Doing a back of the envelope calculation...
In 2012, MME saw the following passengers to Spanish airports
Alicante - 7,758 (all charter)
Palma - 9,058 (all charter)
Tenerife - 8,000 (all charter)
The planned Ibiza flights will like the Palma flights be a 1x weekly trip for the same number of weeks in the summer. We might guess that the Ibiza route will perhaps have roughly the same number of passengers as Palma - so MME-Spain in 2014 might gain about 9,058 passengers in summer, while MME loses 15,758 passengers in the winter, compared to 2012
Net result - loss of about 6,750 passengers to/from Spain or about 4% of annual passenger load in 2014 compared to 2012