Broncano M.J., Riba M., Retana J. (1998) Seed germination and seedling performance of two Mediterranean tree species, holm oak (Quercus ilex L.) and Aleppo pine (Pinus halepensis Mill.): A multifactor experimental approach. Plant Ecology. 138: 17-26.EnlaceDoi: 10.1023/A:1009784215900
A two-level multifactor experimental approach was used to compare seed germination and seedling performance of two Mediterranean tree species: the early successional aleppo pine (Pinus halepensis Mill.) and the late successional holm oak (Quercus ilex L.). In a first experiment germination rate was evaluated under the combined effects of shade, nitrogen availability, and pine or holm oak leaf litter. In a second experiment we tested for the effects of shade, nutrient availability, and litter type on seedling survival, growth and biomass allocation. Holm oak showed higher germination rates under shaded than under unshaded conditions, while Aleppo pine showed no differences between shaded and unshaded conditions. Nitrogen availability and litter type had no significant effect on germination of either species. Both species showed increased RGR, but also higher mortality rates, when grown in an enriched nutrient environment. While Aleppo pine showed no differences in RGR and mortality rate under different shading levels, RGR decreased and mortality increased for holm oak in full light. Increased radiation decreased LAR, SLA and height:diameter ratio, and increased RWR and R/S in both species, although Aleppo pine showed more pronounced changes. Unlike Aleppo pine, holm oak responded to increased nutrient availability by decreasing R/S and increasing LAR. From these results, no seed-seedling conflicts were found in either species, but a trade-off does seem to exist for holm oak between biomass allocation traits deployed in response to increased nutrient availability and radiation. Aleppo pine outperformed holm oak under most environmental conditions tested and showed a wider regeneration niche.
Cerdá X., Retana J. (1998) Interference interactions and nest usurpation between two subordinate ant species. Oecologia. 113: 577-583.EnlaceDoi: 10.1007/s004420050411
Camponotus foreli (Emery) and Cataglyphis iberica (Emery) are two sympatric, subordinate ant species that have been found to fight in attacks that usually conclude with the death of many workers of both species and with nest abandonment by C. iberica. These harassment episodes have been observed in two different areas and over many years of study. No such attacks of C. foreli were observed in the study areas against any other ant species, nor did any other ants attack C. iberica nests, and laboratory confrontations confirmed this specificity. These attacks neither eliminated C. iberica colonies, nor distanced them from C. foreli nests. Moreover, there was no real competition for food between the species: in an experiment where all C. iberica colonies were eliminated from an area, rates of prey and liquid food collection by C. foreli nests in the exclusion zone were similar to those found in the control zone with C. iberica, and the activity rhythms of C. foreli did not change in the absence of C. iberica. The hypothesis of competition for a nest site is more consistent. Both in the laboratory and the field, the most frequent outcome of these aggressive interactions was the occupation of the C. iberica nest by C. foreli. This behavior may be advantageous for C. foreli, because it is much less skilful at excavating than C. iberica. One of the chief concerns of this study is to show that such interference interactions, typical especially of dominant, very aggressive species, are also found between subordinate, apparently nonaggressive species.
Cerdá X., Retana J., Cros S. (1998) Critical thermal limits in Mediterranean ant species: Trade-off between mortality risk and foraging performance. Functional Ecology. 12: 45-55.EnlaceDoi: 10.1046/j.1365-2435.1998.00160.x
1. In Mediterranean ant communities, a close relationship has been found between activity rhythm in the period of maximum activity and position in the dominance hierarchy: subordinate species are active during the day, when conditions are more severe, while dominants are active during the afternoon and the night. 2. Results obtained in this study confirmed that the species foraging at higher temperatures were closer to their critical thermal limits than the species foraging at lower temperatures. 3. This enabled two extreme strategies of foraging in relation to temperature to be distinguished: (1) heat-intolerant ant species behaved as risk-averse species, foraging at temperatures very far from their critical thermal limits; and (2) heat-tolerant ant species behaved as risk-prone species, foraging very near their critical thermal limits and running a high heat mortality risk. 4. Heat-tolerant species benefited from this strategy by having better foraging performance at high temperatures. 5. This wide range of thermal niches may be one reason why Mediterranean ant faunas are so diverse in the face of limited diversity in vegetation and habitat structure: the daily range of temperature may be sufficiently great to meet the requirement both of heat-adapted and cold-adapted species as well as a spectrum of intermediate forms.
Cerdá X., Retana J., Manzaneda A. (1998) The role of competition by dominants and temperature in the foraging of subordinate species in Mediterranean ant communities. Oecologia. 117: 404-412.EnlaceDoi: 10.1007/s004420050674
In this paper we test the influence of temperature and interference competition by dominant species on the foraging of subordinate species in Mediterranean ant communities. We have analyzed the changes in resource use by subordinate species in plots with different abundances of dominant ants, and in different periods of the day and the year, i.e., at different temperatures. The expected effects of competition by dominant species on foraging of subordinates were only detected for two species in the number of baits occupied per day, and for one species in the number of foragers at pitfall traps. In all three cases, subordinate species were less represented at baits or in traps in plots with a high density of dominants than in plots with a medium or low density of dominants. The number of workers per bait, and the foraging efficiency of subordinate species did not differ in plots differing in dominant abundance. Daily activity rhythms and curves of temperature versus foraging activity of subordinate species were also similar in plots with different abundance of dominant species, indicating no effect of dominants on the foraging times of subordinates. Instead, temperature had a considerable effect on the foraging of subordinate species. A significant relationship was found between maximum daily temperature and several variables related to foraging (the number of foragers at pitfall traps, the number of baits occupied per day, and the number of workers per bait) of a number subordinate species, both in summer and autumn. These results suggest that the foraging of subordinate ant species in open Mediterranean habitats is influenced more by temperature than by competition of dominants, although an effect of dominants on subordinates has been shown in a few cases. In ant communities living in these severe and variable environments, thermal tolerance reduces the importance of competition, and the mutual exclusion usually found between dominant and subordinate species appears to be the result of physiological specialization to different temperature ranges.
Cerdà X, Retana J, Cros S (1998) Prey size reverses the outcome of interference interactions of scavenger ants. Oikos 82:99-110.
Savé R, de Herralde F, Retana J, Espelta JM, Biel C (1998) Effect of elevated CO2 on plant productivity and hardening under Mediterranean conditions. The Earth's Changing Land GCTE-LUCC Open Science Conference on Global Change Abstracts. Institut Cartogràfic de Catalunya, Barcelona, pp. 52-53.
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