Untangling Food Webs The factors affecting the stability of food webs are important in conservation and ecological restoration. Gross et al. (p. 747 ) used a generalized modeling approach to evaluate billions of replicates of food webs in order to reveal the properties that stabilize (or destabilize) food webs. Variability in the strength of trophic links between predator and prey strength affected stability in different ways depending on the size of the web—stabilizing only in relatively small food webs and destabilizing in larger ones. Universal topological rules were extracted for the patterns of network links that enhance food-web stability.

The vertical distribution of phytoplankton is of fundamental importance for the dynamics and structure of aquatic communities. Here, using an advection-reaction-diffusion model, we investigate the distribution and competition of phytoplankton species in a water column, in which inverse resource gradients of light and a nutrient can limit growth of the biomass. This problem poses a challenge for ecologists, as the location of a production layer is not fixed, but rather depends on many internal parameters and environmental factors. In particular, we study the influence of an upper mixed layer (UML) in this system and show that it leads to a variety of dynamic effects: (i) Our model predicts alternative density profiles with a maximum of biomass either within or below the UML, thereby the system may be bistable or the relaxation from an unstable state may require a long-lasting transition. (ii) Reduced mixing in the deep layer can induce oscillations of the biomass; we show that a UML can sustain these oscillations even if the diffusivity is less than the critical mixing for a sinking phytoplankton population. (iii) A UML can strongly modify the outcome of competition between different phytoplankton species, yielding bistability both in the spatial distribution and in the species composition. (iv) A light limited species can obtain a competitive advantage if the diffusivity in the deep layers is reduced below a critical value. This yields a subtle competitive exclusion effect, where the oscillatory states in the deep layers are displaced by steady solutions in the UML. Finally, we present a novel graphical approach for deducing the competition outcome and for the analysis of the role of a UML in aquatic systems.

Predator-prey cycles rank among the most fundamental concepts in ecology, are predicted by the simplest ecological models and enable, theoretically, the indefinite persistence of predator and prey [1][2][3][4] . However, it remains an open question for how long cyclic dynamics can be self-sustained in real communities. Field observations have been restricted to a few cycle periods [5][6][7][8] and experimental studies indicate that oscillations may be short-lived without external stabilizing factors [9][10][11][12][13][14][15][16][17][18][19] . Here we performed microcosm experiments with a planktonic predator-prey system and repeatedly observed oscillatory time series of unprecedented length that persisted for up to around 50 cycles or approximately 300 predator generations. The dominant type of dynamics was characterized by regular, coherent oscillations with a nearly constant predator-prey phase difference. Despite constant experimental conditions, we also observed shorter episodes of irregular, non-coherent oscillations without any significant phase relationship. However, the predator-prey system showed a strong tendency to return to the dominant dynamical regime with a defined phase relationship. A mathematical model suggests that stochasticity is probably responsible for the reversible shift from coherent to non-coherent oscillations, a notion that was supported by experiments with external forcing by pulsed nutrient supply. Our findings empirically demonstrate the potential for infinite persistence of predator and prey populations in a cyclic dynamic regime that shows resilience in the presence of stochastic events.Cyclic dynamics are one of the most notable phenomena in population biology and are known to occur in a large range of communities both in the wild [5][6][7][8]13,14 and the laboratory [9][10][11][12][15][16][17][18] . A number of mechanisms that cause populations to oscillate have previously been identified 3,4,20 ; the most highly investigated, however, are the cyclic dynamics that arise from trophic interactions between populations of predator and prey organisms. The nearly century-old fascination with this type of dynamics is rooted in the predictions of simple mathematical models, which suggest that the predator and prey may coexist on recurring cyclic trajectories over indefinitely long periods of time 1,2 . Empirical data that support the sustained nature of predator-prey cycles in the field are difficult to come by because tractable populations typically have cycle periods of three to ten years 4,5,13 . An alternative data source are laboratory studies with fast-reproducing organisms, with which long-running experiments can be realized under strictly controlled conditions 9,10,12,16,21 . In a seminal experimental study, population numbers of weevils and their larval parasites were recorded for a time span of about 20 cycles but oscillations could not be sustained for the whole duration of the experiment 10 . This observation is paradigmatic for many studies, which showed that cycles in c...

The dynamics of ecosystem collapse are fundamental to determining how and why biological communities change through time, as well as the potential effects of extinctions on ecosystems. Here, we integrate depictions of mammals from Egyptian antiquity with direct lines of paleontological and archeological evidence to infer local extinctions and community dynamics over a 6,000-y span. The unprecedented temporal resolution of this dataset enables examination of how the tandem effects of human population growth and climate change can disrupt mammalian communities. We show that the extinctions of mammals in Egypt were nonrandom and that destabilizing changes in community composition coincided with abrupt aridification events and the attendant collapses of some complex societies. We also show that the roles of species in a community can change over time and that persistence is predicted by measures of species sensitivity, a function of local dynamic stability. To our knowledge, our study is the first high-resolution analysis of the ecological impacts of environmental change on predator-prey networks over millennial timescales and sheds light on the historical events that have shaped modern animal communities.community stability | historical ecology | trophic interactions | dynamic sensitivity | redundancy M odern biological communities are vestiges, with rich ecological ancestries shaped by evolutionary, climatic, and more recently anthropogenic effects. Determining the consequences of past ecological disturbance will inform predictions of how modern communities may respond to ongoing anthropogenic or climatic pressures. Of particular importance are extinction cascades (1, 2), which can lead to trophic downgrading and community collapse by altering the structure (2) and relative strengths of interactions between species (3). Examining the long-term effects of extinctions on communities can only be accomplished by studying past ecosystems (4). The paleontological record and the remarkable historical record of species occurrences in Egypt document a biological community changing in the face of increasing aridification and human population densities (5). The timing and pattern of animal extinctions in Egypt are thus well suited to illuminate our understanding of how the structure and functioning of biotic communities are altered by changing climatic and anthropogenic impacts.The Nile Valley north of Aswan is known for its intense heat, low rainfall, and relatively sparse vegetation. In fact, the last 2,750 km of the Nile is devoid of water-bearing tributaries and surrounded by desert with an average rainfall of 3.4 cm/y. The Egyptian landscape in the Late Pleistocene/early Holocene was very different; during the African Humid Period (AHP) (14,800-5,500 y B.P.), the region had a cooler, wetter climate driven by heavy monsoonal rains (5). These factors contributed to a diverse assemblage of mammals that bears a strong resemblance to communities in East Africa today.Termination of the AHP was associated with increasingly wea...