Analyzing the importance of top-down and bottom-up controls in food webs of Chinese lakes through structural equation modeling

Abstract

Zooplankton is generally affected by both top-down and bottom-up regulations in aquatic ecosystems. However, the relative strength of topdown and bottom-up control's on zooplankton assemblages is not well understood. Here, we analyzed this question in five lakes of the Yangtze River basin, an area with high population density and thousands of lakes, many of them suffering multiple environmental pressures. We sampled the whole communities of five lakes in the middle reaches of Yangtze River basin from 2006 to 2011 and used structural equation modeling to evaluate the relative importance of joint top-down and bottom-up effects. With increasing total phosphorous (TP), a major shift occurred in trophic structure. Biomass of phytoplankton, rotifers, cyclopoids, and planktivorous fish significantly increased, while cladocerans and calanoids were negatively correlated with increasing TP. The bottom-up effects were strongest at the bottom of the food web (e.g., effects of TP on phytoplankton). Direct bottom-up effects of phytoplankton and other food resources (latent variable) on rotifers and cyclopoids were greater than top-down controls from planktivores. The predation pressure on crustacean zooplankton by planktivores was higher than that on rotifers. In planktivore-dominated systems, piscivores only played a marginal role, whereas they seem affected by water quality. These results suggest not only in the food web processes the important role of nutrient pollution in affecting the top of the food web in these lakes, but also that the impacts and relative strength of bottom-up and top-down controls may vary with zooplankton assemblages, indicating the complexity of food webs in degraded lakes in China.

Bottom-up and top-down effects on phytoplankton communities in two freshwater lakes

“…Our study showed that the bottom-up effects of physical-chemical variables on phytoplankton were stronger than the top-down effects of zooplankton in both Nansi and Dongping lakes, which is consistent with previous studies [17,18,26]. The growth and reproduction of phytoplankton were influenced by physical-chemical factors in the water [27,28], among which water temperature and nutrient concentration contributed more than the other factors.…”

Quantifying relative importance: computing standardized effects in models with binary outcomes

“…These more complex models often contain numerous multivariable pathways of scientific interest and place a premium on broad, general questions about the relative importance of different ecological flows, such as top‐down vs. bottom‐up control in ecosystems (Du et al. ), the relative importance of environmental vs. biotic drivers in community organization (Laughlin and Abella , Byrnes et al. , Cronin et al.…”

“…However, there is also an increasingly recognized need to work with complex models that correspond more closely with system-level questions (Mitchell 2001, Forister et al 2011, Lamb et al 2014, Fan et al 2016. These more complex models often contain numerous multivariable pathways of scientific interest and place a premium on broad, general questions about the relative importance of different ecological flows, such as top-down vs. bottom-up control in ecosystems (Du et al 2015), the relative importance of environmental vs. biotic drivers in community organization (Laughlin and Abella 2007, Byrnes et al 2011, Cronin et al 2014, Gama-Rodrigues et al 2014, Jing et al 2015, or the relative importance of different biotic processes (Cardinale et al 2009, Scherber et al 2010, Mazancourt et al 2013. For applications such as these, standardization methods that provide clear comparability among coefficients and also that can summarize indirect and total effects become of great importance.…”

Toward an ecological understanding of a flood-pulse system lake in a tropical ecosystem: Food web structure and ecosystem health

“…From the basic estimate of the model, the biomass of fish groups in the TSL ecosystem was estimated at 58 t km −2 higher than the estimated biomass of fish groups in Lake Victoria (43 t km −2 ) and Lake Tanganyika (35 t km −2 ) (Christensen and Pauly, 1993). The relatively high biomass of the top predators in TSL could produce a predation pressure on its forage species through the top-down controls in the food web (Du et al, 2015). The catch statistics from the lake showed that snakehead and catfish were found to be abundant in the TSL system since TSL and its floodplains are suitable feeding and spawning grounds for many carnivore species .…”