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The last ice age covers Canada and Northern United States with ice. It ends about 10,000 years ago. Glacial grinding leaves clay, peat, soil, and sand devoid of earthworms. Latitudes between 45° and 69° lack native earthworms. Starting in the 1600s, European immigrants bring to North America invasive night crawlers among other earthworm species in root balls, soil bulbs or dry ship ballast. Earthworms (soil engineers) epitomize a keystone species. They redistribute nutrients, mix layers, and create pores in soil. Their lifestyle benefits farm crops. They deposit castings (source of fertilizer). Burrowing depths of six feet and extraction of nutrients from decomposing organic matter enriches garden soil.

United statesCarl saganNorthern united statesNorth americaஒன்றுபட்டது மாநிலங்களில்கார்ல் சாகன்வடக்கு ஒன்றுபட்டது மாநிலங்களில்வடக்கு அமெரிக்கா