaws, taxis) from diesel to CNG. CNG burns more efficiently and little is left unburnt
(ii) CNG is cheaper than petrol/diesel and cannot be siphoned off by thiefs or adultered like petrol or diesel.
(b) Government has introduced Euro II norms.This norm stipulates that sulphur be controlled at 350 ppm in diesel and 150 ppm in petrol.
Aromatic hydrocarbons are to be contained at 42% of the concerned fuel.It has significantly improved the air quality.

Flocs are the masses of bacteria associated with fungal filaments to form mesh-like structure. 

Biodiversity is very important for ecosystem functioning. More diverse ecosystems are considered more stable. Stable communities are resilient and resistant to change.
Hence, diversity stabilises the community and also helps in increasing the productivity. Higher the biodiversity of an ecosystem, higher the number of species and therefore, lower is the rate of extinction. While, in case of low biodiversity, there are higher chances of extinction.
Extinction of even one or two key species has negative impacts on the entire ecosystem.
Thus, the diversity ensures the stability and proper functioning of the entire ecosystem as explained via the 'Rivet popper hypothesis'. 
Biodiversity is important for ecosystem functioning.
(i) Ecologists believe that communities with more species diversity tend to be more stable than those with less species diversity.
(ii) Plots with increased species diversity show, higher productivity.
(iii) The ecosystem with more diversity becomes resistant to seasonal disturbances.
(iv) They also show resistant to invasion by alien species.

(i) Speciation is a function of time: the temperate regions were subjected to frequent glaciation in the past, while the tropics have remained undisturbed and hence had longer time to evolve more species diversity.
(ii) The tropical environments are less seasonal and relatively more constant and more predictable than temperate regions; speciation has been promoted/favoured by such constant environments and hence there is greater species diversity.
(iii) There is more solar radiation available in the tropical region; this contributes directly to more productivity and indirectly to greater species diversity.

Ecologists make a statistical comparison of the species richness of systematically studied groups of insects of the temperate and tropical regions and extrapolate this ratio to the other groups of animals and plants. This is how they calculate the gross estimate of the total number of species existing on this planet.