Question

what are two ways that seedless land plants shaped the Earth's ecology?

Answer 1

One is spores, don't know anyelse.

Answer 2

The only other one i could find was through human intervention.

"Plants also reproduce without seed via human intervention, such as through cuttings and grafts or by a gardener physically dividing a root clump with a shovel."

Source: http://www.ehow.com/how-does_5147600_can-plants-reproduce-seeds.html#ixzz1nuBEw7mW

Answer 3

Actually there were/are many plants that evolved before the seed plants. Like the brophytes, ferns and fern allies. Anyway that doesn't really have much to do with your question does it?

These early plants were responsible for (basically) the colonization of earth, they would have made up the earliest forests and then provided food for the other organisms.

The most important thing that happened at around about the same time (carboniferous period) was that the increase in photosynthesis caused by the increase in land plants caused a massive drop in CO2 levels and a rise in Oxygen which essentially allowed other animals to grow. However I think there were a few seed bearing plants around at that time..