Mammalian herbivory exacerbates plant community responses to long-term increased soil nutrients in two Alaskan tundra plant communities

Abstract

Abstract:The interaction between bottom-up and top-down forces in regulating plant communities is a long-standing topic of interest in ecology. Factorial field experiments examining these factors have been relatively few, but recent meta-analyses provide predictions that can be tested in a range of ecosystems. We tested the prediction that added nutrients would reduce species richness and evenness, while herbivore activity would offset those changes in two tundra plant communities after 11 years. In moist acidic tundra (MAT), herbivores reduced richness more in fertilized plots when mammals were present compared with fertilized plots without herbivores. In dry heath (DH), evenness was significantly reduced in fertilized plots only when herbivores were present, also providing evidence that herbivores enhanced community changes caused by greater nutrient availability. The difference in response between MAT and the meta-analysis predictions appears to be driven by Betula nana, the species that dramatically increased with added nutrients in MAT. Unlike in similar studies and in DH, B. nana is not as palatable as most of the species in the community and is generally avoided by herbivores. These results highlight how the effects of herbivory and nutrients differ across communities and can be affected by the traits of the species present.Key words: herbivory, nutrients, tundra, species richness, species evenness.Résumé : L'interaction entre les forces ascendantes (ressources) et descendantes (prédateurs) dans la régulation de communautés végétales est depuis longtemps un sujet d'intérêt en écologie. Les expériences factorielles sur le terrain examinant ces facteurs ont été relativement peu nombreuses, mais les méta-analyses récentes fournissent des prédictions qui peuvent être vérifiées au niveau de divers écosystèmes. Nous avons vérifié la prédiction selon laquelle des substances nutritives ajoutées réduiraient la richesse et la régularité des espèces tandis que les activités des herbivores pallieraient ces changements chez deux communautés végétales de toundra après onze ans. Dans la toundra acide humide (TAH), les herbivores ont davantage réduit la richesse dans les parcelles fertilisées lorsque les mammifères étaient présents en comparaison des parcelles fertilisées sans herbivores. Dans les landes sèches (LS), la régularité a été significativement réduite au niveau des parcelles fertilisées seulement lorsque les herbivores étaient présents, fournissant aussi la preuve que les herbivores ont amélioré les changements des communautés végétales en raison de plus grande disponibilité nutritive. La différence de la réponse entre la TAH et les prédictions des méta-analyses semble être déterminée par Betula nana, l'espèce qui a dramatiquement augmenté avec l'ajout de substances nutritives dans la TAH.

Small herbivores with big impacts: Tundra voles (Microtus oeconomus) alter post‐fire ecosystem dynamics

“…Elevated nutrient conditions can also impact plant community composition. Under fertilization experiments, dwarf birch ( Betula nana ) becomes more dominant in MAT tundra (Gough et al, 2012; Gough & Johnson, 2018; Klupar et al, 2021). Our study suggests that selective foraging by tundra voles may play a secondary role to post‐fire nutrient release, further decreasing graminoid abundance.…”

Above- and belowground responses to long-term herbivore exclusion

“…Our sampling is part of an ongoing effort examining how herbivores influence ecosystem dynamics in these long-term exclosures (Gough, Ramsey, and Johnson 2007;Gough et al 2008Gough et al , 2012. Our data show little change from previous samplings, which also show relatively few changes over time (Gough and Johnson 2018), and provide valuable additional time points for this experiment and the examination of ecosystem functions in a changing arctic environment. Interestingly, in a study design similar to ours in another heath community but also including fertilization (Stark and Grellmann 2002), slower nutrient cycling under grazing after seven years of exclusion was reported.…”