Desert plant defense mechanisms

[Sincere]

Hi,

First time poster, long time reader checking in. I was recently watching an episode of Survivor Man, in which he was in a dry desert environment. I noticed that all the plants he used for food, had alot of defenses that seemed to deter grazers. These were mostly thorns, barbs, and little seeds that would embed in a grazers tounge.

The weird thing was the lack of grazing animals in the environment. So the question is, why do desert plants have so many defenses for so few natural enemies; where as grass and wheat are defenseless and live in an environment with many more threats?

[Colibri]

There are plenty of browsing animals in that environment, although they may not often be seen. They include desert Bighorn Sheep, White-lipped Peccaries, and Mule Deer.

But the main reason that desert plants have so many defenses is that the environment is so harsh that any tissue they lose is very hard to replace by regrowth. In milder environments, it's not worth the plant's extra effort (including water and energy) to invest in manufacturing structures like thorns and spines or defenses like toxins when they can instead rapidly re-grow lost tissue. There's a trade off in whether you invest in growth or in defense; desert plants can't grow fast so they need to invest in defense.

[CalMeacham]

The weird thing was the lack of grazing animals in the environment. So the question is, why do desert plants have so many defenses for so few natural enemies; where as grass and wheat are defenseless and live in an environment with many more threats?

1.) There are plenty of potential grazing animals -- the fact that you don't see them at any time doesn't mean they're not there.

2.) grasses and other "defenceless" plants on an Eastern grassland are so numerous that any one stands only a small chance of being irretrievably ruined (and grasses do grow back if cut, after all). It'dsonly one of a sea of plants in a water-rich environment. Cacti, on the other hand, are few in number, limited by the available water (which is pretty scarce). For them, it is a big deal if they get eaten, because that can cut the gene pool pretty severely. Worse, since the plants act as water reservoirs, any animal eating it is going to do a pretty thorough job that it'd be hard to come back from.

3.) Any plants without such protective mechanisms pretty much aren't there anymore -- which indicates the need for them.