There is no doubt that the number of individuals across species of mammals, birds, reptiles, amphibians, arthropods (insects and arachnids), fish, crustaceans, corals and other cnidarians, and plants have declined, in many cases severely, and that human civilization has had a negative impact on most living things, particularly since the industrial revolution. The devastation of genetically unique populations is an irreversible biodiversity loss. The evidence all points to a global tragedy with a profound loss of biodiversity. 
Many of us have seen images depicting large herds of bison running in the US open prairies that no longer exist. Remember enormous flocks of birds congregated in marshes and lagoons that have seen their numbers reduced dramatically, or knew of millions of wild animals participating in The Great Migration of the wildebeest, zebras and Thomson’s gazelles in Africa, along with beautiful and impressive animals such as elephants, giraffes, and rhinos, which -in some cases- are in danger of becoming extinct.
Other people have cherished memories of less imposing animals that nonetheless bring deeply felt emotions, such as the sound of thousands of frogs croaking in the middle of the night, birds visiting a backyard feeder year after year; or millions of bats flying to their resting place at dusk. Others might remember that when traveling by car through the countryside, the windshield ended up covered with hundreds of dead insects, which somehow gave us a sense of abundance, and now hardly ever happens. And bees and bumble bees swarming in considerable numbers very often to our gardens, or in large meadows covered with wildflowers, and now in many places, we hardly see any.
If you lived close to the ocean or spend much time there, you were possibly privy to conversations about abundant fish stocks and a bountiful ocean. And, now you will often hear that fish stocks have declined dramatically or read stories about whales, dolphins and other marine mammals washed up death in beaches, occasionally in large numbers.
In the last decades, we have also heard of new species of plants or animals being discovered in tropical forests across the globe, giving us a sense of wonder and possibility; and also that millions of acres of natural forests are being destroyed every year.
Let’s check some facts:
As reported in the Guardian in 2014, “the biggest declines in animal numbers have been seen in low-income, developing nations, while conservation efforts in rich nations have seen small improvements overall. But the big declines in wildlife in rich nations had already occurred long before the new report’s baseline year of 1970 – the last wolf in the UK was shot in 1680. Also, by importing food and other goods produced via habitat destruction in developing nations, rich nations are “outsourcing” wildlife decline to those countries, said Norris. For example, a third of all the products of deforestation such as timber, beef, and soya were exported to the EU between 1990 and 2008.”
Academics and others dedicated to the study of species and the natural world debate if we are already facing a new process of extinction, such as the ones the world has experienced before. Even if that is not the case, we know that besides the species that most people and organizations are focused on because they are in danger to be extinct, all land animals, sea creatures, and plants -for the most part- have seen their numbers severely reduced, with few exceptions.
Many species have disappeared, and many more are following the same path. As reported by The World Conservation Union (IUCN), there are 849 cases of species that have disappeared in the wild since 1500 A.D., and this does not account for thousands of species that disappear before scientists can even describe them. “As of 2006, more than 16,000 species worldwide were threatened with extinction, but this is likely a gross underestimate because fewer than 3 percent of the world’s 1.9 million described species have been assessed by IUCN’s Red List of Threatened Species.”
We also know that some people have argued that species have disappeared before and that this is just part of a natural process. We also know that all other processes of extinctions in the history of the planet happened because of a catastrophic natural event. In some ways what we are experiencing today, says professor Peter Ward from the University of Washington in Seattle, is very much like the dinosaur-killing event of 65 million years ago, when a planet already stressed by rapid changes in climate and sea level was knocked into mass extinction by the impact of asteroids. A very similar scenario has been unfolding in which a single species is having an outsized negative impact over all other species: us the Homo sapiens. In others words, human activity is directly related or has caused the conditions under which most species on Earth have seen their numbers dramatically reduced, almost a thousand of them have already disappeared and approximately 16,000 species worldwide are threatened with extinction.
The marine extinction crisis is not as widely grasped as the crises in tropical forests and other terrestrial biome, says Tundi Agardy. We do not know how many species are in the ocean, and therefore we do not know how many have disappeared or how many are in danger of disappearing. We know that “overfishing is a major global concern. Current assessments cover only 20% of the world’s fish stocks, so the true state of most of the world’s fish populations is still murky. Now, a new method based on catch data and fish characteristics suggests that those unstudied stocks are declining.”
Nearly three-quarters of the world’s commercially fished stocks are overharvested and at risk. Furthermore, the bulk of marine species are undiscovered -we are losing species or unique types within a species- before we even know of them. As a result, the number of known marine extinctions is small due to our lack of knowledge.
What is driving the dramatic reduction of individuals/populations across all species?