The lecture and the reading passage havprovide opposing views about the mainprimary causes of the sea cows extinction.

Your sentence is correct. However, 'provide' is better than 'have' because 'have' is very definite. And we will never be 100% sure of the causes of the extinction. 'Causes' should plural because you are talking about several possibilities. And 'primary' is better than 'main' when you are dealing with several possibilities, all of which likely had some effect.

Firstly, the reading mentionclaims that sea cows had been overhunted by native people whichand that caused the main extinction, since sea cows were a good source of food.

'Mentions' implies saying something in passing. You are discussing the main cause of the extinction. So it better to use a stronger word, like 'claims'. If you want to use 'which' there needs to be a comma before it. 'and that' helps to keeps to focus on the core of your argument, which is the possible causes of extinction.

YetHowever, the lecturer confirms that a sea cow was a massive creature.

You need the article 'a' before 'sea cow'.

Sea cows could feed the native people, but they were not large.

This sentence implies that sea cows 'were not large', which contradicts the previous sentence. I'm not sure who is 'not large'. Do you mean that there weren't many native people so a few sea cows could feed the entire community?

So they certainly did hunt the sea cows, but they did not need a lot of them.

'the sea cows' implies a specific group of sea cows. If you are talking about the population in general, there is no article beforehand.

Secondly, the author thinks that ecosystem disturbances caused of the extinction forby reducing the availability of the sea cows’ main source of food— kelp decreasing.

The way your sentence reads, it implies that it is the kelp that became extinct.

In contrast, the speaker from the listening passage indicatesuggests that if ecosystem disturbances really had happened, that notwould not have just affected kelp, but would have impacted other parts of the ecosystem as well.

For example, it would have caused a decrease in the whale population.

However, there didare not reports of such a decline.

SinceIt appears there wasere no ecosystem problems, so the kelp population was just fine and thenprobably unaffected and the sea cows did not experience a food shortage.

Problems plural, so 'were' not 'was'. 'just fine' is too casual for this kind of formal writing. All of this research is well-informed theorising, hence it is better to use words that acknowledge that, like 'appears' and 'probably' rather than words that imply known facts.

Lastly, the reading says that that the main cause of sea cowsthe extinction of the sea cows is that they were hunted by European fur traders, who had weapons that were able to kill a large animal quickeasily.

You can write 'the sea cows' extinction' but when this concept is the heart of your argument, it is better to write it out as 'the extinction of the sea cows'.

InsteadHowever, the listening asserts that the sea cow population was already smallreduced before the traders arrived.

'reduced' is better because it follows the argument that the decline started before the traders arrived.

The audio lecture and the reading passage (that I listened to and read) have opposing views about the main cause of the sea cow's extinction.

I added the part in parentheses since it feels a bit weird to use "the" without having referenced anything prior.

Firstly, the reading mentions that sea cows hadve been overhunted by native people which caused the main extinction, since sea cows were a good source of foodsearching for a good source of food, which mainly caused their extinction.

Yet, the lecturer confirms that the sea cow was a massive creature.

Sea cows could feed the native people, but theyre were not largemany native people to feed.

So, they certainly did hunt the sea cows, but they did not need a lot of them.

Secondly, the author thinks that ecosystem disturbances caused of the extinction fora reduction in kelp, the sea cow's’ main source of food— kelp decreasing.

In contrast, the speaker from the listening passage indicates that if ecosystem disturbances really happened that not just affected kelp but other parts of ecosystem as well, other parts of the ecosystem beside the kelp would have been affected.

For example, it would have caused a decrease in the whale population.

However, there did not report a declineno such decline was reported.

Since there wasere no ecosystem problems, so the kelp population was just fine and then, so the sea cows did not experience a food shortage.

LastFinally, the reading says that thate main cause of the sea cow's extinction is that they were hunted by European fur traders, who had weapons that were able to kill a large animals quickly.

InsteaOn the other hand, the listening asserts that sea cow population was already small before the traders arrived.