Projecting community trophic structures for the last 120,000 years

Studying past community dynamics can provide valuable insights for
anticipating future changes in the world's biota. However, the
existing fossil record is too sparse to enable continuous temporal
reconstructions of wholesale community dynamics. In this study, we utilise
machine learning to reconstruct Late Quaternary community structure,
leveraging the climate-trophic structure relationship. We followed a
four-stage approach: 1) identify and map trophic structure units (TSUs) at
the global scale based on the guild richness and composition of extinct
and extant terrestrial mammal species weighing over 3 kg; 2) train a
random forest model to predict TSUs based on contemporary climatic
conditions; 3) hindcast the global distribution of TSUs using climatic
conditions as reconstructed over the past 120,000 years; and 4) compare
TSU hindcasts against elements of community trophic structure as estimated
with the fossil record. Models project significant shifts in the
geographical distribution of community trophic structures, with more
pronounced changes occurring during the Pleistocene-Holocene transition.
These shifts exhibit regional variations, particularly in Eurasia and
North America, where the models project notable reductions in the
distribution of less complex trophic structures over the last 24,000
years. Hindcasts partially identified the alterations in community
structure seen in the fossil record, demonstrating a match between the
observed and predicted times of change in mammal community structure
(between 24 and 8ka BP). However, climate-driven projections of trophic
guilds diverged from fossil records during the Holocene. While the fossil
record indicated a decrease in the number of grazers and carnivores, our
models projected an increase in these numbers. Characterising
community-wide responses to climatic changes is essential to address key
questions about past and future impacts of such changes. Although further
research is needed to refine the models, our approach offers a perspective
for addressing the complex interactions among climate and trophic
community organisation across ecosystems.