The keystone roles of mega-fauna in many terrestrial ecosystems have been lost to defaunation. Large predators and herbivores often play keystone roles in their native ranges, and some have established invasive populations in new biogeographic regions. However, few empirical examples are available to guide expectations about how mega-fauna affect ecosystems in novel environmental and evolutionary contexts. We examined the impacts on aquatic ecosystems of an emerging population of hippopotamus (Hippopotamus amphibus) that has been growing in Colombia over the last 25 yr. Hippos in Africa fertilize lakes and rivers by grazing on land and excreting wastes in the water. Stable isotopes indicate that terrestrial sources contribute more carbon in Colombian lakes containing hippo populations, and daily dissolved oxygen cycles suggest that their presence stimulates ecosystem metabolism. Phytoplankton communities were more dominated by cyanobacteria in lakes with hippos, and bacteria, zooplankton, and benthic invertebrate communities were similar regardless of hippo presence. Our results suggest that hippos recapitulate their role as ecosystem engineers in Colombia, importing terrestrial organic matter and nutrients with detectable impacts on ecosystem metabolism and community structure in the early stages of invasion. Ongoing range expansion may pose a threat to water resources.

RESUMENUna forma fundamental y efectiva de visualizar los patrones y propiedades dinámicas de una comunidad, es a través de las redes tróficas, las cuales son entendidas como el mapa de las interrelaciones entre la estructura de las comunidades con los procesos que ocurren dentro del ecosistema. En este estudio se describe una red trófica acuática, conformada por invertebrados acuáticos, detritus y perifiton, asociada con la planta Egeria densa. Se realizaron muestreos en periodos de transición y lluvias, en cuatro estaciones en la zona litoral del Lago de Tota. En cada sitio se recolectaron tres muestras de 300 g de material vegetal. Se midieron in situ los siguientes parámetros: conductividad eléctrica, pH, oxígeno disuelto y temperatura. Por medio de información bibliográfica y en algunos casos mediante observación, se asignaron los hábitos alimenticios. Finalmente, se desarrolló un modelo de trama trófica tipo estático. Se registraron 112866 individuos distribuidos en 26 taxones que fueron clasificados según sus hábitos alimenticios en: dos trofoespecies tope, 16 intermedias y dos basales. Esta red trófica presentó una conectancia media de 0,13 y una densidad de uniones de 2,95. En conjunto, estos resultados muestran a E. densa como generadora de detritus y sustrato de perifiton, consideradas en el modelo como trofoespecies basales que sustentan la comunidad. Además de la presencia de 10 trofoespecies consumidoras primarias que generan alta redundancia ecológica y a la vez garantizan el flujo de recursos hacia niveles tróficos superiores. ABSTRACTA fundamental and effective way to view the dynamic patterns and properties of a community is through the trophic network, understood as a map of the interrelationships between the structure of an ecological community and the processes that occur within the ecosystem. In this study, we describe an aquatic food web composed by aquatic invertebrates, detritus and periphyton associated with the plant Egeria densa. Samples were collected during the transitional and rainy season at four stations in the littoral zone of Tota Lake. Three samples (300 g each) of plant material at every point were randomly collected. Electrical conductivity, pH, dissolved oxygen and temperature were measured in situ. The eating habits were assigned by means of bibliographic information and in some cases observation. Finally, we developed a static type model that included 112866 individuals distributed among 26 taxon, which were classified according to their eating habits in: two top trophic species, 16 intermediate and two basal species. We found that this trophic network represents a connectance of 0.13 and density links of 2.95. The model also revealed that E. densa is a basal trophic species. Together, these results demonstrate that E. densa is an important generator of detritus and periphyton substrate, supporting other trophic levels in the community. Moreover, the presence of ten trophic species of primary consumers generates high ecological redundancy that at the same time guarantees t...