As far as I know, the word "species" can pronounced either as spee-sheez or as spee-seez. I understand that neither of these is incorrect: they're just two different ways to say the same thing. I also know that the second one is pretty much only used in the US. Not being a speaker myself, I'd like to know: is this a regional variation, or is it just something that depends on the speaker?
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.
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
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.
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".
My undergrad is in Biology and I have worked in science and engineering for more than 40 years. I travel extensively within the Americas, Europe and China.
Here's my take.
I NEVER EVER say Spee-shees and I never will. To me, that pronunciation is fingernails on a blackboard. Uncultured. Seriously, I can't stand hearing it.
Spee-cees is what I say and over the course of my life I've said it A LOT.
My thought - and I have thought about it...
Someone, somewhere about ten or fifteen years ago - perhaps a broadcast newsperson of some kind - said "Spee-shees". Some dip-sh*t person heard it and thought to themself "Say, here's a way for me to demonstrate how VERY SUPERIOR I am because Spee-Shees sounds soooo much smarter. And that's me ... I are smart!"
Fast forward. Now the pox is pandemic. Every moron say Spee-shees and increasingly I hear even life-sciences professionals saying it that ugly way too. Sad. Yet another example of the Decline of Human Civilization.
The final insult is that David Attenborough - that's SIR David Attenborough (and the guy I wish I was) now says Spee-shees. However, his vocalization is a slightly less horrific as he does not emphasize the repellent 'SHee' part.
Citing various dictionaries doesn't settle anything. Dictionaries are actually pretty quick to accept common mal-pronounciations. (Hmmm. Dictionary sales maybe???)
Do your part.
FRIENDS DON'T LET FRIENDS SAY SPEE-SHEES !!!