GENE FLOW

In population genetics, gene flow (also known as gene migration) is the transfer of alleles or genes from one population to another.

Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of members carrying a particular variant of a gene). Immigration may also result in the addition of new genetic variants to the established gene pool of a particular species or population.

There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind.

Gene flow is the transfer of alleles from one population to another population through immigration of individuals. In this example, one of the birds from population A immigrates to population B, which has fewer of the dominant alleles, and through mating incorporates its alleles into the other population.

Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic differentiation between the two groups. It is for this reason that gene flow strongly acts against speciation, by recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species.

For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side to the other and vice versa. If this pollen is able to fertilize the plant where it ends up and produce viable offspring, then the alleles in the pollen have effectively been able to move from the population on one side of the highway to the other.

Barriers to Gene Flow

Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba,Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China where Ulmus pumila grows. This is because Ulmus pumilahas wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools.

Barriers to gene flow need not always be physical. Species can live in the same environment, yet show very limited gene flow due to limited hybridization or hybridization yielding unfit hybrids.

Female choice can also play a role in hindering gene flow. Asymmetric recognition of local and nonlocal songs has been found between two populations of black-throated blue warblers in the United States, one in the northern United States (New Hampshire) and the other in the southern United States (North Carolina). Males in the northern population respond strongly to the local male songs but relatively weakly to the nonlocal songs of southern males. In contrast, southern males respond equally to both local and nonlocal songs. The fact that northern males exhibit differential recognition indicates that northern females tend not to mate with “heterospecific” males from the south; thus it is not necessary for the northern males to respond strongly to the song from a southern challenger. A barrier to gene flow exists from South to North as a result of the female preference.

Gene Flow in Humans

Gene flow has been observed in humans. For example, in the United States, gene flow was observed between a white European population and a black West African population, which were recently brought together. In West Africa, where malaria is prevalent, the Duffy antigen provides some resistance to the disease, and this allele is thus present in nearly all of the West African population. In contrast, Europeans have either the allele Fy^a or Fy^b, because malaria is almost non-existent. By measuring the frequencies of the West African and European groups, scientists found that the allele frequencies became mixed in each population because of movement of individuals. It was also found that this gene flow between European and West African groups is much greater in the Northern U.S. than in the South.

Gene Flow Between Species

Horizontal gene transfer (HGT) refers to the transfer of genes between organisms, either through hybridization, antigenic shift, or reassortment is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can incorporate genes from other dead bacteria, exchange genes with living bacteria, and can exchange plasmids across species boundaries.^[5] "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes."

Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists [should] instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.

"Using single genes as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT. Combining the simple coalescence model of cladogenesis with rare HGT events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life. Each contemporary molecule has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times."

Genetic Pollution

Naturally-evolved, region-specific species can be threatened with extinction through genetic pollution, potentially causing uncontrolled hybridization, introgression and genetic swamping. These processes can lead to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal. Nonnative species can threaten native plants and animals with extinction by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that supplant the native stock. The extent of this phenomenon is not always apparent from outward appearance alone. While some degree of gene flow occurs in the course of normal evolution, hybridization with or without introgression may threaten a rare species' existence. For example, the Mallard is an abundant species of duck that interbreeds readily with a wide range of other ducks and poses a threat to the integrity of some species.

