This is the weakest part of your essay.
- Deterioration of ecosystem will give a negative impacts to the world such as landslides, climate change and floods
There are eight nouns in this sentence. We could change some to adjectives and replace some with verbs. However, what does the sentence really say?
- The world has environmental problems.
- Landslides, climate change and floods are major environmental problems.
- Our environment is changing. We now have more X, Y and Z.
You could leave it out completely, or put it in the body. There's a bigger problem, however, apart from its vagueness. It’s off-topic (see the word ‘tangential’ in Task Response in the official descriptors for IELTS Task 2 Writing (public version).
Your essay really starts at the third sentence. Get to the point quickly. Mention the topic in the first sentence. Animals, plants - work them in.
A balanced ecosystem is an essential need for a better tomorrow for mankind. Deterioration of ecosystem will give a negative impacts to the world such as landslides, climate change and floods. However, large number of animals and plants are threatened with extinction these days. We depend on them not just to have our daily food but in many ways especially in environmental and ecology control. So that, causes and preventions steps should be taken into account before it become worse.
What’s the difference between ‘environmental’ and ‘ecology’? What does ‘in environmental and ecology control’ mean? Could you specify?
You don’t have one. You need one. A thesis sentence helps the reader to know what is coming in your essay. It reminds you of how your essay will be laid out, and it makes it very easy for the examiner to give you a good mark for Coherence and Cohesion (see “presents a clear central topic within each paragraph”) in the descriptors.
This is not a good thesis sentence:
So that, causes and preventions steps should be taken into account before it become worse.
It could be used in a million essays. What does ‘it’ refer to? What are the causes? What are the steps? Why not specify? Where are the keywords (‘animal’, ‘plants’)? Here’s one possible rewrite:
This essay suggests that human activity is the main cause. It also points out we need to stop hunting and polluting and instead educate our children about protecting wildlife.
Word Choice/Word Form/Usage
- So that, those plants and animals cannot survive → Because of that, many plants and animals cannot survive
The body of your essay is fine. Good!
The last sentence in your conclusion needs work. You wrote:
This the only way for us to have a better tomorrow for our next generation.
Yes, it’s nice, but examiners will ignore it. It’s not directly relevant to the topic. It could fit into many essays, perhaps not a million, but several thousand.
Where are the animals? Where are the birds and the plants and the forests? Make sure every sentence is relevant to the topic. It's a test of how you can answer the question, not how many nice phrases and sentences you can produce at random!