If seeds are light and small, they can be dispersed by wind. The problem with this system is that the plant has to make a lot of seeds, because most of them end up in the wrong place.
Having an animal disperse your seeds can solve this problem, though it may introduce others (the animal might eat your seeds). Animals can move larger seeds, and they provide a bit more specificity in where the seeds end up. If a particular bird is eating a particular fruit, that bird is known to visit places where that particular kind of plant can grow and may end up depositing the seeds in a similar habitat. This system is how mistletoe seeds get dispersed. Mistletoe is a parasitic plant that grows on other plants. Mistletoe fruit is eaten and the seeds dispersed by perching birds, which conveniently deposit the seeds right onto another potential host plant. The fruit is the lure that gets the birds to eat the seed.
Not all relationships that result in seed dispersal are mutualisms, of course. Some plants are very good at getting animals to disperse their seeds without giving any reward. If you've ever had to pull seeds out of your socks or off your dog after a hike, you know exactly what I'm talking about. Some plants have evolved quite clever mechanisms that allow their seeds to be attached to animals.
Not all seed or fruit eaters disperse the seeds; some just digest those as well, setting up another co-evolutionary interaction that has resulted in plants producing seeds with toxins and animals developing the ability to cope with these toxins.
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