Long-term aboveground and belowground consequences of red wood ant exclusion in boreal forest

Abstract

Despite their ubiquity, the role of ants in driving ecosystem processes both aboveground and belowground has been seldom explored, except within the nest. During 1995 we established 16 ant exclusion plots of approximately 1.1 x 1.1 m, together with paired control plots, in the understory layer of a boreal forest ecosystem in northern Sweden that supports high densities of the mound-forming ant Formica aquilonia, a red wood ant species of the Formica rufa group. Aboveground and belowground measurements were then made on destructively sampled subplots in 2001 and 2008, i.e., 6 and 13 years after set-up. While ant exclusion had no effect on total understory plant biomass, it did greatly increase the relative contribution of herbaceous species, most likely through preventing ants from removing their seeds. This in turn led to higher quality resources entering the belowground subsystem, which in turn stimulated soil microbial biomass and activity and the rates of loss of mass and carbon (C) and nitrogen (N) from litter in litterbags placed in the plots. This was accompanied by losses of approximately 15% of N and C stored in the humus on a per area basis. Ant exclusion also had some effects on foliar stable isotope ratios for both C and N, most probably as a consequence of greater soil fertility. Further, exclusion of ants had multitrophic effects on a microbe-nematode soil food web with three consumer trophic levels and after six years promoted the bacterial-based relative to the fungal-based energy channel in this food web. Our results point to a major role of red wood ants in determining forest floor vegetation and thereby exerting wide-ranging effects on belowground properties and processes. Given that the boreal forest occupies 11% of the Earth's terrestrial surface and stores more C than any other forest biome, our results suggest that this role of ants could potentially be of widespread significance for biogeochemical nutrient cycling, soil nutrient capital, and sequestration of belowground carbon.

The mixed effects of experimental ant removal on seedling distribution, belowground invertebrates, and soil nutrients

“…Indeed, there is some evidence that ants play key roles in shaping plant population dynamics (Kalisz et al 1999, Rodriguez-Cabal et al 2009), altering arthropod community structure both above and belowground (Wimp and Whitham 2001, Moya-Laraño and Wise 2007, Wardle et al 2011, Sanders and Frank van Veen 2011, or modulating ecosystem processes (Folgarait 1998). However, surprisingly few studies have experimentally removed ants from an intact understory community to assess simultaneously their long-term impacts on plant population structure, the structure of belowground arthropod communities, and soil nutrient dynamics (but see Wardle et al 2011). Where ants have been removed, the effort has tended to focus on behaviorally dominant species (see Schmitz et al 2000, Gibb and Hochuli 2004, Wardle et al 2011), which may not necessarily be the species with the greatest potential community effects.…”

“…However, surprisingly few studies have experimentally removed ants from an intact understory community to assess simultaneously their long-term impacts on plant population structure, the structure of belowground arthropod communities, and soil nutrient dynamics (but see Wardle et al 2011). Where ants have been removed, the effort has tended to focus on behaviorally dominant species (see Schmitz et al 2000, Gibb and Hochuli 2004, Wardle et al 2011), which may not necessarily be the species with the greatest potential community effects. No study to date has examined the cascading effects of removing seed dispersing ants on the plants they disperse and on belowground processes.…”

“…Ants prey upon soil-and litter-dwelling invertebrates, and, at least in come cases, affect the abundance of particular soil taxa and the composition of soil communities (Wagner et al 1997, Laakso 1999, Laakso and Setala 2000, Lenoir et al 2003, Wilson 2005, MoyaLorano and Wise 2007, Wardle et al 2011, Sanders and Frank van Veen 2011. Ants may indirectly influence soil nutrients by suppressing populations of microbe-feeding mites and Collembolans.…”

Manna from heaven: Refuse from an arboreal ant links aboveground and belowground processes in a lowland tropical forest

“…The role of social insect nutrient translocation at ecosystem-level spatial scales has received surprisingly little attention (Wagner 1997, Whitford 2000, O'Dowd et al 2003, Freymann et al 2010, Wardle et al 2011) despite other well documented excretion impacts on ecosystem function by consumers occurring in high densities (McNaughton et al 1997, Beard et al 2002, Feeley 2005, McIntyre et al 2008, Atkinson et al 2013. In kind and similar to insect frass additions in temperate forests (Lovett and Ruesink 1995, Reynolds and Hunter 2001, Kagata and Ohgushi 2012, our common garden experiment confirmed the stimulatory effects of A. trigona refuse on artificial and natural substrate decomposition (Fig.…”

“…However, surprisingly few studies have examined their ecosystem-level impacts (Wagner 1997, Whitford 2000, O'Dowd et al 2003, Freymann et al 2010, Wardle et al 2011, despite their prominence as consumers in many systems including tropical forest canopies (Fittkau and Klinge 1973) where they dominate in terms of numbers and animal biomass (Tobin 1997, Dial et al 2006. Ants have considerable promise as conduits between canopy and litter; they are voracious consumers that concentrate excreta, carcasses, and other 'trash' into refuse piles in or near their nests, which can create nutrient hot spots that increase nutrient mineralization and modify soil biota communities (Rojas 1989, Wagner 1997, Weiss 2006, Fox-Dobbs et al 2010.…”