Thanks for posting the links @jeremie
2 articles about Parsa from Panthera and ZSL, and one about Bardia from WWF.
Reading the links about Parsa some things struck me, they don't mention actual numbers only percentages, and the percentages between the articles didn't match. I wondered why, and since I love numbers I looked a bit deeped into it.
The 2016 survey confirms that Parsa specifically has seen around a 45% annual increase in its tiger population.
In 2013 Nepal was estimated to support 198 Bengal tigers; the latest survey confirms that Parsa is now home to approximately 90% more.
Let's just ignore that grammatically the quote from ZSL doesn't make sense, as if Parsa alone would have 90% more tigers now than the whole of Nepal in 2013.
A 45% annual increase over 2-3 years doesn't equal a 90% total increase, odd.
But how many tigers are we talking about in Parsa?
Here is an article which mentions the number of tigers in Parsa:
Parsa Wildlife Reserve increased tigers to seven from four
Three(!) tigers! Is that really reason for big news? And these are estimates and estimates come with uncertainty. So I looked into the 2013 report (found here) to see what numbers were estimated and what the uncertainty in those estimates were.
In that report they use two different software programs to estimate the number of tigers (CAPTURE and DENSITY), these are the reported estimates:
CAPTURE: 5, ranging from 5 to 11 (95% confidence interval)
DENSITY: 4, ranging from 4 to 9 (95% confidence interval)
In other words, in 2013 they were 95% certain that there were between 5 and 11 tigers in Parsa (or between 4 and 9, depending on the software program used). Now, they estimate 7 tigers. So can they claim with certainty that this is an increase from 2013? I don't think so...there could just as well have been 11 in 2013.
Looking at the article about Bardia from WWF. Bardia has more tigers and the article states:
A tiger monitoring study conducted in Bardia National Park in Nepal's Terai Arc Landscape has estimated 56 wild tigers, an increase of six from the 2013 estimate for the national park.
Again, only the best estimate is given, no indication of the uncertainty in the estimate. But the numbers can be found in the same report again.
The current reported number of 56 is lies well within the confidence limits of the estimate in 2013, so you can't make any claims of an increasing population.
So based on this data there is not really any evidence of growing tiger populations between 2013 and 2016 in those areas.
Are claims like this valid: Panthera:
Tiger Population Rebounds in Parsa, Nepal, Instilling Hope for the Species
Roaring success as tiger population rises in Nepal
As there is not really any evidence of a growing tiger population in Parsa, and as it only concerns a handful of tigers anyway, I don't think such claims can be made. In fact, both Panthera and ZSL have good scientists, who know what estimates are, and what confidence intervals around estimates mean. But with tiger day in mind, it seemed that the need to publish a success story was bigger than to publish truth.
What would be more truthful? The populations of tigers in Parsa and Bardia seem to be stable and are possibly increasing (but the latter depends on the confidence intervals, which aren't published yet, around the published estimates).