This can be explained taking examples of several countries belonging to different geographical regions in the world. First and foremost the overall size of the population in Sri Lanka in particular and the geographical size of our island nation in general play a big role in deciding it. Take for example our giant neighbor India, home to the second largest Muslim population in the world after Indonesia. The number of children in an average Muslim family in India is around 05 while in Sri Lanka it is around 10 to 12 in the East and 06 or 07 in other parts of the country. Given the sheer size of non-Muslim populations of India with equally the geographical size of it, the family size of the Muslims in India and their growth rate hardly make any adverse effect on other communities. However, when it comes to Sri Lanka the present growth rate of Muslims make a horrible impact given the non-Muslim families' average number of children hardly pass the number 02. And also the sheer geographical size of India hardly make any negative effect while in Sri Lanka it bears terrible consequences with non-Muslims being evicted in their traditional places of habitations giving way to the disproportionately large exclusive Muslim enclaves as result of Muslim expansionism.
Tension among the communities has already become a reality in Sri Lanka with the land greedy Muslim community acquiring lands at an amazing rate resulting in other communities becoming almost refugees. This has become a common phenomenon in the North- East region as well which was under war for 30 years. With the conclusion of war Muslims have been settling in in the former war zones at such a rate that there have been frequent agitations by Tamils who were the owners of those lands before the war. The latter community claims that the lands owned by them are frequently seized to settle Muslims. This has been rampantly happening with a Muslim Government Minster has been totally biased towards the Muslims when settling them. There has been a simmering row going on in Mullaiveli in Mulaitivu District which I highlighted in one of my posts too. So it is plainly clear that the demographical changes that are taking placing in Sri Lanka due to Muslim population explosion have a terrible bearing on other communities.
How other countries like Singapore, Japan and Australia cope up with the problem:
The countries which have the potential to face similar consequences like Sri Lanka have taken timely actions to prevent it from happening. A case in point is Singapore which took actions to impose a law prohibiting Muslim families not to raise more than two children going in line as same as families of other communities. Singapore would have been facing disastrous consequences given the small size in its population and the land mass in a few years, had not such a law been imposed. Had this law not been effective, the ethnic composition of Singapore which comprises Chinese and Tamils as the first and second largest communities would have had a completely different outlook with Muslims becoming the majority community in a few years.
Take another example of Japan which never gives citizenship to Muslims immigrating to Japan from other countries. Given the small geographical size of Japan and the very slow birth rate among Japanese population, the island nation would have become a sitting duck to prey to the huge Muslim population explosion that would have been unleashed by the Muslim immigrants as they have exactly done in many European countries.
Learning lessons from other countries Australian government has been quite strict in its immigration laws when it comes to Muslim immigrants having known well in advance the terrible consequences that Australian society would have had to face when the population explosion is unleashed by the Muslim immigrants. (Australians can't take chances given its small population size which is similar to Sri Lanka's and very law growth rate). Australia has resorted to take such sever actions despite the fact that the country is blessed with a massive land mass and being a continent. What would have obviously made Australian government to have been cautious was not to disturb the ethnic composition in the country despite the fact the availability of lands has never been a problem to them.
Terrible consequences that European countries are currently facing:
Most of the European countries such as Netherlands, France, the UK, Norway, Belgium and numerous other Scandinavian countries which were lax in their immigration policies towards Muslim immigrants are now reeling with the terrible consequences of Muslim expansionism and Fundamentalism. Muslim immigrants from Asian and African countries have unleashed such a huge population explosion that the original European inhabitants face the danger of becoming minority communities giving way to Muslims in a few decades. The very low population growth rate among their populations similar to non-Muslim ethnicities in Sri Lanka has led to the emergence of numerous far right movements in Europe solely campaigning against Muslim expansionism and Fundamentalism. So the situation that Sri Lanka faces now is similar to a certain extent what some European countries face now.
Vulnerability faced by Sri Lanka
Sri Lanka's unique situation in demographically as well as in geographical size makes it vulnerable to face a serious imbalance in ethnic composition in a few years' time if Muslim expansionism in Sri Lanka is not checked. By encouraging non- Muslim communities to increase the number of children in their families will aggravate problems as the Sri Lanka's land availability has been fast depleting even to accommodate present population.
PS: The fact that Kaththankudi in Batticaloa District has been identified as having the largest population density per square Kilometer in the entire world underscores the terrible situation that Sri Lanka is in. (Population in Kaththankudi is 100% Muslims and this figure was first reported a few decades ago).