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.

Trophic cascade direction and flow determine network flow stability

“…All of these indirect pathways are likely to naturally occur in the webs we examined and all depend to some degree on strong predator competition for food (Wootton, 1994). In the flow models examined, strong indirect effects present as trophic cascades that permeate the effects of species extinction along food chains and severely impacting other species, thereby reducing robustness (May, 1971;Saint-Béat et al, 2015;Zhao et al, 2016). A system's total through-flow is composed of boundary, direct and indirect flows with indirect flows being dependent on the direct and boundary flows (Borrett et al, 2006).…”

“…Here we consider stability to be a general term for the maintenance of food web structure and function over time, it includes various aspects such as equilibrium, persistence, resilience, robustness and resistance (Dunne et al, 2005). Complexity (i.e., aspects including diversity, connectance, pathway proliferation and link distribution) promote stability (by supporting predators with multiple energy pathways and dampening disruptive oscillations) or destabilize it by spreading disruptive oscillations (McCann, 2000;Rooney and McCann, 2012;Saint-Béat et al, 2015;Zhao et al, 2016). It is unclear whether long food chains (network paths) are always unstable or whether the stability is dependent on the underlying trophic dynamics in those food chains (Rooney and McCann, 2012;Saint-Béat et al, 2015).…”

“…Recently, Zhao et al (2016) applied dynamical robustness and found that food webs are most sensitive to secondary extinction when species are removed by their link weights rather than connectivity. Both diversity and connectance reduced robustness, in contrast to topological studies that found connectance increased robustness (Dunne et al, 2002;Zhao et al, 2016). They also found donor control cascades caused only half of the secondary extinctions, highlighting the importance for also assessing predator control along with donor control.…”

Identifying compartments in ecological networks based on energy channels

“…Ecologists have tried to apply these methods to ecological networks (Krause et al., ; Leger, Daudin, & Vacher, ; Stouffer & Bascompte, ); however, food webs have at least two special characteristics: (1) The distribution of the link weights is largely uneven (O'Gorman, Jacob, Jonsson, & Emmerson, ); and (2) food web links have explicit directions from low to high trophic levels. These characteristics provide a cautionary note when interpreting results using methods designed for undirected and unweighted networks (Zhao et al., ). Moreover, the methods borrowed from network theory do little to improve our understanding of the underlying mechanisms of compartmentalization in food webs.…”

Effects of Environmental Factors on the Temporal Stability of Phytoplankton Biomass in a Eutrophic Man-Made Lake

“…Recently, more and more lakes face eutrophication caused by human activities and global climate warming [5][6][7][8][9]. As a result, catastrophic consequences such as species turnover and algae blooms are common in aquatic ecosystems [3,5,6,10]. Thus, studies on the stability of phytoplankton community and its influencing factors are essential to develop appropriate conservation strategies in aquatic ecosystems [11][12][13][14].…”