Life on a roller-coaster

Life is not easy. It may not always be okay. The history of Life also had ups and downs like a roller-coaster, some moments with a great turn up of diversity and some moments with a breathtaking fall. Literally. These moments of abrupt decline in diversity are called mass extinctions when most species disappear.

 

Not all species survive the journey.

There is usually a normal rate of extinctions in Nature. This rate is called background extinction. Some species are extinguished by natural factors. However, mass extinctions are events in which at least half of the lineages are extinguished in a short period of time. Recalling that “short” on the geological scale can mean up to 15 million years. These events mark an important change in the formation of the biota. There were five big events of this size and they were called “Big Five”. The end of the Ordovician (440 m.y.a) and the end of the Devonian (375 m.y.a) had extinctions caused by glaciations; the extinctions of the end of Permian (250 m.y.a) and the end of the Triassic (200 m.a.a.) were caused by great volcanic activity; the famous extinction of the late Cretaceous (65 m.y.a.) was caused by the impacts of meteorites.

 

The geological eras. Pterosaurs not included.

The image of a destructive meteor is a very common in the popular imaginary, although it was officially proposed only in 1980. Despite this extinction that decimated the dinosaurs to be more striking, the mass extinction of the Permian was the worst of all. Between 80-90% of all species that existed died, ecosystems such as forests and corals were decimated and the Life as we know it almost got eliminated. Important lineages like sea scorpions and trilobites disappeared. This was the most dangerous time to go on Earth, but we survived.

 

R.I.P. – Rest in Permian

Illustrations of the past eras point too dramatic titles such as “age of the fishes”, “age of the dinosaurs”, “age of men” as if it were a succession of which team is winning. In fact, we are still in the “age of bacteria” that remains surviving all extinctions. There were still bacteria in the Ordovician, there were fish in the Jurassic, and there were birds in the Eocene. There are no evidences about obligatory progress and gain of complexity, it is just survival. In 5/6 of the history, Life was unicellular and interpreting large animals as powerful survivors is a bias of human observation. Diversity today is a consequence of several lineages that have survived in their own way, changing some anatomic details. There were several Noah’s arks going through mass extinctions. The ark of the vertebrates still survives since the Cambrian, but some of us ended up being left behind.

 

When bacteria ruled the Earth

Mass extinctions also bring an important lesson in humility. It does not matter much your skills, you need to be lucky. Chance plays a large role in the survival of species. A hairy lineage would do very well if the climate changed in a great glaciation, but would be bad if the climate changed to a desert. And it would certainly not matter if he lived in the slopes of an erupting volcano. It’s not always blind luck: large, rare, ultra-specialist species restricted to one local becomes an easier target. But there are no precise answers. Chance also causes the winners. Mammals coexisted with dinosaurs for 100 million years until a meteor exterminated the best competitors. And the dinosaurs themselves were already decreasing in the meteor epoch. In addition, a lineage of dinosaurs also survived the meteor, diversified and took advantage of the extinction of others: the birds. Birds are dinosaurs that survived, not for its skills, but for a great portion of luck.

 

This is not how good luck is achieved

Life is a branched pattern of ephemeral components in which lineages matter more than the represents. This is a story punctuated by tragedies, but always (for now) recovered the roles, even if it is with new actors. We are living in a time of great loss of diversity. In the Anthropocene, the nowadays age, there are destruction of ecosystems, global warming, pollution, and unbridled hunting. In relation to birds, we have lost 1.3% of all known species in the last 500 years. This rate is 26 times bigger than a normal background extinction rate. Apocalypse Now. Whether it is or not a sixth mass extinction (now, by our fault) it does not matter. We must confront the problem of loss of a large number of species in a much faster period. With the knowledge of the other extinctions we can guard us against what can happen. Our species is here for a time crumb and we can be extinguished much more easily than many other ex-winners. Trilobites, dinosaurs, Ammonites, they all reigned much longer than humans. If we do not put ourselves in our place, soon we will be in another.

 

“Those who cannot remember the past, are condemned to repeat it”

 

 

References:

Benton, Michael J., and Richard J. Twitchett. “How to kill (almost) all life: the end-Permian extinction event.” Trends in Ecology & Evolution 18.7 (2003): 358-365.

Gould, Stephen Jay. “The evolution of life on the earth.” Scientific American 271.4 (1994): 84-91.

 

 

What about historical biogeography in the 21st century?

            As life and Earth evolve together, biogeography can be considered as the area of biology that pursuits to establish patterns of biotic distribution and connections between the biotas, both resulting from the evolutionary process, being closely related to the triad: form, space and time. In this context, biogeography seeks to understand such patterns by addressing a set of biological and geographic information in order to describe the distribution of organisms on the planet and to give explanations for the history that would have led to such spatial configurations. On its larger scale, biogeography provides the historical perspective required to understand the evolution of biota as well as the geological evolution of the planet.

           Traditionally, biogeography has been divided into ecological and historical. Ecological biogeography studies how ecological processes (such as climate or other physical or environmental factors) interfere in the distribution of organisms in a short period of time, while history biogeography comprises the action of processes, based on causes that existed in the past, over a long period of time in this pattern. Historical biogeography is in the midst of a scientific revolution and that it is a rapidly evolving discipline. As a result of this a great number of approaches and methodologies arose in the most recent decades (second half of the 20th century and the first years of the 21st century).

