Testing food web theory in a large lake: The role of body size in habitat coupling in Lake Michigan.

The landscape theory of food web architecture (LTFWA) describes relationships among body size, trophic position, mobility, and energy channels that serve to couple heterogenous habitats, which in turn promotes long-term system stability. However, empirical tests of the LTFWA are rare and support differs among terrestrial, freshwater, and marine systems. Further, it is unclear whether the theory applies in highly altered ecosystems dominated by introduced species such as the Laurentian Great Lakes. Here, we provide an empirical test of the LTFWA by relating body size, trophic position, and the coupling of different energy channels using stable isotope data from species throughout the Lake Michigan food web. We found that body size was positively related to trophic position, but for a given trophic position, organisms predominately supported by pelagic energy had smaller body sizes than organisms predominately supported by nearshore benthic energy. We also found a hump-shaped trophic relationship in the food web where there is a gradual increase in the coupling of pelagic and nearshore energy channels with larger body sizes as well as higher trophic positions. This highlights the important role of body size and connectivity among habitats in structuring food webs. However, important deviations from expectations are suggestive of how species introductions and other anthropogenic impacts can affect food web structure in large lakes. First, native top predators appear to be flexible couplers that may provide food web resilience, whereas introduced top predators may confer less stability when they specialize on a single energy pathway. Second, some smaller bodied prey fish and invertebrates, in addition to mobile predators, coupled energy from pelagic and nearshore energy channels, which suggests that some prey species may also be important integrators of energy pathways in the system. We conclude that patterns predicted by the LTFWA are present in the face of species introductions and other anthropogenic stressors to a degree, but time-series evaluations are needed to fully understand the mechanisms that promote stability.

Early impacts of the largest Amazonian hydropower project on fish communities.

Hydropower is a threat to freshwater fishes. Despite a recent boom in dam construction, few studies have assessed their impact on mega-diverse tropical rivers. Using a before-after study design, we investigated the early impacts of the Belo Monte hydroelectric complex, the third-largest hydropower project in the world, on fishes of the Xingu River, a major clear-water tributary of the lower Amazon. We explored impacts across different river sectors (upstream, reservoir, reduced flow sector, and downstream) and spatial scales (individual sectors vs. all sectors combined) using joint species distribution models and different facets of diversity (taxonomic, functional, and phylogenetic). After 5 years of the Belo Monte operation, species richness declined ~12% in lentic and ~16% in lotic environments. Changes in abundance were of less magnitude (<4%). Effects were particularly negative for species of the families Serrasalmidae (mainly pacus), Anostomidae (headstanders), Auchenipteridae, and Pimelodidae (catfishes), whereas no taxonomic group consistently increased in richness or abundance. The reservoir and downstream sectors were the most impacted, with declines of ~24-29% in fish species richness, overall reductions in fish body size and trophic level, and a change in average body shape. Richness and abundance also declined in the reduced river flow, and changes in size, shape, and position of fins were observed. Relatively minor changes were found in the upstream sector. Variation in functional and phylogenetic diversity following river impoundment was subtle; however, across sectors, we found a reduction in functional divergence, indicating a decline in the abundance of species located near the extremities of community functional space. This may be the first sign of an environmental filtering process reducing functional diversity in the region. Greater changes in flow and habitats are expected as hydropower operations ramp up, and continued monitoring is warranted to understand the full scope and magnitude of ecological impacts.

Can ancestry and morphology be used as surrogates for species niche relationships?

Species interactions are difficult to quantify, and, consequently, many studies have used species traits and phylogeny as proxies under an assumption of niche conservatism (i.e., closely related and morphologically similar species should have similar niches). However, few studies have investigated whether niches actually are conserved within and across diverse communities. Here, we tested the degree to which phylogenetic relatedness and morphological similarity predict diets and stable isotopic ratios (δ 15N and δ 13C), two common descriptors of the trophic niche, in fish assemblages of two small streams in the Neotropics. We also tested the strength of the association between isotopic ratios and diet composition and found significant correlations implying that isotopic signals reveal trophic structure despite error associated with estimates of trophic enrichment and variation associated with tissue preservation, metabolism, and other factors affecting isotopic ratios. Morphological traits yielded a significant phylogenetic signal, and both morphological traits and phylogeny were correlated with diet composition, with morphological traits being a stronger predictor. We infer that functionally relevant morphological traits of fish can be used to infer trophic niches for certain kinds of questions and analyses when trophic data are lacking. However, we highlight that using phylogenetic and morphological data in combination with dietary and/or isotopic data can improve resolution of assemblage trophic structure and niche diversification.

