 Sometimes the effects that humans have on populations are good especially when we do conservation actions and these have often been the places where we get to witness things called a trophic cascade or a cascading effect which is where you impact one part of the system and that has flow on to multiple parts of the system and even other systems that are connected to that and the classic example of a trophic cascade happened in Yellowstone National Park in the United States. Wolves, gray wolves went extinct in the park in the 1920s or 30s because humans got rid of them. We feared them, we thought they were ordained to us in our livestock so we got rid of them and this happens to lots of predators around the world. So in 1995 scientists put wolves back into the park knowing that the park needed that predator but what ended up happening was a rippling effect through the system. So the wolves they controlled populations of elk directly by killing the elk and eating them but also indirectly because having a predator makes prey items fearful. So even when they weren't hunting the elk the elk would stick to the forest they wouldn't come out in risky places like the open grasslands and therefore the elk ate less of some types of trees and that allowed trees like aspens, olders and willows to re-establish these important forest types and that had flow on effects. The willows and the olders on the riverbank stabilized the streams and provided habitats for beavers and the beavers have been bouncing back and the wolves killing and eating things provided carrion and this provided food for coyotes, ravens and other birds of prey. So one animal, one action the wolves ended up changing the entire system and the fundamental composition and makeup of it and those kind of cascades can happen whether we mean to in a conservation effect and whether we don't just because of human behaviors, trade and commercialization. Disturbance is a natural part of ecosystems. You need little changes and movement in order to make the system complex and like a mosaic. So around me here we're seeing that we have some open grasslands, some closed woodlands, some small shrubs coming up, acacias who have died and these little tiny pieces make the system diverse and beautiful but in the same way that systems have evolved to have disturbance, they also have a threshold at which they cannot handle it and the reason that human activities are bad and we're in this anthropogenic extinction is because humans are pushing the envelope of what natural systems can survive. So in Australia the main actions that are causing the extinction and collapse of ecosystems at the moment are climate change, changed fire regimes, introduced species, especially foxes and cats and rabbits and cantoes, introduced pathogens like kittred and overabundant grazes, whether these are introduced species like bison and pigs or even overabundant native macropods like kangaroos and also clearing and logging which are causing habitat loss. So all of these things are pushing the resilience of natural systems. Globally it's much the same, the biggest threats to species globally are climate change, introduced species, over exploitation, things like fishing and hunting and introduced diseases around populations. So in order to allow systems to recover we have to take a step back and make sure we're taking within the limit, living within the proverbial donut of what humanity needs but what ecosystems can sustain.