References

1. ^ ^a b c Su H, Qu LJ, He K, Zhang Z, Wang J, Chen Z, Gu H (March 2003). "The Great Wall of China: a physical barrier to gene flow?". Heredity 90 (3): 212–9. doi:10.1038/sj.hdy.6800237, PMID 12634804.
2. ^ Colbeck, G.J.; Sillett, T.S.; Webster, M.S. (2010). "Asymmetric discrimination of geographical variation in song in a migratory passerine". Animal Behaviour 80 (2): 311–318. doi:10.1016/j.anbehav.2010.05.013.
3. ^ "Brain & Ecology Deep Structure Lab". Brain & Ecology Comparative Group. Brain & Ecology Deepstruc. System Co., Ltd. 2010. Retrieved March 13, 2011.
4. ^ http://webcache.googleusercontent.com/search?q=cache:tpICVNWaTbgJ:non.fiction.org/lj/community/ref_courses/3484/enmicro.pdf+sex+evolution+%22Horizontal+gene+transfer%22+-human+Conjugation+RNA+DNA&hl=en.
5. ^ http://www2.nau.edu/~bah/BIO471/Reader/Pennisi_2003.pdf.
6. ^ http://opbs.okstate.edu/~melcher/MG/MGW3/MG334.html.
7. ^ Horizontal Gene Transfer - A New Paradigm for Biology (from Evolutionary Theory Conference Summary), Esalen Center for Theory & Research.
8. ^ http://web.uconn.edu/gogarten/articles/TIG2004_cladogenesis_paper.pdf.
9. ^ Mooney, H. A.; Cleland, E. E. (2001). "The evolutionary impact of invasive species". PNAS 98 (10): 5446–5451, doi:10.1073/pnas.091093398  PMC 33232 PMID 11344292.
10. ^ Aubry, C.; Shoal, R.; Erickson, V. (2005). "Glossary". Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. Corvallis, OR: USDA Forest Service; Native Seed Network (NSN), Institute for Applied Ecology.

- Wikipedia 

ELAIOSOMES

Elaiosomes (Greek élaion "oil" and sóma "body") are fleshy structures that are attached to the seeds of many plant species. The elaiosome is rich in lipids and proteins, and may be variously shaped. Many plants have elaiosomes that attract ants, which take the seed to their nest and feed the elaiosome to their larvae. After the larvae have consumed the elaiosome, the ants take the seed to their waste disposal area, which is rich in nutrients from the ant frass and dead bodies, where the seeds germinate. This type of seed dispersal is termed myrmecochory from the Greek "ant" (myrmex) and "dispersal" (kore). This type of symbiotic relationship appears to be mutualistic, more specifically dispersive mutualism according to Ricklefs, R.E. (2001), as the plant benefits because its seeds are dispersed to favorable germination sites, and also because it is planted (carried underground) by the ants.

Elaiosomes develop in various ways either from seed tissues (chalaza, funiculus, hilum, raphe-antiraphe) or from fruit tissues (exocarp, receptacle, flower tube, perigonium, style or spicule). The various origins and developmental pathways apparently all serve the same main function, i.e. attracting ants. Because elaiosomes are present in at least 11,000, but possibly up to 23,000 species of plants, elaiosomes are a dramatic example of convergent evolution in flowering plants.

Caruncle

Caruncle

The particular elaiosome in the spurge family Euphorbiaceae is called caruncle (Latin caruncula "wart"). Seeds that have a caruncle are carunculate, seed that do not have a caruncle are ecarunculate.

Carunculate seeds of Ricinus communis (Castor beans)

List

A fully referenced current list of plants that have seeds with elaiosomes can be found in Lengyel et al. (2010).

• Chelidonium majus (greater celandine)
• Claytonia virginica
• Cnidoscolus urens
• Corydalis
• Dicentra (bleeding-heart, Dutchman's breeches)
• Hyacinthus (hyacinth)
• Myrtus (myrtle)
• Ricinus communis castor oil plant
• Sanguinaria canadensis (Bloodroot)
• Trillium
• Viola (violet)

References

1. ^ Gorb, E. and Gorb, S. (2003). Seed Dispersal by Ants in a Deciduous Ecosystem. Kluwer Academic Publishers, Dordrecht, the Netherlands.
2. ^ ^a b Lengyel, S.; et al. (2010). "Convergent evolution of seed dispersal by ants, and phylogeny and biogeography in flowering plants: a global survey". Perspectives in Plant Ecology, Evolution and Systematics 12 (1): 43–55. doi:10.1016/j.ppees.2009.08.001.

