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The Extent of Contemporary Species Loss and the Effects of Local Extinction in Spatial Population Networks

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The Extent of Contemporary Species Loss and the Effects of Local Extinction in Spatial Population Networks A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Biological Sciences of the College of Arts and Sciences by Megan Lamkin M.S. Purdue University June 1998 Committee Chair: Stephen F. Matter, Ph.D. Abstract Since the development of conservation science nearly four decades ago, leading conservation biologists have warned that human activities are increasingly setting the stage for a loss of life so grand that the mark on the fossil record will register as a mass extinction on par with the previous “big five” mass extinctions, including that which wiped out the dinosaurs 65 million years ago. The idea that a “sixth mass extinction” was in progress motivated me to explore the extent of recent extinction and the underpinning of the widely iterated statement that current rates of extinction are 100-1,000 times greater than the background rate. In Chapter 2, I show that the estimated difference between contemporary and background extinction does not align with the number of documented extinctions from which the estimates are extrapolated. For example, the estimate that current extinction rates are 100-1,000 times higher than background corresponds with an estimated loss of 1-10 named eukaryotic species every two days. In contrast, fewer than 1,000 extinctions have been documented over the last 500 years. Given this discrepancy, it may prove politically imprudent to use extraordinarily high rates of contemporary extinction to justify conservation efforts. Conservation efforts are sufficiently justified based on the proportion of habitat that has been destroyed or degraded in recent decades and the proportion of species threatened with extinction. In addition to examining the current extinction crisis, I evaluated potential mechanisms of extinction. Although mechanisms of population-level extinction and species-level extinction are well-resolved, little is known regarding the effects of extinction in spatial population networks. A fascinating question that I was surprised had not been thoroughly investigated concerned potential effects of population-level extinction on surrounding populations of the same species: How does the extinction of one population affect the risk of additional extinction within a ii network and the persistence of the network as a whole? Chapters 3, 4, and 5 document findings from my experimental approach to evaluating the effects of population extinction and recolonization on the abundance, synchrony, and stability of surrounding populations. In various tests of extinction and recolonization in experimental protozoan networks, I found that population-level extinction does not necessarily increase the risk of additional extinction beyond the simple loss of a population. Although the loss of an important source of immigrants may reduce the abundance of surrounding populations, for various reasons, extinction is unlikely to increase the risk of network-wide extinction through synchronized dynamics. However, exponential growth of a population during its recovery from extinction can generate a degree of dispersal that bolsters surrounding populations beyond what would be expected in the absence of extinction. Because the bolstering is temporary, these results by no means suggest extinction- recolonization dynamics as a potential conservation strategy. Rather, they emphasize the importance of habitat restoration and population recovery for the sustainability of population networks and the persistence of species at risk of extinction. The results of this dissertation illustrate several mechanisms promoting resiliency in spatial population networks which may help them persist despite continuing anthropogenic onslaught. iii This Page Intentionally Left Blank iv Acknowledgments My “Cerebral Sensei”, Dr. Stephen F. Matter, guided my academic and professional development over the last five and a half years. Just as a sensei of martial arts trains his/her students in all aspects of the combative arts, Dr. Matter trained me in all aspects of the art of science. I am forever grateful of his mentorship, particularly for his patience while I fumbled. Dr. Edna Kaneshiro was a strong source of academic and moral support over the years. She was always generous with her space, equipment, expertise, and culinary delights. I will always appreciate her simultaneous toughness and warmth. Dr. Arnold Miller inspired my interest in paleontology and encouraged me to pick back up after a few failures through his steadfast belief in my ability to “do anything”. Research advisory committee members Dr. Ken Petren, Dr. Eric Maurer, and Dr. Thomas Crist challenged me to improve my presentation skills and expand my breadth of knowledge. Mom, Dad, Simone, Diane, Aunt Pat, Eric, Lucia, Rick and Paulette: you kept me grounded along the way. Chris, I couldn’t have done it without your help and support with Simone. Family and friendship are everything. Love and gratitude. v Table of Contents Abstract……………………………………………………………………. …………….. ii Acknowledgments……………………………………………………………………….... v Chapter 1………………………………………………………………………………….. 