The Species They Are A-Changin’

Some people worry about keeping up with the endless revisions to species names and meaningful field marks. “Too hard? Should I just give up?”

Might such people be reassured by an influential article in the journal Nature? There, Reisenberg et al. wrote,

"Many botanists doubt the existence of plant species, viewing them as arbitrary constructs of the human mind, as opposed to discrete, objective entities that represent reproductively independent lineages. … Zoologists ... argue that botanists have been overly influenced by a few 'botanical horror stories', such as dandelions, blackberries and oaks ... We show that although discrete phenotypic clusters exist in most genera (>80%), the correspondence of taxonomic species to these clusters is poor (<60%)."

In other words, Reisenberg (see References, below) seems to be saying that groupings akin to plant ‘species’ do exist, but the currently accepted classifications accurately represent them less than 60% of the time. Should that make me feel better or worse about my level of plant knowledge?

This blog recently discussed the intimidatingly new Flora of the Chicago Region. In a comment on that post “JWPboss” declared independence from both microscopes and the new flora. He wrote: “Mohlenbrock and others have created species based on morphological differences visible only with good magnification … Until ecological or phylogenetic differences are demonstrated, I will ignore them, i.e. use older texts …”

Having spent my life in biodiversity conservation, I can attest that species distinctions are important. But, at least in my experience, disagreements about them rarely cause much trouble. Let’s consider yellow violets and “hybrid” oaks, as examples.

Does Chicago Wilderness have one or two species of yellow woodland violets?

The 1979 Swink and Wilhelm considered ‘smooth yellow violet’ and ‘downy yellow violet’ to be separate species. The 1994 Swink and Wilhelm lumped them together as one species with the comment:

“At once reluctantly and at once with some sense of relief, we are … [lumping the two species] … Our specimens display every imaginable combination [of characteristics] to the point that consistent distinctions become virtually impossible. … [Downy yellow violet] is more likely to grow in the very closed … beech forests, while [smooth yellow violet] is the more common element in the timbered prairies and oak savannas. … It will be interesting, as savanna and woodland restoration efforts mature in the coming decades, to see if these two elements will sort themselves more discretely.”

But, in the 2017 Flora, Wilhelm and Rericha revise the yellow violets again – as subspecies this time – with the comment that “the two elements sort rather well” if you only consider the number of leaves that rise directly from the root – and length of the leaves that don’t.

Yellow violets have inspired contention and angst among taxonomists.
Beautiful and conservative, increasing dramatically with good stewardship,
they inspire only sweet pleasure among conservationists.

Those conflicting approaches in 1979, 1994, and 2017 also differ from all my many botany books and plant keys. Indeed, no two of them agree. The various texts separate out downy from smooth yellow violet according to various combination of:

Lengths of the leaves

Widths of the leaves

Amounts of hair or down on various parts of the plant

Numbers of leaves coming directly from the root

Numbers of leaves not coming directly from the root

Whether or not the stipules have distinct teeth

So, if you’re working on biodiversity conservation, does it make any difference which names you use? No. (Though, if you’re writing a report or making a site inventory, it’s best to follow one source and cite in the case of questionable species.)

Smooth yellow violet (left) was once said to be distinguished from downy yellow violet
by its more numerous basal leaves and smooth(er) stems.
But that key didn’t work. Apparently none do.

Another way to answer the “does it make a difference” question is hinted at in the 1994 quote above. The biodiversity of the fire-dependent ecosystems of the Midwest is currently in a shambles. Our ecosystems have been degrading into chaos under the impact of fire-suppression, invasive native and alien species, pollution, fragmentation, and more. We conservationists want to save all the genetic diversity of the formerly healthier ecosystems. Perhaps in time we’ll learn that big, diverse sites have a complete enough gene pool such that yellow violets will in time “sort themselves” back into distinct species. Or not. Perhaps large degraded sites restored with seeds from many nearby sources will save parts of the gene pool unrepresented elsewhere – and over time even develop unexpected new ‘species’ or ‘subspecies’ or ‘varieties’ under the influence of climate change, nitrogen pollution, and other modernisms.

