Changes in the diet and body size of a small herbivorous mammal (hispid cotton rat,Sigmodon hispidus) following the late Pleistocene megafauna extinction

Abstract

The catastrophic loss of large‐bodied mammals during the terminal Pleistocene likely led to cascading effects within communities. While the extinction of the top consumers probably expanded the resources available to survivors of all body sizes, little work has focused on the responses of the smallest mammals. Here, we use a detailed fossil record from the southwestern United States to examine the response of the hispid cotton rat Sigmodon hispidus to biodiversity loss and climatic change over the late Quaternary. In particular, we focus on changes in diet and body size. We characterize diet through carbon (δ13C) and nitrogen (δ15N) isotope analysis of bone collagen in fossil jaws and body size through measurement of fossil teeth; the abundance of material allows us to examine population level responses at millennial scale for the past 16 ka. Sigmodon was not present at the cave during the full glacial, first appearing at ~16 ka after ice sheets were in retreat. It remained relatively rare until ~12 ka when warming temperatures allowed it to expand its species range northward. We find variation in both diet and body size of Sigmodon hispidus over time: the average body size of the population varied by ~20% (90–110 g) and mean δ13C and δ15N values ranged between −13.5 to −16.5‰ and 5.5 to 7.4‰ respectively. A state–space model suggested changes in mass were influenced by diet, maximum temperature and community structure, while the modest changes in diet were most influenced by community structure. Sigmodon maintained a fairly similar dietary niche over time despite contemporaneous changes in climate and herbivore community composition that followed the megafauna extinction. Broadly, our results suggest that small mammals may be as sensitive to shifts in local biotic interactions within their ecosystem as they are to changes in climate and large‐scale biodiversity loss.

The sensitivity of Neotoma to climate change and biodiversity loss over the late Quaternary

“…4B, 5). Tomé et al (2020a) found a similar pattern in the δ 15 N values for Sigmodon hispidus (hispid cotton rat) in the Hall's Cave record, an herbivorous rodent generally inhabiting grassland habitats (Cameron and Spencer, 1981;Kincaid and Cameron, 1985;Randolph et al, 1991;. Sigmodon δ 15 N values ranged between 5.5-7.4‰, and throughout the 14 time intervals that Sigmodon and Neotoma co-occur, their mean δ 15 N values were within 2‰ of each other and they had a similar degree of within-interval variance, suggesting they were likely feeding at similar trophic levels.…”

“…Sigmodon δ 15 N values ranged between 5.5-7.4‰, and throughout the 14 time intervals that Sigmodon and Neotoma co-occur, their mean δ 15 N values were within 2‰ of each other and they had a similar degree of within-interval variance, suggesting they were likely feeding at similar trophic levels. Sigmodon was strongly influenced by shifts in community composition and resource availability such that the decrease in δ 15 N values in this omnivorous generalist corresponded with an increase in the proportion of insectivores on the landscape (Tomé et al, 2020a). Interestingly, a significant correlation between Neotoma δ 15 N and turnover (negative correlation) and similarity to modern (positive correlation) species composition in the community suggest that changes in resource use by Neotoma were linked to biotic interactions (i.e., competition).…”

A guide to state–space modeling of ecological time series

“…State-space models are now used for paleoecological research. For example, Tomé et al (2020) used SSMs to identify the drivers of changes in the mass and diet of a small mammal during the late Pleistocene. They use three separate linear Gaussian SSMs to model temporal changes in mass (as estimated from molar size) and in two stable isotopes (extracted from jaw bone collagen) as responses to each other and of a set of covariates related to climate (e.g., maximum temperature) and community structure (e.g., species richness).…”

The hidden legacy of megafaunal extinction: Loss of functional diversity and resilience over the Late Quaternary at Hall’s Cave

“…Well-defined horizontal layers deposited from repeated flooding show minimal bioturbation and have been described chronologically using an age model developed from 44 published Accelerator Mass Spectrometry 14 C measurements from the site (Tomé et al, 2020).…”