Summary 1.We analyse diet and propagule selection by the harvester ant Messor barbarus (L.) in Mediterranean grassland and scrubland in central Spain. 2. Diet was estimated by the identification of worker-transported prey in 34 colonies per system type, and compared with seed and fruit availability in the foraging areas. Propagules were characterized by six morphological traits: total weight; weight of seed content; the three main dimensions; and shape. 3. The effect of propagule attributes on selectivity was analysed after transforming data into phylogenetically independent contrasts. Propagules from a small number of species dominate the diet of M. barbarus in the study area, in terms of both frequency and contribution in seed weight. In grassland, prey selection depends on ln(prey length) and ln(prey weight) (R 2 = 0·57). In scrubland, ln(prey length) explains 64% of selection. Long and heavy propagules are preferred. 4. This pattern of selection can be a mere effect of a time-saving foraging strategy, as apparent preference for long propagules can be expected even if workers forage in a non-selective way. 5. Messor ants are likely to play a role in the plant composition of Mediterranean grassland and scrubland, limiting the abundance of long propagules and thus indirectly favouring small-seeded species without dispersal appendages.
We analysed the relationship between seed txaits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same área. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Múltiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small-seeded species without specialised dispersal structures are over-represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and smallseeded species were relatively more abundant in the upper than the lower slope zones. This pattem is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability.
Abstract. We analysed the relationship between seed txaits (weight, shape and dispersal structures) and the abundance and habitat segregation of Mediterranean grassland species. To take into account possible correlations with other plant traits, the study also includes 5 vegetative traits (growth form, plant longevity, clonality, onset of flowering and plant size) of commonly accepted functional importance. Data were recorded for 85 species from dehesa grasslands in central Spain. Species abundance was measured in upper (dry and less productive, high stress) and lower (moist and more productive, low stress) slope zones in the same área. Habitat segregation was estimated using an index based on the relative frequencies of species in upper and lower slope zones. Múltiple regression models were fitted using species, as well as phylogenetically independent contrasts, as data points. Annual small-seeded species without specialised dispersal structures are over-represented in dehesa grasslands. Abundance was negatively related to seed weight in upper slope zones. None of the recorded plant traits were related to abundance in the lower slope zones. Habitat segregation was mainly related to seed weight, but also to some vegetative traits. Annual, early flowering and smallseeded species were relatively more abundant in the upper than the lower slope zones. This pattem is independent of phylogeny. Our results suggest that in dry Mediterranean grasslands, abundance of many species is determined by dispersal (production of numerous small seeds) rather than by competitive ability.
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