Underrated past herbivore densities could lead to misoriented sustainability policies

Abstract

Knowing the carrying capacity of the Earth’s grazed ecosystems, and the relevance of herbivory, is important for many scientific disciplines, as well as for policy. Current herbivore levels are estimated to be four to five times larger than at the Pleistocene–Holocene transition or the start of the industrial revolution. While this estimate can lead the general public and the scientific community to predict severe, widespread environmental impacts by livestock in terms of deforestation, biodiversity loss, and climate change, it ignores the inherent uncertainty of such calculations. We revise the evidence published during the last decade regarding Late Pleistocene herbivore abundance, along with contemporary and some pre-industrial data on herbivore density in grazed ecosystems. Both Late Pleistocene and pre-industrial herbivore levels are likely to be consistently higher than what has generally been assumed, confirming increasing awareness on the importance of herbivory as a widespread ecological process. We therefore call for more refined research in this field to have the reliable baselines currently demanded by society and policy. These baselines should orient sound action toward policies on biodiversity conservation, ecosystem restoration, food systems, and climate change.

Herbivory baseline estimates in Spanish Protected Areas, and environmental implications.

“…In the Spanish plateaus and mountain ranges (occupying most areas of the country), migrations are particularly necessary, because they are greatly affected by seasonally extreme climatic conditions (dry and cold) that limit plant growth (Garzón, 1992;Manzano -Baena & Casas, 2010;San Miguel et al, 2017). Hence, carrying capacities are signi cantly higher in other areas with similar bioclimatic conditions and where migrations are possible (Rodríguez et al 2014;Fløjgaard et al 2022;Manzano et al 2023a), similar to what we here describe for Cabañeros National Park. In Spain, a great carrying capacity was observed during periods of high livestock migratory systems, like trashumancia (long distance) and transterminancia (short-distance i.e., < 100 km).…”

“…In Spain, a great carrying capacity was observed during periods of high livestock migratory systems, like trashumancia (long distance) and transterminancia (short-distance i.e., < 100 km). A census from 1750 (Sanz 1994;San Miguel et al 2017) shows grazing livestock biomass of 10.7 •10 3 kg/km 2 , a number closer to Pleistocenic abundances than to current ones (Manzano et al 2023a). Such migration patterns are logical in wild systems too (Garzón 1992).…”

“…are indeed adapted to bottom-up regulation, i.e., an open landscape (Adie and Lawes 2023). With human expansion, livestock progressively replaced wild herbivores, maintaining high densities that ful l part of megafauna's previous ecological role (Smith et al 2016;Manzano et al 2023a). Therefore, baseline herbivory intensities at landscape level in Spain can be very signi cant.…”

“…Environmental accounting tools applied to agricultural systems have therefore regarded all environmental impacts from livestock as anthropogenic, leading to a high direct impact attribution of GHG emissions per kg of product (Gerber et al 2011). But domestic grazing herbivores can indeed ful l ecological roles that are equivalent to wild herbivores, and their densities can also be equivalent (Manzano et al 2023a). At the same time, some authors have demanded that the consideration of baselines can be important in environmental assessment (Pardo et al 2023) and policymaking (Moncrieff et al 2016), for example, when supporting or limiting pastoralism (Manzano et al 2023a).…”

“…But domestic grazing herbivores can indeed ful l ecological roles that are equivalent to wild herbivores, and their densities can also be equivalent (Manzano et al 2023a). At the same time, some authors have demanded that the consideration of baselines can be important in environmental assessment (Pardo et al 2023) and policymaking (Moncrieff et al 2016), for example, when supporting or limiting pastoralism (Manzano et al 2023a). In sum, new methodological approaches demand delving into a speci c framework for Open Ecosystems that explicitly includes and quanti es baseline values (Manzano and White 2019).…”