How do we know that scientists are not counting more species than there really are?
Well, you can always question any statement anyone makes, but is it a reasonable question? If you don't want to take science's estimate of the number of species on earth
"As of 2009 the estimated number of total species is ~11,327,630
However the total number of species for some taxa may be much higher.
10-30 million insects;
5-10 million bacteria;
1.5 million fungi;
~1 million mites
~1 million protists
source
[The number of possible chordates: mammals, fish, birds etc, is ~80,500]
then so be it. However, the rest of us do. So you can either stick to your doubts and excuse yourself from the discussion (if you can't accept the premise then what else do you have to contribute?), or play along with what science says and remain a participant.
If two animals can produce a fertile offspring, then they are the same species.
Not at all. Wolves,
Canis lupus, and coyotes
Canis latrans, two different species, have often interbred to produce fertile offspring. Then there's the grapefruit, which is a cross between two plant species, a sweet orange,
Citrus sinesis and a pomelo,
Citrus maxima.
In fact, animals from different genera have been known to interbreed and produce fertile offspring, such as cattle,
Bos taurus and bison
Bison bison, which produce beefalo.
Short of trying to crossbreed all the so called "species" on earth and testing for fertility, we must assume the estimated number of species is at the high end. There could be actually a lot less.
Of course you must, because if you don't it makes explaining all the animals
and plants Noah had to take aboard all the more difficult. (The number of identified plants is 326,175, and they had no way to travel cross country to walk on board the ark.)
And let's not forget about all the fish Noah and crew would have had to gather and take on board. If the flood water was saline then about 15,000 species of freshwater fish would have to have been saved---freshwater fish will die if put in salt water. Or if the flood water was fresh then approximately 15,300 marine fish would be in jeopardy of dying---saltwater fish die if put in freshwater. And if the water was mix of the two, only half as saline, then just about all the fish would have had to be taken aboard. Only a handful of fish, such as the bull shark, salmon, striped bass, and rainbow trout, can tolerate both waters.
Trouble is, the more one thinks about the particulars of the story the more ludicrous it sounds.
.