To what extent did the locals suffer in short and longterm because of the eruption? Did it have specific economical and political consequences? How much were the Dutch involved in helping to restore the order in what must have been an incredible humanitarian crisis?
TL;DR: Locals suffered a lot in many places and benefited in others. The Dutch were not involved because the British controlled their colonies until 1816. In any case the British didn't achieve much.
It was April 1815 and cannot shots ringed through Indonesia. Princes and sultans rallied their armies, only to find that there had never been an enemy army firing at their gates. What on earth had just happened?
The next three days darkness filled the sky and ashes fell from heaven. Nothing was wrong, the Javanese tried to assure themselves, the Goddess of the Southern Ocean just happened to be in a celebratory mood. Others were more apprehensive. This had to be allah taala marah - the wrath of God Almighty. Then there were people who started collecting the ashes to sell as magic powder.
Mount Tambora had just exploded.
Mount Tambora is located on the island of Sumbawa, and naturally it was this island that was the most devastated. Everyone in the Tambora Peninsula itself - every 11,000 of them - died almost immediately. The mysterious Tambora language instantly went extinct, much to the sadness of future linguists. An enormous tidal wave devastated the north coast and reduced the biggest port to rubble.
Volcanic ash sixty centimeters deep coated the ground, destroying 95% of the Sumbawan rice crop. It was not until 1820 that rice could be grown again. People began to eat anything - dry leaves, poisonous plants, even horses, Sumbawa's main export. People sold themselves as slaves to foreigners, the only way to leave the accursed island. Parents sold their children to buy three kilograms of rice; one village dug up the local cemetery to sell the grave goods. Ash made water undrinkable, leading to an epidemic of diarrhea and fever. The king of Bima himself, the most powerful lord in Sumbawa, fell victim to the disease. British ships brought aid, but this was too little, too late. (The British controlled Dutch possessions in Indonesia until 1816.)
All in all the population of the island fell from 170,000 to 86,000. Animals had it even worse; 75% of the livestock had died and bees and birds were wiped out.
Recovery was slow to come. The devastation of beehives, birds, and livestock meant that there was nothing for the Sumbawans to sell (besides themselves). One commodity, sappanwood, remained common, but it couldn't be exported because there weren't enough people to collect the wood. By 1847 the animal population (and consequently trade) had recovered in some parts of the island, but other areas remained in utter ruins.
In the long term, the explosion of Tambora led to an overall decline in the area under cultivation in Sumbawa. But this didn't mean that the forest returned or biodiversity rose. The forests were annihilated by the volcanic ashes and replaced by vast plains of cogon grass and by mountain farms cleared by refugees fleeing the devastation in the lowlands. Politically, the eruption was blamed on the impiety of the king of Tambora who had murdered a pilgrim from Mecca, and a fundamentalist Islamic movement sprang up.
Today, Sumbawa's population is 430,000 - less than three times the pre-eruption population. In Indonesia as a whole, the population is twenty-five times what it was in 1815.
Bali and Lombok were also badly affected. So much volcanic ash fell from the sky that buildings collapsed under the weight, crushing everyone inside. The rice plants in the paddies were almost completely obliterated. In November there was an earthquake in Bali that burst a lake, washing away more than a dozen villages. To add insult to injury, a swarm of rats ate up what little food remained. All this forced the Balinese and Lombokese to rely on imported food - but the main exports of both islands had been slaves, and now no food was arriving thanks to the British administration's misguided humanitarian policy of banning the slave trade.
A fourth (or possibly three-fourths) of the population of Lombok died, while at least 25,000 people in Bali were killed. Horror stalked the isles. One Dutchman traveled on a 25-kilometer road in Bali, passing by 34 rotting corpses on the way. Dead children lay prone in the beaches. The Balinese said that their parents had killed them so that they would die quickly rather than starve to death.
But in the long term, the eruption was a blessing in disguise (I feel horrible saying this considering how many people died horribly). Tambora's ashes were absorbed into the soil of Lombok and Bali and made the land more fertile than ever before. By 1846 Bali had become the "chief granary for Singapore."
The eruption disrupted existing political arrangements and some kingdoms were able to exploit this to their advantage. One small kingdom in the island of Flores had long been subject to Bima in Sumbawa. With Sumbawa annihilated, this little kingdom could revolt against Bima and remain independent until the 1850s.
But by far the greatest beneficiary was Java, the population center of Indonesia. Unlike in Sumbawa or Bali, the volcanic ashes that fell on Java weren't enough to smother the rice plants. This meant that the rice plants were safe and sound and could soak up the volcanic nutrients that had just fallen down from heaven. The British reported that "such a season of abundance as the present has not been known for many years."
Tambora blowing up was just the Goddess of the Southern Ocean celebrating a marriage, the Javanese had said. Well, maybe she was. The Goddess had always been the guardian spirit of Java. Perhaps it was she who was behind making such a calamity a blessing for the Javanese. Who knows?
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