Disturbance and the (surprising?) role of ecosystem engineering in explaining spatial patterns of non-native plant establishment

The Intermediate Disturbance Hypothesis is widely considered to be wrong
but is rarely tested against alternative hypotheses. It predicts that soil
disturbances and herbivory have identical impacts on species richness via
identical mechanisms (reduction in biomass and in competition). An
alternative hypothesis is that the specific traits of disturbance agents
(small mammals) and plants differentially affects richness or abundance of
different plant groups. We tested these hypotheses on a degu (Octodon
degus) colony in central Chile. We ask whether native and non-native forbs
respond differently to degu bioturbation on runways vs. herbivory on
grazing lawns. We ask whether this can explain the increase in non-native
plants on degu colonies. We found that biopedturbation did not explain the
locations of non-native plants. We did not find direct evidence of grazing
increasing non-native herbs either, but a grazing effect appears to be
mediated by grass, which is the dominant cover. Further, we provide
supplementary evidence to support our interpretation that a key mechanism
of non-native spread is the formation of dry soil conditions on grazing
lawns. Thus ecosystem engineering (alteration of soil qualities) may be an
outcome of disturbances, which each interact with specific plant traits,
to create the observed pattern of non-native spread in the colony. Based
on these results we propose to extend Jentsch & White’s (2019)
concept of combined pulse/ disturbance events to the long-term process
duality of ecosystem engineering/ disturbance.