So this drives to the question: What is the real present state of historical biogeography?

               According to a study made by Posadas et al. (2013), based on the papers published in the Journal of Biogeography (JB) during 1998–2010, there is a clear tendency to increase the number of contributions per volume, which seems to indicate that biogeography as a whole is an expanding research field. However, historical biogeographical contributions have shown a more marked increase. While the number of total contributions published in JB more than doubled from 1998 to 2010 (from 90 in 1998 to 200 in 2010), the historical biogeographical contributions increased almost 10 times during the same period (from 11 in 1998 to 105 in 2010).

Number of articles published in each volume of the Journal of Biogeography during 1998–2010 (total = black bars; historical biogeography = gray bars) (Posadas et al., 2013).

            Out of a total of 2095 papers analyzed, 610 papers (29%) deal with historical biogeography, wich were written by 2018 authors, with a mean of 3.3 authors per paper. A trend toward more authors per paper is evident. This could be an indicator of increased cooperation/collaboration in historical biogeographical work, a pattern not exclusive to historical biogeography but found in most of the contemporary scientific journals. The increasing specialization of researchers, the emergence of more complex techniques to address problems, the presence of multidisciplinary teams in research institutions and  the significant impacts on productivity and citation rates could explain in part this growth.

Percentage of papers authored by one (black bars), two (diagonal line bars), three (horizontal line bars), and four or more authors (gray bars) per year. Trend lines added for papers authored by a single author (black) and by four or more authors (gray) (Posadas et al., 2013).

               The analysis shows a strong trend favoring works devoted to terrestrial habitats (75% terrestrial, 12% freshwater, and 13% marine habitats). A possible explanation for this particular distribution of papers is that it is strongly biased by the degree of difficulty involving access to different habitats, as well as to the smaller average cost of accomplishing taxonomic works in continental environments (including freshwater) relative to marine environments. An alternative explanation for this pattern is related to the distribution of species diversity on continents (including freshwater habitats) and oceans (87% inhabit continental habitats and the remaining 13% inhabit marine). Thus, the study of different habitats seems to be more influenced by taxon diversity than by the surface covered by each habitat.

                  From a taxonomic point of view, at the kingdom level  it was observed that 59% of the papers dealt with animal taxa, 35% with plant taxa, and 4% with taxa from more than one kingdom; only 2% of the papers dealt with taxa from other kingdoms (Protoctista, Fungi). The percentage of papers devoted to higher taxonomic groups correlates better with ‘the inefficiently distributed’  labor force in taxonomy than with taxonomic diversity. The distribution of historical biogeographical papers does not reflect the real diversity of these phyla, the megadiverse animal groups, for example, such as arthropods (particularly insects), nematodes, and marine invertebrates are underrepresented.

                   In relation with approaches and techniques, five approaches were used in almost 72% of the papers considered: phylogeography (35%),   biota similarity and PAE approaches (13%), molecular biogeography (12%), and cladistic biogeography and event-based methods (6% each). The great surge in phylogeographical papers seems to lead the evolution of historical biogeography over the last years. The increase in the number of historical biogeographical contributions per year has accompanied the growth of phylogeographical papers . The fact that phylogeography is the main force which is driving historical biogeographical research in recent years is reflected, for example, in the increase in papers dealing with infraspecific taxa and in those that use molecular clocks. Thus, the taxonomic scale is showing increasing representation of those papers focused on a single species or a few closely related species.

Distribution of papers applying the five most used approaches per year (Posadas et al., 2013).

               Historical biogeography is a research field that has increased its representation throughout the analyzed period. The emergence of new approaches, the combination of techniques, and the increase in questions indicates that historical biogeography is an active discipline. However there are many challenges in the future to understand with more completeness the the infinite variables that influence the existing patterns and processes. Some of them are:(1) to increase the study of those taxa underrepresented according to the part of the biodiversity they represent; (2) to balance the amount of work devoted to different biogeographical regions; (3) to increase biogeographical knowledge of marine and freshwater habitats; (4) to maintain the diversity of approaches, preventing the reduction of time, spatial, and taxonomic scales addressed by the discipline; and (5) to continue integrating historical biogeography along with other sources of information from other disciplines (ecology, paleontology, geology) into a richer context for explaining past, present, and future patterns of biodiversity on Earth.

Literature:

Almeida, E.A.B. & Santos, C.M.D. 2011. Lógica da biogeografia de vicariância. In: Carvalho, C.J.B. & Almeida, E.A.B. (ed.), Biogeografia América do Sul:Padrões e Processos. São Paulo: Roca, pp. 52-62.

Crisci, J.V., Katinas, L. & Posadas, P. 2003. Historical Biogeography: an introduction. Cambridge: Harvard University Press.

Posadas, P; Grossi, M.A. &  Ortiz-Jaureguizar, E. 2013. Where is historical biogeography going? The evolution of the discipline in the first decade of the 21st century? Progress in Physical Geography, 37(3): 377–396.

Santos, C.M.D. & Amorim, D.S. 2007. Why biogeographical hypotheses need a well supported phylogenetic framework: a conceptual evaluation. Papéis Avulsos de Zoologia, 47(4): 63-73.