How could seeds alter the carrying capacity of an ecosystem?

Carrying capacity may be defined as the maximum population that a given ecosystem can support. Depending on whether the seeds are brought into the ecosystem or taken out of the ecosystem, the carrying capacity of an ecosystem can be positively or negatively affected. 

Seeds are generally dispersed by a number of agents, including wind, water, animals, insects, etc. If the seeds are brought into the ecosystem, there is a chance of new plants growing which will provide oxygen and food to more organisms. This will likely increase the carrying capacity of the ecosystem. On the other hand, if the seeds produced by the plants (within the ecosystem) are dispersed out of the ecosystem, then potential new plants are lost and the carrying capacity of the ecosystem decreases. 

Hope this helps.