Introduction:
    The genus Drosera consists of plants commonly known as sundews.  There are more than 160 species of plants in this genus, and they are scattered around the globe.  The cladistic analysis of the genus is as follows:
 Kingdom Plantae
  Division Magnoliophyta
   Class Magnoliopsida
   Subclass Dilleniidae
    Order Nepenthales
     Family Droseraceae
      Genus Drosera
    Sundews are carnivorous plants that trap their prey with glandular hairs, or trichomes.  The trichomes, which look similar to tentacles, have glands at the top.  The glands secrete a sticky fluid that traps the insects when they land on the leaf.  This fluid consists of nectar, adhesive compounds and digestive enzymes.  The leaves then fold inward to surround the insect, and the plant secretes proteolytic enzymes to digest the insect.  All that is left of the insect after digestion is fragments of the insect’s hard exoskeleton.  This is dropped from the leaf when it opens again, as the glands do not start secreting the sticky fluid immediately.
    Sundews grow in peaty areas such as bogs, fens, and the edges of ponds and rivers.  These sites all have nutrient poor soil.  The plants are herbaceous, have fibrous roots, a basal rosette, and small perfect flowers.  The sundew gets its name from the drops of what look like dew on the tips of its leaves.  These drops are biggest when the sun is overhead, and they shimmer like dew to attract insects.  Droseros is Greek for dewy or watery, explaining the family and genus names.  The sundew reproduces either by seed or vegetatively.
    Drosera rotundifolia has round leaves with red trichomes.  Drosera intermedia has more narrow leaves that are in a spatula shape.  The flowers of both species are white or light pink..  While D. intermedia seems to thrive better in nearly waterlogged soils, D. rotundifolia prefers a less inundated environment.  Both species require a cold dormancy period.  D. rotundifolia needs no less than six months of dormancy, and can take up to nine.  D. intermedia, on the other hand, requires only a three-month dormancy period.

Reproduction:
    As mentioned earlier, the sundew reproduces either by seed or vegetatively.  The wind and insects may pollinate flowers, or they may self-pollinate.  Seeds in D. rotundifolia are not released until the fruit, a capsule, rots. The seeds have trapped air in their testa, so they can float in water for a number of days.  This helps distribution along streams and rivers.
    Reproduction in D. intermedia results in a dry capsule fruit type.  When reproducing vegetatively, the sundew can form plantlets from leaf buds.  Auxiliary buds below the rosette can also create a secondary rosette.  In either case, the stem connecting the two parts decays, and they separate.

Habitat:
    As stated in the introduction, sundews are found in peaty areas such as bogs, fens, and the edges of ponds and rivers.  The soils in these areas usually lack Nitrogen, Phosphorus and Calcium.  The sundew can live on soils with a pH range from neutral (7) to acidic (3).  This is important, because peaty areas have low decomposition rates, and the dead organic matter makes the soil acidic.  Sundews are also tolerant of flooding for up to about two months, but they cannot survive drought.  They die after just a few weeks without enough water.  Sundews can survive a wide temperature range.  D. rotundifolia and D. intermedia can stand up to about 30 degrees C in the summer, and as low as 5 degrees C in the winter.  They need direct sunlight, and are shade intolerant.

Distribution:
    The Sundew family is found almost worldwide.  It is much more common in the Northern Hemisphere.  D. rotundifolia and D. intermedia are the two sundews with northern distributions.  They range into Northern Europe, Canada, and Siberia.

Factors Affecting Distribution:

Soil Conditions
    As mentioned before, D. rotundifolia and D. intermedia require low nutrient, acidic soils.  These conditions are only found in wet areas such as bogs, fens, and along streams and rivers, which have low decomposition rates.  These areas are currently being drained for agricultural use or for building sites.  Drainage decreases the amount of water at the site, and so the sundew cannot survive.
    Run-off from agricultural sites nearby has also had a devastating effect on the soil, and so also on the sundew.  It would seem like extra nutrients would help the sundew grow better, but this is not the case.  In fact, Stewart Neal and Erik Nilsen have found that “nutrient additions reduced D. rotundifolia vegetative growth. . . [and] allocations to reproduction (inflorescences) decreased by 98% when N was added to the soil” (Neal).   It is not clear why nutrient availability in soil reduces growth.
    The current problems with soil draining and run-off are causing small populations of D. rotundifolia and D. intermedia, as well as other sundews, to become extinct.  This is decreasing their range, especially in those areas at the edges of their distributions.

Temperature
    This is probably the most obvious limiting factor in D. rotundifolia and D. intermedia distributions.  These two species of Drosera have northern distributions, and they account for most of the sundews found in the northern latitudes.  They can withstand temperatures of up to 30 degrees C in the summer and down to 5 degrees C in the winter.  These two species need a winter dormancy period, something that no other sundew does.  This limits how far south they can grow, because if they do not have a dormancy period, they will not thrive.  Their northern limit is determined by the 5 degree C limit.  This can be maintained even when the temperature outside is below freezing because the plants form hibernacula, or protective cases, and because the snow can insulate the plants.

