The reasons that caused land degradation are shown in the pie chart.
One problem here is that reason is closely synonymous with cause, and so it’s as though you’re saying:
Another problem is that reason has stronger collocates:
Notice that in these last two examples there is no cause effect signal (cause). If you use a cause effect signal then you need a more suitable substitute word for the cause or reason:
In your IELTS Task 1 essay you will go on to name and describe factors and conditions, and this is easier to do elegantly if you call them ‘factors’ and ‘conditions’ than if you call them ‘reasons’.