Recovering palaeo-distributions from rock art

Corroborating, refining, or refuting species’ modelled historical distributions can require innovation in datatypes and the synthesis of diverse kinds of information from sometimes unexpected sources.

Ecological niche and species distribution models are tools currently applied in several fields of biology and other disciplines. These models use algorithms to correlate species presence data with environmental layers to reconstruct ecological niches. Ecological niches can then be projected into a geographic space and time, which enables us to make a hypothesis about the potential species distribution.

(Above) Rock art depiction of a desert bighorn sheep
from Sierra de San Francisco, Baja California, Mexico.

We have long been interested in the reconstruction of vertebrate distributions at different spatial and temporal scales. Obtaining their current potential distributions is relatively simple since it is the period where we have more data on the species presence. However, when faced with palaeo-distributions, we can only reconstruct the ecological niches of species in the present and project them to past climate scenarios. This has drawbacks, for example we do not know if the reconstruction of the current niches is well represented. Additionally, we must assume those niches did not change over the projected period of time (also known as niche conservatism).

EDITORS‘ CHOICE: (Read for free until Feb 2021):
Gámez‐Brunswick, C, Rojas‐Soto, O. New insights into palaeo‐distributions based on Holocene rock art. J Biogeogr. 2020; 47 (12): 2543–2553.

One day we stumbled upon a magazine that had a piece about rock art and realized that we have had information from the past for a long time. Rock art has been there for thousands of years, talking to us, static, but telling very interesting stories. They tell us about the interaction of those prehistoric humans with species and their environment through incredible paintings and engravings in stone. Therefore, we could use rock art sites as records of the species represented in that time frame, and then model their distribution with modern day tools.

This seemed possible, but we had not yet realized all the difficulties; there is an enormous number of rock art sites in North America, and at each one there are several species depicted with varying degrees of detail. How is it that we could choose our species and reduce the identification error to a minimum? And moreover, how could we set a time range for all of the representations? Then, once inside the wonderful world of rock art, we found drawings of an ungulate that was very meaningful to hunters, with easily identifiable backward-twisted horns and the only species in that range … the bighorn sheep.

Potential distribution of the desert bighorn sheep from Mid-Holocene (red) and present (grey) in two environmental dimensions (PCs) (left) and in a geographic space (right), as reconstructed from rock art (light orange squares) and current records (black points) respectively.

The bighorn sheep has a wide representation in North American rock art; its importance for so many cultures allowed us to obtain records captured in rocks both from northern Mexico and the southwestern United States. The complex task was to assign a date to all these depictions, and it was there that biology, anthropology, and archaeology collided. Thanks to the study of all those prehistoric human settlements, we were able to assign them a space in our timeline, and thus achieve a match with the environmental layers of the Mid Holocene (6,000 years ago). With this, we were able to model the niche of the bighorn sheep in the past and make comparisons with its data from the present, which we already had very well represented.

The importance of this analysis is in the use of records that were illustrated in rock art for so long and had been overlooked. Furthermore, it allowed us to demonstrate for the first time that niche conservatism does exist in time periods such as the one that separates us from the Mid Holocene. With this, we open the opportunity to start a line of study where we can delve more precisely into palaeo-distributions; particularly in the face of an extremely scarce fossil record. The analysis of species niches and their distributions over time gives us the possibility to understand their temporal dynamics and with this, to better understand the environmental changes that we are already beginning to suffer and will only increase in the future with climate change.

Written by:
Carolina Gámez-Brunswick & Octavio Rojas-Soto
Laboratorio de Bioclimatología, Red de Biología Evolutiva, Instituto de Ecología A. C.

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