That is a very important question that no one can answer.
It's hard enough to figure out how many big, obvious things there are. There are somewhere around 10,000 species of birds, for example. There are around 5,500 species of mammals. Even with these species, we may not have found the ones that are small, rare, hidden, or look like other species.
Insects are a huge group. Scientists have identified nearly a million species, but there are probably many that we haven't identified yet. There are lots more other invertebrates, too. Worms, arachnids (spiders and mites), corals, and all sorts of other animals don't get as much attention, but there are lots of them.
I've only mentioned the animals so far. There are also plants, fungi, protists (lake amoebas and paramecium), and bacteria.
About 1.3 million living things have been named, but there's no way to know how many we haven't discovered. Some scientists estimate that this might be only 15% of the actual number of species.
The number of species there are is called biodiversity. Scientists like to look for patterns, so we wonder about patterns in biodiversity. Why do you think there are lots of species in the rainforest, but not a lot in the desert?
Thanks for asking,
That is a VERY difficult question to answer, especially because there are still plenty of species left to discover (think insects & deep marine life)!! As of 2011, there were ~8.7 million different species recognized on Earth: ~6.5 million of these species live on land, and ~2.2 million of these species live in the oceans.
If you include bacteria (as a more recent 2017 study did), that number gets MUCH larger - to the tune of ~2 BILLION different species (the majority of which are different bacteria; source: here ).
I realize that this doesn't directly answer your question, but for the majority of these species, we don't have an accurate count of their populations. Unfortunately, the species we do have good population data for are likely either at risk of becoming an endangered species, or are already an endangered species, possibly even on the brink of extinction (like the Northern white rhinoceros, which is extinct in the wild).
We don't know how many different kinds of living things there are on Earth - people keep finding new kinds - mostly tiny new kinds that are hard to see.
We don't know how many living things there are on Earth, counting the populations of all the different kinds of living things. But here's a link you could read, and add up the numbers for the different types of living things. For the most common species of bacteria, it says: "It is estimated that the oceans contain about 2.4 × 1028 (24 billion billion billion) SAR11 cells".
In terms of diversity, there are more than 1.5 million species of plants, animals, fungi, lichen, and bacteria documented on Earth. And we are discovering more species every day! In terms of mass, there are currently about 75 billion tons of living things (biomass) on Earth.
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