Why are there so many kinds of plants?

(first published at biology-online.org, updated with additional photos here)

Estimates vary, but there are about 300,000 named species of plants, with more being discovered daily.  There may ultimately be as many as 500,000, if and when all are catalogued.  Some botanists include some 10,000 species of red and green algae in such estimates, but others include only the land plants.  Either way, it�s a lot.   Theoretically, each species differs from every other enough to create a unique niche for itself.  Species with very similar niches in the same location will compete with each other for resources, and the stronger species will drive the other to adapt to a different niche or else go extinct.   Hence, �no two species can occupy the same niche.�   Yet it is sometimes difficult to tell what is unique about each species, and why each maintains a seat at life�s table. 

I�d like to do a thought experiment to get a picture of how many possible plant niches there might be.  Imagine a multidimensional �niche space,� each dimension representing a variable plant characteristic.  Suppose first that there were just one dimension, such as a gradient of temperature regimes running from the equator to the pole.  For simplicity, let�s say that this gradient is occupied by 10 different species, each specialized to make the best use of a particular combination of summer warmth and winter cold.  In reality of course there would probably be more, but we�ll keep all of our hypothetical niche dimensions at 10 for easy calculation.

Now let�s add another dimension, let�s say a moisture gradient.  At the equator, we might have 10 different plants, adapted to habitats ranging from evergreen rain forest to barren desert.  There would likewise be 10 such possible niches for each of the other temperature regimes, and so in this 2-dimensionsal niche space there would be a possibility of 100 different plant species worldwide.  Remember that in this simple array the plants differ only in their temperature and moisture requirements.

Now we can add a 3^rd  dimension for growth form: trees, vines, shrubs, epiphytes, etc.   In biological communities one growth form creates subhabitats for other growth forms.  Trees create shade for understory plants, as well as support for light-seeking vines and epiphytes.  So, multiply our previous niche matrix by 10 again, and you have room for 1,000 species of plants in the world. 

Adaptations for pollination and seed dispersal create still more niche dimensions.  For flowers adapted for specific pollination vectors (e.g. by bird, butterfly, bumblebee, wind, etc.) and for seeds adapted for different means of dispersal (bird, wind, mammal fur, rodents, etc.) add two more dimensions, bringing us to 100,000 different possible kinds of plants. 

There are many other ways in which plants differ from one another: how they protect themselves from herbivores (spines, fuzzy coverings, toxic secretions, etc.), leaf shape and texture (for balancing light reception,  CO₂ absorption, keeping cool, and avoiding water loss).  Each of these could be another niche dimension, but if we throw in just one, we multiply our possibilities by 10 again to have over a million.

We�re not quite done yet, however.  We also need to take into consideration geographical isolation, for similar species occupying similar niches tend to occur on different continents and major islands. Assuming just 10 more-or-less isolated geographical regions, each with warm wet lowlands, dry deserts, and cold mountain tops, we�re up to a possible 10,000,000 species in our matrix. In reality, individual mountain tops and isolated valleys frequently contain unique endemic species, adding even more diversity. 

 Even if this estimate is only moderately accurate, we can see that the number of potential plant niches vastly exceeds what actually exist.  Using 10 spots within each niche dimension is of course a simplification; for some dimensions there will be more, and for some there may be fewer.  Subarctic environments, for example, don�t support trees, vines, etc., so the number of growth forms is much fewer. Our model may seriously underestimate the options for pollination.  In a tropical rainforest, for example, there may be hundreds of possible pollinators. And there are niche dimensions that I just brushed to the side.  Nevertheless, the exercise provides a good perspective on the vast range of possibilities.  We in fact start to wonder why there are so few species of plants! 

Most likely, not every possible kind of plant can exist because the total resource �pie� of the ecosystem can only be sliced into so many pieces.  There is a minimum amount of energy, mineral resources, etc., required to maintain each species as a viable, genetically diverse population.  Those that are more aggressive, or that happen to be at the right place at the right time, are the ones that have actually succeeded.

The flowers of Bahia grass, Paspalum notatum, are specialized for pollination by the wind.  The feathery,
Christmas tree-like stigmas stand upright, while the pendant stamens dangle below.

For flowering plants, it is therefore not difficult to see how different sets of adaptations allow so many different kinds to coexist on our planet.  For example, consider three different kinds of plants from the monocot clade.  Their common ancestor was a creeping perennial herb with long, strap-like leaves with parallel veins, and embryos with a single cotyledon.  It had flower parts (sepals, petals, stamens and carpels) in sets of three.   Epiphytic orchids, savanna grasses, and succulent aloes are descendants of the ancestral monocot with quite different sets of adaptations. 

The butterfly orchid, Epidendrum radicans is adapted for
pollination by butterflies with well-developed color vision. Each orchid
species has uniquely shaped and colored flowers that attract a specific
bee, butterfly, hummingbird or other specialized pollinator.

They differ not only in where they grow (tree limbs or rocks for orchids, seasonally dry savannas for grasses, and deserts for aloes), but also most conspicuously in their mode of pollination.  The Epidendrum orchid pictured is pollinated by butterflies, and its tiny seeds are dispersed by the wind.  The grass is pollinated by the wind, and its seeds dispersed by grain-hoarding mammals and birds (and sometimes ants).   The Aloe dichotoma  (a relative of the familiar Aloe vera) is not only a succulent, but has also evolved a tree-like form. Its flowers are pollinated by birds and its flat seeds dispersed by the wind. 

The giant Aloe dichotoma has evolved a unique way to continue thickening its stem, allowing it to become tree-like.

Aloes in general have red, orange or
yellow flowers adapted for
bird-pollination.  The flowers of
Aloe dichotoma are yellow.

The other 60,000 species of monocots exhibit various combinations of these and many other adaptations. Agaves and Yuccas greatly resemble the African Aloes, but evolved in the Americas with different pollinators.  Palms and screwpines (Pandanus) are tree-like but with very different leaf structure and flowers from Aloe, Yucca or Agave, and prefer moister habitats.   Members of the ginger family have flowers that are often as elaborate as orchid flowers, but with a different growth habit and larger, animal-dispersed seeds. The orchid family itself has more than 20,000 species, each with a unique pollinator.  Sedges, cat-tails, and rushes are wind-pollinated like grasses, but adapted to different habitats and modes of seed dispersal.

Yuccas and their relatives live in similar climates
and have similar growth forms as Aloes,
 but  have colorless flowers adapted for pollination by moths.

We don�t have time to consider the other major groups of plants: eudicots, gymnosperms, ferns, mosses, etc., but you get the point.  There are seemingly endless ways for plants to vary � a multidimensional hyperspace of possibilities.  The half million or so living today are the fraction of those possibilities that through aggressiveness or chance maintain a tenuous foothold on our planet.