Relative ascendency predicts food web robustness

Abstract

Threats to ecosystems globally from anthropogenic disturbance and climate change requires us to urgently identify the most sensitive biological communities to ensure they are effectively preserved. It is for this reason that understanding and predicting food web stability has been topical within ecology. Food web stability is a multi‐faceted concept that represents the ability of a food web to maintain its integrity following disturbance, it includes resistance, resilience and fragility. In this study, we examine the ability of four food web metrics to predict the fragility to random species extinctions in 120 qualitative food webs. We show that three information‐based indices out performed food web connectance in predicting fragility, with relative ascendency having the strongest relationship. Relative ascendency was a much stronger predictor of fragility than MacArthur's stability metric, Average Mutual Information and connectance as it accounted for both the distribution and number of links between species. We also find that most qualitative food webs persist around a central tendency of relative ascendency.

The influence of nutrient enrichment on riverine food web function and stability

“…It is plausible that when DIN concentrations exceed 0.55 mg/L that hypoxic events occur and disturb the system sufficiently to drop relative ascendency. Ulanowicz (1997b) found that most weighted food webs have very similar relative ascendency values, which has been suggested to represent a common trade‐off between adaptable yet inefficient communities and efficient yet brittle communities—termed the window of vitality (Canning & Death, 2018; Ulanowicz, 2009; Ulanowicz et al., 2009). Interestingly, all the webs we assessed were within the window of vitality, with the webs of highest relative ascendency occurring approximately at the upper boundary of the established window.…”

“…and efficient yet brittle communities-termed the window of vitality (Canning & Death, 2018;Ulanowicz, 2009;Ulanowicz et al, 2009).…”

“…When disturbances are species‐specific, secondary extinctions are most likely to occur when species with the highest link weight (both direct and indirect effects) are perturbed (Zhang et al., 2016). Analysis of un‐weighted food webs also suggested species connectivity had high influence on secondary species extinctions (Canning & Death, 2018; Dunne et al., 2002; Montoya et al., 2006); however, connectivity has since been shown to be unimportant in weighted food webs (Zhang et al., 2016). Species with high influence on food web stability are often termed as being keystone (Mills et al., 1993).…”

“…Analysis of un-weighted food webs also suggested species connectivity had high influence on secondary species extinctions (Canning & Death, 2018;Dunne et al, 2002;Montoya et al, 2006); however, connectivity has since been shown to be unimportant in weighted food webs (Zhang et al, 2016). Species with high influence on food web stability are often termed as being keystone (Mills et al, 1993).…”

Using Multiple Ecological Models to Inform Environmental Decision-Making

“…Ascendency is a measure of the average mutual information in a system. That is, Ascendency is a measure of the amount of specialist pathways and the size of a food web (Canning and Death, 2018). Ascendancy is inversely proportional to the uncertainty of where a unit of energy will flow next in a food web.…”