Fish metacommunity structure in Caño Maraca, an important nursery habitat in the Western Llanos of Venezuela

We investigated spatial and seasonal variation of fish assemblages of Caño Maraca, a creek in Venezuela's Western Llanos, a region with strong wet-dry seasonality. Fishes were surveyed over a 19-year period at three sites along the longitudinal gradient: a headwater site with a narrow channel, a middle site with shallow channels traversing a seasonal wetland, and a lower site where the channel has higher banks. Assemblage composition and presence of species with juveniles and various life history strategies were compared during wet and dry seasons. Overall, fish species richness was lowest at the headwater site and highest at the downstream site. During the wet season, however, species richness is greatest at the middle site, a pattern associated with migration into the site for reproduction and use of the wetland as a nursery. During the dry season, species richness is greater at the downstream site where habitat quality is sufficient to provide suitable habitat for many species. Fish movements and population dynamics in Caño Maraca respond to seasonal environmental changes, and the fish metacommunity appears influenced by species sorting (habitat selection), mass effects (source-sink dynamics), patch dynamics (interspecific differences in colonization and species interaction) as well as random factors (dry-season strandings).(AU)

Investigamos la variación espacial y temporal de los ensambles de peces en el Caño Maraca, un arroyo localizado en los Llanos Occidentales de Venezuela, región caracterizada por una fuerte estacionalidad húmedo-seco. La ictiofauna fue evaluada en intervalos durante un período de 19 años, en tres sitios a lo largo del gradiente fluvial longitudinal: El primer sitio en la cabecera con un canal estrecho e hidrología estable, el segundo, intermedio con canales poco profundos atravesando un humedal estacional y el ultimo en la zona baja donde el canal tiene bancos más altos. La composición del ensamblaje y la presencia de especies con juveniles y diversas estrategias de historia de vida fueron comparadas durante las estaciones húmedas y secas. En general, la riqueza de especies de peces fue menor en el sitio de la cabecera y más alta en el sitio aguas abajo. Sin embargo, durante la época húmeda, la riqueza de especies fue mayor en el sitio central, patrón asociado con la reproducción de peces y el uso del humedal como criadero. Durante la época seca, la riqueza de especies fue mayor en la zona baja donde la calidad del hábitat es suficiente para proporcionar un hábitat adecuado para muchas especies. El movimiento de los peces y la dinámica poblacional en Caño Maraca están relacionadas a cambios ambientales estacionales. Adicionalmente la metacomunidad de peces parece estar influenciada por el ordenamiento de especies (selección de hábitat), efectos de masas (dinámica fuente-sumidero), dinámica de parches (involucrando diferencias interespecíficas en la tasa de colonización y capacidad competitiva) y factores aleatorios (relacionados con la duración de la estación seca).(AU)

Seasonal variation in fish trophic networks in two clear-water streams in the Central Llanos region, Venezuela

Food webs are not static entities; consumer resource interactions vary in both time and space, which complicates depiction and comparisons of food web structures. We estimated fish assemblage structure and diets in two clear-water streams in the Venezuelan Llanos region (Charcote and Charcotico) and constructed trophic networks (sub-webs defined by fishes as the principal consumers) during four periods of the annual flood pulse. As stream conditions changed from high-water to low-water, we hypothesized that: 1) the piscivore-non-piscivore ratio would increase; 2) dietary diversity would decrease; 3) interspecific dietary overlap would decline; 4) fewer allochthonous food items would be consumed; and 5) food-web connectance would increase. The piscivore-non-piscivore abundance ratio was higher in both streams during the low-water period. Dietary diversity declined as water levels dropped and availability of aquatic habitats and resources declined, but interspecific dietary overlap was not lower. Contrary to our hypothesis, average interspecific dietary overlap increased at Charcote as the dry season progressed, even though dietary overlap among species was significantly lower than expected by chance. We did not find strong support for our hypotheses regarding seasonal patterns of consumption of allochthonous resources and food web connectance, both of which revealed little seasonal variation.(AU)

Redes alimentares não são entidades estáticas; interações entre consumidores e recursos variam no tempo e no espaço, o que complica as representações das estruturas de redes alimentares. Nós estimamos a estrutura da assembléia de peixes e dieta em dois riachos de água clara na região venezuelana dos Llanos (Charcote e Charcotico) e construímos redes tróficas (sub-redes definidas por peixes como os principais consumidores) para quatro períodos do pulso anual de inundação. À medida que as condições dos riachos mudam de águas altas para águas baixas, nós hipotetizamos que: 1) a razão piscívoros e não piscívoros irá aumentar; 2) a diversidade da dieta irá diminuir; 3) a sobreposição alimentar interespecífica irá diminuir; 4) menos itens alimentares alóctones serão consumidos; e 5) a conectância da teia alimentar irá aumentar. A razão da abundância de piscívoros e não piscívoros foi maior em ambos riachos durante a estação de águas baixas. A diversidade da dieta declinou à medida que as águas baixavam e a disponibilidade de habitats aquáticos e recursos declinavam, mas a sobreposição alimentar interespecífica não foi menor. Contrária à nossa hipótese, a média de sobreposição alimentar interespecífica aumentou no Charcote à medida que a estação cheia progrediu, mesmo com a sobreposição alimentar significativamente menor do que o esperado ao acaso. Não encontramos forte suporte para nossas hipóteses relacionadas aos padrões sazonais de consumo de recursos alóctones e conectância da cadeia trófica, os quais revelaram pouca variação sazonal.(AU)