Forb diversity globally is harmed by nutrient enrichment but can be rescued by large mammalian herbivory

Nelson, Rebecca A. and Sullivan, Lauren L. and Hersch-Green, Erika I. and Seabloom, Eric W. and Borer, Elizabeth T. and Tognetti, Pedro M. and Adler, Peter B. and Biederman, Lori and Bugalho, Miguel N. and Caldeira, Maria C. and Cancela, Juan P. and Carvalheiro, Luísa G. and Catford, Jane A. and Dickman, Chris R. and Dolezal, Aleksandra J. and Donohue, Ian and Ebeling, Anne and Eisenhauer, Nico and Elgersma, Kenneth J. and Eskelinen, Anu and Estrada, Catalina and Garbowski, Magda and Graff, Pamela and Gruner, Daniel S. and Hagenah, Nicole and Haider, Sylvia and Harpole, W. Stanley and Hautier, Yann and Jentsch, Anke and Johanson, Nicolina and Koerner, Sally E. and Lannes, Lucíola S. and MacDougall, Andrew S. and Martinson, Holly and Morgan, John W. and Olde Venterink, Harry and Orr, Devyn and Osborne, Brooke B. and Peri, Pablo L. and Power, Sally A. and Raynaud, Xavier and Risch, Anita C. and Shrestha, Mani and Smith, Nicholas G. and Stevens, Carly J. and Veen, G. F. Ciska and Virtanen, Risto and Wardle, Glenda M. and Wolf, Amelia A. and Young, Alyssa L. and Harrison, Susan P. (2025) Forb diversity globally is harmed by nutrient enrichment but can be rescued by large mammalian herbivory. Communications Biology, 8 (1): 444. ISSN 2399-3642

Abstract

Forbs (“wildflowers”) are important contributors to grassland biodiversity but are vulnerable to environmental changes. In a factorial experiment at 94 sites on 6 continents, we test the global generality of several broad predictions: (1) Forb cover and richness decline under nutrient enrichment, particularly nitrogen enrichment. (2) Forb cover and richness increase under herbivory by large mammals. (3) Forb richness and cover are less affected by nutrient enrichment and herbivory in more arid climates, because water limitation reduces the impacts of competition with grasses. (4) Forb families will respond differently to nutrient enrichment and mammalian herbivory due to differences in nutrient requirements. We find strong evidence for the first, partial support for the second, no support for the third, and support for the fourth prediction. Our results underscore that anthropogenic nitrogen addition is a major threat to grassland forbs, but grazing under high herbivore intensity can offset these nutrient effects.