Abstract
To understand omnivore function in food webs, we must know the contributions of resources from different trophic levels and how resource use changes through space and time. We investigated the spatial and temporal dynamics of pine (Pinus ponderosa) food webs that included the omnivorous ant, Formica podzolica, using direct observation and stable isotopes. Formica podzolica is a predator of herbivorous and predatory arthropods, and a mutualist with some aphids. Observations in 2001 of foragers showed that in early summer (June) ants fed upon equal parts non-mutualist herbivores (31% prey biomass), mutualist aphids (27%), and predators (42%); ant trophic position was thus between that of primary and secondary predator (trophic level = 3.4). In late summer (September), ant feeding remained relatively constant upon non-mutualist herbivores (53%) and mutualist aphids (43%), but ant feeding upon predators fell (4%), thus shifting ant trophic position to that of a primary predator (trophic level = 3.0). Feeding on honeydew increased from 25% of ants in early summer to 55% in late summer. By increasing the frequency of their interactions with mutualist aphids, ants maintained a constant supply of arthropod prey through the summer, despite a two-thirds decline in arthropod biomass in pine canopies. Stable isotope analysis (δ15N, δ13C) of six pine food webs dispersed over 150 ha placed ant trophic level at 3.3 for early summer in 2002. There was significant variation among these trees in ant trophic position (range 3.2–3.6), but no indication of positive spatial autocorrelation. The combined results from this work shows that, across two years, F. podzolica fed, on average, at or slightly above the trophic position of primary predator, but trophic positioning varied both temporally and spatially by ∼0.4 trophic levels.
Keywords
aphid tending,
canopy,
Cinara,
Essigella,
food web,
Formica podzolica,
intraguild predation,
mutualism,
omnivory,
Schizolachnus,
spatial autocorrelation,
stable isotope
Citation
Mooney, Kailen A.; Tillberg, Chadwick V. 2005. Temporal and spatial variation to ant omnivory in pine forests. Ecology. 86(5): 1225-1235.
