One word: Acacia. For many people, that’s all it takes to conjure up the image of a grand African savannah rolling off into the endless distance. Expansive, untamed and the essence of wilderness, with the occasional flat-topped thorn tree standing watch over the ceaseless red horizon like silent, inexplicable guardians of this land.
Despite being some of the most iconic and widespread tree species in Africa, it has recently become ‘technically’ incorrect to describe any Acacia species whatsoever as native to Africa – a point that many people, scientists and laymen alike, find deeply dissatisfying and distressing. In this popular piece, I will address why it is technically incorrect to do this, outlining the primary reason for the reclassification of the legume genus Acacia in 2011 and describing how Africa ultimately lost its grip on this iconic Latin genus name to Australia. I will also provide some evidence from my own research to highlight the importance of moving past the petty ‘stick-it-to-the-man’ attitude of many scientists and laymen who aggressively refuse the name changes to Acacia following its reclassification. In doing so, I believe that many people have blinded themselves to the fact that we, in Africa, have been referring to two distantly related, and probably quite different, plant lineages by the same name for over 200 years – a terrible misunderstanding of some of the most important tree species on the continent.
A genus (plural = genera) is the taxonomic rank above the species level used by scientists to describe and classify a group of species who share a hypothesised common ancestor. Traditionally, this common ancestry was assumed based on physiological studies uncovering morphological and/or cellular similarities between species. However, with the advent of molecular (genetic) techniques, there are now vastly superior ways to infer the evolutionary relationships between species that compare, base-for-base, the DNA sequences of conserved regions in a plant’s genome. Molecular studies have radically altered many of the originally hypothesised evolutionary relationships between species and precipitated many taxonomic reclassifications, particularly for very large genera that are found to contain disparate, and evolutionarily independent, groups of closely-related species. This turned out to be true for the genus Acacia and is the primary reason for its reclassification.
In the event of a genus being reclassified, classically, the group of related species that contained the type-specimen (the first formally described and catalogued species of a genus) would maintain the original generic name. However, this proved not to be the case when Acacia was reclassified in 2011. Because A. nilotica (or more commonly known as sweet thorn), a widespread African species, was the type specimen for Acacia, formally recognised by Phillip Miller in 1754, by all accounts at least some of the African species should have stayed in the original genus Acacia. But, for the first time in systematic history, the type specimen was changed!
Scientists in Africa were unable to resist the Australians’ plea to the International Botanical Congress (IBC) in 2005 to change the type specimen to A. penninervis, an Australian species, to save them the supposedly arduous chore of changing their 1010 species to ‘Racosperma’ pernninervis etc. and the pain of losing the wattle (Acacia) as their national emblem. The decision to change the type specimen of Acacia was ratified at the XVIII IBC held in Melbourne in 2011, at which point Africa lost its grip on this iconic Latin genus name but gained two new ones instead, Vachellia (after the prominent 19th century French politician and entomologist, Joseph Vachal) & Senegalia (after the country of origin of the type specimen S. senegal).
Despite tremendous conflict and argument around ownership of the Latin genus name Acacia and the great emotion and furore generated internationally by the controversy, there is a very good reason this genus was reclassified. This is a point many people seem to be missing in Africa because bruised egos would rather ‘stick-it-to-the-man’ and go on like the reclassification never happened than accept the ‘unfair’ name changes. Unfortunately, many scientific journals, recognising the distressing nature of the name changes for many scientists, continue to allow authors to publish using Acacia to describe African species – a terrible mistake! I feel this has stunted research and led to considerable oversight of the most interesting questions that faces us following the reclassification of Acacia in Africa.
The genetic data clearly demonstrate that African ‘Acacia’ species are in fact representatives of two distantly related legume lineages, which diverged approximately 30 million years ago (mya) in tropical African lowland forests. The split occurred when species of Vachellia moved into open habitats and species of Senegalia remained in the ancestral forests until approximately 10 mya when they too moved out into the ancient open savanna-like habitats of Africa. Strong evidence of the deep divergence and independent transitions of Vachellia & Senegalia into the ancient African savanna greatly challenges the long-held belief that African ‘Acacia’ species are closely related. Therefore, their previously assumed ecological equivalence now seems to be highly unlikely.
