Even though most organisms and people fight to stay alive, they do not always fight against each other.  Organisms have very different jobs.  If they have the same job, they can do it in different ways.  People can be very different too.  Some people are funny.  Some are very good at sports, art, or music.  Millions of organisms like cockroaches, trees, mice, cats, dogs, birds, and many more live around people.  They all need different things to survive.  When different organisms live together in the same place, we can say they coexist.



In the same house, people can coexist with their family, their pets, and other organisms that live around them.  This works because they live in different parts of the environment and need different things.  A bird might need a tree to build a nest.  A mouse will try to hide in a hole in a wall.  A dog might be able to sleep anywhere on the floor.  The bird might eat bugs and worms, the mouse might eat very small pieces of food, and the dog might eat larger pieces of food that people feed it.  Each of these animals can live together because each one has a place.



It might work like this: a dog eats big food pieces and leaves small ones on the floor.  A mouse comes in and eats the small pieces that people dropped on the floor, and some that the dog left behind.  The mouse goes back into the wall and hides some of the food it found.  Bugs sneak in when the mouse is gone and eat some of the mouse's food.  The bugs crawl outside, where the bird eats them up.
 


Every living thing fits where it lives.  Each of their lives helps each other.  They need each other to live.  They are like puzzle pieces that fit together.  Each organism fits into a niche.   A niche is the place an organism finds to survive with the other organisms that live around them.



Everyone can do different jobs, but you have probably noticed that it is easier for you to do some things than other things.  For example, some people are really good dancers.  They see someone else dance and they can do the same thing.  They do not even need to think about it because their body moves however they want it to.  Most people see someone dance, and try to get their body to do the same thing, but it is too hard.  Their body does not listen to them as well.


Dancers are usually flexible, and strong too.  They can bend more than most people and they have the strength to move their bodies however they want to.  Some people are born strong, flexible, and with a body that listens to what they ask it to do.  An advantage is when something about you makes you better at something, or makes something easier for you.  If a dancer were trying out for a dance team, it would be an advantage to be strong and flexible.  That person would be able to do dance moves better than someone who was not strong or flexible.



Unlike people, organisms have to worry about being eaten or killed.  In most places, every animal has another animal trying to eat it.  This is true!  In the ocean, there is always a bigger fish that would love to eat a smaller fish, or a human that would love to catch the fish.  On the land, there is always a bigger bird, reptile, or any kind of animal who is walking around hungry.  Bigger animals aren't the only things that can kill other animals.  Sometimes smaller and faster animals can kill larger animals.  A large group of one kind of an organism can bring down a bigger animal.  A virus is very, very, very small, but it can kill the largest animals on the planet.


Finding food and staying away from other organisms who want to eat them is very important, but there is more than that to life for most organisms.  They also have to worry about finding a place to sleep or rest.  If they live in a place where it gets very cold at night or very hot during the day, they need to make sure they are also safe from the weather.  Staying away from organisms that want to eat them, keeping safe from weather, and finding food are all environmental pressures.



Pressure means to push on something.  These pressures push on organisms and make it hard for them to live.  Many scientists think this is the main reason why organisms live and some die.  Traits that give an organism an advantage will help them live longer.  If it is not too hard for an organism to stay alive with the environmental pressures where they live, they have a better chance to live long enough to have babies.  Their genes will be passed on and the traits that helped them stay alive will be passed on too.  In this way, nature chooses what traits help organisms survive and what traits they will have in the future.