Bursera simaruba has always been one of my favourite tree species. It’s a dry-season deciduous tree with compound leaves and a coppery peeling outer bark and a green (presumably photosynthetic) inner bark.  It’s a conspicuous element of tropical dry forests in Trinidad and Tobago, Puerto Rico and parts of southern Florida (where they call it the ‘gumbo limbo’ tree).  In all these places it’s the only representative of its genus.  In my experience, Bursera was Bursera simaruba, so I was surprised when I came across a Bursera that was grown from seed collected in Costa Rica that was obviously not B. simaruba. 

Nonetheless, I still thought of Bursera as a relatively small genus.  Then I came across some information on the genus in Mexico which turned my picture of Bursera completely on its head.  There are 84 species of Bursera in Mexico - 80 of which are endemic - out of a total of approximately 100 species in the genus.  So why are 80% of the species of Bursera - a genus which ranges from Florida to Argentina - restricted to Mexico?

Species diversity patterns reflect several underlying processes - those that generate diversity, and those that maintain that diversity. When species are grouped into a genus, the assumption is that they are more closely related to one-another than are they to any species in a different genus.  To get from that one ancestral species to its modern descendants, something must occur that allows the single ancestral lineage to split into several daughter lineages (a process known as speciation).  This figure from the Wikipedia article on speciation summarises the different modes of speciation.

In order to generate the type of pattern seen in Bursera, you need one of two evolutionary processes to be active.  Either Bursera originally diversified in Mexico, and a few species have spread beyond that ancestral range (giving rise to their own daughter species along the way) or something happened in Mexico that led to the diversification in a limited portion of the range of a widespread genus.  In the former case, Mexican diversity should be old, and the splits between the Mexican species should lie deep in the ancestry of the genus.  In the latter case, Mexican diversity is newer, and the splits between the Mexican species are likely to have been derived from more widespread species.

In a paper published in PLoS ONE in October, Judith Becerra and Lawrence Venable of the University of Arizona looked at the case of Bursera in Mexico.¹ Bursera is an old genus - molecular phylogenies based on ribosomal DNA suggest that modern species share a common ancestor about 66-74 million years ago, and fossil evidence suggests that the genus was once ranged over a much wider portion of North America.² It turns out that most of the Mexican species are more recent.  The number of lineages increased substantially within the last 30 million years³ and peaked between 10 and 17 million years ago (which coincides with the formation of the Western Sierra Madre and the Neovolcanic belt).¹ Becerra suggested that the diversification of Bursera is likely to have coincided with the expansion of dry forests in central and southern Mexico.³ These dry forests were made possible by the uplift of the mountains which provided appropriate climatic conditions for the establishment of tropical dry forests by sheltering them from northern cold fronts.¹

In previous work, Becerra has built a detailed phylogeny of the Mexican species of Bursera. Using this phylogeny, she was able to show that the diversification of these species coincided with the formation of the Western Sierra Madre and the Neovolcanic belt.  In the PLoS ONE article she and Venable used this phylogeny and the distribution of existing Bursera species to predict where the various species are likely to have originated. 

Despite the fact that it ranks third in Bursera species richness today, they found that the Southwest was actually the source of the largest number of species.  The Balsas River basin, on the other hand, has the most species (and the largest number of endemic species), but was the site fo relatively few diversifications.  Continued mountain-building led to an expansion of dry forest, into which new species wer able to spread.  Other new species were able to invade the Mexican highlands, the Sonoran Desert, upland oak forests or subhumid tropical forests.

Becerra and Venable termed the diversity-generators “source” areas and the non-dry forest habitats as “diversity sinks”.  Personally, this bothered me, as it felt like they were borrowing terminology from population ecology (where it applies to individuals within populations) and applying it to species in a way that is likely to bring unwanted baggage.  Sink populations recruit fewer individuals than are required to replace losses to the population, and as such will go extinct if they don’t continue to receive immigrants from the source population.  There’s nothing to indicate that Becerra and Venable are using 'sinks' to mean anything beyond the fact that these areas are occupied by species that evolved elsewhere. Using this borrowed terminology is likely to mislead readers who are more familiar with the concept of source-sink dynamics in population ecology.

Since certain areas have been superior generators of diversity, Becerra and Venable suggest that prioritising them for conservation should yield superior long-term outcomes.  Protecting areas that can generate diversity should be more important than simply protecting areas that harbour greater diversity.  They write:

The differences between diversity and diversification mean that this may be transitory in the long run, analogous to protecting species in zoos. While it might sound unusual to try to conserve diversity based on events happened in the past, there may be cases in which the aerographic patterns of diversification have occurred repeatedly for a long time, giving us some kind of assurance that it will continue happening in the same way for at least the near future. In the case of Bursera, diversification seems to have been higher in one area for a long time, starting 15 million years ago or perhaps even longer. If not greatly perturbed, there is no reason not to believe that these same patterns of diversification will continue. This approach could be especially useful if there are no other stronger criteria to decide where conservation efforts should be directed. If we had to choose between conserving one of two areas and everything is equal except their history of being sinks or sources of diversification, there would be no harm and perhaps much gain in choosing the source. The long-term maintenance of biodiversity require us preserve its sources, to the extent that these can be accurately determined [8].

[Emphasis added]

I’m not sure if I agree, or disagree.  On one hand, there’s a lot of evidence that suggests that species assemblages are more transient than they were assumed to be in the past.  The simple fact that an area supports a large assemblage of species may reflect chance as much as some special property of the site.  So from that perspective, the areas that have generated diversity should be more important than the areas that harbour diversity.  On the other hand, why should we assume that an area that generated a lot of diversity in the past will continue to do so in the future?  The rate at which new species are being generated appears to have declined sharply in past 10 million years.³ If, as has been suggested, the generation of diversity was related to mountain-forming, is it reasonable to expect the process to continue?  It’s difficult to say what it is that generates a species flock in one area and not in another.

The other big question I found myself with was what is the purpose of conservation?  At what point will we be able to stop protecting species and environments?  When will the threats recede, or will they recede at all?  What will the world look like when the current human-driven extinction event has run its course?

1. Becerra, Judith X. and D. Lawrence Venable 2008. Sources and Sinks of Diversification and Conservation Priorities for the Mexican Tropical Dry Forest PLoS ONE 3(10) DOI:10.1371/journal.pone.0003436



2. Becerra, Judith X. 2003. Synchronous coadaptation in an ancient case of herbivory.  Proceedings of the National Academy of Sciences USA 100(22):12804-12807 DOI:10.1073/pnas.2133013100



3. Becerra, Judith X. 2005. Timing the origin and expansion of the Mexican tropical dry forests.  Proceedings of the National Academy of Sciences USA 102 (31):10919-10023 DOI:10.1073/pnas.0409127102