Orcas, or killer whales, are natural predators of sea otters. it has been observed that in regions where the sea otters are not present, sea urchins grow rapidly and destroy much of the natural kelp vegetation. which of these statements is correct in this context? a) kelp is the keystone species because their survival is key to the survival of the entire ecosystem. b) sea otters cannot be considered the keystone species because they are not the top predators in the ecosystem. c) sea urchins are the keystone species because they multiply rapidly in the absence of their natural predators. d) sea otters are a keystone species as they prevent a herbivorous species from completely destroying a plant species.

The correct answer will be option-D

Explanation:

Keystone species refers to the species which disproportionately affect the ecosystem irrespective of their abundance and number.

The species are keystone species as they directly affect the ecosystem thus they maintain the structure of the community. The absence of these species modifies or change the structure of the community.

In the given, since the sea presence of sea otters maintains the structure of the community whereas the absence of the sea otters affect the vegetation of the kelp and thus food chain or food web of the ecosystem, therefore, sea otters are the keystone species.

Thus, option-D is the correct answer.

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a fossil fuel is a fuel formed by natural processes, such as anaerobic decomposition of buried dead organisms, containing energy originating in ancient photosynthesis.[1] the age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.[2] fossil fuels contain high percentages of carbon and include petroleum, coal, and natural gas.[3] other commonly used derivatives include kerosene and propane. fossil fuels range from volatile materials with low carbon to hydrogen ratios like methane, to liquids like petroleum, to nonvolatile materials composed of almost pure carbon, like anthracite coal. methane can be found in hydrocarbon fields either alone, associated with oil, or in the form of methane clathrates.

the theory that fossil fuels formed from the fossilized remains of dead plants by exposure to heat and pressure in the earth's crust over millions of years was first introduced by andreas libavius "in his 1597 alchemia [alchymia]" and later by mikhail lomonosov "as early as 1757 and certainly by 1763".[4] the first use of the term "fossil fuel" was by the german chemist caspar neumann, in english translation in 1759.[5]

in 2017 the world's primary energy sources consisted of petroleum (34%), coal (28%), natural gas (23%), amounting to an 85% share for fossil fuels in primary energy consumption in the world.[6] non-fossil sources in 2006 included nuclear (8.5%), hydroelectric (6.3%), and others (geothermal, solar, tidal, wind, wood, waste) amounting to 0.9%.[7] world energy consumption was growing at about 2.3% per year. in 2015 about 18% of worldwide consumption was from renewable sources.[8]