Harvester ants strongly limit seed dispersal by sheep in mediterranean grasslands.
Manzano, Pablo*,1, Azcárate, Francisco M.1, Peco, Begoña1, Malo, Juan E.1, 1 Universidad Autónoma de Madrid., Madrid, Spain
ABSTRACT- Endozoochory is considered one of the main seed dispersal mechanisms in Mediterranean type systems. In particular, sheep are regarded as an efficient endozoochory vector in grasslands of the Mediterranean basin. On the other hand, harvester ants are known to predate a high proportion of the seed production in these systems. To date, there is no research on the interaction between both processes. Here we present data from a one-year survey aimed to monitore secondary predation on sheep faeces by Messor barbarus (L.) harvester ants. We excluded ants from twenty 40-cm diameter plots, and placed inside 20 pellets distributed in two treatments: 10 entire pellets and 10 disgregated ones, simulating sheep trampling. The experiment design was completed with a similar series of 20 control plots, accesible to ant activity. The pellets were placed once a month and were left for 48 hours. Faeces were then collected, counted and weighted. Our results show: (i) secondary predation shows a clearly seasonal pattern, which peaks in summer months; (ii) disgregated pellets are preferred to entire ones and (iii) in the months of higher predation rates, ants deplete near 90% of the disgregated excrement mass and more than 70% of the entire ones. These findings confirm that secondary predation by ants has been underestimated in the studied system. The phenomenon is likely to have a major impact on seed dispersal by sheep due to at least two facts: First, summer is a critical time in the survival of Mediterranean annual seeds, after spring production and before autumn germination. Second, summer is also the peak time in sheep mediated endozoochory. Hence, we conclude that secondary predation by ants on faeces seriously interferes in the success of herbivore seed dispersal in Mediterranean grasslands.
Key words: seed predation, secondary predation, endozoochory
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