Declining agricultural jobs in rural areas, alongside better opportunities in cities, have driven urbanization since the mid-twentieth century. Currently, 55% of the population resides in urban areas, a figure projected to rise to approximately 70% by 2050 (source: UN World Urbanization Prospects). Urbanization contributes to the loss of native plant species, reduced species diversity, habitat fragmentation and isolation, altered microclimates, rising temperatures, and increased pollution across various levels. Anthropogenic activities further impact plant growth (de Barros Ruas et al., 2022). While plants have adapted morphologically and physiologically to urban environments, research on their coping mechanisms remains limited. Urbanization's impact on adaptive evolution has been understudied.
Plants, having evolved over millions of years to adapt to their environments, face significant challenges from changing conditions, climate shifts, and anthropogenic activities. Phenotypic plasticity emerges as a crucial mechanism enabling rapid responses to environmental changes. It allows a single genotype to produce different phenotypes based on encountered environmental conditions (Bradshaw, 1965). Phenotypic plasticity influences morphological, physiological, and behavioral aspects of an organism's phenotype (Sommer, 2020). Some plastic responses are heritable, contributing to adaptation over evolutionary timescales. This adaptability through plasticity is a remarkable feature of plants, enabling them to colonize diverse habitats and effectively respond to environmental challenges.
Dandelions, non-native to the Americas, are the earliest spring flowers with uses in food and medicine. Considered invasive weeds, they compete with agriculture crops, reducing food production (McPeek and Wang, 2007). In suburban settings, dandelions impact the aesthetics of lawns and gardens (Molina-Montenegro et al., 2018a). Plants have evolved various adaptations to evade herbivory. Fujita and Koda (2015) examined grazing patterns by livestock on the capitula and rosette leaves of two Taraxacum species in Mongolia. They found that in Mongolia, naturally short capitula at the green bud and brown corolla stages, as well as rosette leaves of T. collinum, were left untouched by grazing. However, artificially elevated capitula and rosette leaves of T. collinum, as well as the naturally tall capitula and leaves of T. ceratophorum, were grazed. The study revealed that shorter capitulum and leaf heights were effective in deterring livestock grazing in Mongolia.
Dandelions exhibit superior physiological performance, growth, and survival under unfavorable abiotic conditions compared to native plant species (Molina-Montenegro et al., 2012). In Japan, the invasive T. officinale replaces the native T. japonicum, attracting more pollinator visits possibly due to its increased nectar production. This results in reduced seed set for T. japonicum (Kandori et al., 2009). Struik (1967) observed dandelion behavior in various grassland conditions in New Zealand, noting that dandelions in uncut grasslands had greater weight in flowers, leaves, and roots compared to those in stressed environments. He also found that dandelion buds were closer to the ground in mowed and grazed grasslands. Our study delves into the morphological adaptations of dandelions in suburban areas and playgrounds under stress conditions.
Dandelions were introduced in the Americas from Europe by the early European settlers (Iaffaldano et al. 2017). They have since spread all over the continent. The plants are highly successful in adapting to different conditions and environments. In this paper we demonstrate that the dandelion uses different strategies to adapt to the suburban conditions.
Our study areas were confined to the counties west of Philadelphia, Pennsylvania. Urbanization has reduced areas for the habitat of dandelions. The taller dandelions (“wild”) are seen in nature reserves, wastelands, or peripheral areas of the suburbs (“wild areas”) with few human activities. The dandelions of the nature reserves are taller with larger flowers, flower stalks, and leaves. The mean angle of the dandelion’s flower stalks of the wild areas was calculated to be around 70o, and the angle of the seed head stalks was calculated to be 90o (Fig. 7).
