One of the grasses I saw as I first stepped into the streets of Lima, Peru last month was a species that I had seen many times in many different places. There was no mistaking the distinctive digitate inflorescence, nor the wiry stoloniferous habit that was one factor that allowed the grass to become a fearsome competitor and so valuable to turf grass managers. I had seen the same species in New Jersey, in Florida, in several Caribbean Islands, and even in the Philippines. It was the common Cynodon dactylon
(called Bermuda grass in the USA), and its success is one that is repeated time and time again in the Poaceae.
|Cynodon dactylon in Peru|
The discovery of this species in Peru was not even a surprise to me, because I have grown accustomed to seeing grass species that are extremely cosmopolitan in their distribution.
In an earlier article, I had summarized some of the reasons why grasses can be considered the most successful plant family in the world, even not taking into account their economic importance to humanity (e.g. rice, wheat, maize/corn, etc).
Grasses are only the fifth most species-rich angiosperm family with about 12,000 species (Clayton et al., 2015). And yet ecologically, grasses are by far the most dominant plant family.
Areas dominated by grasses cover up to 43% of the surface of the world (Gibson, 2009), and they are found in almost every ecological habitat, including Antarctica. Up to 37% of the land area of the USA
is dominated by grasses! Entire ecosystems of animals and other plants depend on grasses for their continued health and existence.
In fact, although grasses account for only 3% of plant species on Earth, grass-dominated landscapes contribute 33% of global primary productivity, the amount of CO2 removed from the atmosphere every year to fuel photosynthesis (Beer et al., 2010).
The overwhelming success of the Poaceae is not simply because of their well known habit of growing from the base, unlike many other plants. It is also because they exhibit a suite of invasive traits that when taken together, researchers have termed the "Viking Syndrome," in honor of the ancient group that ranged far and wide, colonized many places, and had profound effects in areas where they landed (Linder et al, 2018).
Specifically, grasses have:
- Efficient dispersal,
- Rapid population growth,
- Environmental flexibility
- Flexible growth forms and phenotypic plasticity
- The ability to transform environments to benefit themselves
It is this combination of invasive traits that have allowed grasses to become the most successful plant family in the world today.
1. Efficient Dispersal
Grasses certainly are able to disperse far and wide. The method of dispersal after pollination varies among species, but include wind dispersal (anemochory), dispersal via animal coats
(epizoochory), and dispersal after ingestion by an animal (endozoochory). The ability of grass species to be efficiently dispersed is almost everywhere in evidence.
For example, the dune grass Leymus arenarius was one of only 4 plants to appear during the first decade after the formation of the volcanic island of Surtsey in 1963 (Magnusson et al, 2014). Some grass species like Phragmites australis are also hailed as being perhaps the most widely distributed angiosperm in the world, with a range that extends from 70 degrees N to the tropics (Holm et al, 1977).
The unit of dispersal is the inflorescence in whole or part. This "diaspore" includes the basic unit of the grass flowers, which are the spikelets. In many grasses these are frequently tiny and lend themselves to easy dispersal, but beyond that grass spikelets can show various modifications that enable them to be carried far and wide. The most common have long awns that help in wind dispersal, but some Cenchrus spp have sharp barbed "thorns" that can easily catch on animal fur or clothing.
|The deadly spines of Cenchrus sp|
In some others like Oplismenus undulatifolius
the awns exude sticky droplets of liquid which can also adhere to passing traffic
. Studies have found that a dog running through this grass could pick up 12000 or more seeds on its fur! People are also used as vehicles for dispersal, with one study showing that around 800 seeds adhered to fleece within 30 seconds, while even denim pulled in around 300 seeds within the same timespan (Beauchamp et al, 2013)!
|The sticky awns of Oplismenus undulatifolius helps in its dispersaL|
2. Rapid Population Growth
The ability to create enormous populations very quickly might be another trait that helps in their invasive success. Such rapid population growth can be the result of shorter generation times and early reproduction. This allows pioneer populations in new habitats to quickly overrun the area, and the shorter generation times also creates more rapid responses to directional selection.
