"He has been a stalwart of the park FOR THE LAST TEN YEARS." "He has been a stalwart of the park FOR TEN YEARS." The first sentence emphasizes the amount of time that he has been a stalwart: ten years. The second sentence focuses on what he has been: a stalwart of the park. You can tell this because additional language details are present in the first sentence: "For the last 10 years." It is the part of the sentence that the author wants you to focus on.

I wouldn't say that there has is a difference in meaning between the two.

Now, let's look at one more pair of sentences... A3) "He WAS a stalwart of the park FOR THE LAST TWO YEARS of his university years. B3) "He WAS a stalwart of the park FOR TWO YEARS when he was at university." In (A3), you know which ten years is being discussed (more or less). In (B3), you still don't know which ten years of his childhood is being referred to. It could be the first two years, the last two years or the middle two years. In summary, "FOR THE LAST TEN YEARS" is more specific than "FOR TEN YEARS" and only the use of the verb tense "has been" makes them have the same meaning in the original pair of sentences.

However, "for the last ten years" ALWAYS means "the ten years immediately preceding the time that the sentence is spoken" or "ten years preceding another time period that is specified" (e.g. "for the last ten years that he was able to walk") In contrast, "for ten years" could mean any period of ten years. and without additional information, it is not clear which specific ten year period is being referred to. The reason "for ten years" and "for the last ten years" have the same meaning is because of the verb tense "has been". If a different verb tense is used, the two phrases could mean different things. So...one could write A1) "He WAS a stalwart of the park FOR THE LAST TEN YEARS. B1) "He WAS a stalwart of the park FOR TEN YEARS." In this case, (A) sounds a little awkward ("has been a stalwart of the park" would be better). But... (A) means "the ten years preceding the time the sentence was spoken". In (B), you don't know which ten year period is being referred to. Now , let's add some more information.... A2) "He WAS a stalwart of the park FOR THE LAST TWO YEARS of his high school studies. B2) "He WAS a stalwart of the park FOR TWO YEARS before he went to university." Here the two sentences mean mostly the same thing but only because of the extra information added to each sentence. If you change the extra information, you change the meaning of the sentence. NOTE: Already (B2) has some ambiguity whether the TWO YEARS are the two years immediately preceding university but the strong implication is that they are.

As Su.Ki. says, there isn't a difference in meaning between the two but it may be helpful to understand that the reason there isn't is because of the verb tense used (i.e. "has been") and the lack of any additional information in the sentence. In the case of the asker's original pair of sentences, "for the last ten years" and "for ten years" refers to the same period of time i.e. the ten years immediately preceding the time that the sentence is spoken