Introduced Herbivores Restore Late Pleistocene Ecological Functions
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Introduced herbivores restore Late Pleistocene ecological functions Erick J. Lundgrena,1, Daniel Rampa, John Rowanb, Owen Middletonc, Simon D. Schowanekd,e, Oscar Sanisidrof,g, Scott P. Carrollh,i, Matt Davisj, Christopher J. Sandomc, Jens-Christian Svenningd,e, and Arian D. Wallacha aCentre for Compassionate Conservation, Faculty of Science, University of Technology Sydney, Ultimo 2007, NSW, Australia; bOrganismic and Evolutionary Biology, University of Massachusetts Amherst, Amherst, MA 01003; cSchool of Life Sciences, University of Sussex, Brighton BN1 9RH, United Kingdom; dSection for Ecoinformatics and Biodiversity, Department of Biology, Aarhus University, DK 8000 Aarhus C, Denmark; eCenter for Biodiversity Dynamics in a Changing World (BIOCHANGE), Department of Biology, Aarhus University, DK 8000 Aarhus C, Denmark; fDepartamento Ciencias de la Vida, Universidad de Alcalá, 28801 Alcalá de Henares, Spain; gDepartment of Vertebrate Paleontology, Biodiversity Institute, University of Kansas, KS 66045; hDepartment of Entomology and Nematology, University of California, Davis, CA 95616; iInstitute for Contemporary Evolution, Davis, CA 95616; and jNatural History Museum of Los Angeles County, Los Angeles, CA 90007 Edited by James A. Estes, University of California, Santa Cruz, CA, and approved February 21, 2020 (received for review September 10, 2019) Large-bodied mammalian herbivores dominated Earth’s terrestrial diet (12) (SI Appendix, Fig. S1). For example, large-bodied ecosystems for several million years before undergoing substan- hindgut grazers have the unique capacity to bulk graze large tial extinctions and declines during the Late Pleistocene (LP) due to quantities of low-nutrient grasses (8, 13, 14). However, the prehistoric human impacts. The decline of large herbivores led to downstream ecological effects of this function vary with ecological widespread ecological changes due to the loss of their ecological context (e.g., precipitation, soil type, and predation pressure). For functions, as driven by their unique combinations of traits. How- example, bulk grazing can lead to the formation of high produc- ever, recently, humans have significantly increased herbivore spe- tivity grazing lawns, but this process is shaped by interactions cies richness through introductions in many parts of the world, among soil nutrients, rainfall, and herbivore densities (15). potentially counteracting LP losses. Here, we assessed the extent Most extant plant and animal species evolved in the context to which introduced herbivore species restore lost—or contribute of diverse large-bodied herbivore assemblages from the early — novel functions relative to preextinction LP assemblages. We Cenozoic (30–40 million ybp) until the LP extinctions (16). constructed multidimensional trait spaces using a trait database However, most research on introduced herbivores has been ECOLOGY ≥ for all extant and extinct mammalian herbivores 10 kg known conducted under the premise that they are ecologically novel ∼ from the earliest LP ( 130,000 ybp) to the present day. Extinction- and thereby disadvantage resident species (e.g., ref. 17). The driven contractions of LP trait space have been offset through possibility that introduced herbivores may, in part, restore the introductions by ∼39% globally. Analysis of trait space overlap ecological functions that characterized the past several million reveals that assemblages with introduced species are overall more years until the LP extinctions has been suggested (18–21) but has similar to those of the LP than native-only assemblages. This is not been rigorously evaluated. because 64% of introduced species are more similar to extinct Here, we analyze how the twin anthropogenic forces of ex- rather than extant species within their respective continents. Many introduced herbivores restore trait combinations that have tinction and introduction have shaped herbivore functional di- the capacity to influence ecosystem processes, such as wildfire and versity and the extent to which introduced herbivores restore lost, shrub expansion in drylands. Although introduced species have or introduce novel, ecological functions relative to preextinction LP long been a source of contention, our findings indicate that they may, in part, restore ecological functions reflective of the past Significance several million years before widespread human-driven extinctions. Humans have caused extinctions of large-bodied mammalian megafauna | novel ecosystems | functional ecology | restoration ecology | herbivores over the past ∼100,000 y, leading to cascading changes invasion in ecosystems. Conversely, introductions of herbivores have, in part, numerically