It has been demonstrated that the peculiar structure of the dandelion seed makes them remain airborne for longer periods, increasing the distance it can travel. Studies have shown that under optimal wind conditions, dandelion seeds can travel distances of up to 150 kilometers (Cummins et al. 2018). This efficient wind dispersal mechanism allows dandelions to colonize new areas and spread their seeds over a wide range, contributing to their success as a weed and their ability to populate various environments. This study demonstrates a direct correlation between seed stalk size and flower stalk length. Specifically, a lengthy flower stalk corresponds to an extended seed stalk, while a shorter flower stalk is associated with a shorter seed stalk. The length of the seed stalk does not influence its angle; overall, the seed stalk had an angle of 90o. The 90o angle of the seed stalks facilitates the wide dispersal of dandelion seeds.
In the suburbs, the dandelions occupy lawns and gardens. They grow in spaces between the lawn grasses. The suburban dandelions have smaller leaves, short flower stalks that are at an angle of 30o or lesser, smaller flowers that grows parallel to the lawn grass and early blooming; features that can escape mowing. However, the seed stalks were significantly longer than the flower stalks to aid in wind dispersal (Fig. 8).
In the suburbs, the dandelion blooms, set seeds and disperses the seeds usually before the first lawn mowing. The lawns are mowed regularly when the grass grows during mid-spring. The seed head stalks elongate before the mowing, or after the first mowing. The seed head stalks of the dandelions in the wild, or in the lawn is 90o. The seeds of the dandelions are dispersed by wind. Wind can transport the seeds to long distances (Cummins et al. 2018). A taller seed head stalk can influence the transport of the seeds. Rarely have we observed short seed head stalks in dandelions. Seeds from the short seed head stalks fall in the vicinity of the dandelion plants thereby impacting its germination (Fig. 9). Hence, we speculate that longer seed head stalks are an adaptation favoring transport of the seeds to long distances and its survival.
Though the dandelions in the lawn of the suburbs are shorter, those plants growing at the periphery of the lawns, with no human activities are comparable to the plants as seen in the nature reserves with longer flower stalks and larger leaves. Thus, a favorable environment elicits larger vegetative growth of the plants demonstrating phenotypic plasticity.
Dandelion is a prime example of how certain plant species can thrive in human-altered habitats due to their ability to exhibit phenotypic plasticity. This plasticity allows them to adjust their traits and behaviors to better exploit the opportunities and challenges presented by these environments, ultimately contributing to their success as invasive species. In the study we demonstrated changes in morphology of the dandelions in the suburban lawns in response to mowing. The dandelions with their large size and floral stalk can not withstand mowing conditions. Hence they have adapted to the surroundings by growing parallel to the lawn; with smaller leaves and floral stalk. To investigate whether heightened stress could further influence their morphology, we examined their adaptation in playgrounds. In these areas, dandelions exhibited the smallest vegetative features compared to other locations, showcasing adaptations that could withstand the trampling by children. Dandelions on the margin of lawns had large leaves and floral stalks with wider angles demonstrating that absence of stress could change their phenotype. Our study provided evidence of the morphological plasticity of dandelions in response to environmental stressors. The adaptation of the habit of dandelion in response to human (anthropogenic) activities in the suburbs is an example of anthroprosarmogi (human induced adaption; Greek; Anthropos, human; Prosarmogi, adaption).
Dandelions exhibit remarkable plasticity in their morphology and behavior, allowing them to thrive in human-altered habitats. Their small leaves, short floral stalks angled at 30 degrees or less, early blooming, and efficient seed dispersal mechanisms are advantageous for survival, particularly in environments managed by humans. The elongation of their seed head stalks facilitates wind dispersal, enabling seeds to travel long distances. These combined traits make dandelions well-adapted to human-modified landscapes such as lawns and gardens. They can withstand frequent disturbances, competition from other plants, and habitat changes resulting from human activities. Consequently, dandelions persist and reproduce successfully in urban and suburban settings, establishing themselves as invasive species. As a result, dandelions are an ideal subject for researchers and ecologists investigating invasive plant species. Their adaptable traits and behaviors offer valuable insights into the mechanisms driving invasive success in human-altered environments.