Rainfall and water
    Both D. rotundifolia and D. intermedia need a high humidity level to survive.  They need about seventy to eighty percent humidity to survive in a terrarium.  While D. intermedia prefers nearly waterlogged soils, D. rotundifolia thrives in a more moderate environment.  This might help to explain why D. rotundifolia is more widespread than D. intermedia.  D. rotundifolia can live in almost waterlogged soils, but also does well in other sites.  D. intermedia has a stronger need for wet soils, and so is limited by the availability of these areas.  Indeed, one can see by looking at the distribution maps that D. intermedia has a smaller range than D. rotundifolia.

Quaternary and Holocene migrations
     D. rotundifolia and D. intermedia would have been able to survive during the glaciation of North America.  They would be closer to the equator than they currently are, but there would have been adequate habitats for them to live in.  However, the water requirements for the two species might have contributed to a wider distribution of D. rotundifolia.
    During glaciation, wind patterns were very different.  The large ice sheet over Canada and part of America split the jet stream in half.  This caused a clockwise circle of air on top of the sheet.  As a result, the dry cold air coming off of the ice sheet absorbed water from the land below it.  This left the upper Eastern part of the continent drier than usual.  Since D. rotundifolia can tolerate these drier conditions better than D. intermedia, it makes sense that D. rotundifolia would become more widespread.  However, there is an important inconsistency with this theory.  The westerlies were bringing wet air into California and the West coast, and according to the theory stated above, this means that D. intermedia would be present in greater numbers than D. rotundifolia.  In fact, it is not found there at all.  This could be due to inferior seed dispersal mechanisms, though.

Role of fire and disturbances
    Before colonization, fires were prevalent across much of North America.  The sundew population is estimated to have been bigger at that time.  North America is used as a reference here because there are data for the periods before, during, and after colonization.  The population was bigger in this earlier time because fire is an important factor in sundew growth.  Stephen Brewer found that removing dead biomass increased Drosera seedling density.  This is true both in open areas (where Drosera normally grows) and in shrub thickets.  Seedlings survived better in open areas, however.  This supports a hypothesis that sundews recolonize well after fire, and may rely on them to maintain their habitat.  Brewster also found in a different study that fires “increased the density, growth and establishment of seedlings”.
     Recent decline in the amount of fire in the wild is due to fire suppression.  Although the idea was originally developed to prevent the loss of life and property, fire suppression has drastically harmed the plant communities that depend on fire to survive.  Without fire, many bogs are undergoing succession.  Larger trees and shrubs are coming to dominate the area, and they shade out the sundew.  Also, without a removal of dead biomass, it is hard for the seedlings to take root.  They have so much dead biomass in the way that they spend most of their energy trying to get through it.
     There is some confusion about fire, however.  Catastrophic burn of an entire bog will not help the sundew.  Drosera relies on smaller fires that, while they may kill a few sundews, will create open spaces for recolonization by nearby sundew plants.  These plants are not fire resistant, they just come in quickly and effectively after fire.  Some who have not researched the topic very well have come to the conclusion that fire is nature’s own disturbance, so sundew should recolonize areas disturbed by humans.  The sundew cannot do this, however, for the areas colonized by humans usually involve drainage or agricultural run-off, which, as discussed earlier, push the local populations into extinction.
Activities of man
     As seen above, humans are drastically changing the environment in which the sundew lives.  Ninety-five percent of carnivorous plant wetland habitat has been destroyed in the United States.  This destruction is irreversible.  Humans have drained wetland areas for cultivation in agriculture or for building projects.  Drosera cannot live in sites with less water.  Sites that still do exist are suffering from agricultural run-off.  This enriches the soil with extra nitrogen and phosphorus.  As discussed previously, Stewart Neal has proven that increased amounts of these nutrients in the soil actually decreases Drosera growth and reproduction.
     Simply stopping development and run-off will not solve the problem.  Fire suppression is detrimental to the sundew as well.  Drosera needs fire to kill shading shrubs and trees, as well as to clear the dead biomass off the ground.  If this does not happen, as proven by Brewer, the seedlings cannot germinate very well.  The destruction of the sundew’s natural habitat is not the only problem humans create, however.
     Carnivorous plants have become very popular in flower arrangements and to collectors.  Many collectors are amateurs who want to try to grow a sundew in a terrarium or near their house.  They find a site that has many Drosera, and pick most or all of them.  Galambosi Bertalan has done a study where he collected D. rotundifolia regularly and measured the populations in following years.  He found that with regular collection, population density and fresh weight decreased with collection.  He says 4H developed a new policy as a result of this information.  “Collectors were obliged to leave five to ten flowering plants to spread seed and to assure natural regeneration of sundew populations”.  He does not talk about the possible harm this policy could incur, however.  If the largest and heartiest plants are selected each year, and so cannot reproduce, there will definitely be a shift in the genetic makeup of the future populations at that site, since some genes may be lost.  Also, having a policy of collection makes people feel it is acceptable to collect rare plants.  This is not a good idea to propagate.
    Weather it is due to changes is soil conditions, fire suppression or collection, man has caused some Drosera populations to go extinct.  This is evident in Arkansas.  Older records indicate that D. rotundifolia was found in one or two isolated populations.  Newer records usually explain these occurrences as incorrect identification, but this student wonders if the small populations have become extinct, possibly due to the activities of humans
 
