The primary difference between marine and terrestrial ecosystems lies in their habitat characteristics. Marine ecosystems, such as oceans and coral reefs, are water-based environments containing saltwater, while terrestrial ecosystems, such as forests and grasslands, are land-based with variable moisture levels. These differences lead to distinct biodiversity and adaptations for organisms in each ecosystem. Marine species often have adaptations for buoyancy and salt regulation, while terrestrial species evolve strategies to cope with variable climates and resource availability.