Correct?

Hi. Is the tense of the underlined verbs correct? Thank you in advance.

A long time ago, history was made in the deserts. At least two great civilisations were established there. These civilisations were built in deserts because people knew how to live in harmony with nature. Today, about 140 million people are threatened by the spread of the desert caused through the interaction of climatic fluctuations and man's abuse of his environment. Many people think that desertification is caused only by a change in climate, but the truth is that deserts are provoked by human actions. Throughout the world, deserts are created because pastures near arid lands are heavily grazed and trampled. More productive plants are intoduced into semi-arid lands. As a result, indigenous plants necessary for the maintenance of the soil are nudged out. Because of the intensive production of cash crops like cotton and groundnuts, soil nutrients are dissipated and the topsoil is eroded by wind and water. Around towns, adjacent forest belts are denuded by people in their search for firewood. Desertification must be stopped now, otherwise deserts will transform into a curse for man soon.