Many plants do have defensive measures to ward off herbivores. For example, when grass is damaged it releases numerous chemical signals into the air. This is wh

Many plants do have defensive measures to ward off herbivores. For example, when grass is damaged it releases numerous chemical signals into the air. This is what causes the freshly-mown lawn smell: http://mentalfloss.com/article/30573/what-causes-fresh-cut-grass-smell http://mentalfloss.com/article/30573/what-causes-fresh-cut-g... These defensive measures have a huge range. Some might only increase healing, others might attract life higher up in the food chain to attack the herbivores, and still others might be able to actively damage the herbivore. My point is that eating plants may be less passive than we think. All of life has had billions of years to evolve, so the interactions can be pretty sophisticated all around.