- Wikipedia 

- wikipedi

DORMANCY

Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be closely associated with environmental conditions. Organisms can synchronize entry to a dormant phase with their environment through predictive or consequential means. Predictive dormancy occurs when an organism enters a dormant phase before the onset of adverse conditions. For example, photoperiod and decreasing temperature are used by many plants to predict the onset of winter. Consequential dormancyoccurs when organisms enter a dormant phase afteradverse conditions have arisen. This is commonly found in areas with an unpredictable climate. While very sudden changes in conditions may lead to a high mortality rate among animals relying on consequential dormancy, its use can be advantageous, as organisms remain active longer and are therefore able to make greater use of available resources.

During winter dormancy, plant metabolism virtually comes to a standstill due, in part, to low temperatures that slow chemical activity.

Animals

Hibernation

Hibernation is a mechanism used by many mammals to escape cold weather and food shortage over the winter. Hibernation may be predictive or consequential. An animal prepares for hibernation by building up a thick layer of body fat, during late summer and autumn that will provide it with energy during the dormant period. During hibernation, the animal undergoes many physiological changes, including decreased heart rate (by as much as 95%) and decreased body temperature. Animals that hibernate include bats, ground squirrels and other rodents, mouse lemurs, the European hedgehog and other insectivores, monotremes and marsupials. Although hibernation is almost exclusively seen in mammals, some birds, such as the common poorwill, may hibernate.

Diapause

Diapause is a predictive strategy that is predetermined by an animal's genotype. Diapause is common in insects allowing them to suspend development between autumn and spring, and in mammals such as the roe deer (Capreolus capreolus, the only ungulate with embryonic diapause), where a delay in attachment of the embryo to the uterine lining ensures that offspring are born in spring, when conditions are most favorable.

Aestivation

Aestivation, also spelled estivation, is an example of consequential dormancy in response to very hot or dry conditions. It is common in invertebrates such as the garden snail and worm but also occurs in other animals such as the lungfish.

Brumation

Brumation is an example of dormancy in reptiles that is similar to hibernation. It differs from hibernation in the metabolic processes involved.

Reptiles generally begin brumation in late autumn (more specific times depend on the species). They often wake up to drink water and return to "sleep". They can go for months without food. Reptiles may want to eat more than usual before the brumation time but eat less or refuse food as the temperature drops. However, they do need to drink water. The brumation period is anywhere from one to eight months depending on the air temperature and the size, age, and health of the reptile. During the first year of life, many small reptiles do not fully brumate, but rather slow down and eat less often. Brumation is triggered by lack of heat and the decrease in the hours of daylight in winter, similar to hibernation.

Viruses

Dormancy is not relevant for viruses as they are not metabolically active themselves. Some viruses are physically stable and may remain infectious for long periods of time, such as poxviruses and picornaviruses. Some viruses may infect cells and then remain inactive for some time, for example Herpesviruses.

Plants

In plant physiology, dormancy is a period of arrested plant growth. It is a survival strategy exhibited by many plant species, which enables them to survive in climates where part of the year is unsuitable for growth, such as winter or dry seasons.

Many plant species that exhibit dormancy have a biological clock that tells them when to slow activity and to prepare soft tissues for a period of freezing temperatures or water shortage. On the other hand, dormancy can be triggered after a normal growing season by decreasing temperatures, shortened day length, and/or a reduction in rainfall. Chemical treatment on dormant plants has been proven to an effective method to break dormancy, particularly in woody plants such as grapes, berries, apples, peaches and kiwis. Specifically, hydrogen cyanamide stimulates cell division and growth in dormant plants, causing budbreak when the plant is on the edge of breaking dormancy. Slight injury of cells may play a role in the mechanism of action. The injury is thought to result in increased permeability of cellular membranes. The injury is associated with the inhibition of catalase, which in turn stimulates the pentose phosphate cycle. Hydrogen cyanamide interacts with the cytokinin metabolic cycle, which results in triggering a new growth cycle. The images below show two particularly widespread dormancy patterns amongst sympodially growing orchids:

Annual life cycle of sympodially growing orchids with dormancy after completion of new growth/pseudobulb, e.g., Miltonia, or Odontoglossum.