This is the crux. If we continue to ignore the name changes to this genus, we will forever fail to make the distinction between these two leguminous lineages, leading to even more studies with preposterously unbalanced species sampling and spurious predictions of the future based on mean data for both genera. However, despite six years passing since the formal reclassification of Acacia, no one has yet addressed the most tantalising question we are faced with in Africa: have we been incorrect to believe in the ecological equivalence and close relationship of these species for over 200 years?
To tackle this question, we need to begin by defining what we mean by ecological equivalence. To describe two lineages (e.g. genera) as ecologically equivalent implies that they are of the same ‘functional type’ and so possess physiological traits that allow them to exploit their environment and provide ecosystem services in a similar manner and to a comparable extent.However, measuring these traits for many species over many years to quantify the exact differences in resource assimilation and ecosystem service between genera is not only daunting, but often completely infeasible. Because of this difficulty, niche theory, often becomes essential in uncovering the ecological equivalence of species or higher plant taxa.
The central axiom of niche theory was developed by Georgy Gause in 1934, when he stated that complete competitors cannot co-exist based on his experiments with Paramecium. These experiments demonstrated that species with similar resource requirements could never maintain their population sizes when they were forced to co-exist in vitro; the superior competitor always drove the inferior one to extinction given enough time. This principle was later developed into an ecological context by MacArthur & Levins in 1967 when they proposed their principle of limiting similarity, which stated that closely related species that share too much ecological similarity will never be able to co-exist in the same habitat. So, if Vachellia & Senegalia are ecologically equivalent – as we have assumed – then we would expect them to occupy the same niche and be unable to co-exist in the same habitats… but, is this the case?
To address this question, I analysed three datasets of differing scales ranging from the continental scale (> 1000 km) right down to the local scale (< 20 m) to determine if there were any significant differences in habitat preference between Vachellia & Senegalia. Furthermore, I used my local dataset to test if there was any significant relationship between species co-existence and evolutionary relatedness at this scale – the scale at which competition becomes the dominant force in determining species presence, or absence, in communities.
What did I find? Well, no significant difference in habitat preference between Vachellia & Senegalia at the continental scale. Therefore, So, at this scale Vachellia & Senegalia are ecologically equivalent (i.e. occupy the same niche). However, when I analysed my dataset at a local scale, which included 851 (20 X 20 m) plots distributed throughout the Kruger National Park, South Africa, collected by Freek Venter in the 1980’s, astonishingly, I found a strong positive relationship between evolutionary relatedness and species co-existence in these plots. In other words, the more closely related species were to each other the less likely they were to co-exist in these small plots and so species of the distantly related Vachellia and Senegalia were in fact more likely to co-exist than expected by chance!
This implies that we have been completely wrong in our understanding of ‘Acacia’ in Africa for over 200 years. Rather than species of Vachellia & Senegalia occupying the same niche – in which case they could never co-exist in small plots in the same habitats – we now see that, in fact, they are more likely to co-occur in small plots in the same habitat! This is great evidence demonstrating that these two genera, because of their deep evolutionary divergence, have managed to accumulate physiological differences that allow them to co-exist at a local scale more than would be expected by chance.
Vachellia and Senegalia are therefore, clearly, ecologically non-equivalent and occupy different niches at the local scale – this is a complete revision of what was once believed. If this really is true, then their difference in niche should allow them to co-exist at a very fine scale but, do we see any evidence of this in the field? The answer is yes, there is in fact some evidence that species of these genera can co-exist at an astonishingly fine scale (Fig 1).
This result makes it critical to distinguish between these two genera in Africa when we refer to species of acacia as scientists or laymen. The fine-scale co-existence of Vachellia & Senegalia in the same habitats completely invalidates any claims of ecological equivalence between these species.
To be sure, the use of the new generic names, especially for laymen, will be slow and difficult in the beginning. However, no amount of kicking and screaming is going to change the fact that the reclassification is now formalised and is not going to be reversed (at least in terms of Africa losing Acacia), so we best make peace with it now. As passionately as I feel about using the correct generic name to refer to specific species of ‘Acacia’ in Africa, especially in the scientific literature, I do believe that the informal term “acacia” has no substitute for reference to the iconic thorn trees of the continent. If we do continue to use this term though, emphasis must be placed on Africa as the place of origin and the fact that “acacia” is just a colloquial blanket-term for two plant genera that only appear to be very similar.
I might add that the similarity between genera isn’t as great as one might imagine. In reality, there are easy ways to determine which respective genus an African acacia falls into – but, I will leave that discussion for another time, hopefully soon…