I am not a verbal expert, but will try to answer the qs.
Qtn1- The population of tigers in the National Park is increasing steadily: which is a source of encouragement to those who have worked so hard to fund the conservation effort.
" is increasing steadily: which" , i never saw any official sentence using ":". In general as per GMAT rules "," should precede the which because it is a non-essential modifier.
Qtn2- In the engineering sector at the moment there are no jobs for those without experience, which makes it difficult for us recent graduates to get started on our careers.
hah, this a classical example where which is wrongly used. in fact this kind of sentence cannot be modified using touch modifiers such as that or which
Noun modifiers are good at those modification.
why this sentence is wrong for that you have to answer one qs : " which makes it difficult for us recent graduates" actually what is the reason for this difficulty. it is the fact that they dont have work experience , but which is touching only work experience. it doesn't convey the meaning.
that and which cannot modify this complex idea , they can modify only simple things.
For ex: Ravi used a pen, which is used by sachin, for his entrance exam.
Look closely at the above sentence here we just added one specific info about pen.
But in our early case which is trying to modify a idea.
with the above explanation i have provide definitely you will not able to understand fully.
if you are practicing
OG question, you can see maagoosh video. They have provided explanation.
And there are few articles about noun modifiers by
egmat. it will be available in SC section. if you read that you will get good idea.