The evolution of terrestrial ecosystems began with the advent of land plants. Land plants affected multiple aspects of the early terrestrial environment such as the reduction of atmospheric CO2 concentrations and creating inhabitable habitats for other organisms to survive in. The transition from an aqueous to gaseous medium exposed plants to new physical conditions, resulting in vital physiological and structural changes.
Earth’s First Plant
The first evidence of colonization of land occurred in the mid-Ordovician by plants what were known as eoembryophytic. These very small plants were gametophyte dominant, spending the majority of their life in the haploid phase. Eoembroyphytes were dependent on water for both survival and reproduction. The innovation of individually, dispersed spores did not increase until the early Silurian with the rise of eotracheophytic plants such as hornworts, mosses, and early vascular plants. Unlike eoembroyphytes, the principal generation phase of eotracheophytes was the sporophyte (diploid), and thus the increasingly use of spores instead of tetrads. Consequently, the Silurian was beginning of the reduction of the gametophyte phase; the arrival of spore morphology provided evidence of increasing land plant diversity, forming several basal groups. In addition, plants of this time began to build larger and more complex structures; however, most were still generally small at about 10 – 20 cm tall. Furthermore, eotracheophytic fertilization was still limited to water.
Modern Plants
True vascular plants-eutrachiophytes-did not rise until the Devonian. It was in this group of plants that two vital innovations evolved: seeds and lignin. The invention of seeds led to the reduction of the gametophyte generation and thus the need for water in reproduction. As a result, land plants could begin to colonize the interior of continents for the first time. Furthermore, plants perfected the reproduction through seeds with the invention of flowers (angiosperms) in the Cretaceous. Lignin is an essential compound used to make wood, a crucial substance for plant structure and complex development. With lignin and seed dispersal, plants were able to build enormous and diverse biogenic habitats that eventually led to the rise of terrestrial animals.
Plant habitats allowed animals to begin colonizing terrestrial environments as well. However, out of the 35-40 phyla of animals that exist, only 3 – arthropods, chordates, and pulmonates – were successful on land. Animals moving onto land from sea experience profound changes in all aspects of life such as low densities of air, effects of gravity, high oxygen content, intense sunlight, desiccation, and temperature fluctuations. These aspects did not affect marine ecosystems as they do terrestrial ones. As a result, organisms that became terrestrial had to develop physiological adaptations to overcome these hardships. Nevertheless, terrestrial organisms were successful as they make up over 70% of all animal species today (Selden).