Annual life cycle of sympodially growing orchids with dormancy after blooming, e.g., Cycnoches ventricosum, Dendrobium nobile,  or Laelia.

Seeds

When a mature and viable seed under a favorable condition fails to germinate, it is said to be dormant. Seed dormancy is referred to as embryo dormancy or internal dormancy and is caused by endogenous characteristics of the embryo that prevent germination (Black M, Butler J, Hughes M. 1987). The oldest seed that has been germinated into a viable plant was an approximately 1,300-year-old lotus fruit recovered from a dry lakebed in northeastern China.  Dormancy should not be confused with seed coat dormancy, external dormancy, or hardseededness, which is caused by the presence of a hard seed covering or seed coat that prevents water and oxygen from reaching and activating the embryo. It is a physical barrier to germination, not a true form of dormancy (Quinliven, 1971; Quinliven and Nichol, 1971).

Trees

Typically, temperate woody perennial plants require chilling temperatures to overcome winter dormancy (rest). The effect of chilling temperatures depends on species and growth stage (Fuchigami et al. 1987). In some species, rest can be broken within hours at any stage of dormancy, with either chemicals, heat, or freezing temperatures, effective dosages of which would seem to be a function of sublethal stress, which results in stimulation of ethylene production and increased cell membrane permeability.

Dormancy is a general term applicable to any instance in which a tissue predisposed to elongate or grow in some other manner does not do so (Nienstaedt 1966). Quiescence is dormancy imposed by the external environment. Correlated inhibition is a kind of physiological dormancy maintained by agents or conditions originating within the plant, but not within the dormant tissue itself. Rest (winter dormancy) is a kind of physiological dormancy maintained by agents or conditions within the organ itself. However, physiological subdivisions of dormancy do not coincide with the morphological dormancy found in white spruce (Picea glauca) and other conifers (Owens et al. 1977). Physiological dormancy often includes early stages of bud-scale initiation before measurable shoot elongation or before flushing. It may also include late leaf initiation after shoot elongation has been completed. In either of those cases, buds that appear to be dormant are nevertheless very active morphologically and physiologically.

Dormancy of various kinds is expressed in white spruce (Romberger 1963). White spruce, like many woody plants in temperate and cooler regions, requires exposure to low temperature for a period of weeks before it can resume normal growth and development. This “chilling requirement” for white spruce is satisfied by uninterrupted exposure to temperatures below 7°C for 4 to 8 weeks, depending on physiological condition (Nienstaedt 1966, 1967).

Tree species that have well-developed dormancy needs may be tricked to some degree, but not completely. For instance, if a Japanese Maple (Acer palmatum) is given an "eternal summer" through exposure to additional daylight, it grows continuously for as long as two years. Eventually, however, a temperate-climate plant automatically goes dormant, no matter what environmental conditions it experiences. Deciduous plants lose their leaves; evergreens curtail all new growth. Going through an "eternal summer" and the resultant automatic dormancy is stressful to the plant and usually fatal. The fatality rate increases to 100% if the plant does not receive the necessary period of cold temperatures required to break the dormancy. Most plants require a certain number of hours of "chilling" at temperatures between about 0°C and 10°C to be able to break dormancy (Bewley, Black, K.D kendawg 1994).

Short photoperiods induce dormancy and permit the formation of needle primordia. Primordia formation requires 8 to 10 weeks and must be followed by 6 weeks of chilling at 2°C. Bud break occurs promptly if seedlings are then exposed to 16-hour photoperiods at the 25°C/20°C temperature regime. The free growth mode, a juvenile characteristic that is lost after 5 years or so, ceases in seedlings experiencing environmental stress (Logan and Pollard 1976, Logan 1977).