Although no definitive study has been made to compare entire plant families, many grasses have extremely short generations times. For example, the weedy and cosmopolitan Poa annua can flower just six weeks after germination (Cope et al, 2009) . In the same way, the weedy and extremely invasive Bromus tectorum (called cheatgrass in the USA) also can flower within 5 weeks (Meyer et al, 2004).
Interestingly enough, the grass embryo might be one factor that contributes to their reproductive success. Unlike other plants in their order (the Poales), which consist of undifferentiated cells, the embryos of grasses is already differentiated into specialized tissues, including the root, a shoot with leaf initials, and a specialized structure called the scutellum, which digests endosperm and moves it to the rest of the embryo.
|Bromus tectorum can flower within 5 weeks of germination |
3. Environmental Flexibility
The Poaceae as a whole occupy all the environmental niches available to angiosperms. This ability is related to the amazing flexibility of the grasses when it comes to adapting to various environments.
Another example of the ability of members of the Poaceae to exist in extreme environments is the large number of halophytes in the family. Halophytes are organisms that are very tolerant of high salt concentrations, and the grass family is second only to the Amaranthaceae in the number of species that are halophytes (Moray et al, 2015), with studies indicating that salt tolerance evolved independently at least 70 times in the family (Bennett et al, 2013).
4. Flexible growth forms and phenotypic plasticity
Grasses have evolved a variety of growth forms that allow them to adapt to almost any situation or event, whether it be (a) continuous defoliation, such as by herbivores, (b) periodic defoliation, such as in seasonal climates, or (c) competition against other plants such as in forests.
In terms of vertical reach, most grasses are low lying organisms, but some species are tall, and form towering grasslands that reach high above the height of a man (e.g. Saccharum spontaneum grasslands). Bamboos are even taller, and their woody stems allow them to reach the height of mature trees in shaded forests.
In terms of horizontal spread, some grasses are bunch grasses that form tussocks, while others spread horizontally via stolons (above ground horizontal shoots) or rhizomes (below ground horizontal shoots) to form (for example) the typical lawns. This type of growth is controlled mainly by whether the species has intravaginal innovation (new shoots originate from axillary buds within the leaf sheaths) or extravaginal innovation (new shoots grow from axillary buds outside the leaf sheaths). In the latter case, the result is the ability of the grass to spread horizontally and blanket an entire area.
The vast majority of grasses also exhibit hemicryptophyte growth, which means that their buds are at or near the soil surface. This is one of the key mechanisms that allows grasses to withstand repeated defoliation via grazing or fire, and thus create climax communities that demonstrate an alternative biome state. This trait also means grasses do not necessarily need to maintain above ground structures during periods of extreme stress, such as droughts and cold, and it is the common reason that people assume grasses are so successful.
|Bamboo sp (probably Bambusa vulgaris)|
5. Transformation of Environments
The ability of members of the Poaceae to significantly transform their environment is perhaps the primary key to their success. By changing their surroundings, grasses create a hostile environment for other plants (including trees and shrubs) that may usurp their dominance, and even relegate them to minor components of the biota.
The way grasses transform the environment is rooted in their makeup. The reproductive fecundity of the grasses allows them to exist in numberless hordes, and their growth forms enable them to blanket entire habitats in contiguous swards, denying food, water and sunlight to competing plants. Their enormous populations, enabled via wind pollination, also allows them to use biotic feedback mechanisms that utilize fire and herbivore grazing to transform closed canopy forests into grasslands, and later maintain this alternative climax state.
|African Tropical Grassland (Savanna), by Gossipguy)|
All the factors above combine to give to the Poaceae an invasive and aggressive quality that is perhaps unmatched in the plant kingdom. It allowed the grasses to range far and wide, colonizing all four corners of the world, and transforming vast lands into the wide open spaces that we see today.
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