Effects of humans on the environment

A large area of my research concerns the extinction of large mammals that occurred during the late Pleistocene. I am interested in the causes and consequences of this extinction event. With Felisa Smith and Jim Brown, I have compared the body size distributions of mammals on four continents before and after the extinction of the megafauna at the end of the Pleistocene.  We conclude that human hunting played a large role in this extinction event.  On each continent, human arrival occurred just prior to the extinction, whereas significant climate change was coincident with the extinction on only two continents.  Moreover, present day patterns of extinction risk indicate that only large-bodied species are affected by human hunting practices (Lyons et al., 2004a). We have several follow-up papers examining the potential causes of this extinction event (Smith and Lyons 2011, Lyons and Smith 2013).


Most recently, we were interested in understanding whether there was a detectable effect of this extinction event earlier in the fossil record and what effect this event had on mammalian faunas on different continents. We found that this extinction event began as early as 125,000 years ago in Africa, that the event is time transgressive and follows the path of human migration. Moreover, we show that this event is unique in mammalian history. Prior to the terminal Pleistocene, mammalian extinctions do not show strong size selectivity, nor is extinction across the Cenozoic correlated with climate change. We conclude that “body size downgrading” of mammalian faunas is a characteristic of hominin activity (Smith et al., 2018 Science). A follow-up paper examines the impact of human activity over the last 125,000 years on multiple macroecological patterns and finds that most have been altered by humans (Smith et al. in review).


In order to understand the consequences for mammal communities of losing the vast majority of our large mammal fauna, I investigated the role of body size in structuring communities through time and across the globe.  Examination of ~300 local assemblages over the last 40,000 years indicates that despite the considerable turnover in community composition, ~60% of the communities have body size distributions similar to modern temperate communities.  Such results imply that body size is an important factor in determining membership in communities and that there is stability in community structure despite the loss of the large mammals. Extension of this work to include ~300 modern communities across the globe indicates a significant latitudinal gradient in the shape of local body size distributions (Lyons and Smith, 2013).   


We have also been interested in whether the extinction of the megafauna could have had an effect on global biogeochemical cycles.  For example, we know that the modern livestock industry contributes a great deal to the global methane budget.  We were interested in asking whether the large mammals could have played a similar role and whether their loss would have a measureable effect (Smith et al. 2010, 2011, 2015, 2016).  We estimated the amount of methane produced per year by the extinct megafauna in the New World and estimated that it was minimum of 9 Tg. This is sufficient to account for ~12% of the drop in methane at the Younger Dryas and suggests that more research into the role of animal and plant biota on biogeochemical cycling is needed.


A major part of my current research concerns the consequences of the loss of tens of millions of large-bodied mammals on the structure and functioning of ecosystems in the Americas. I have a collaborative NSF grant with Felisa Smith and Seth Newsome of the University of New Mexico to address this question using a cave site in Texas called Hall’s Cave. Hall’s cave is an amazing cave site with extremely exquisite sampling and a number of radiocarbon dates that allows us to refine the community in the cave to close to 1000 year time bins, a fine scale sampling that is almost unheard of in the fossil record. We are characterizing patterns of abundance, distribution, diet and morphology in the surviving mammals both before and after the extinction event. In an initial publication, we have found significant changes in composition, species associations, and body size distributions across the extinction boundary (Smith et al. 2016). Additional work being led by graduate students at the University of New Mexico is finding that there have been dietary shifts in canids at the site (Elliott Smith et al., in prep) and in cotton rats (Tóme et al., in prep), but that the shifts appear to be in response to underlying baseline shifts in the vegetation rather than a change in response to loss of the megafauna. In contrast, I project being led by our postdoc, Amelia Villaseñor examining dietary shifts in the surviving large herbivores is finding dietary shifts that may be explained by the extinction of competitors (Villaseñor et al. in prep). One of the more interesting results from this work involves Onycomys, the grasshopper mouse. Prior to the extinction of the megafauna, this species was almost twice as large as it is now, yet the stable isotopes do not indicate a major shift in diet over this interval (Keller et al., in prep).


With Advait Jukar, a graduate student from George Mason University that I co-avised with Mark Uhen, I have been examining the megafaunal extinction in the Indian sub-continent. The Pleistocene mammal record in India is dominated by elephants, but the fossil remains generally consist of skull material and teeth. Body size reconstruction in fossil proboscideans typically requires that long bones be preserved. We developed an equation that will allow us to estimate body size in extinct elephants using the occipital condyles, a set of bones that allow the skull to articulate with the neck (Jukar et al. 2018). Using this information, we were able to reconstruct the body sizes of extinct Indian megafauna. One of the most striking findings of this work is that India has experienced very little extinction of its megafuana since the arrival of humans 70000-90000 years ago. We speculate that differences in the effects of climate change in India coupled with a potential rescue effect from the rest of Eurasia may have muted the effects of humans on large mammals in this area (Jukar et al. in prep).