Bacteria

Many bacteria can survive adverse conditions such as temperature, desiccation, and antibiotics by endospores, cysts, conidia or states of reduced metabolic activity lacking specialized cellular structures. Up to 80% of the bacteria in samples from the wild appear to be metabolically inactive —many of which can be resuscitated. Such dormancy is responsible for the high diversity levels of most natural ecosystems.

Recent research has characterized the bacterial cytoplasm as a glass forming fluid approaching the liquid-glass transition, such that large cytoplasmic components require the aid of metabolic activity to fluidize the surrounding cytoplasm, allowing them to move through a viscous, glass-like cytoplasm. During dormancy, when such metabolic activities are put on hold, the cytoplasm behaves like a solid glass, freezing subcellular structures in place and perhaps protecting them, while allowing small molecules like metabolites to move freely through the cell, which may be helpful in cells transitioning out of dormancy.

Notes

1. ^ Capon, Brian (2005). Botany for gardeners. Timber Press: Timber Press. p. 146. ISBN 0-88192-655-8. Retrieved 2009-09-12.
2. ^ Reptilian Brumation.
3. ^ Brumation.
4. ^ Brumation.
5. ^ Long-living lotus: germination and soil {gamma}-irradiation of centuries-old fruits, and cultivation, growth, and phenotypic abnormalities of offspring, 2002, American Journal of Botany Vol. 89:236-247.
6. ^ Fuchigami, L.H., Nee, C.C., Tanino, K., Chen, T.H.H., Gusta, L.V., and Weiser, C.J. 1987. Woody Plant Growth in a Changing Chemical and Physical Environment. Proc. Workshop IUFRO Working Party on Shoot Growth Physiology, Vancouver BC, July 1987, Lavender, D.P. (Compiler,& Ed.), Univ. B.C., For. Sci. Dep., Vancouver BC,:265–282.
7. ^ ^a b Nienstaedt, H. 1966. Dormancy and dormancy release in white spruce. For. Sci. 12:374–384.
8. ^ Owens, J.N.; Molder, M.; Langer, H. 1977. Bud development in Picea glauca. 1. Annual growth cycle of vegetative buds and shoot elongation as they relate to date and temperature sums. Can. J. Bot. 55:2728–2745.
9. ^ Romberger, J.A. 1963. Meristems, Growth, and Development in Woody Plants. USDA, For. Serv., Washington DC, Tech. Bull. 1293. 214 p.
10. ^ Nienstaedt, H. 1967. Chilling requirements in seven Picea species. Silvae Genetica 16(2):65–68.
11. ^ Logan, K.T.; Pollard, D.F.W. 1976. Growth acceleration of tree seedlings in controlled environments at Petawawa. Environ. Can., For. Serv., Petawawa For. Exp. Sta., Chalk River ON, Inf. Rep. PS-X-62. 11 p.
12. ^ Logan, K.T. 1977. Photoperiodic induction of free growth in juvenile white spruce and black spruce. Can. Dep. Fish. & Environ., Can. For. Serv., Ottawa ON, Bi-mo. Res. Notes 33(4):29–30.

References

• Bewley, J. D. and Black, M. (1994). Seeds: physiology of development and germination, 2nd edn. New York, London: Plenum Press.
• Black, M.; Butler, J. and Hughes, M. (1987). "Control and development of dormancy in cereals". In: Mares DJ, ed. Fourth International Symposium on Pre-Harvest Sprouting in Cereals, Boulder, Co., USA: Westview Press, 379-92.
• Quinlivan, B. J. and Nicol, H. I. (1971). "Embryo dormancy in subterranean clover seeds. I. Environmental control". Australian Journal of Agricultural Research, 1971, 599-606.
• Quinlivan, B. J. (1971). "Seed coat impermeability in legumes". Journal of the Australian Institute of Agricultural Science, 37, 283-295.
• Scholar team. (2002). "SQA Adv. Higher Biology". Environmental Biology. Heriot-Watt University, 93–95.

