Scientists have addressed the question of what caused the biodiversity crisis in the Late Devonian period in an effort to analyse the roots of speciation rate reductions, the best method for collecting data on these events, and the impact of these depressions on the overall health of an ecosystem. In this field, the time period most generally associated with the Late Devonian is the Frasnian-Famennian Period (382.7-372.2 million years ago), which is considered one of the five mass extinctions (Lamsdell and Selden, 2017).
This is in large part due to extremely depressed levels of speciation, or the process of forming new and distinct species, which have been recognized in the Late Devonian since at least 1989 (Stigall, 2010; Bambach et al., 2004). More recently, the importance of vicariant speciation and its reduction during the Late Devonian have become of particular interest to this field of study (Lam et al., 2018; Stigall 2010). Vicariance is speciation that takes place when two populations are geographically isolated for a long period of time, and it is the primary driver behind speciation in an ecosystem (Stigall, 2010).
However, there are competing views on the cause of this decrease in vicariant speciation. Largely, the views come down to a debate between biotic causes that were worsened by abiotic factors, such as invasive species, and abiotic causes worsened by biotic factors, such as reduced ocean oxygen levels during this time (Huang, et al., 2018; Stigall, 2010).
The significance of this study is the parallels it considers between the Late Devonian and today’s world, such as climate change and our own human-created invasive species problem.
It provides important context to consider the possible impacts of these factors on modern ecosystems, and how negative effects can be mitigated (Stigall 2010; Stigall et al., 2017; Huang et al., 2018).The combination of decreased speciation rates with climate change, increased biogeographic ranges, and abiotic factors such as ocean levels and tectonic movements all work to create complex and competing explanations for a devastating natural event (Huang et al, 2018; Abe and Lieberman, 2009).
On one side of the debate, there is the argument that the cause of the Late Devonian biodiversity crisis is primarily biotic, with abiotic factors exacerbating the problem.
What makes the Late Devonian unique is that it is widely considered a biodiversity crisis, or a sharp decline in the number of species in an ecosystem, rather than an extinction crisis (Bambach et al., 2004). The main driving force behind this crisis was a sharp decline in speciation rates during the late Devonian, dropping close to zero percent (Stigall, 2010; Bambach et al, 2004; Alroy, 2008). Historically, volcanic island arcs and shallow-water basins have been vital for speciation and healthy diversity because they provide the necessary geographic isolation for vicariant speciation (Lam et al., 2018).
However, rising sea levels during this period expanded species’ geographic ranges and decreased geographic isolation (Abe and Lieberman, 2009). This was devastating because ecosystems rely on a balance between biotic immigration, or an influx of species not native to an area, followed by periods of isolation to prompt vicariance and create healthy diversity (Stigall et al., 2017). The lack of isolation resulted in uncontrolled biotic immigration, which has been shown to decrease speciation and increase extinction rates.
This isolation also provided an environment in which broadly adapted species were able to thrive and invade non-native areas to compete with more specifically adapted organisms (Stigall, 2010). As these invasive species pushed out native ones, a lack of speciation meant that ecological roles were left unfilled, and ecosystems were destabilized (Alroy, 2008; Stigall, 2010). The result, as previously discussed, is that researchers such as Stigall, Lam, and Lamsdell all attribute decreased speciation in the Late Devonian to biotic factors such as invasive species that hindered speciation and destabilized ecosystems.
Other studies, however, argue that the cause of the Late Devonian crisis lies in primarily abiotic factors such as climate change and decreased oceanic oxygen levels, while biotic factors played a smaller role (Frey et al., 2018; Huang et al., 2018). Studies of climate during this period have analyzed the presence of oxygen isotope O-18 in the fossil record of Late Devonian oceans, which is a good indicator of climate change because it is typically in higher content in colder oceans (Huang et al., 2018).
What this study showed was that during the late Devonian period, there were two increases in the presence of this isotope suggesting two cooling periods in the Frasnian Crisis, which occurred as part of the crisis in the late Devonian (Huang et al., 2018). Further analysis reveals that during these periods of global cooling, environmental conditions were mostly oxygen-depleted in Late Devonian oceans, and the fossil record reflects this with species well-adapted to colder, anoxic conditions being abundant (Frey et al., 2018).