Bibliography
Bertalan, Galambosi.  2000.  The effect of regular collection of Droera rotundifolia in natural peatlands in Finalnd.
Brown, M.L. and R.G. Brown. 1984. Herbaceous plants of Maryland. Univ. Maryland.
Colorado Native Plant Society. 1997.  Rare plants of Colorado. Second Edition.
Deam, C.C. 1940. Flora of Indiana.
Freeman, J.D., A.S. Causey, J.W. Short and R.R. Haynes. 1979. Endangered, threatened, and special concern plants of Alabama. Auburn Univ.
Hitchcock, C.L., A. Cronquist, M. Ownbey and J.W. Thompson. 1955-1969. Vascular plants of the Pacific Northwest. U. Washington Press.
Hulten, E. 1986. Atlas of North European vascular plants north of the Tropic of Cancer. Koeltz Sci. Books.
Lowe, E.N. 1921. Plants of Mississippi: a list of flowering plants and ferns. Miss. State Geological Survey. Bull. 17.
MacRoberts, D.T. 1989. A documented checklist and atlas of the vascular flora of Louisiana. Bull. Mus. Life Sciences #7-9, La. State Univ. Shreveport. 3 vols.
McCance, R.M. and J.F. Burns. 1984. Ohio endangered and threatened vascular plants: abstracts of state-listed taxa. Div. Nat. Areas, Dept. Nat. Res., Columbus, OH.
McGregor, R.L. & T.M. Barkley (eds.). 1977. Atlas of the flora of the Great Plains. Iowa State Univ. Press, Ames.
Neal, Stewart C Jr.  1992.  Drosera rotundifolia growth and nutrition in a natural population with special reference to the significance of insectivory.
New York Flora Association. 1990. Preliminary vouchered atlas of New York state flora. New York State Museum Institute.
Ownbey, G.B. and T. Morely. 1991. Vascular plants of Minnesota: a checklist and atlas. Univ. Minn. Press, Minneapolis.
Perring, F.H. & S,M. Walters (eds.). 1962. Atlas of the British flora. (Also see supplement) Nelson, Norwich.
Radford, A.E., H.E. Ahles & C.R. Bell. 1968. Manual of the vascular flora of the Carolinas. Univ. North Carolina Press, Chapel Hill.
Scott, R.W. 1995. The alpine flora of the Rocky Mountains. Vol. 1. Middle Rockies. U. Utah Press.
Stephen, Brewer J.  1998.  Effects of competition and litter on a carnivorous plant, Drosera capillaris.
Stephen, Brewer J.  1999.  Effects of fire, competition and soil disturbances on regeneration of a carnivorous plant.
Smith, E.B. 1988. An atlas and annotated list of the vascular plants of Arkansas (and supplements). Private printing. Second edition.
Voss, E.G. 1972, 1985, 1996. Michigan flora. Cranbrook Inst. Sci.
 

Websites used:
http://chili.rt66.com/hrbmoore/Maps/Drosera_rotundifolia.gif
http://cricket.biol.sc.edu/herb/
http://fs.fed.us/database/feis/plants/forb/dorot
http://galaxy.cs.berkeley.edu/calflora/
http://gudjons.com/Mittel/Drosera-rotun.html
http://ibs.uel.ac.uk/ibs/
http://plants.usda.gov/plants/
http://wiscinfo.doit.wisc.edu/herbarium/
http://www.bio.utk.edu/botany/herbarium/vascular/atlas.html
http://www.carniverousplants.org
http://www.csdl.tamu.edu/FLORA/arkansas/
http://www.csdl.tamu.edu/FLORA/b98/check98.htm
http://www.csdl.tamu.edu/FLORA/newgate/gateopen.htm
http://www.csdl.tamu.edu/FLORA/tracy/main1.html
http://www.fs.fed.us/ne/delaware/ilpin.html
http://www.herbaria.harvard.edu/~rangelo/Neatlas0/WebIntro.html
http://www.huntana.com/feis/plants/forb/drorot/
http://www.natureserve.org/
http://www.orst.edu/dept/botany/herbarium/projects/oap.html
http://www.plantatlas.usf.edu/
http://www.rook.org/earl/bwca/nature/acquatic
http://www.sarracenia.com/faq/faq524.html
http://www.upenn.edu/paflora/