Abstract
Simple SummaryAlpine grasslands are among the few terrestrial habitats not obviously dominated by ants. Yet few studies have addressed how ant communities change along tree-line ecotones on mountains. We combined five survey methods to assess ant assemblages along the tree line at five mountains in the south-central Alps of Italy. Ant species richness peaked directly at the tree line, but this was not due to a mixing of forest with grassland species. In subalpine forest and at the tree line, ant assemblages were dominated by mound-building red wood ants. Community composition and functional species traits indicated competition as a potential effect of community assembly in subalpine forest. Further, habitat features such as elevation, dwarf shrub cover, and the extent of a soil humus layer shaped species composition of ant communities around the tree line.Ants are crucial for the functioning of many terrestrial ecosystems, but detailed knowledge of their ecological role is often lacking. This is true for high mountains where a steep environmental gradient exists from mountainous forest, densely populated by ants, to grassland habitats above the tree line, harboring a sparse ant community. We assessed ant communities in and around the tree line ecotone on five slopes in the southern-central Alps, focusing on their species diversity, community composition, and functional dimensions. Species richness and functional diversity were highest directly at the ecotone. Ant community composition was shaped by elevation and shrub cover. Further, the abundance of the dominant mound-building red wood ants (Formica s. str.) influenced the community composition of the subordinate species. We conclude that over the tree line ecotone a shift in predominance from biotic limitations in the forest to abiotic filters in the alpine environment takes place.
Highlights
Ants have a predominantly thermophile geographic distribution and their centers of origin as well as of current diversity are located in the tropics [1,2]
Reymond et al [5] observed a reduction of functional diversity with increasing elevation in the European Alps, and a later study [13] found a similar pattern for ant phylogenetic diversity along three elevation gradients in Europe and North America
In a previous study over the same alpine tree line ecotones, we showed that the availability of trophobionts and the abundance of wood ants decreased from the forest to the alpine habitat above the tree line [31]
Summary
Ants have a predominantly thermophile geographic distribution and their centers of origin as well as of current diversity are located in the tropics [1,2]. Since temperature is the main limiting factor for the position of the tree line ecotone [22] and for the occurrence of ants at high elevations, elevational distributions of trees as well as ants will probably change in the few decades [23]. The two forces that shape ant community composition over this ecotone are expected to shift from a prevalence of biotic competition among ants (niche partitioning effect) in the forest to environmental abiotic limitation above the tree line (niche filtering effect). SSaammpplliinngg wwaass ccoonndduucctteedd oonn ffiivvee ssllooppeess ooff tthhee ssoouutthheerrnn ppaarrtt ooff tthhee CCeennttrraall ssiilliicceeoouuss AAllppss,, iinn SSoouutthh TTyyrrooll ((IIttaallyy)). A list of all vascular plant species for each site was compiled and their Landolt indicator values of light and soil humus were extracted from [37] (Table S1; for further habitat descriptors and the plant species lists of each site see [38])
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