Adaptation in Weeds - Dandelion

A characteristic of most weedy species is the ability to adapt to management practices. As soon as we think we have developed the perfect control program, a new weed or weed biotype emerges that possesses a trait that allows it to survive this tactic. Recently, much of the focus has been on the selection of herbicide resistant biotypes, but it is important to remember that weeds are also capable of adapting to other management tactics. 

Dandelion is somewhat unique among weeds in that it is successful at colonizing habitats with widely varying environments (e.g. no-till corn fields and high maintenance turf). The biological traits that favor survival in these habitats are often contradictory, thus a trait that favors survival in an agricultural field would be unfavorable for survival in a turf setting. Research conducted in the 1970's provides insight on how dandelion accomplishes this feat, and is an excellent example of the adaptive nature of weeds (Solbrig and Simpson,  1974;  Solbrig and Simpson, 1977).

The two scientists identified three turf habitats on the University of Michigan campus characterized by varying levels of disturbance. The highly disturbed habitat was used as a pedestrian pathway and was mowed weekly. The moderately disturbed habitat was managed similarly to the highly disturbed area, but received little foot traffic. The final setting was an area along a creek that was only mowed once annually. All sites were within 1/4 mile of each other and had moderate to heavy dandelion infestations. One hundred dandelions were collected from each habitat and biotypes¹ among these plants were characterized using isozyme analysis. Four distinct dandelion biotypes (A-D) were identified, and the biotypes demonstrated strong preferences for the different habitats (Table 1).  Biotype A dominated the highly disturbed habitat (73% of the population) but only made up 17% of the dandelion population in the area with a low level of disturbance. Biotype D had the opposite distribution of Biotype A, comprising 65% of the population in the low disturbance habitat but was absent in the highly disturbed habitat. Biotype C did not appear to demonstrate any habitat preference, whereas Biotype B was 3 to 4 times more prevalent in the moderately disturbed habitat than the other sites.

Table 1.  Distribution of dandelion biotypes among three habitats on University of Michigan campus. 
(Solbrig and Simpson. 1974. J. Ecol.)

The next step was to characterize the growth parameters of the biotypes that might determine their habitat preference. These studies were done only with Biotypes A and B since they demonstrated a greater habitat preference than the others. The researchers evaluated reproductive characteristics since these traits are extremely important in the ability of weeds to colonize disturbed areas.  In one experiment the number of seedheads produced per plant was tracked for the first three years after planting under non-competitive conditions. Plants of Biotype A averaged 15 seedheads/plant in the first year, whereas less than 10% of Biotype D plants produced a single flower. When the two biotypes were planted under competitive conditions (adjacent to other dandelions), 75% of Biotype A produced flowers whereas only 5% of D produced seedheads. 

In addition to reproductive characteristics, the biomass accumulation of the two biotypes was studied. When the two biotypes were grown separately they accumulated similar amounts of dry matter. However, when they were grown together Biotype D produced more biomass and also had lower mortality rates than biotype A. This  indicates that D is a better competitor than A when grown in close proximity with other plants. On the other hand, the prolific seed production of Biotype A enhances its ability to survive under conditions where frequent disturbance eliminates existing vegetation and creates vacant areas where new plants can develop from seed. 

In a second paper the authors further evaluated the ability of the two biotypes to survive under different disturbance conditions. Four dandelions (2 of each biotype) were planted in 1 m² plots in 1970. In mid-summer of 1971 and 1972 three disturbance treatments were imposed:
        1) No disturbance
        2) All foliage removed from dandelions
        3) Plots tilled to remove all foliage and roots.
At the end of the fourth year (1974) all dandelions in the plots were harvested and the biomass and biotype of each plant was determined.

As was observed on the University of Michigan campus, Biotype D dominated the plots with no disturbance, whereas Biotype A was the predominant plant in disturbed plots (Table 2). In the non-disturbed plots, dandelion density increased from 4 plants in 1970 to 84 plants in 1974, with 85% of the plants being Biotype D. Thus, even though this biotype produces fewer seeds than A, its seedlings were better adapted to survival under the competitive conditions created by the lack of disturbance and relatively high plant densities. Total dandelion density was much lower in the disturbed plots (less than 40 plants) and the increased seed production of Biotype A allowed this biotype to dominate these areas.

Table 2.  Effect of disturbance on the occurrence and biomass accumulation of two dandelion biotypes.
(Solbrig and Simpson. 1977. J. Ecol.)

This research illustrates how diversity within dandelion results in plants that are adapted to survival in widely varying habitats. Biotype A emphasizes seed production like many annual weeds, and thus is adapted to areas where disturbance may eliminate much of the existing vegetation. It is likely that dandelions found in corn and soybean fields have characteristics similar to Biotype A, whereas dandelions occurring in home lawns where the dandelion must compete with a dense sod are similar to Biotype D. 

Unfortunately, this type of adaptation is common among weed species - this is why weeds persist in spite of the intense efforts we use to control them. Whenever we over- utilize a weed control tactic, some weed which has a trait that allows it to survive that strategy will develop into a serious problem.  This is why it is important to utilize integrated weed management programs. The use of multiple control tactics reduces the likelihood that any single weed species will reach densities that pose economic problems. Thus, even though we will never eradicate weeds, economical control programs can be implemented that eliminate the rapid fluctuations in weed populations that can occur when simple management programs are utilized.

¹A biotype is a group of individuals within a species that share genetic traits. Lambsquarter plants that are resistant to atrazine would be considered a biotype since they all possess an altered binding site for the triazine herbicides. A specific soybean variety would also be considered a biotype since individuals in this variety share traits not common among other varieties. Analyzing isozyes is a relatively simple method of determining genetic similarities and differences among plants within a species. 

Solbrig, O. T. and B. B. Simpson. 1972. Components of regulation of a population of dandelions in Michigan. J. Ecol. 62:473-486.

Solbrig, O. T. and B. B. Simpson. 1976. A garden experiment on competition between biotypes of the common dandelion. J. Ecol. 65:427-430.