For example Syrian refugees get much too much newsmedia attention to be used in for example Portugal for the abusive work in the agricultural sector that Portugal "needs" for export.
IDK reliable figures of undocumented migrant workers, but it is huge, and growing with the growing agricultural export, and anyway "needed" for the Portugese economy as it is handled now.
The gig workers needed in the tourism sector can be added (another migrant profile AFAIK, and the "domestic help" workers (undocumented elderly care and household) another profile).
I guess when all undocumented migrants would leave Portugal, the economy will collapse (at least for a while). Other South European countries in a similar situation depending on region.
This is a totally other situation than East-Europe (stone-walled) or the North-West of Europe.
The agricultural sector is diverse here and the use of migrant workers is
very much a new thing. It started with the romanians and bulgarians some 10 years ago and is now based on importing people from the indian subcontinent. Always on a temporary basis, with the full connivance and indeed guiding of the government.
As (most of) eastern Europe progresses and we keep sinking those eastern europeans were starting to get expensive! Portuguese are already emigrating away into Hungary, for one example. And it's for low-paying jobs, not the tech professionals who think they have no country.
This import of migrants scheme is nor really necessary, and is mostly tied into some sectors where the business is mostly foreign-owned: irrigation in Alentejo for crops such as olives or certain vegetables for export northwards in Europe. Starting to get into wine also. The portuguese profiting from it are actually the landowners who rent the land to foreigners (mostly but not only spanish) doing these operations, and the local farmers complain that the intensive use made of it leaves the land barren and abandoned after a few years. Easier to rent another plot than to pay for lost of fertilizer. This kind of agriculture is not really a sane economic bet. As for tax receipts and employment: if the products are exported and the corporations are foreign-registered, what taxes are paid here? If the migrants workers are exploited and not even declared? Only the tax on land rents I guess. It's not even useful for the commercial balance because locked into the euro as we are that's not an issue - at present. And this agriculture is completely short-termist, it's not creating a local infrastructure/knowlege base that could be useful in the future. We're a country run by corrupt idiots.
The (mostly imported also) gig workers for tourism and now things such as food delivery are other things that build no foundation for anything, no structure for progress.
No the economy would
not collapse if all this ended overnight, if all undocumented migrants left (or just ceased coming - and the documented ones for seasonal labour also). The local people benefiting from this are very few indeed, the wealth spent and remaining here likewise. And in any case
tourism has been killed by the idiots who run my country thinking they were "saving tourism" by refusing to handle this pandemic properly. Even now they pretend to control the borders while refusing to actually control them. Border crossings have been reduced because
the rest of the world is treating us as if we have the plague (we do!), but no strict and enforced quarantine and test scheme was put in place. It's all theatrics, to avoid spending more from the state budget and to "reopen" as soon as the government can get away with it. Because they're very concerned with not writing off a few billion that the local air company is supposedly still worth. In their imagination, because of course air travel companies are bankrupt and will have to downsize enormously and continue to exist based on state support for years to come. There is no strategy, there is only denialism of the problems, and therefore inability to produce solutions.
What
is collapsing is the ability to produce capital intensive anything because investment is pushed into get-rich-quick schemes based on exploitation of cheap labour. Both state investment (
austerity good!) and private investment for anything
hard and productive has been cut over the last decade.