Everywhere around the world habitat destruction continues and we don't know what this is going to do because other extinctions have not been like this one. They haven't been wholesale removal of habitats because of the activities of one species. The previous great extinctions have been the result of often external disasters - a meteorite impact, huge desertification - but never before we had one where one species has done for so many others.
The Earth's climate has suffered successive changes which have determined the number and variety of species in each period. The concept of biodiversity specifically refers to the richness of life in a determined time and space. It's a calculation that measures the variety of species, keeping in mind that those species are capable of reproducing among themselves. There is little difference between insects, plants and mammals because every living being counts... and not only because it's alive but because life also depends on it.
The number of species that exist today is unknown. There are about 2 million classified although no one really knows the percentage that this figure represents with respect to the total number of species inhabiting the Earth. Scientists do not agree on approximations because some believe that there are at least 4 million species while others who are more optimistic estimate that there are more than 100 million. Between the two extremes the most accepted figure is around 14 million different living beings.
The predominance and diversity of what is small is not a whim of nature... small creatures correspond to a need. Without insects the machinery of life would not work. They rid the ecosystems of biological remains, they oxygenate the soil and serve as food for hundreds of species of birds, reptiles and mammals. But most of all they're essential for plants whose reproduction depends almost exclusively on the millions of insects that transfer the pollen from one flower to another.