Types of dispersal

At some time during its life, an organism, whether animal or plant, moves, or is moved, so that it or its offspring do not die exactly where they were born. Such movement is called dispersal. Some organisms are motile throughout their lives, but others are adapted to move or be moved at precise, limited phases of their life cycles. This is commonly called the dispersive phase of the life cycle. The strategies of organisms' entire life cycles often are predicated on the nature and circumstances of their dispersive phases. In general there are two basic types of dispersal: Density independent dispersal Organisms have evolved adaptations for dispersal that take advantage of various forms of kinetic energy occurring naturally in the environment. This is referred to as density independent or passive dispersal and operates on many groups of organisms (some invertebrates, fish, insects and sessile organisms such as plants) that depend on animal vectors, wind, gravity or current for dispersal.[7][8] Density dependent dispersal Density dependent or active dispersal for many animals largely depends on factors such as local population size, resource competition, habitat quality, and habitat size.[9][10] Due to population density, dispersal may relieve pressure for resources in an ecosystem, and competition for these resources may be a selection factor for dispersal mechanisms.[11] Dispersal of organisms is a critical process for understanding both geographic isolation in evolution through gene flow and the broad patterns of current geographic distributions (biogeography). Invertebrates are animal species that do not develop a vertebral column. This in effect includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include insects, worms, clams, crabs, octopus, snails, and starfish. Taxonomically speaking "invertebrate" is no more than a term of convenience. The vast majority of animal species are invertebrates, since only about 3% of animal species include a vertebral column in their anatomy.[1] In other words all animals except those in the chordate subphylum Vertebrata (fish, amphibians, reptiles, birds, and mammals) are regarded as invertebrates. What is more, many an individual invertebrate taxon has a greater number and variety of species than the entire subphylum Vertebrata,[2] In fact some of the so-called invertebrata, such as the Chaetognatha and Hemichordata, are more closely related the Chordata than to other invertebrate phyla. The division of the entire Kingdom Animalia into vertebrates (about 65000 species in part of one phylum) and invertebrates certainly is convenient in some practical contexts, but to put it into taxonomic perspective, it is roughly on the same scale as dividing the animal kingdom into Gastropoda (perhaps 60000 species in part of one phylum) and non-Gastropoda; worthwhile only in certain constrained contexts. Invertebrates accordingly form a massively paraphyletic group. It is generally accepted that the phyla comprising modern Metazoa share a common multicellular ancestor, but with the sole exception of one subphylum of the phylum Chordata, all those phyla are classified as invertebrates along with two of the three subphyla in the Chordata: Tunicata and Cephalochordata. These two, plus all the other known invertebrates, have only one cluster of Hox genes, while the vertebrates have duplicated their original cluster more than once.[citation needed] Within palaeozoology and palaeobiology, invertebrates are often studied within the fossil discipline called invertebrate palaeontology.