Dispersal In Space And Time

Another layer of realism is added to models of population dynamics by considering the spatial processes of organismal movement (immigration, emigration). Species are often distributed in space in multiple populations that are linked by migration. The first attempt to deal with this situation by Levins (1969) ignored population dynamics within each habitat patch, assuming that each patch was of the same quality and had the same probabilities of local population extinction and emigration. The scenario described is the definition of a "metapopulation." Migration creates significant feedback between populations, resulting in persistence of a species whenever migration rate is greater than population extinction rate.

Populations of species that are not in decline often correspond with a different spatial distribution called a source-sink metapopulation (Harrison, 1991). In this distribution, there is a large habitat with a thriving population in no danger of local extinction and other smaller populations in less suitable habitats. Since emigration is often density-dependent, the thriving population regularly produces emigrants while the other populations do not. Hence, the less suitable habitats are kept populated, over the long term, by emigration from the large "source" population(s). These spatially distributed patterns for species populations have been shown to play an important role in the global population dynamics of species and are now a fundamental part of population viability analysis in conservation ecology.

Active dispersal involves the expenditure of energy by the organism. Passive dispersal occurs due to the movement of material the organism is attached to or caught in (e.g., wind or water). Passive dispersal can be truly passive with no energy expended, or an organism may prepare morphologically and physiologically for passive dispersal by entering a new life stage. Stages for dispersal are typically more resistant, dormant, or mobile than growth stages. The fruiting bodies and spores of fungi and myxococci are examples of elaborate life stages for passive dispersal. Passive dispersal of bacteria with water flow can occur if the bacteria are not adsorbed onto immobile soil particles or protected by soil structure. Fecal coliforms applied to the soil surface with manure can move several meters and contaminate ground or surface waters in some situations, particularly when preferential flow paths limit interaction of the water and cells with the soil. Cell size limits the passive movement of organisms with water in soil due to the sieving effect of soil particles. Larger-celled microbes such as yeasts and protozoans do not experience passive dispersal to as great a degree as bacteria and viruses; however, nondormant protozoans are typically engaged in active dispersal to obtain food. Plant roots, seeds, fungal spores, and chemical substrates found within several centimeters of particular soil bacteria have been shown to induce chemotactic responses (active dispersal) although the extent to which this occurs naturally is unclear (Murphy and Tate, 1996).

Passive dispersal of hyphal fungi is normally restricted to spores. Spores may be produced in the soil, such as by arbuscular mycorrhizal fungi, or in sporocarps (fruiting bodies) above the soil surface. Sporulation is often induced by environmental cues such as moisture. Spores are also dispersed by animal activity both above and below ground. Vegetative growth of fungal hyphae can also be considered a form of active dispersal since new areas are being explored. Fungi often have distinct forms of hyphal growth for nutrient acquisition versus dispersal. Hyphae for dispersal, such as rhizomorphs, grow more rapidly, are thicker and tougher, and may be formed by anastomosis (cellular fusion) of multiple smaller hyphae (Rayner et al., 1999). The strategy is to invest little and maintain impermeable surfaces during exploration of a resource-poor environment until a resource-rich patch is encountered. Upon encountering such a patch, there is a proliferation of thinner, more permeable hyphae with a higher surface-to-volume ratio.

The ability of an organism to enter a dormant phase can also be seen as a dispersal mechanism, but through time rather than space. This form of dispersal is one version of the "storage effect," by which reproductive potential is stored across time, resulting in higher reproduction rates under favorable environmental conditions. Entrance of individuals into a dormant life stage may be a develop-mentally programmed event for some species or may be induced to avoid density-dependent competition. For many organisms, life stages that facilitate passive dispersal in space are also optimal for dispersal in time. This is true of plant seeds and fungal spores. A general state of dormancy for soil microorganisms is indicated by the increase in numbers and metabolic activity when soil is amended with water or nutrients. Bacterial cells entering a dormant stage are known to undergo a suite of biochemical and morphological changes, including reduction in size. "Dwarf' cells (<0.07 pm3 biovolume or <0.3 pm diameter) make up the majority of bacterial cells in soil (Kieft, 2000).

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