Microclimate and seed availability regulate seed harvesting by ants in mediterranean grassland and scrubland.
Azcárate, Francisco1, Kovacs, Eva2, Peco, Begoña1, 1 Universidad Autónoma de Madrid, Madrid, Spain2 Kiskunsag National Park, Kecskemét, Hungary
ABSTRACT- Messor barbarus harvester ants are the main seed predators in Mediterranean grassland and scrubland, and hence constitute a major source of seed loss. Wether harvesting intensity is dependent on seed density or microclimate is relevant for the estimation of the effects on plant populations, and also to interpret which plant strategies are expectable in response to granivory. This research analyses the effect of seed availability and microclimate conditions on harvesting activity by Messor barbarus in Mediterranean grassland and scrubland of central Spain, following a three-stage approach. First, we studied seasonal dynamics of seed removal rates using bait trays placed periodically in ten 20 m x 40 m plots from march to november 1996. Second, we analysed the role of microclimate factors by monitoring activity in 44 colonies between March 1998 and September 1999. Activity was analyzed by different descriptors (activation of the colony, flows of exiting and returning workers, worker speed and success of the foraging expeditions) which were related to internal soil temperature, surface soil temperature and surface relative humidity. Third, we studied the role of resource availability in the same ten plots, by periodically collecting the seed content from the upper soil layer, which was used to predict the abundance of foragers and the total seed harvest. Results show that (i) seed harvesting activity peaks in summer; (ii) microclimatic factors largely suffice to predict the activation of the colonies and the flows of exiting workers, but fail to explain the success of foraging expeditions; and (iii) seed availability is a good predictor of forager abundance and total seed harvest. According to these results, summer must be considered a critical season for seed bank survival, as this season provides the most favourable microclimate conditions for colony activity, and also offers the highest seed availability. We claim that summer seed predation by harvester ants should be considered as a major structuring force in Mediterranean grassland and scrubland, in contrast with the low importance given to this season by previous research.
Key words: granivory, seasonal dynamics, seed removal rates, foraging
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