- Wikipedia 

DISTURBANCE ( ECOLOGY)

In biology, a disturbance is a temporary change in environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass. Major ecological disturbances may include fires, flooding, windstorms, insect outbreaks and trampling. Earthquakes, various types of volcanic eruptions,  tsunami, firestorms, impact events, climate change and the devastating effects of human impact on the environment (anthropogenic disturbances) such as clearcutting, forest clearing and the introduction of invasive species can be considered major disturbances. Disturbance forces can have profound immediate effects on ecosystems and can, accordingly, greatly alter the natural community. Because of these and the impacts on populations, these effects can continue for an extended period of time.

Damages of storm Kyrill in Wittgenstein, Germany.

Criteria

Conditions under which natural disturbances occur are influenced mainly by climate, weather, and location. Fire disturbances will only occur in areas where there is low precipitation, some form of ignition (typically lightning), and enough flammable biomass to allow fire to spread. Conditions often occur as part of a cycle and disturbances may be periodic. Other disturbances, such as those caused by humans, invasive species or impact events, can occur anywhere and are not necessarily cyclic. Extinction vortices may result in multiple disturbances or a greater frequency of a single disturbance.

Cyclic Disturbance

Often, when disturbances occur naturally, they provide conditions that favor the success of different species over pre-disturbance organisms. This can be attributed to physical changes in the abiotic conditions of an ecosystem in combination with reduced levels of competition. Because of this, a disturbance force can change an ecosystem for significantly longer than the period over which the immediate effects persist. However, in the absence of further disturbance forces, many ecosystems will trend back toward pre-disturbance conditions. Such alteration, accompanied by changes in the abundance of different species over time, is called ecological succession. Succession often leads to conditions that will once again predispose an ecosystem to disturbance.

Pine forests in the western North America provide a good example of such a cycle involving insect outbreaks. The mountain pine beetle (Dendroctonus ponderosae) play an important role in limiting pine trees like lodgepole pine in forests of western North America. In 2004 the beetles affected more than 90,000 square kilometres. The beetles exist in endemic and epidemic phases. During epidemic phases swarms of beetles kill large numbers of old pines. This mortality creates openings in the forest for new vegetation. Spruce, fir, and younger pines, which are unaffected by the beetles, thrive in canopy openings. Eventually pines grow into the canopy and replace those lost. Younger pines are often able to ward off beetle attacks but, as they grow older, pines become less vigorous and more susceptible to infestation. This cycle of death and re-growth creates a temporal mosaic of pines in the forest. Similar cycles occur in association with other disturbances such as fire and windthrow.

Compound disturbances

When multiple disturbance events affect the same location in quick succession, this often results in a "compound disturbance," an event which, due to the combination of forces, creates a new situation which is more than the sum of its parts. For example, windstorms followed by fire can create fire temperatures and durations that are not expected in even severe wildfires, and may have surprising effects on post-fire succession.

Species adapted to disturbance

A disturbance may change a forests significantly. Afterwards, the forest floor is often littered with dead material. This decaying matter and abundant sunlight promote an abundance of new growth. In the case of forest fires a portion of the nutrients previously held in plant biomass is returned quickly to the soil as biomass burns. Many plants and animals benefit from disturbance conditions. Some species are particularly suited for exploiting recently disturbed sites. Vegetation with the potential for rapid growth can quickly take advantage of the lack of competition. In the northeastern United States, shade-intolerant trees like pin cherry and aspen quickly fill in forest gaps created by fire or windthrow (or human disturbance). Silver maple and eastern sycamore are similarly well adapted to floodplains. They are highly tolerant of standing water and will frequently dominate floodplains where other species are periodically wiped out.

Forest fire burns on the island of Zakynthos in Greece on July 25th, 2007.

