Cattail marsh at Apostle Islands National Lakeshore, Wisconsin.
Hybridization is a genetic process in which
individual organisms from two genetically distinct populations mate and produce offspring. It can occur between distinct populations of the same species or between two different species. Plant hybrids of different species in the same genus are called interspecific hybrids or crosses. Hybridization between two closely related species is actually a common occurrence in nature with regard to plants, but is also being greatly influenced by human activities (Allendorf et al. 2001). Hybrids can produce either sterile (not viable) or fertile (viable) seed. They can also exhibit “hybrid vigor” or heterosis, which results in more robust growth than exhibited by the parent species. This phenomenon determines the kind of effect that a hybrid will have on its own population and others with which it interacts. Hybrid zones occur where the ranges of two species meet; hybrids are continually produced in great numbers in these zones. Hybrid zones are useful as biological model systems for studying the mechanisms of speciation confirmed by DNA analysis.
The main harmful genetic effect of hybrids on native species is the loss of both genetic diversity and locally adapted populations, such as rare and threatened species (Rieseberg 1991; Ellstrand and Elam 1993). From a conservation perspective, hybrids can negatively affect biodiversity if they spread aggressively in a community. The spread of aggressive hybrid groups can reduce the growth of, or replace, native species (Vilà et al. 2000). The main anthropogenic factors promoting hybridization of species are increased species dispersal by humans, landscape fragmentation, and land disturbance. Enhanced cross-pollination of two species and increased range expansion of an exotic species into native ranges are the result primarily of human activity.
Certain differences in flowering times, pollination, and seed dispersal patterns differentiate parental species from hybrids (Vilà et al. 2000); however, the barriers to crossing can break down if life history traits overlap. For example, production of fertile hybrids can result when the pollen cells of one species can fertilize ovules (immature seeds) in the other species and the chromosomal barriers (such as pairing during meiosis) are overcome by similarity of the chromosomes or through polyploidy (multiple sets of paired chromosomes).
Introgressive hybridization (transfer of traits between species) can also increase genetic biodiversity of a taxon (Vilà et al. 2000) and can be the source of new adaptations. However, in many cases the genetic integrity of a common native species can be threatened. This is the case with the hybridization of the California cordgrass Spartina foliosa with the introduced S. alterniflora (Daehler and Strong 1997) in San Francisco salt marshes.
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This page updated:
13 January 2014
Suggested citation for this article:
Marburger, J., and S. Travis. 2013. Cattail hybridization in national parks: An example of cryptic plant invasions. Park Science 30(2):58–68.
Available at http://www.nature.nps.gov/ParkScience/archive/PDF/Article_PDFs/ParkScience30(2)Fall2013_58-68_MarburgerTravis_3668.pdf.
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