Are we in a modern mass extinction?

Introductory historical geology or paleontology course. Suitable for undergraduate majors and non-majors.

Students should:

• Understand the concepts of extinction and mass extinctions
• Recognize the fossil record as a sample of past biodiversity
• Understand some of the potential sampling biases in the fossil record in comparison to modern diversity data (e.g., taphonomy, collections bias)
• Possess quantitative skills in basic arithmetic, percentages, and rates

This activity fits in historical geology near the end of the semester, where it highlights the usefulness of fossil data for addressing present and future challenges. This also provides time for students to acquire the necessary background knowledge about mass extinctions and the fossil record. The activity is designed as a stand-alone exercise that can be completed within one lab period, and it can also be assigned as a homework assignment.

• Mass extinctions'
• Extinction rates, including background rates in the fossil record
• Biases in the fossil record

• Compare quantitative metrics for defining a mass extinction
• Compare modern and fossil extinction data to evaluate whether the present "sixth extinction" meets the criteria for a mass extinction (hypothesis testing using real-world data)
• Identify possible sources of error and uncertainty in both modern and fossil biodiversity data and assess their impact on the results of the analysis

• Calculating percentages and rates
• Reading data tables

In this exercise, students are introduced to the hypothesis that the modern loss of global biodiversity represents a mass extinction in progress. Students use real data on species extinctions in the modern world to compute extinction percentages and rates, then compare these to observed extinction percentages and rates in the fossil record. In the end, students reach their own interpretations about the present biodiversity crisis, consider the limitations of the data, and propose possible future avenues of research.

The activity begins by introducing the "Sixth Extinction" hypothesis and encouraging students to engage in open-ended dialog about their initial expectations regarding the hypothesis. In Part 1, students examine real modern biodiversity data consisting of extinction counts and percentages for different vertebrate groups. They also consider the effects of using different ways to count the total number of recent extinctions. Students are then provided with the first metric for defining a mass extinction: the percentage of species on Earth that went extinct during the event. Students compare modern and fossil data, evaluating whether the modern extinction percentage matches mass extinctions known in the fossil record.

In Part 2, students learn about a second metric for defining a mass extinction, extinction rates that significantly exceed background or "normal" rates. Students calculate extinction rates for the modern era and compare them to background rates calculated from the fossil record.

In Part 3, students break the data down further into the last 500 years and the last 100 years so that they discover how extinction rates have accelerated in the recent past.

In Part 4, students build on their understanding of the current extinction trends to make predictions about the future likelihood of a mass extinction.

In Part 5, students reevaluate their original hypothesis in light of their results and propose directions for future research. All required materials are contained within the exercise itself. The data used are originally from Ceballos et al. (2015) and Barnosky et al. (2011); citations are found below in Resources. These primary sources are not required reading but could be assigned for more advanced students (e.g., upper-division paleontology courses, graduate students) to provide additional discussion. However, Ceballos et al. (2015) contains completed calculations that would provide answers to the problems in the exercise, so students should not read the whole article ahead of time.

Student assignment for sixth extinction exercise (Microsoft Word 2007 (.docx) 31kB Oct8 21) 
Instructor key for sixth extinction lab exercise (Microsoft Word 2007 (.docx) 33kB Oct8 21) 

Discussion opportunities: This activity is designed to foster discussion among students. It works well when students are in pairs or small groups. The open-ended questions are suitable for the think-pair-share technique. These smaller and more private discussions provide space for a greater diversity of perspectives to be explored and debated, whereas large class discussions can be susceptible to the false appearance of consensus if students do not want to be seen challenging a perceived majority view or conflict if the subject matter is seen as politically or emotionally charged.

Materials: In order to complete the calculations, students need access to devices such as calculators, computers, or phones. An instructor who prefers that students calculate by hand may wish to provide additional time or reduce the number of calculations. As a variant, students could be required to graph the number of extinctions over time to visualize rates; examples of these types of plots can be found in Ceballos et al. (2015).

Calculations: An instructor may offer students the opportunity to show their calculations to receive partial credit and to reinforce arithmetic skills.

Formative assessment: When conducted during class time, the instructor can check in throughout the activity to provide feedback on progress and understanding. In-class discussions allow students to receive feedback and gain alternative perspectives from their peers as well as the instructor.

Summative assessment: For this graded lab activity, assessment is based on the completed worksheet. Students are evaluated on quantitative analytical skills and reasoning in justifying their interpretations. Success in quantitative analysis is recognized based on correct numerical responses. The interpretive questions allow greater room for students to develop their own ideas. Successful responses demonstrate an understanding of key course concepts (e.g., mass extinction, bias in the fossil record) and meaningfully connect these to the observed data.

Published works cited in the exercise:

Barnosky, A. D., Matzke, N., Tomiya, S., Wogan, G. O. U., Swartz, B., Quental, T. B., Marshall, C., McGuire, J. L., Lindsey, E. L., Maguire, K. C., Mersey, B., and Ferrer, E. A. 2011. Has the Earth's sixth mass extinction already arrived? Nature 471:51–57. DOI: 10.1038/nature09678.

Ceballos, G., Ehrlich, P. R., Barnosky, A. D., García, A., Pringle, R. M., and Palmer, T. M. 2015. Accelerated modern human-induced species losses: entering the sixth mass extinction. Science Advances 1:e1400253. DOI: 10.1126/sciadv.1400253.