2 Introduction Chapter 2…………………………………………………………………………………..10 On the challenge of comparing contemporary and deep-time biological extinction rates Chapter 3…………………………………………………………………………………. 26 An experimental test of local population extinction in spatial population networks Chapter 4………………………………………………………………………………......48 Stochastic extinction of a large population stabilizes but does not synchronize dynamics of small populations in experimental networks Chapter 5…………………………………………………..................................................77 Recolonization of a large population temporarily counters effects of long-term habitat degradation in a spatial population network Chapter 6……………………………………………………………….…………………107 Conclusions Supplemental Information (SI) I…….…………………………………………………..115 Supplemental Tables Supplemental Information (SI) II…………………..…………………………………....119 Supplemental Figures vi Chapter 1 Introduction As the global human population continues to grow in abundance, so too does the human capacity to influence the abundance and distribution of millions of other species on Earth (MEA 2005, Freedman 2014). Anthropogenic impacts on species’ fates (excluding impacts on fellow hominids) date back to more than 50,000 ago when colonization of Pacific islands resulted in the extinction of various large mammals and flightless birds (Diamond et al. 1989, Braje and Erlandson 2013). Despite a global population of less than one million, hunting and trapping facilitated humankind’s first marks of anthropogenic extinction on the fossil record. Through advances in agriculture, engineering, and medicine over the recent 10,000-15,000-year Holocene epoch, the global population increased from approximately one million to more than 7.3 billion individuals (US Census 2016). During that time, humans have influenced the abundance and distribution of species worldwide (Diamond et al. 1989, Braje and Erlandson 2013). Human mutualists (pets, crops, and livestock) and synanthropes (weeds, rodents) have increased in abundance and distribution, often to the detriment of other species (Simberloff 1990), whereas many other species that have failed to adapt to anthropogenic change have decreased in abundance, often to extinction (Diamond et al. 1989, Freedman 2014, IUCN 2015). Considering the number of species that humans promote is small compared to the number of species humans negatively impact (Diamond 1997), it is not surprising that large proportions of the earth’s major taxonomic groups are threatened with extinction (IUCN 2015). Despite the long history of anthropogenic extinction, humans have only recently recognized their impact on the natural world. For example, extinction was not accepted until the early 1800s (Sepkoski 2016). Prior to Cuvier’s incontrovertible demonstration that fossilized remains of mammoth and mastodon were those of extinct species and not elephants (Cuvier 2 1796), all fossils were presumed to belong to extant species that, if not locally present, had relocated (Sepkoski 2016). Despite widespread acceptance within Cuvier’s lifetime that extinction was a natural phenomenon, it was not accepted that extinction could be caused by humans. Rather, leading scientists including Charles Darwin believed that extinction merely offset origination and so was an inevitable component of natural law. That is, extinction was required to maintain constancy in the number of species on earth (Sepkoski 2016). In America, pioneering efforts by Theodore Roosevelt and John Muir in the late 1800s-early 1900s advanced the protection of natural resources, but did not prevent the rapid extinction of species such as the passenger pigeon (Ectopistes migratorius). The lack of scientific understanding of ecological principles made it easy to dismiss fears by ornithologists that humans were hunting the bird to extinction (Schulz et al. 2014). Advances in ecology during the 1950s and 1960s (Hutchinson 1959, MacArthur and Wilson 1963), showed the tremendous impact human actions can have on species’ fates (Wilson 1988, Barnosky et al. 2011, Sepkoski 2016). Despite recent widespread recognition among scientists and world leaders of the importance of biodiversity for the economic, social, and medical well-being of earth’s burgeoning
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