Why do plants and fungi often contain psychoactive substances but not animals?

(*Note that immobile animals like sea sponges - are exceptions in that they are animals in which you can find an uncharacteristically high frequency of occurrence of psychoactive substances.)

I find that addressing this question is worthwhile - It seems that the clincher here which is mobility. Mobility is driven by specialized molecules - specifically the neurotransmitters that control movement. An organism's movement isn't just random - it's a reaction to what the organism experiences around it.

But what if you are the kind of organism that can't move (like plants or fungi)? For example - let's suppose that you're an acacia tree on the African Savannah and you don't have access to very much water and therefore, the fleshy part of your leaves are very precious to you, and you would rather not have harvester ants cut them up and take them away. From an evolutionary point of view, one way to respond would be to biosynthesize toxins that kill the irksome herbivorous ants. However, the actual strategy adopted by some African acacia’s is to sample the DNA in the saliva of the munching ants, determine what species they belong to, and then to very quickly biosynthesize and exude the pheromone of that species of ant’s most voracious predator! Just to be clear - it is not that the acacia trees have a taste for drama… it is simply more energy-efficient to summon an insect to do the hard work by way of biosynthesizing one of the most potent chemical signals in biology - the pheromone.

Cultivated Grapevine

Energy efficiency is a very common cut-off criterion of natural selection because the main source of energy used by organisms on this planet is photons coming from the Sun - there is only a finite amount of photon-carried energy coming to Earth per day. Since evolution favours energy-efficient designs over those that are less energy-efficient, plants find it more advantageous to ward off herbivores with psychoactives than with toxins and even more advantageous still to use pheromones for nature's version of biological control (by summoning the herbivores’ predators). In fact, if you remember the post about the invention of the Mephedrone, https://www.facebook.com/share/p/154rEjLRQi/ 

this phenomenon was mentioned there (and was one of the topics addressed by my PhD research).

Humans have a practice of domesticating plants, a process that may also be termed human-assisted evolution. The best example of human-assisted evolution (or domestication) in plants would probably be the grapevine (Vitis vinifera) that ~13,000 years ago was already being used by humans ceremoniously. Today wine and stronger spirits such as brandy are the foundation of (probably) thousands of different products meant for human consumption. The incorporation of drinks made from the fruit of this plant has gone so far as to make its way into the rituals of Judaism and Christianity.

Another important example of human domestication is that of the cannabis plant that 10,000 years ago occurred in mountainous and isolated areas of Asia and had a natural THC content of less than 1% (which was enough to deter its natural enemies after all: the weight of an adult human is equivalent to the weight of 20,000,000 ants), but today, thanks to our domestication, the cannabis plant grows all over the world and in some cases can contain even 30% THC or more!