Where I’ve worked, we’ve gathered yellow violet seeds for woodland restoration where needed. We were and are not troubled about what name(s) people use for them – or if we mix seeds with all the various characteristics. We appreciate the efforts of ecologists and taxonomists to help us understand them better, and we keep doing our effective and rewarding stewardship work.  

Are our oaks different species – or just two big hybrid swarms?

Decades ago we stewards learned something fundamental and initially confusing from the region’s classic oak expert from the Morton Arboretum, Dr. George Ware. As with all the biodiversity experts at the dawn of volunteer stewardship, Dr. Ware was generous with his time and led field seminars with stewards to answer questions. But at first it seemed that every difficult tree we challenged him with elicited the same straightforward, clear, and unhelpful response.

He would say, ‘This tree is expressing Hill’s oak in its branching and red oak in its bark and leaves.’ Or he’d say, ‘This tree is expressing bur oak in its leaf shape, white oak in its lack of pubescence, and swamp white oak in the flakey bark on its younger branches.’ People tried to pin him down to an expert opinion on what species it was – or at least satisfying us by confirming that the tree was a hybrid, but he wouldn’t use our words. Dr. Ware seemed not to be troubled by our insistence. He just kept answering in his way, tree after tree. At one point, he finally referred vaguely to that troubling ‘species concept’ issue with words that I remember as being something like: ‘Some people have these numbering systems, and they need to have definite species lists to make that system work.’ Then he chuckled. Was the tone of that chuckle dismissive? complaining? lamentational? It did not seem so. He seemed to be a sincere scientist, trying to teach us to see and understand oaks as best he could, while steering clear of academic conflict as much as possible.

It seems like all the oaks in the white oak group (including bur and swamp white oak) pollinate each other freely and produce endless combinations of characteristics. This is also true within the red or black oaks (red, black, Hill’s, scarlet, and other oaks with points on their leaves). So, does Chicago Wilderness have 26 oak species and hybrids (as in Wilhelm and Rericha’s key) or 12 (as in Swink and Wilhelm) – or just two hybrid swarms called the white oaks and the black oaks? And how much difference does it make for the people working to conserve ecosystems?

The twigs of some young bur oaks are heavily armored with cork.
Other bur oaks grow faster but put little energy into such armor.
Stewards might want to thin some of those fast-growing burs, if that’s the only way
not to lose those corky, more fire-adapted ones. Genetic differences within species
may be as ecologically significant (and as significant to restoration)
as the differences between species. 

Many stewards notice that, as John Curtis pointed out in the 60s, bur oaks die or fail to reproduce in the shade of white oaks, and white oaks die or fail to reproduce in the shade of red oak, basswood, and, the worst tree-thug of all, sugar maple. So we thin and burn to try to restore light and natural function. We are delighted (and motivated) to see hundreds of species of formerly declining animals and plants recover within a few years or decades. Subsequently, the trees will continue their promiscuous sex life, and we’ll appreciate the evolving results.  

It is not a problem that we cannot provide every tree with a simple name. But if, overall, the trees that look the most like classically defined bur oaks are getting shaded out and failing to reproduce, then perhaps we need to burn or thin more. Changing names or plant keys don’t impact those decisions much.

Stewards increasingly work with teams that rely on ongoing feedback from animal and plant biologists, ecosystem ecologists, and, especially, diverse expert land managers including other stewards, who understand best practices and listen to all kinds of helpful input. We can’t say, “Don’t worry. Be happy.” But about species disagreements we can certainly say, don’t worry too much. And do definitely be inspired and happy.   

REFERENCES

Loren H. Rieseberg, Troy E. Wood, & Eric J. Baack. The nature of plant species. Nature 440, 524-527 (23 March 2006)

Strategies for Stewards: Blog post on the Flora of the Chicago Region http://woodsandprairie.blogspot.com/2017/04/flora-of-chicago-region.html

Floyd Swink and Gerould Wilhelm, Plants of the Chicago Region, Morton Arboretum, 1994

Gerould Wilhelm and Laura Rericha, Flora of the Chicago Region, Indiana Academy of Science, 2017

Violet drawings are from Henry A. Gleason’s New Britton and Brown Illustrated Flora, New York Botanical Garden, 1952