Browsing by Person "Zarfl, Christiane"

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Competitive hierarchies in bryozoan assemblages mitigate network instability by keeping short and long feedback loops weak
(2023) Koch, Franziska; Neutel, Anje-Margriet; Barnes, David K. A.; Tielbӧrger, Katja; Zarfl, Christiane; Allhoff, Korinna T.
Competitive hierarchies in diverse ecological communities have long been thought to lead to instability and prevent coexistence. However, system stability has never been tested, and the relation between hierarchy and instability has never been explained in complex competition networks parameterised with data from direct observation. Here we test model stability of 30 multispecies bryozoan assemblages, using estimates of energy loss from observed interference competition to parameterise both the inter- and intraspecific interactions in the competition networks. We find that all competition networks are unstable. However, instability is mitigated considerably by asymmetries in the energy loss rates brought about by hierarchies of strong and weak competitors. This asymmetric organisation results in asymmetries in the interaction strengths, which reduces instability by keeping the weight of short (positive) and longer (positive and negative) feedback loops low. Our results support the idea that interference competition leads to instability and exclusion but demonstrate that this is not because of, but despite, competitive hierarchy.
Eco-evolutionary dynamics in two-species mutualistic systems: one-sided population decline triggers joint interaction disinvestment
(2023) Weyerer, Franz; Weinbach, Avril; Zarfl, Christiane; Allhoff, Korinna T.
The interplay between ecological and evolutionary dynamics can create feedback that reinforces external disturbances and potentially threatens species’ coexistence. For example, plants might invest less into attracting insect pollinators (decreased flower or nectar production) and more into independence (increased selfing or vegetative reproduction) when faced with pollinator decline. This adaptive response saves plant resources at the cost of further threatening the pollinator population. We ask under which conditions such self-reinforcing feedback occurs in two-species mutualistic systems when considering one-sided population decline and whether it can be counteracted by self-dampening feedback if co-evolution of both interaction partners is considered. Based on a mathematical model and in line with previous studies, we find that the described pattern of accelerated population decline occurs for a wide range of parameter values if a concave allocation trade-off between independent growth and interaction investment is assumed. The undisturbed population typically disinvests first, which then forces the declining population to also disinvest, in favour of other energy sources. However, a decelerated population decline can occur if the adaptation of the undisturbed partner is relatively slow compared to environmental decay, reducing the speed of its disinvestment, or if the initial investment into the interaction was very high. Our results suggest that if actions are taken to save endangered populations, not only the evolution of the target species but also of their interaction partner, as well as the interaction between them should be considered.