The problem was worsened by the instability of the invertebrate ecosystem, which the vertebrate ecosystem was directly dependent on, generated as a result of climate change and decreased biodiversity (Frey et al., 2018). This means that sharp declines in invertebrate biodiversity caused a chain reaction of destabilisation that was felt throughout food chains and detrimentally impacted ecosystems (Frey et al., 2018; Huang et al., 2018). The resulting conclusion of this side of the argument is that while biotic factors did play some role in the decreased biodiversity rates, the predominant driver behind the biodiversity decline was abiotic factors such as climate change and ocean anoxia (Huang et al., 2018, Frey et al., 2018).
While both hypotheses overlap to some degree with regard to their claims, they should not be confused with one another. The reason for this lies in what factor, biotic or abiotic, each hypothesis considers the main cause of the biodiversity crisis in the Late Devonian. While the biotic hypothesis does discuss the impact of abiotic factors such as tectonic movements, ocean levels, and currents, it is through the lense of how these components made ecosystems more vulnerable to other biotic factors (Abe and Lieberman, 2009; Lam et al., 2018; Stigall, 2010).
For example, rising ocean levels during the Late Devonian aren’t seen as the cause of the biodiversity crisis, but rather as a part in a larger, more biotic cause such as reduced vicariant speciation and thriving invasive species (Lam et al., 2018; Stigall, 2010). Similarly, the abiotic hypothesis considers factors such as ecosystem instability and the survival of species adapted to oxygen-depleted environments, but it views them as a result of other abiotic causes. These factors include anoxia and climate change that killed off non-opportunistic species and destabilized ecosystems (Huang et al., 2018; Frey et al., 2018). This is in contrast to the other side of the argument, which asserts that ecosystem instability was a result of decreased speciation caused by invasive species (Stigall, 2010; Stigall et al., 2017). The result is that these two competing hypotheses both view the Late Devonian biodiversity crisis as the result of biotic and abiotic factors, but differ in the importance they give each side of the argument.
The subject that both sides of the argument agree on is the parallels between the Late Devonian and today’s world. These parallels are particularly evident when comparing the possible impacts of invasive species on modern ecosystems (Stigall, 2010). Studies of biotic immigration events, such as the human created invasive species epidemic, have been shown to decrease biodiversity rates to almost zero (Stigall et al. 2017). This has the potential to have substantial impacts because ecosystems rely on oscillations between periods of connectivity, or increased geographic ranges, and isolation in order to create a mechanism for healthy vicariant speciation (Stigall et al., 2017). Furthermore, if the Late Devonian is analogous to modern ecosystems, human-introduced invasive species could have similar long-term impacts on biodiversity and ecosystem health (Stigall, 2010).
This problem could potentially be compounded by the fact that climate change, also a modern, anthropogenic problem, is typically a time in which the most strain is put on an ecosystem and the most evolutionary change occurs (Lam et al., 2018). This is especially worrying considering that modern extinction rates are greater than those in the Late Devonian (Stigall, 2010). The role of this field of science, then, is to provide an important, deep-time perspective on biodiversity crises throughout history (ScienceDaily, 2018).
This perspective gives researchers an ecological understanding of the potential impact of today’s biodiversity crisis, allows them to identify the most unstable ecosystems, and provides a model for long-term, sustainable preservation of biodiversity and ecosystem health (Stigall et al., 2017). I believe that the public is obligated to know the ramifications of this study. As seen in the popular science article by ScienceDaily, the findings of this study is readily available to the public in a comprehensible way (ScienceDaily, 2018). Moreover, the intersection of this field’s area of interest with real-world ecological issues make it necessary for the public to be educated on the possible long-term impacts of the problems facing our ecosystems today so they can make informed decisions on issues pertaining to this subject.
The study of the Late Devonian biodiversity crisis is one that is forced to constantly evolve due to the complex and wide-ranging issues pertaining to it. It must take into account the intricate interworking of biotic and abiotic relationships that all contributed to decreases in speciation and caused a massive biodiversity crisis. This field of study is extremely relevant to today’s world, given the historical data it provides on ecosystem health, speciation rates, and overall causes of the biodiversity crisis (Stigall et al. 2017). This data can be useful in predicting and mitigating harmful, long-term impacts on global ecosystems produced by humans (Stigall, 2010; Stigall et al. 2017).