There's only so much of our planet to go around and so as the human population has accelerated in growth and as human consumption has increased - and intensified per person especially in the rich world - so biodiversity has declined. Its estimated that there are 1.4 to 1.7 million living species that we have named and described and that there may be as many as 10 or even 100 million species in total - and we are responsible for hundreds of extinctions, more likely thousands if you count in species we did not even know existed.
We hunt species directly - for food or medicine or sport - cutting numbers and sometimes wiping out, as with the dodo, or nearly wiping out species, as with the blue whale. We take large areas of land, wiping out habitats and ecosystems eg by deforestation, wetland drainage...to create farmland, mine resources, build roads, airports, towns and cities...We dig and drill into the ground and under the sea to extract resources, like coal, oil and gas, that have taken millions of years to form. We consume the resources we've extracted and pour waste and pollution into the air, oceans and onto land.
We grow our economies as fast as we can - that's how we measure our progress, by the increase in gross domestic product (GDP). GDP treats loss of biodiversity and loss of ecosystems and the services provided as a benefit not a cost eg the current oil spill in the Gulf of Mexico or the Exxon Valdez disaster or all the other oil spills are counted not as a cost but as a benefit - yet we are in truth impoverished by it.
I've previously posted on the gross deficiencies of GDP as an indicator here and on the need for a new kind of economics here. See here for the wiki entry on why GDP is very bad as an indicator of general welfare and wellbeing.