Seed dispersal networks are often organized in nested structures in a way that a few core species can disproportionally affect the remaining species in a network, in both the ecological and evolutionary sense. Yet, the relative importance of core and peripheral species has not been properly tested in seed dispersal network studies. We determined core species from 10 local seed dispersal networks composed by fleshy-fruited plants and frugivorous birds. Each of those local quantitative networks was characterized with a core quality value, a core score for each species and a threshold value between core and peripheral species. From a total of 52 bird and 69 plant species that interacted in the study area, only 8 and 15, respectively, were identified as core. Each local network had a core that comprised a small number of birds and plants, always lower than 30% of the interacting species. There was no difference in the quantitative component of seed dispersal effectiveness (QC) provided by the frugivorous bird assemblage to plant functional groups clustered according to their growth form and fruit characteristics. Core birds dispersed seeds from each of these plant functional groups with a higher QC than peripheral species. Thus, we empirically demonstrate for the first time that seed dispersal networks at a regional scale have a small core set of fruit-eating birds, upon which heavily rely most fleshy-fruited plants for their seed removal. Hence, the activity of just a few core frugivores could deeply impact the demography of an entire assemblage of fleshy-fruited plants.

a b s t r a c tMovement patterns of Akodon azarae and Calomys venustus in linear habitats were studied in relation to sex, season and abundance. Movement distances were measured by seasonal capture-mark-recapture samples from November 2005 to August 2007 in field borders in agro-ecosystems of Central Argentina. Movement distances in A. azarae varied by season and sex. During the breeding period, movement distances of males were longer than those of females, meanwhile in non-breeding period movement distances became similar between sexes. The main effect of sex on movement distances is consistent with the promiscuous / polygynous mating system suggested for A. azarae. Abundance and sex were the most important factor affecting movement distances in C. venustus, males moved larger distances than females and the higher the abundance, the shorter the movement distances of individuals. The results of this study may have implications in the management of non-crop linear habitats in agriculture landscapes since the maintenance of field borders would be important to support rodent populations that are important food sources for birds and mammals.

Patterns of space use and sexual dimorphism are frequently used to infer mating systems. We examined body size and home range size and intra-and intersexual overlap degree in order to elucidate mating strategy of Akodon azarae males. We studied spacing patterns using 113 and 129 home ranges established by males and females, respectively, in four 0.25 ha enclosures during the breeding season. Significant differences between sexes in home range size and overlap degree were found. Male home ranges were always larger than those of females. We observed exclusive space use among males and among females. Considering only those males that shared their home ranges with females, average intersexual overlap value was about 50%. Males mainly overlap their home ranges with home ranges of two or three females. Significant differences in body size were found between males and females, with males being larger. We concluded that space use and sexual dimorphism in this species is consistent with patterns characteristic of polygynous rodents, and we propose a polygynous system in A. azarae.

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Theoretical models predict that the relative importance of competition and facilitation vary inversely along gradients of abiotic stress, with facilitation dominating under harsh conditions (the so called ''stress-gradient hypothesis''). To date, very few studies have tested this hypothesis in the framework of succession. Moreover, the generality of the hypothesis is currently under debate and the necessity to examine responses at the community level and using different stress levels has been emphasized. We evaluate the mechanisms of succession across the emersion gradient in a rocky shore system. After complete clearing of experimental plots at four shore heights, we removed two separate components of the assemblage: ephemeral green algae, early colonists and canopy-forming species, main space holders in undisturbed assemblages. We aimed to test two hypotheses: (a) the net effect of ephemeral algae on canopy-forming species shift along the emersion gradient according to the predictions of the stress-gradient hypothesis and (b) the net effect of the canopy-forming group on associate species change, from neutral/negative to positive, as succession progresses and plants grow, especially at the highest shore heights. Our results did not support the predictions of the stress gradient hypothesis in the framework of succession. We did detect changes through time in the effect of canopy-forming species, from negative to positive, but this was not dependent on the shore height. The outcome of interactions depended on the identity and life-stage of the species. Moreover, indirect interactions among species could also create a less predictable relationship between successional mechanisms and the environmental gradient.

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