Another species which is well adapted to a particular disturbance is the Jack Pine in boreal forests exposed to crown fires. They, as well as some other pine species, have specialized serotinous cones that only open and disperse seeds with sufficient heat generated by fire. As a result, this species often dominates in areas where competition has been reduced by fire.

Species that are well adapted for exploiting disturbance sites are referred to as pioneers or early successional species. These shade-intolerant species are able to photosynthesize at high rates and as a result grow quickly. Their fast growth is usually balanced by short life spans. Furthermore, although these species often dominate immediately following a disturbance, they are unable to compete with shade-tolerant species later on and replaced by these species through succession.

While plants must deal directly with disturbances, many animals are not as immediately affected by them. Most can successfully evade fires, and many thrive afterwards on abundant new growth on the forest floor. New conditions support a wider variety of plants, often rich in nutrients compared to pre-disturbance vegetation. The plants in turn support a variety of wildlife, temporarily increasing biological diversity in the forest.

Importance

Biological diversity is dependent on natural disturbance. The success of a wide range of species from all taxonomic groups is closely tied to natural disturbance events such as fire, flooding, and windstorm. As an example, many shade-intolerant plant species rely on disturbances for successful establishment and to limit competition. Without this perpetual thinning, diversity of forest flora can decline, affecting animals dependent on those plants as well.

A good example of this role of disturbance is in ponderosa pine (Pinus ponderosa) forests in the western United States, where surface fires frequently thin existing vegetation allowing for new growth. If fire is suppressed, Douglas fir (Pesudotsuga menziesii), a shade tolerant species, eventually replaces the pines. Douglas firs, having dense crowns, severely limit the amount of sunlight reaching the forest floor. Without sufficient light new growth is severely limited. As the diversity of surface plants decreases, animal species that rely on them diminish as well. Fire, in this case, is important not only to the species directly affected but also to many other organisms whose survival depends on those key plants.

References

1. ^ ^a b Dale, V., Joyce, L., McNulty, S., Neilson, R., Ayres, M., Flannigan, M., Hanson, P., Irland, L., Lugo, A., Peterson, C., Simberloff, D., Swanson, F., Stocks, B., Wotton, B. (September 2001). "Climate Change and Forest Disturbances". BioScience 51 (9): 723–734. doi:10.1641/0006-3568(2001)051[0723:CCAFD]2.0.CO;2.
2. ^ Mock, K.E., Bentz, B.J., O’Neill, E.M., Chong, J.P., Orwin, J., Pfrender, M.E. 2007. Landscape-scale genetic variation in a forest outbreak species, the mountain pine beetle (Dendroctonus ponderosae). Molecular Ecology 16: 553–568.
3. ^ Ham, D.L., Hertel, G.D.. 1984. Integrated Pest Management of the Southern Pine Beetle in the Urban Setting. Journal of Arboriculture 10, 10: 279-282.
4. ^ Forest Practices Board. 2007. Lodgepole Pine Stand Structure 25 Years after Mountain Pine Beetle Attack. http://www.fpb.gov.bc.ca/special/reports/SR32/Lodgepole_Pine_Stand_Structure_25_Years_after_Mountain_Pine_Beetle_Attack.pdf.
5. ^ Buma and Wessman 2011 Disturbance interactions can impact resilience mechanisms of forests. Ecosphere. http://www.esajournals.org/doi/abs/10.1890/ES11-00038.1.
6. ^ ^a b Pringle, L. 1979. Natural Fire: Its Ecology in Forests. William Morrow and Company, New York. 27-29.
7. ^ Marks, P.L. 1974. The Role of Pin Cherry (Prunus pensylvanica) in the Maintenance of Stability in Northern Hardwood Ecosystems. Ecological Monographs 44, no. 1: 73-88.
8. ^ Schwilk, D., Ackerly, D. 2001. Flammability and serotiny as strategies: correlated evolution in pines. Oikos 94: 326-336
9. ^ Pringle, L. 1979. Natural Fire: Its Ecology in Forests. William Morrow and Company, New York. 22-25.

- Wikipedia