RESUMO
Understanding how biodiversity loss influences plant litter decomposition-that is, the biologically mediated conversion of coarse to fine particulate organic matter-is crucial to predict changes in the functioning of many stream ecosystems, where detrital food webs are dominant. Rates of litter decomposition are influenced by detritivore diversity, but the mechanisms behind this relationship are uncertain. As differences in detritivore body size are a major determinant of interspecific interactions, they should be key for predicting effects of detritivore diversity on decomposition. To explore this question, we manipulated detritivore diversity and body size simultaneously in a microcosm experiment using two small (Leuctra geniculata and Lepidostoma hirtum) and two large detritivore species (Sericostoma pyrenaicum and Echinogammarus berilloni) in all possible 1-, 2- and 4-species combinations, and litter discs of Alnus glutinosa. We expected that larger species would facilitate smaller species through the production of smaller litter fragments, resulting in faster decomposition and greater growth of smaller species in polycultures containing species of different body size. To examine this hypothesis, we used a set of "diversity-interaction" models that explored how decomposition was affected by different interspecific interactions and the role of body size, and quantified the magnitude of such effect through ratios of decomposition rates and detritivore growth between polycultures and monocultures. We found a clear positive effect of detritivore diversity on decomposition, which was mainly explained by facilitation and niche partitioning. Facilitation of small animals by larger ones was evidenced by a 12% increase in decomposition rates in polycultures compared to monocultures and the higher growth (20%) of small species, which partly fed on fine particulate organic matter produced by larger animals. When the large species were together in polycultures, decomposition was enhanced by 19%, but there were no changes in growth; niche partitioning was a plausible mechanism behind the increase in decomposition rates, as both species fed on different parts of litter discs, only one species being able to eat less palatable parts. Our study demonstrates that interspecific differences in body size should be taken into account in diversity-decomposition studies. Future studies should also consider differences in species' vulnerability to extinction depending on body size and how this might affect ecosystem functioning in different scenarios of detritivore diversity and more complex food webs.
Assuntos
Ecossistema , Folhas de Planta , Animais , Biodiversidade , Cadeia Alimentar , RiosRESUMO
The majority of insulin-like growth factor (IGF)-I and IGF-II circulate in the serum as a complex with the insulin-like growth factor binding protein (IGFBP)-3 or IGFBP-5, and an acid-labile subunit (ALS). The function of ALS is to prolong the half-life of the IGF-I-IGFBP-3/IGFBP-5 binary complexes. Fourteen different mutations of the human IGFALS gene have been identified in 17 patients, suggesting that ALS deficiency may be prevalent in a subset of patients with extraordinarily low serum levels of IGF-I and IGFBP-3 that remain abnormally low upon growth hormone stimulation. Postnatal growth was clearly affected. Commonly, the height standard deviation score before puberty was between -2 and -3, and approximately 1.4 SD shorter than the midparental height SDS. Pubertal delay was found in 50% of the patients. Circulating IGF-II, IGFBP-1, -2 and -3 levels were reduced, with the greatest reduction observed for IGFBP-3. Insulin insensitivity was a common finding, and some patients presented low bone mineral density. Human ALS deficiency represents a unique condition in which the lack of ALS proteins results in the disruption of the entire IGF circulating system. Despite a profound circulating IGF-I deficiency, there is only a mild impact on postnatal growth. The preserved expression of locally produced IGF-I might be responsible for the preservation of linear growth near normal limits.