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Irob K, Blaum N, Weiss‐Aparicio A, Hauptfleisch M, Hering R, Uiseb K, Tietjen B. Savanna resilience to droughts increases with the proportion of browsing wild herbivores and plant functional diversity. J Appl Ecol 2023. [DOI: 10.1111/1365-2664.14351] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Affiliation(s)
- Katja Irob
- Freie Universität Berlin, Theoretical Ecology Institute of Biology Berlin Germany
| | - Niels Blaum
- Plant Ecology and Nature Conservation University of Potsdam Potsdam Germany
| | - Alex Weiss‐Aparicio
- Freie Universität Berlin, Theoretical Ecology Institute of Biology Berlin Germany
| | - Morgan Hauptfleisch
- Biodiversity Research Centre Namibia University of Science and Technology Windhoek Namibia
| | - Robert Hering
- Plant Ecology and Nature Conservation University of Potsdam Potsdam Germany
| | - Kenneth Uiseb
- Ministry of Environment, Forestry and Tourism Windhoek Namibia
| | - Britta Tietjen
- Freie Universität Berlin, Theoretical Ecology Institute of Biology Berlin Germany
- Berlin‐Brandenburg Institute of Advanced Biodiversity Research (BBIB) Berlin Germany
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Donaldson JE, Holdo R, Sarakikya J, Anderson TM. Fire, grazers, and browsers interact with grass competition to determine tree establishment in an African savanna. Ecology 2022; 103:e3715. [PMID: 35388482 DOI: 10.1002/ecy.3715] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/22/2021] [Accepted: 02/10/2022] [Indexed: 11/11/2022]
Abstract
In savanna ecosystems, fire and herbivory alter the competitive relationship between trees and grasses. Mechanistically, grazing herbivores favor trees by removing grass, which reduces tree-grass competition and limits fire. Conversely, browsing herbivores consume trees and limit their recovery from fire. Herbivore feeding decisions are in turn shaped by risk-resource trade-offs that potentially determine the spatial patterns of herbivory. Identifying the dominant mechanistic pathways by which fire and herbivores control tree cover remains challenging, but is essential for understanding savanna dynamics. We used an experiment in the Serengeti ecosystem and a simple simulation driven by experimental results to address two main aims: (1) determine the importance of direct and indirect effects of grass, fire and herbivory on seedling establishment; and (2) establish whether predators determine the spatial pattern of successful seedling establishment via effects on mesoherbivore distribution. We transplanted tree seedlings into plots with a factorial combination of grass and herbivores (present/absent) across a lion kill-risk gradient in the Serengeti, burning half of the plots near the end of the experiment. Ungrazed grass limited tree seedling survival directly via competition, indirectly via fire, and by slowing seedling growth, which drove higher seedling mortality during fires. These effects restricted seedling establishment to below 18% and, in conjunction with browsing, resulted in seedling establishment dropping below 5%. In the absence of browsing and fire, grazing drove a 7.5-fold increase in seedling establishment. Lion predation risk had no observable impact on herbivore effects on seedling establishment. The severe negative effects of grass on seedling mortality suggests that regional patterns of tree cover and fire may overestimate the role of fire in limiting tree cover, with regular fires representing a proxy for the competitive effects of grass.
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Affiliation(s)
| | - Ricardo Holdo
- Odum School of Ecology, University of Georgia, Athens, GA, USA
| | | | - T Michael Anderson
- Department of Biology, Wake Forest University, Winston Salem, North Carolina, USA
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3
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Hays BR, Riginos C, Palmer TM, Doak DF, Gituku BC, Maiyo NJ, Mutisya S, Musila S, Goheen JR. Demographic consequences of mutualism disruption: Browsing and big-headed ant invasion drive acacia population declines. Ecology 2022; 103:e3655. [PMID: 35132627 DOI: 10.1002/ecy.3655] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/31/2021] [Revised: 10/14/2021] [Accepted: 10/26/2021] [Indexed: 11/09/2022]
Abstract
Across the globe, biological invasions have disrupted mutualisms, producing reverberating consequences for ecosystems. Although invasive species frequently trigger mutualism disruptions, few studies have quantified the demographic mechanisms by which mutualism breakdown may generate population effects. In a Kenyan savanna, the invasive big-headed ant (Pheidole megacephala) has disrupted a foundational mutualism between the monodominant whistling-thorn tree (Acacia drepanolobium) and native ants (Crematogaster spp.) that deter browsing by large mammalian herbivores. We conducted experiments to quantify the demographic consequences of this mutualism disruption in the presence and absence of large mammalian herbivores. Invasion by P. megacephala exacerbated population declines of A. drepanolobium, primarily through decreased survival and reproduction of adult trees. However, these fitness reductions were small compared to those resulting from the presence of large mammalian herbivores, which negatively impacted growth and survival. Contrary to expectation, the expulsion of metabolically costly Crematogaster mutualists by P. megacephala did not result in higher population growth rates for trees protected from large mammalian herbivores. Our results suggest that invasive P. megacephala may impose a direct metabolic cost to trees exceeding that of native mutualists while providing no protection from browsing by large mammalian herbivores. Across landscapes, we expect that invasion by P. megacephala will reduce A. drepanolobium populations, but that the magnitude and demographic pathways of this effect will hinge on the presence and abundance of browsers.
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Affiliation(s)
- Brandon R Hays
- Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, USA
| | - Corinna Riginos
- Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, USA.,The Nature Conservancy, 258 Main Street, Lander, Wyoming, USA
| | - Todd M Palmer
- Department of Biology, University of Florida, Florida, USA
| | - Daniel F Doak
- Environmental Studies Program, University of Colorado, Boulder, Colorado, USA
| | - Benard C Gituku
- Department of Land Resource Management & Agricultural Technology, University of Nairobi, Nairobi, Kenya.,Conservation Department, Ol Pejeta Conservancy, Nanyuki, Kenya
| | - Nelly J Maiyo
- Conservation Department, Ol Pejeta Conservancy, Nanyuki, Kenya
| | - Samuel Mutisya
- Conservation Department, Ol Pejeta Conservancy, Nanyuki, Kenya
| | - Simon Musila
- Mammalogy Section, National Museums of Kenya, Nairobi, Kenya
| | - Jacob R Goheen
- Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, USA
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Mochi LS, Mazía N, Biganzoli F, Aguiar MR. Contrasting effects of grazing on the early stages of woody encroachment in a Neotropical savanna. Basic Appl Ecol 2022. [DOI: 10.1016/j.baae.2022.01.007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
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