Caribou Survived the Late Pleistocene Extinction, but Can They Avoid Extinction in the Twenty-First Century?
by Mark Lawler
The impacts of contemporary climate change on biodiversity are undeniable and emphasize the critical need for understanding biotic responses to past shifts in climate, and in applying that knowledge to guide current land management decisions [1, 2]. The earth is experiencing human-induced global ecological transformations. These changes have led to a global biodiversity crisis where the species lost rate exceeds the current background rate of 0.1–1.0 extinctions/million species years [3, 4, 5, 6]. Mathematically speaking, this means that if there are a million species on the earth, one would go extinct every year, while if there was only one species it would go extinct in one million years, etc. [6]. One of the species currently threatened by these ecosystem changes is caribou, which are members of the cervid (deer) family, known in Europe as reindeer (Figure 1). Caribou have a Holarctic distribution and comprise five recognized subspecies: R. t. granti; R. t. groenlandicus; R. t. pearyi; R. t. tarandus and R. t. caribou, which has three ecotypes the migratory barren-ground, the mountain or woodland (montane), and the forest-dwelling [7].
Caribou survived the late Pleistocene extinction, but today, in the face of a warming planet which increases the sensibility of the area together with the induced ecosystem changes, it has an uncertain future. Over the past 30 years, important changes were observed in the caribou community of Canada. For example, the Labrador George River herd had 800,000 animals at their peak in the late 1990s, but now, 30 years later the number declined by 99% to just 5,500 animals. In Newfoundland, numbers of woodland caribou have declined from 95,000 to near 30,000 today [8, 9]. What is causing this decline? Is it a natural "boom/bust" cycle, anthropogenically driven, or a combination of anthropogenic and natural causes? How can the fossil record help shed light on these questions?
Fig. 1: Woodland caribou (Ryan Hodnett / CC BY-SA )
Caribou Decline Today
The woodland (boreal) caribou (R. t. caribou) is listed as a threatened species in Newfoundland and Labrador [9]. A 2015 report produced by the Department of Environment and Conservation of Newfoundland concluded that the rapid decline and slow-to-recover numbers of Newfoundland's caribou are part of a long, natural cycle of unsustainably high populations during the 1990s followed by a rapid decline [8]. The reasons for these changes were the broad shifts in habitat use, e.g., diet became less selective with more poor-quality foods. Body size was reduced, and in some herds, calf recruitment (the percentage of calves that survive to one year of age) decreased. Within ten years, the Newfoundland caribou population was reduced by half.
The National Recovery Strategy for Labrador's caribou developed by the government of Labrador suggests that ranges (areas in Labrador where caribou are found today) with less than 10% disturbance have a greater than 90% likelihood of stable or increasing population growth based on the disturbance–recruitment relationship derived from ranges throughout Canada [9]. In other words, the lesser the amount of human disturbance, the greater the odds that caribou populations can increase and stabilize. Despite range disturbance between 5–9%, four out of five populations of woodland caribou in Labrador were in decline [9]. A combination of factors from disturbance to competition and range-specific environmental conditions may have the most explanatory power.
Caribou in Prehistorical Times
The rapidly changing climates of the Pleistocene-Holocene Transition (PHT) re-organized ecosystems, altered species abundances, and forced large-scale movements of taxa, thereby changing biodiversity distribution patterns [10, 11]. The PHT is defined here as the time between 15 ka cal BP (thousand years before present) and 11 ka cal BP. While direct analogs cannot be made between the global environmental changes that took place at the PHT and what is occurring today, assessments of how fauna reacted to these rapid ecosystem changes can be made.
Fig. 2: Late Pleistocene fossil caribou localities, courtesy of Mark Lawler
In North America, caribou are believed to have congregated in a refugium (favorable area where species have survived periods of glaciation and ecological transitions during the late Pleistocene) south of the Laurentide ice sheet (LIS) during the last glacial maximum (LGM) and early retreat of the LIS (20 ka cal BP-2 ka cal BP) (Fig. 2). At approximately 12 ka cal BP, the population split into a northern and southern ecotype as it recolonized recently deglaciated terrain [13]. It had been presumed that this late-glacial refugium was located along the southern edge of the LIS within the tundra/taiga ecozone. However, few fossils have been found in this location, with most caribou remains found hundreds of kilometers south of the LIS, some as far south as Alabama [14]. Late-glacial caribou remains have been found within the Appalachians leading some to postulate that the main concentration of caribou prior to 11 ka cal BP was in an Appalachian refugium [13]. A montane strategy of altitude migrations was employed to avoid competition and predation at lower elevations decreasing the potential of resource competition and alternative prey scenarios [7; 13; B. Gordon, personal communication, February 18, 2019].
Outlook & Future Work
We may not be able to determine how all species, populations, and clades will respond to
anthropogenic perturbations, but we can predict how some species can respond to
anthropogenically forced ecological change by examining those that survived the PHT [12]. Caribou in eastern North America can be considered one of these extant winners. Analysis of the paleobiology and ecology of caribou is needed to understand why this species survived the PHT while other megafauna (mastodon and mammoth, for example) did not.
What was unique about caribou that allowed them to survive the terminal Pleistocene extinctions in eastern North America and survive there until being extirpated from the region in the 19th century? Unlike mastodons, with which caribou shared the landscape in the late Pleistocene of eastern North America, caribou remains are far less numerous, fragmentary, and often consist of calcined bone [15]. The lack of well-provenienced, viable (for analysis), late Pleistocene caribou remains makes it challenging to answer the question above.
The question of caribou persistence at the PHT continues to be investigated as part of my dissertation research. In a future blog post, I will discuss how using the fossil record can help address the question of caribou persistence in the past, and their survival into the future in a rapidly warming world. I will discuss the importance of assessing the energetic requirements of R. t. caribou, an essential metric as temperatures continue to rise, placing high latitude taxa such as caribou under more significant threat of ecological disturbance including loss of habitat, increased harassment from oestrid flies leading to reduced calf recruitment, increased competition from other megaherbivores, and increased predation. I will also discuss how analyzing the life-history traits (e.g., fecundity, gestation length, dispersal, adult size, mass) will lead to a better understanding of how these traits helped caribou survive the late Pleistocene extinction and can help them and other taxa persist through the current biodiversity crisis.
Stay tuned…
Mark Lawler, a paleobiologist/paleoecologist completing his doctorate in Environmental Studies
at Antioch University New England, USA
If you have questions or comments concerning Mark's post, please leave a comment below, or send him an email. You can also follow his research on ResearchGate, LinkedIn or Twitter.
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