Are the different pronunciations of "species" regional differences?

Solution 1:

The Received Pronunciation (which is the "British Standard", in a way, the one which is also exported) suggests the first one "spee-sheez". Source: Oxford English Dictionary

The US pronunciation accepts both. Source: Macmillan Dictionary

Probably the biologists would use the second one, as internationally there is more consensus on that variant. Also, it comes from ecclesiastical Latin, where the /spetʃies/ pronunciation was used.

There is more likely a professional/academical difference in pronunciation, than a geographical one, as the word is not everyday language.

Solution 2:

It pretty much depends on the speaker, aside from the general factors you already mentioned. An older pronunciation that I don't think is used anymore was /ˈspiːʃiiːz/ ("speeshy-eez"); this stems from the same variation in syllabification that affects words like fascia (which can be pronounced /ˈfeɪʃiə/, /ˈfeɪʃə/, /ˈfæʃiə/, /ˈfæʃə/).

The pronunciation ending in /siːz/ (or in British English, sometimes /sɪz/), is a bit irregular from an etymological standpoint, although it actually doesn't have anything to do with Ecclesiastical Latin.

"-ies" was originally pronounced with two separate vowel sounds

In Latin, -ies was pronounced as two syllables. It is pronounced with two syllables in some English words, such as sanies ("sayny-eez") and paries ("pairy-eez").

However, in other words, such as rabies, scabies, and series, it is pronounced with a single vowel /iː/ (in British English, sometimes /ɪ/). This is actually an irregular correspondence between Latin and English pronunciation: the Oxford English Dictionary (OED) says that the N.E.D. (1903) listed a trisyllabic pronunciation for rabies, and the N.E.D. (1912) listed /ˈsɪərɪiːz/ ("seery-eez") for series.

The OED still lists a trisyllabic pronunciation for caries, but all the other dictionaries I've checked give the two-syllable pronunciation.

simplification to a single vowel sound might be due to analogy?

I suspect that the pronunciation of these words has been affected by analogy, either with plural words that end in monosyllabic -ies such as cities, or simply with words where "ie" is used as a digraph to represent /iː/ (e.g. field, belief, chief). A re-analysis of rabies and scabies as plural forms seems somewhat likely since there are a number of other disease names in English that are morphologically plural (such as measles). Series also seems similar to a plural in meaning (some people, although I think mostly non-native speakers, back-form a singular serie).

That said, in "RP" British English plurals like prophecies generally are transcribed with a lax vowel (/ɪz/), so in British English pronunciations of rabies, scabies and the like with a tense vowel (/iːz/) do not actually necessarily sound like the plural forms of hypothetical singular forms "raby", "scaby".

in "-cies" words, this reanalysis affects the consonant used

In words like species, facies, superficies, it's actually pretty regular for the i to not be pronounced as a separate syllable, but we would still expect it to palatalize the preceding consonant, resulting in /ʃiːz/, due to the phenomenon of "yod coalescence" that gives us /ʃəl/ in special (as opposed to, e.g., /ri.əl~rɪ.əl/ in material). The pronunciations with unpalatalized monosyllabic "-cies" /siːz/ are irregular and would have to stem from some process of reanalysis, like the pronunciations of rabies, scabies and series that were mentioned above.

Solution 3:

As an American, I pronounce it similar to [spee-sheez]. However, the "sh" sound in species is different from how I pronounce the "sh" in most other words. I am not sure if this is normal or not for an American. I pronounce the "sh" in species with my tongue in the normal position for "s" with the back of my tongue near where it would be for saying the y sound.

I always pronounce the normal "sh" sound with my tongue between where it would be for saying an "s" and saying a "y".