Dung beetle activity did not increase foliar nutrients or performance of established rainforest seedlings

Lina Adonay Urrea-Galeano, Ellen Andresen, Francisco Mora, Ek del-Val and Maribel Nava Mendoza, Instituto de Investigaciones en Ecosistemas y Sustentabilidad, Universidad Nacional Autónoma de México, Morelia, Mexico, Rosamond Coates, Estación de Biología Tropical Los Tuxtlas, Instituto de Biología, Universidad Nacional Autónoma de México, San Andrés Tuxtla, Mexico

Background/Question/Methods
Dung beetles cause physical and chemical changes in the soil when they relocate dung. These changes may have positive effects on plants, as has been shown for temperate crops and grasslands through controlled experiments. However, in tropical forests under natural conditions, these effects remain practically unstudied. We assessed whether dung burial by dung beetles has any positive effect on nutrient assimilation and performance of understory seedlings in a tropical rainforest. We predicted that dung beetle activity would have a positive effect on nitrogen and phosphorous concentrations, growth and seedling survival, and that it would modify seedling resource allocation (favoring allocation to shoot biomass). In Los Tuxtlas Biological Station (Veracruz, Mexico), we performed field experiments with seedlings of six tree species in understory plots where the presence of feces and dung beetles were manipulated. Seedlings were exposed to three treatment levels: feces added and access to beetles, feces added and exclusion of beetles, and without feces or beetles. We analyzed data at two levels: community (pooling all plant species) and individual species.

Results/Conclusions
Dung beetle activity had no positive effect on nutrient concentration or seedling performance for any of the species. Furthermore, at the community level and for two plant species, dung beetle activity had a negative effect on the probability of seedlings growing in height. For one species, dung beetle activity modified resource allocation as predicted. We propose that in rainforests with relatively fertile soils, such as our study site, dung beetle activity may not confer a nutrient boost to seedlings, whereas such an effect might occur under nutrient-poor conditions. Future studies will need to test this hypothesis. Also, future research is needed to determine whether the occurrence of the negative effects found in our study occurs for other species/contexts and identify the mechanisms that cause them. At this point, we have no evidence to assert that dung beetle activity, through its effects on soil attributes, benefits tropical seedlings established in the understory. But neither can we generalize that such effects do not occur in other contexts. Our study shows that reaching conclusions on the ecological functions of dung beetles in tropical forests, based on results obtained through controlled experiments in temperate systems, can be problematic.