Alexander V. Christianini

The role of ants in the removal of non-myrmecochorous diaspores and seed germination in a neotropical savanna

Ants frequently interact with non-myrmecochorous diaspores on the ground in neotropical savannas. Our objective was to quantify the removal rate of these diaspores by ants and vertebrates in order to test the predator avoidance hypothesis, and to test how diaspore traits influence removal by ants and dispersal distance. We also investigated whether seed cleaning (removal of fruit matter simulating ant activity) can influence seed germination. We performed removal experiments with nine diaspore species in a reserve of cerrado savanna in south-east Brazil. Considerable differences in removal rates were found among the nine species. We found a positive linear relationship between lipid content and removal rates for five diaspore species. Vertebrate predation pressure was low for most species, limiting the benefits that ants can provide to plants to escape predators. Ants displace diaspores up to 25 m, which may increase the chance of a seed hitting a safe site. Smaller diaspores attain longer distances of dispersal than large ones. Seed cleaning increased the germination rate for five out of six species tested in greenhouse experiments. Ant activity can have relevant and possibly lasting effects on seed fate of plants adapted for vertebrate dispersal in the cerrado savanna.

The relevance of ants as seed rescuers of a primarily bird-dispersed tree in the Neotropical cerrado savanna.

The scale at which seed dispersal operates has many implications for the spatial patterns of plant recruitment and diversity. We investigated the effect of short- (ants) and long-distance (birds) seed dispersal of the fleshy-fruited melastome, Miconia rubiginosa, in the Brazilian savanna. We estimated the contribution of dispersal vectors to the removal of the fruit crop from the canopy (birds), and once seeds have reached the cerrado floor (ants) over two fruiting seasons. Birds (13 species) removed up to 23.7% of the fruit crop from the crown, but dropped a substantial proportion of fruits beneath the parent plant. Birds removed a greater proportion of fruits from trees producing large fruit crops, as predicted by the fruit crop size hypothesis. However, up to 18.9% of the fruit crop fell beneath the parent plant as ripe fruit. Most fallen fruits were removed by ants (seven genera), which are likely to play a relatively important role in terms of the quantity of seeds dispersed, especially for plants producing small fruit crops (a conceptual model is presented). Birds and ants did not influence seed germination, but they differ in terms of the spatial scale of dispersal and deposition patterns. Ants probably play an important role in the local population dynamics of Miconia, whereas birds are responsible for long-distance dispersal associated with the colonization of new patches and metapopulation dynamics. By removing seeds from bird droppings, ants may also reshape at a finer scale the seed rain generated by primary dispersers. Indeed, seedlings and saplings of Miconia are more frequently found around leaf-cutter ant nests than in control areas away from ant nests or around large Miconia trees. The quantitative component of dispersal effectiveness by ants acting as “rescuers” of seeds that fail to be dispersed, or fall under parent trees, is probably more important than currently recognized in other systems.

Edge effects decrease ant-derived benefits to seedlings in a neotropical savanna

Edge effects may lead to changes in mutualistic plant–animal interactions, such as seed dispersal, that are critical to plant regeneration. However, research into edge effects is neglected in the Brazilian cerrado, the largest neotropical savanna. We evaluated the consequences of edge effects in the cerrado for the regeneration of Erythroxylum pelleterianum (Erythroxylaceae), a shrub that benefits from seed dispersal by ants. We compared air temperature, relative humidity, and vapor pressure deficit, as well as the frequency and outcome of ant–diaspore interactions between cerrado edges and interiors. The inner portion of cerrado was likely to be moister than its borders, but seed production and germination did not differ between edge and interior of cerrado. Ants removed more seeds near fragment edges than at the interior. However, Myrmicinae ants dominated ant–fruit interactions at edges. These ants are likely to provide few benefits to the seeds. Seedlings of E. pelleterianum growing close to Ponerinae ant nests showed higher survival than seedlings growing away from nests in the interior of cerrado, but such effect disappeared near edges. Widespread seedling mortality due to a higher evaporative demand at edges may partially account for this effect. Furthermore, Ponerinae’s nests also showed a lower residence time near edges, decreasing possible benefits derived from ant colony activity such as nutrient enrichment and protection against insect herbivores. Edge effects could change the structure and dynamics of vegetation in cerrado fragments, due in part to the collapse of the mutualistic interactions demonstrated here.

The role of very small fragments in conserving genetic diversity of a common tree in a hyper fragmented Brazilian Atlantic forest landscape

In hyper fragmented biomes, conservation of extant biota relies on preservation and proper management of remnants. The maintenance of genetic diversity and functional connectivity in a landscape context is probably key to long-term conservation of remnant populations. We measured the genetic diversity in seedlings and adults of tree Copaifera langsdorffii and evaluated whether edge and density-dependent effects drive natural regeneration in a set of very small and degraded Brazilian Atlantic forest fragments. We evaluated the role of small remnants in the conservation of genetic diversity in a hyper fragmented landscape and discuss the challenge of long-term population sustainability of such altered habitats. High genetic diversity in adults indicated these fragments are valuable targets for C. langsdorffii in situ conservation, but both genetic diversity and divergence among patches decreased in seedlings. In our landscape, regeneration increased as it neared edges and adults; suggesting this population is resilient to fragmentation. However, at a broader scale, current levels of gene flow have not been sufficient to prevent the loss of genetic diversity across generations. Restoration plans, even at a small scale, are necessary to promote fragment connectivity and spatially expand opportunities for the fairly restricted gene flow observed in this severely fragmented Brazilian Atlantic forest region.

Fauna in decline: Meek shall inherit

ALTHOUGH LARGE ANIMALS capture our attention and directly or indirectly play essential ecological and ecosystem functions (1), it is the sheer diversity and abundance of invertebrates that make them “run the world” (2). A recent Review presented evidence of human-induced pervasive defaunation in terrestrial ecosystems (“Defaunation in the Anthropocene,” R. Dirzo et al., special section on Vanishing Fauna, 25 July, p. 401). Given that larger animals are more likely to go extinct [Dirzo et al. and (3)], invertebrates may soon play even more important roles in the dynamics of ecosystems than they already do (1, 4). Many of the vertebrate species that have been locally extirpated or are in decline are herbivores, seed dispersers, or granivores