The example that best demonstrates evolution of the basic structure of the community is that of the development of a terrestrial community of a modern type by early reptiles some 250 million years ago.
Between the time when vertebrates (amphibians) first became able to lead a predominant terrestrial existence some 350 million years ago and the establishment of an essentially modern type food web some 100 million years later, the structure of terrestrial community was decidedly different from what it is now.
Development of the modern type of community structure required not only a complete rearrangement of the niche structure of the community but also the evolution of new species that could fill the new niches (Olson, 1961, 1966).
Attainment of the adaptations needed for terrestrial life by the first amphibians did not in itself establish a land-based vertebrate community. These early amphibians were carnivores, and the only animals inhabiting the land environment were insects. It is inconceivable that the clumsy locomotors system of early amphibians would have allowed them to prey effectively on animals such as insects.
Thus the first communities inhabited by terrestrial vertebrates are best regarded as extensions of aquatic communities, with the land habit as an adaptation to improve the capabilities of organisms whose prime food supply was aquatic invertebrates and fish (Fig 14.4A).
By some 300 million years ago, reptiles had evolved that could feed effectively on terrestrial invertebrates an entirely land-based community was theoretically possible in which all herbivore niches were assumed by invertebrates and some of the carnivore niches by vertebrates.
However, the palaeoecological evidences suggest that most contemporary carnivorous vertebrates were unable as yet to realize an entirely terrestrial carnivore niche, so that the great majority of the energy flow through the community continued to pass through the aquatic route.
The typical food chain to the highest terrestrial vertebrate carnivore was plant-aquatic invertebrate- aquatic-invertebrate-feeding vertebrate-* semi- aquatic predator-terrestrial predator.
By 250 million years ago terrestrial herbivorous vertebrates had evolved and a fully terrestrial vertebrate’s community could come into being. From this time onward the basic structure of the terrestrial community was of an essentially modern sort, with all consumer trophic levels occupied by a wide range of animals, both vertebrate and invertebrate.
Such evolutionary changes in the structure of communities were caused by a large number of factors. One factor was changed in the regional climate. It was becoming progressively drier during the period under consideration, and the development of a land-based community reasonable response to this sort of change.
Indeed many evolutionary changes in community structure can be explained on the basis of responses of major changes in the regional biotic factors of the environment (Axelrod, 1950, 1958). But other chief factors of evolutionary change in community include reorganization of the community’s structure in response to the realization of niches that had not previously existed in the community.