Lei Zhao

Weighting and indirect effects identify keystone species in food webs

Abstract Species extinctions are accelerating globally, yet the mechanisms that maintain local biodiversity remain poorly understood. The extinction of species that feed on or are fed on by many others (i.e. ‘hubs’) has traditionally been thought to cause the greatest threat of further biodiversity loss. Very little attention has been paid to the strength of those feeding links (i.e. link weight) and the prevalence of indirect interactions. Here, we used a dynamical model based on empirical energy budget data to assess changes in ecosystem stability after simulating the loss of species according to various extinction scenarios. Link weight and/or indirect effects had stronger effects on food‐web stability than the simple removal of ‘hubs’, demonstrating that both quantitative fluxes and species dissipating their effects across many links should be of great concern in biodiversity conservation, and the potential for ‘hubs’ to act as keystone species may have been exaggerated to date.

Responses of a phytoplankton community to seasonal and environmental changes in Lake Nansihu, China

Phytoplankton is the primary producer and the basis of most aquatic food webs. Characterising the variations in phytoplankton communities and the factors affecting these variations in a fluctuating environment are central issues in ecology and essential to developing appropriate conservation strategies. In the present study, seasonal variations in the phytoplankton community and the driving environmental factors were analysed based on data from Lake Nansihu in 2013. In all, 138 phytoplankton species were identified. The phytoplankton community exhibited seasonal variations, with a mean abundance that ranged from 5.00×105 cells L–1 in winter to 4.57×106 cells L–1 in summer and a mean biomass that varied from 0.44mgL–1 in winter to 3.75mgL–1 in summer. A spring algal bloom did not appear in this warm, temperate monsoon lake, but an algal bloom did appear in summer when the temperature and nutrient concentrations were high. There were substantial seasonal variations in the dominant phytoplankton taxa, from Chlorophyta, Bacillariophyta and Euglenophyta in spring to Chlorophyta and Bacillariophyta in summer, followed by dominance of Chlorophyta in autumn and Bacillariophyta in winter. Results of canonical correspondence analysis indicated that although the environmental factors affecting the seasonal variations in different phytoplankton species varied, water temperature, total nitrogen, total phosphorus and ammonia nitrogen appeared to be the most dominant. These four variables were also the main environmental factors driving the seasonal variations in the phytoplankton community in the lake. The results of the present study will be useful in guaranteeing the water quality and ecological security of Lake Nansihu.

Biodiversity effects on resource use efficiency and community turnover of plankton in Lake Nansihu, China

The relationship between biodiversity and ecosystem functioning is a central issue in ecology, especially in aquatic ecosystems due to the ecophysiological characteristics of plankton. Recently, ecologists have obtained conflicting conclusions while analyzing the influence of species diversity on plankton resource use efficiency (RUE) and community turnover. In this study, both phytoplankton and zooplankton communities were investigated seasonally from 2011 to 2013 in Lake Nansihu, a meso-eutrophic and recovering lake in China. The effects of phytoplankton diversity on RUE of phytoplankton (RUEPP), zooplankton (RUEZP), and community turnover were analyzed. Results showed that both phytoplankton species richness and evenness were positively correlated with RUEPP. RUEZP had a negative relationship with phytoplankton species richness, but a weak unimodal relationship with phytoplankton evenness. Cyanobacteria community had the opposite influence on RUEPP and RUEZP. Thus, cyanobacteria dominance will benefit RUEPP in eutrophic lakes, but the growth and reproduction of zooplankton are greatly limited. The strong negative relationship between total phosphorus and RUEZP confirmed these results. Phytoplankton community turnover tended to decrease with increasing phytoplankton evenness, which was consistent with most previous studies. The correlation coefficient between phytoplankton species richness and community turnover was negative, but not significant (pxa0>xa00.05). Therefore, phytoplankton community turnover was more sensitive to the variation of evenness than species richness. These results will be helpful in understanding the effects of species diversity on ecosystem functioning in aquatic ecosystems.

Viewing the effects of species loss in complex ecological networks

Species loss is becoming a major threat to ecosystems. An urgent task in ecology is to predict the consequence of species loss which requires an extending of our traditional study of the topology of network structure to the population dynamic analyses in complex food webs. Here, via numerical simulations of the model combining structural networks with nonlinear bioenergetic models of population dynamics, we analyzed the secondary effects of species removal on biomass distribution and population stability, as well as the factors influencing these effects. We found that the biomass of target species, the nutrient supply, and the trophic level of target species were the three most significant determiners for the effects of species loss. Species loss had large negative effect on the biomass of the species with small biomass or intermediate trophic levels, especially in infertile environment. The population stability of the species with large biomass or low trophic level is easily to be influenced especially in nutrient-rich environment. Our findings indicate the species which are easily to be affected by species loss in food webs, which may help ecologists to outline a better conservation policy.

Effects of Nutrient Enrichment on a Realistic Food Web

Nutrients supplies play a crucial role in population distribution of food webs, and it is one of the most important challenges in both theoretical and applied ecology to better understand the ‘bottom-up’ effect. Here we analyzed a realistic food web containing one limiting nutrient and two producer-consumer interactions. Through the simulations of the mean population size and the variability of each species, we found that 1) in infertile environment the coexistence of species increases with nutrient supply; 2) the mean values of species abundance increase dramatically at then remain constant with the increase of nutrient supply; and 3) the stability of each population also increases and then remains constant. These results are inconsistent with the traditional predictions known as the ‘paradox of enrichment’, and our analysis may provide a solution to this paradox.

A Study of Dynamical Model on the Competitive Relationship Between Soil Erosion and Vegetation Growth in Humid Regions

In this study, a new dynamical model is established based on Thornes’ model. Then a detailed competitive and interactive relationship between soil erosion process and vegetation growth process is detected in humid regions. By employing the nonlinear dynamical analyses, a globally asymptotically stable equilibrium point is obtained under given parameters. This stable state indicates the vegetation growth process can coexist with soil erosion process. Furthermore, the transitions among different equilibrium states caused by the variation describe the competitive process in diverse environment. The results show that the soil erosion has effect on the formations of vegetation patterns in humid regions.Copyright

Fragility of Chaos in Multispecies Competition Influenced by Predation

In order to study the stability of chaotic behaviors, a nonlinear dynamical model of the competing multispecies with a predator is investigated. A series of numerical simulations is demonstrated via wave diagram and phase diagram. The results show that the chaos can change into either oscillation or ordinary equilibrium as the attacking rate of the predator increases. Moreover, chaos in the system becomes fragile and even vanishes when the attacking rate reaches 0.0186. This study also exhibits the transformation in phase diagram from a strange attractor to a stable equilibrium.Copyright

A Study on Nonlinear Population Dynamics Based on Energy Flow

In considering energy flow between organism and environment, population dynamics for single species is studied in the present research. When the total available energy of environment is infinite, a model obeying the Malthusian law is obtained. Under the condition of limited total available energy, a model approximate Logistic Model is also established. These two models give a better comprehension on the ecological effects of growth rate and carry capacity of environment.© 2010 ASME