compensated for extinction losses. However, the lobal extinctions and range contractions of large-bodied net outcome of the twin anthropogenic forces of extinction and Gmammalian herbivores have occurred across the world be- introduction on herbivore assemblages has remained unknown. ginning ∼100,000 y ago and peaking toward the end of the Late We found that a primary outcome of introductions has been the Pleistocene (LP) (1). Emerging consensus indicates that LP reintroduction of key ecological functions, making herbivore as- losses were primarily driven by prehistoric human impacts (2, 3) semblages with nonnative species more similar to preextinction either alone or synergistically with climate change (4). On the ones than native-only assemblages are. Our findings support calls other hand, recent introductions of herbivore taxa outside their for renewed research on introduced herbivore ecologies in light of native ranges has increased species richness across much of the paleoecological change and suggest that shifting focus from world, in some continents to levels approaching the LP (5). eradication to landscape and predator protection may have The prehistoric declines of large-bodied herbivores led to broader biodiversity benefits. widespread ecosystem changes, including reduced nutrient cy- Author contributions: E.J.L., D.R., J.R., O.M., S.D.S., S.P.C., M.D., C.J.S., J.-C.S., and A.D.W. cling and dispersal, reduced primary productivity, increased designed research; E.J.L. performed research; E.J.L., J.R., O.M., and S.D.S. analyzed data; wildfire frequency, and intensity, and altered vegetation struc- E.J.L., D.R., J.R., O.M., S.D.S., O.S., S.P.C., M.D., C.J.S., J.-C.S., and A.D.W. wrote the paper; ture (6–8). Likewise, introduced herbivores have been found to and O.S. designed and illustrated figures. drive changes in vegetation structure (9), to increase water The authors declare no competing interest. availability in deserts through grazing and disturbance (10), and This article is a PNAS Direct Submission. to reduce fuel loads and thus wildfire (9, 11). Published under the PNAS license. These effects emerge from the distinct ecological functions of 1To whom correspondence may be addressed. Email: [email protected]. large herbivores. Here, we define “function” as the capacity of This article contains supporting information online at https://www.pnas.org/lookup/suppl/ organisms to affect their environment as determined by their doi:10.1073/pnas.1915769117/-/DCSupplemental. combinations of traits, such as body mass, fermentation type, and First published March 23, 2020. www.pnas.org/cgi/doi/10.1073/pnas.1915769117 PNAS | April 7, 2020 | vol. 117 | no. 14 | 7871–7878 Downloaded by guest on September 25, 2021 A Africa Asia B 100 0.2 50 Arboreal Aquatic 0 Australia Europe Log Mass 100 Browsing/Frugivory 0.0 50 Number of Species PCoA 2 (28% variation) Grazing 0 N. America S. America 100 Terrestrial −0.2 50 Fermentation Efficiency −0.25 0.00 0.25 0 e LP LP v PCoA 1 (34% variation) Inclusive Inclusi Africa Asia Australia C 0.3 0.2 0.1 0.0 −0.1 −0.2 Europe N. America S. America 0.3 PCoA 2 0.2 0.1 0.0 −0.1 −0.2 −0.2 0.0 0.2 0.4 −0.2 0.0 0.2 0.4 −0.2 0.0 0.2 0.4 PCoA 1 Species history Reintroduced Introduced Native Locally Extinct Globally Extinct Functional space/centroid LP Native−only Inclusive Fig. 1. Trait space changes resulting from LP extinctions and recent introductions. (A) Species richness per continent. Introductions have numerically replaced lost species richness by between 11% (Asia) and 50% (Australia and Europe). Fill color indicates species fate with the legend shared with C. Inclusive = native + introduced modern assemblages. (B) Global herbivore trait space. Arrows indicate how particular traits shape trait space axes. The first two PCoA axes (∼62% of variation) of trait space are shown (see the SI Appendix, Fig. S1 for PCoA axes 3 and 4). Points indicate species, and the fill density indicates their density distribution with the legend shared with C.(C) Changes in continental trait space (PCoA 1 and 2) from extinctions and introductions. Crosses indicate centroids of the first two PCoA axes. Locally extinct species went extinct within the respective continent but survived elsewhere. Native only = modern native as- semblages; inclusive = native + introduced modern assemblages. 7872 | www.pnas.org/cgi/doi/10.1073/pnas.1915769117 Lundgren et al. Downloaded by guest on September 25, 2021 Trait space volume gain recorded in terms of fermentation efficiency with ruminants A relative to LP scoring highest. Limb morphology was included as a trait due to 0.0 Native−only its influences on soil disturbance (22), locomotion (e.g., cursor- Inclusive iality and fossoriality), and habitat constraints, which can oth- −0.1 erwise be difficult to infer for extinct