What if we lose a hub? Experimental testing of pollination network resilience to removal of keystone floral resources

Abstract

On the basis of theoretical predictions, pollination networks seem to be resilient to random node elimination but sensitive to targeted exclusion. However, such predictions have a very weak empirical basis. In order to test the robustness of the pollination network to shortterm disturbances, we removed inflorescences of the most connected species occurring in a lowland meadow network using the before-after approach and compared the result with that obtained by network modelling. The manipulated network showed no significant differences for the most commonly used metrics, but was more generalized than control networks, owing to a change in the preferences of pollinators. Furthermore, no secondary extinctions (emigrations) were found, owing to the considerable natural variation found among insect species assemblages. Following elimination of the most linked plant species, a new hub was detected in the experimental meadow, the hub node being a plant species with a similar inflorescence to that removed, and formerly playing the role of a peripheral node. We conclude that exclusion of the main food source forced insects to change their specialized preferences to other plant species that were available. Mostly, these had inflorescences similar to those that were removed.

A landscape experiment of spatial network robustness and space‐use reorganization following habitat fragmentation

“…Predicting the robustness of landscape connectivity from network theory generally assumes the complete disappearance (or functional loss) of network patches (Bunn et al, ; Urban & Keitt, ), and this has been the approach typically employed in real‐world systems (Shimazaki et al, ; Treml et al, ). When the fragmentation process results in the retention of remnant patches, however, they can actually facilitate the rewiring than when patches are completely removed and thus provide more robustness to the network (Goldstein & Zych, ). In our study, the diameter, the CPL and the number of components of caribou networks did not change following the fragmentation of either hub or non‐hub patches.…”

“…Scale‐free spatial networks, for example, tend to be more resistant to node alteration when they include several long‐range connections (Jacob & Mörters, ). Second, individuals can respond to disturbances by creating new links to rewire their network (Goldstein & Zych, ; Ramos‐Jiliberto, Valdovinos, Moisset de Espanés, & Flores, ). Accordingly, the removal of generalist floral resource in a pollination network results in the emergence of new hubs through the rewiring process (Goldstein & Zych, ), with the consequences that the removal of a large number of plants is needed to cause partial extinction of both plants and pollinators (Ramos‐Jiliberto et al, ).…”

“…Second, individuals can respond to disturbances by creating new links to rewire their network (Goldstein & Zych, ; Ramos‐Jiliberto, Valdovinos, Moisset de Espanés, & Flores, ). Accordingly, the removal of generalist floral resource in a pollination network results in the emergence of new hubs through the rewiring process (Goldstein & Zych, ), with the consequences that the removal of a large number of plants is needed to cause partial extinction of both plants and pollinators (Ramos‐Jiliberto et al, ). Despite the potential for rewiring, spatial networks are generally simulated while assuming they are static entities, that is no rewiring following node or edge loss (Bunn, Urban, & Keitt, ; Rhodes et al, ; Urban & Keitt, ), a faulty assumption that can induce spurious expectations of network robustness, and ultimately, biased conclusions on animal distribution dynamics (Martensen, Saura, & Fortin, ).…”

“…Scale-free spatial networks, for example, tend to be more resistant to node alteration when they include several long-range connections (Jacob & Mörters, 2017). Second, individuals can respond to disturbances by creating new links to rewire their network (Goldstein & Zych, 2016; Ramos-Jiliberto, Valdovinos, Moisset de Espanés, & Flores, 2012).…”

“…Accordingly, the removal of generalist floral resource in a pollination network results in the emergence of new hubs through the rewiring process (Goldstein & Zych, 2016), with the consequences that the removal of a large number of plants is needed to cause partial extinction of both plants and pollinators (Ramos-Jiliberto et al, 2012).…”