California Institute of Technology -- Division of Biology -- Bi 11 Organismic Biology

Terrestrial Organisms: Introduction

Important characteristics of terrestrial environments, as contrasted to aquatic environments:

1. Dry. Water vapor pressure of the environment is frequently lower than a value that would be in equilibrium with the aqueous solutions in organisms ( = relative humidity of ~99.4%). Therefore, organisms must control the rate of water loss from the general body surface, and from specialized surfaces involved in food absorption, respiration, and/or excretion.

2. The surrounding fluid medium (air) has much lower density than water. It provides much less buoyancy, but it also offers less resistance to movement. What is the net result?

3. Oxygen is much more readily available than in aquatic environments. But note carefully that this statement means that the oxygen content is higher, and that the diffusion coefficient is greater, but not that the partial pressure is different. In contrast, the carbon dioxide content of air is lower than the carbon dioxide content of sea water and some other aqueous environments.

4. The environmental temperature of terrestrial environments is less stable . But, on the other hand, in air it is easier to maintain a body temperature that is different from the environmental temperature because air has a lower heat capacity and conductivity than water.

5. Macroscopic plants are available as the major food source for animals.
*Terrestrial plants typically obtain nutrients such as N and P from soils, and also water from soils. The evolution of roots for uptake from soils and a vascular system for distribution through the plant body was a major adaptation enabling large terrestrial plants. Diffusion is not an important consideration for nutrient uptake.
*However, diffusion might be more of a problem for carbon dioxide uptake, since its concentration in air is low. Since almost all terrestrial plants have a substratum to attach to, they can utilize air convection to bring carbon dioxide, and therefore do not have to rely entirely on diffusion, and can be large. In many cases, diffusion would be adequate for an isolated plant.
*Since plants are large, animals at the second trophic level (herbivors) can be large, in contrast to the oceans where large animals are only feasible at the 3rd or 4th trophic level (or higher). Since the energy available to the 2nd trophic level is much more than at higher trophic levels, the total biomass of large animals that can be supported in terrestrial environments is larger.

Three groups of organisms have been successful in adapting to dry, terrestrial environments:
Vascular plants
Higher vertebrates
Paleontological data indicate that the invasion of terrestrial environments by these groups occured relatively late -- in the last 400 Ma -- after all of the invertebrate phyla were well established in marine environments. There is at least some sense in which this was a coevolution : the animals dependent upon the plants for food, and the plants dependent upon the animals for pollination and dispersal.
Among the vascular plants, it is the seed-bearing plants, Gymnosperms and Angiosperms, that have been most successful. Before their appearance, the dominant land plants were the ferns and their relatives (Pteridophytes). These plants still require a moist environment for sexual reproduction, and rely upon spores rather than seeds for dispersal. The sporophyte is the dominant form, but the gametophyte is also a free-living organism, rather than a tiny parasitic form as in the Angiosperms.
The non-vascular land plants, mosses and liverworts (Bryophytes) are necessarily smaller. The haploid, gametophyte generation is dominant, with the sporophyte retained in the archegonium. The fossil record doesn't really tell us whether this was a primitive stage in evolution towards vascular plants, or an alternative, less successful, direction of specialization.

Among the animals, complete adaptation to dry environments is believed to have evolved independently in the arthropods and in the vertebrates. In the arthropods, the insects are clearly dominant in terms of numbers, but the arachnids and myriopods are also fully adapted to life in dry environments. Less fully adapted, semi-terrestrial, arthropods are found among the Crustaceans -- some species of isopods and crabs.
In the vertebrates, the reptiles, birds, and mammals have adapted fully to dry environments, while the amphibians represent a stage that is often still dependent upon an aquatic environment for reproduction, and a moist environment so that the skin can be used as an accessory respiratory pathway.
Less successful adaptation to terrestrial environments is also found among gastropod molluscs -- some snails and slugs. In addition, there are many soil-dwelling nematodes, and annelids, flatworms, and nemerteans that are found in moist terrestrial environments. These are physiologically more like fresh-water organisms.

Terrestrial vertebrates and snails usually have body fluids with salt concentrations equivalent to 0.16 M NaCl, or lower. Terrestrial arthropods usually have body fluids with higher salt concentrations -- equivalent to 0.20 to 0.25 M NaCl. This is usually explained by saying that the terrestrial arthropods evolved directly from marine ancestors, while the ancestors of terrestrial vertebrates and snails lived in fresh-water environments. This conclusion is supported by evidence that some of the earliest vertebrate fossils are found in rocks that are believed to have sedimented from fresh waters, and by the existence of crustaceans such as the isopods and crabs seen in dry places near the Corona del Mar tidepools.

There is clearly a major difference in size between the successful terrestrial arthropods -- the insects, and the terrestrial vertebrates. Insects range in size from about 0.001 to 0.1 m, and are small relative to plants. Terrestrial vertebrates range in size from about 0.05 to 5 m, and are equal to or larger than the size of plants. There is only a little overlap between these size ranges. A very few insects get as large as the smallest birds or rodents. This is clearly different from the situation with cephalopods and fish, where these two groups compete in the same size ranges.
Associated with this difference in size is a difference in the number of species -- estimated to be of the order of one million for insects and 20,000 for terrestrial vertebrates. Additionally, the smaller insects tend to have short life cycles.
In spite of their small size, insects are ecologically successful. One official estimate, made several years ago by the Food and Agriculture Organiziation of the UN, indicated that about 1/3 of human agricultural productivity was actually consumed by insects.

How can we explain the size specialization of insects vs. vertebrates?
I suggest that insects are more successful at meeting the problems of being small terrestrial animals, especially the problem of avoiding water loss when the surface/volume ratio is high. Vertebrates, on the other hand, are more successful at meeting the problems of being large terrestrial animals, especially the problem of providing efficient skeletal support for a large body mass.