While some media and political discourses describe migration as a threat or an "avalanche", the data show the opposite: Spain is not experiencing an invasion, but a relationship of functional interdependence with the countries of the Global South. In other words, what is presented as a problem is, in reality, a structural need. The country needs a migrant population to sustain its demographic pyramid, its Economics and its welfare system.

This migratory symbiosis is not a metaphor. It is a demographic reality. Since 1950, the world's population has tripled. In regions such as sub-Saharan Africa and South Asia, growth is constant and sustained, putting pressure on resources, employment and social systems. On the other hand, Europe and North America are facing the opposite phenomenon: population aging, low fertility fees and progressive reduction of the working population.

In the case of Spain, the contrast is clear. The fertility rate has been below 1.3 children per woman for years, far from the generational replacement threshold (2.1). Without constant migratory flows, Spain would lose millions of inhabitants in the coming decades, with direct effects on the tax system, pensions and employment. According to INE and United Nations projections, if the current fertility rate is maintained without migration, the population could fall to 30 million in 2100, compared to the current 47 million.

In all these scenarios, we must bear in mind that babies born today would not begin to pay contributions until 2045. In other words, a fertility rate of three children per woman would mean an additional economic effort for the welfare state for at least two decades, before these new generations would be able to sustain the system.

Against this backdrop, can the birth rate alone make up for the demographic deficit? The answer is no. We have modeled three natality policy scenarios, all of which have structural limitations.

In the best case scenario - with public policies sustained for 20 years - there would be no real impact before 2045. In more intensive scenarios (such as increasing to three or four children per woman in one legislature), the results are ineffective, fiscally unsustainable and socially unfeasible. You cannot compensate for a structural problem with short-term measures or with pressure on women's bodies.

On the other hand, migration does have immediate effects. The arrival of young people of working age has helped to balance the ratio of contributors to retirees in recent decades. One study estimates that without migration the demographic sustainability indicator - which measures how many people of working age there are for every person over 65 - would have been 30% lower, further exacerbating the pressure on the pension system. This means that the pressure on the pension system would have been much greater with fewer workers supporting it.

Far from being a threat, migration has helped to maintain the balance between contributors and retirees, delaying a collapse that would otherwise already be underway.

But the function of migration is not only economic. Human mobility plays a redistributive role at the global level: it moves the working population from regions with excess demographic pressure to others with labor shortages and aging. This relationship of interdependence - albeit unequal - makes it possible to keep essential sectors such as care, agriculture and hospitality in operation. Denying this reality for ideological reasons does not change the facts: it only prevents us from managing it realistically, with planning and justice.

At the same time, aging poses additional challenges. Fewer workers means fewer contributions, but also more health care expense , more dependency and more elderly people living alone. In Spain, life expectancy has increased and the feminization of old age introduces new inequalities: many older women have no pension of their own or depend on increasingly fragile family networks.

Projections show that without a sustained migration policy, the Spanish welfare system will face unsustainable pressure. Discourses that reject migration appeal to the myth of national self-sufficiency, but such a model has never existed. Since the 2000s, Spanish growth has been directly linked to the work and contributions of millions of migrants.

This does not mean that migration is a magic solution. It also needs planning, integration and rights. But it is an essential component of any realistic demographic strategy. In fact, countries like Canada and Germany already have active policies in place to attract and retain skilled and unskilled foreign populations. Spain, however, remains trapped in a narrative of emergency, insecurity and control.

Therefore, we must stop seeing Spain only as a entrance for migration to Europe, and begin to understand its position as a strategic opportunity: to attract talent, correct imbalances and rejuvenate the social fabric. Continuing to be anchored in fear and emotional lies not only harms migrants, but also deprives the country of a tool that is essential for its sustainability.

The migratory symbiosis between North and South is a reality of the 21st century. Denying it does not eliminate the problem, it only prevents us from finding effective solutions.