下列哪项不属于动物在自然界中的作用（　　）

通读全文，根据对短文的理解，选择正确的答案填空。   

    I can't remember when I started collecting litter. But it was when I got tired of 1 litter nearby that I realized no one else was going to pick it up.

    I live near a forest in Ohio, America. I can walk there  2 three minutes. I used to love going there to play with my dog. But one day there was 3 much litter there that I became very unhappy. I decided to clean up the forest, and I wanted to 4 happy when I went again. I made my first trip to clean the forest that afternoon. I took a big black rubbish bag with me. Ten minutes 5  I started to pick up litter, my bag was full! There were cans, bottles, broken glass and newspapers in it.

    From then on, I 6  to the forest four times a year to pick up litter. I often stay there for three hours. It makes me feel 7 to do something for the environment. After each trip, I look through all the litter that I've collected. If 8  of it is recyclable（可回收利用的）, I'll keep it. I can't understand why people drop litter. But I will keep on picking it up  9they stop dropping it. I know I am only doing a small bit  10 the earth, but I still think it is important.