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My first love was conservation biology, and I worked for a time on a master’s in Botany with a  concentration in landscape ecology.  Studying such things,  I found out about the evolution of the Angiosperms during the time of the dinosaurs from Peter Ward’s “On Methuselah’s Trail”.   Every plant up until then, like the gymnosperms and club mosses that dominated Jurassic and Cretaceous Forests, took years to mature to the size that could grow cones , and needed to live another two years to mature seeds for release.

The gymnosperms got around this need for long life with woody growth and resinous leaves, to make them hard to eat for the herbivorous dinosaurs and insects of the era.  In the big, violent world of dinosaurs 75 million years ago, there was ample opportunity for a plant species that could reproduce in the first year, set seeds, and die.  Like the angiosperms.

The large world of the dinosaurs was an opportunity for weedy species of all sorts, including the shrew sized proto-mammals that became us.    We started out as shrews and rats eating insects and carrion, with quick reproductive cycles and high energy needs.  That’s why we were small then.  Being warm blooded had its advantages in foraging and reproduction, but it also required more food.  The dinosaurs solved their thermal needs partially bey being large enough to stay warm long after they exerted their muscles.  We solved it by eating a lot.  Shrews to this day eat something like their own weight in insects every day.

When the dinosaurs were extinguished 65 million years ago, the world was made bare for the spread and diversification of mammals and angiosperms over the world.  Even though warm blooded mammals needed more energy to survive the long winter, their small size meant they could still survive on less energy than dinosaurs weighing hundreds of pounds.  The world was already blanketed with the seeds of the angiosperms, constantly spreading seeds to unused cracks to grow in.  As long as they made it through the winter, the world was theirs.

There were five mega extinctions in the fossil record, where over 50% of the species from the previous million years do not survive into the next million years.  The extinction rate during these events is over ten times the background rate.The most recent of these, the Cretaceous-Paleogene extinction, was the fifth.  It set the stage for life on earth as we know it, with Mammals and Angiosperms the dominant big animals and plants on the landscape.
We are the sixth mega extinction.  In the last few millenia, the extinction rate from overhunting of large megafuana was over ten times background.  While some of this extinction was due to the end of the ice age 40-15 thousand years ago (kya), much of it was abetted by the wily hunters with the better communications and walking range.  The correlation of exticntions with colonization by humans between 60 and 10 kya is particularly damning on this point.

We also began to clear land for domesticated plants and animals.  While this is entirely in our legitimate interest, this inevitably put a strain on many species that happened to live on arable lands, and on predators and herbivores that framers identified as competitors for their livestock.

Within the last few hundred years, we began shipping species all over the world.  While this is usually a benign side effect of commerce, in quite a few case invasive species take to alien landscapes far better than their native lands.  Kudzu, Mile-A-Minute, English Ivy and Privet are some of the plant species around where I live.  Pythons, Rats, and Cats kill millions of animals every year wherever they colonize.  Most of the plant diseases, like Dutch Elm Disease, Chestnut Blight, Hemlock Woody Adelgid and Anthracnose were imported with overseas shipments of food or building materials.  The same goes for Fire Ants, Zebra Mussels, and Asian Grass Carp.  All these species, by eating or outcompeting the places they colonize, nudge native species out, stand by stand, on the way to extinction.

In the last hundred years, we have multiplied the width and number of our paths through nature tenfold.  The survival of species in the remaining fragments between is determined by how well they can cross roads.  Bird diversity declines linearly as a function of the distance to edge in these fragments, and the world is now full of fragments.  Where nature surrounded America’s founding fathers, we now surround nature.

The extinction rate in the last century has been between 100 and 10,000 the background rate, depending on how you figure it.  But all is not lost.

We are encouraging a large and opportunistic clade of weedy species, that will inherit the Earth should we become absent through folly or meteorite.  Civilization, with its roads, plumbing, electricity, internet, landfills, plastics and hydrocarbons, is not so different from the Dinosaurs.  Large, blocky, with lots of spaces in between for the weed looking out for a quick opportunity.