If your question can be answered by any result from the first page of a search-engine, then you asked a stupid question.
You have no right to demand that I do your homework for you. I am under no obligation to provide you with a basic education.
OK I'll play along.
From the first result, Wikipedia - population decline
Causes A reduction over time in a region's population can be caused by several factors including sub-replacement fertility (along with limited immigration), heavy emigration, disease, famine, and war. History is replete with examples of large-scale depopulations. Many wars, for example, have been accompanied by significant depopulations. Before the 20th century, population decline was mostly observed due to disease, starvation or emigration. The Black Death in Europe, the arrival of Old World diseases to the Americas, the tsetse fly invasion of the Waterberg Massif in South Africa, and the Great Irish Famine all caused sizable population declines. In modern times, the AIDS epidemic caused declines in the population of some African countries. Less frequently, population declines are caused by genocide or mass execution; for example, in the 1970s, the population of Cambodia declined because of wide-scale executions by the Khmer Rouge.
More specifically for first world countries.
Changing trends Since the dire predictions of coming population overshoot in the 1960s and 70s, and many other social changes, more couples in many countries have tended to choose to have fewer children. Today, emigration, sub-replacement fertility and high death rates in the former Soviet Union and its former allies are the principal reasons for that region's population decline.[citation needed] However, governments can influence the speed of the decline, including measures to halt, slow or suspend decline. Such measures include pro-birth policies and subsidies, media influence, immigration, bolstering healthcare and laws aimed at reducing death rates. Some of these have been applied in Russia, Armenia, and many Western European nations which have used immigration and other policies to suspend or slow population decline. Therefore, although the long-term trend may be for greater population decline, short term trends may slow the decline or even reverse it, creating seemingly conflicting statistical data. A great example of changing trends occurring over a century is Ireland.
Nowhere is "success" mentioned.
You had to skip over the answer to find the definition of the phrase in wikipedia.
If you're intentionally being stupid, you know I am just going to mock you.
If you feel like being honest, why don't you have a look at those search results again. I'm sure you can find the answer.
This is literally the first result on Google
And if you don't like that one, there are many more there.
Why do you do this? You come off looking stupid each time you try and manufacture an argument with me.
I would expect you to up your game, but you don't. It's just the same low effort swing and miss each time.
<Yawn>
That is just dumb. If you make statements like that you should be prepared to back them up.