Competition

Many different animals compete over different resources to survive. They compete over prey,  shelter and more importantly water.  The water here is a rare resorce recently since most of the heartland and chaparrel areas have been in a drought in recent years. That is most likely why some of the weaker speicies have died  off and stronger plants and animals provailed such as a few speces of cacti that can hold water for a long time. These plants also have a defense mecanism and thats why they survive.