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\begin{document}

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



\author[1]{Meredith Root-Bernstein}%
\author[2]{Cesar Muñoz}%
\author[3]{Juan Armesto}%
\affil[1]{CNRS}%
\affil[2]{Pontificia Universidad Católica de Chile}%
\affil[3]{Universidad de Chile Instituto de Ecología y Biodiversidad}%


\vspace{-1em}



  
  \date{May 25, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
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.%
\end{abstract}%



\sloppy


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

Meredith Root-Bernstein\textsuperscript{1,2,3}, C\selectlanguage{ngerman}ésar
Muñoz\textsuperscript{4}, Juan Armesto\textsuperscript{2}

(1) CNRS, Musée National d'Histoire Naturelle, Paris, France

(2) Instituto de Ecología y Biodiversidad, Santiago, Chile

(3) Center for Sustainability and Applied Ecology, Santiago, Chile

(4) Department of Ecology, Pontificia Universidad Católica de Chile,
Santiago, Chile

Funding Information:

During the preparation of this manuscript MR-B was supported by a
doctoral fellowship from CONICYT (No. 63105446), a Marie Curie FP7
COFUND Agreenskills Post Doctoral Fellow, and the Center of Applied
Ecology and Sustainability (CONICYT PIA/BASAL FB0002).

Abstract

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 (\emph{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.

Keywords: \emph{Octodon degus} , non-native plants, bioturbation,
herbivory, ecosystem engineering, disturbance

Words total: 5643

Introduction

Disturbances and perturbations are key factors determining change in
vegetation composition over time (Pickett \& Cadenasso 2005). Herbivory
and granivory may be considered to be endogenous ``perturbations''
rather than exogenous non-trophic disturbances \emph{per se} (Proulx \&
Mazumender 1998). Others consider perturbations, along with disturbances
e.g. to the soil, as all being forms of intermediate disturbance (Mackie
\& Curry 2001). According to the Intermediate Disturbance Hypothesis
(IDH), the expected outcomes of herbivory are the same as, for example,
those of soil disturbances (e.g. biopedturbation) (Proulx \& Mazumender
1998). The IDH proposes that all forms of disturbance/ perturbation
equivalently allow some species to escape competition through the
destruction of other species' biomass: in theory, two different kinds of
disturbance regimes could be plotted as IDH humps, scaled, and an
equivalence of rate of disturbance per species richness increment could
be calculated. The IDH is widely believed to be wrong or at least
non-predictive, although no alternative theory has been developed to
explain how disturbance affects species composition or richness.

However, an interesting new conceptualization of disturbance is offered
by Jentsch \& White (2019). They proposed that all disturbances are
simultaneously pulses, as every pulse/ disturbance is a multi-factorial
event that increases some variables, decreases other variables, and
perhaps does not affect another set of variables. This implies that
different pulse/ disturbance events produce unique combinations of
multfactorial effects.

In line with Jentsch \& White's (2019) multifactorial pulse/ disturbance
event concept, some data demonstrates that the identities of the
disturbing and disturbed species can affect disturbance outcomes, which
should be expected if each kind of pulse/ disturbance produces a
different set of increases and decreases in a unique set of variables.
The literature on non-native herb establishment or invasion provides
some examples of non-identical effects of disturbance or perturbation.
For example, species identity---native or non-native---of both the
disturbing/ perturbing (animal) and establishing (plant) species, has
been shown to lead to different trajectories of plant community
composition (e.g. invasion of non-native plants) (Parker et al. 2006).
Soil disturbance by a native small mammal, but not herbivory by invasive
herbivores, leads to expansion of a non-native herbaceous plant in
central Chile (Torres-Díaz et al. 2012). A plausible explanation for how
the identities of interacting species drive plant community change is
that disturbance/ perturbation effects are highly sensitive to not only
the type of disturbance/ perturbation (e.g. herbivory vs. mounds vs.
runways), but also to the timing, spatial distribution, scale, etc. in
which a given disturbance is carried out by different species
(Maschinski \& Witham 1989). Plant species tolerate and respond to these
disturbances differently, with different competitive advantages (Grace
1991). For example, non-native plants are often ruderals adapted to
intensive grazing, making them potentially superior competitors to
native plants under herbivory and large mammal trampling (Schiffman
1994, Kean \& Crawley 2002, Fraser \& Madson 2008, Seabloom et al.
2009).

Non-trophic disturbances of small mammals such as burrow digging have
attracted attention since the origins of ecology (Whitford \& Kay 1999;
Kelt 2011). In addition, long-term studies have examined trophic impacts
of small mammals on plant community change, for example in kangaroo rats
(Brown et al. 2001). Small mammal disturbances/ perturbations frequently
lead to increases in plant richness and diversity (Root-Bernstein \&
Ebensperger 2012). Native small mammal disturbances can also present a
threat to native herbaceous plant communities by favouring
grazing-adapted non-native plants (Torres-Díaz et al. 2012).

In this study we ask whether native and non-native herbaceous plants
respond differently to degu runway-related biopedturbation (running up
and down the runways) and to degu herbivory. The degu \emph{Octodon
degus} is a group-living, colonial burrow-dwelling rodent in central
Chile (Ebensperger et al. 2019), that creates runways between burrow
entrances and grazes aboveground (Figure 1; Madrigal et al. 2011). Their
grazing lawns on colonies increase herbaceous plant richness along with
other elements of biodiversity (Root-Bernstein et al. 2013;
Root-Bernstein et al. 2014). Degu colonies exhibit central areas with
high runway density and multiple-entrance burrows \textless{} 10 m
apart, as well as extensive, less-dense peripheries constructed by
dispersing juveniles and occupied during high degu-population years
(Ebensperger et al. 2011, Quirici et al. 2011, Ebensperger et al. 2009).
Biopedturbation is concentrated on runways especially before grazing
lawn creation, but grazing occurs both on and off runways
(Root-Bernstein et al. in submission). Surrounding grassland is expected
to be mainly annual herbs, and at least half non-native (Deil et al.
2007).

We predict that the non-native herbs that are ruderal should be more
common where bioturbation on runways is the highest. We also predict
that non-native herbs adapted to herbivory, should be more common at
sites with higher grazing pressure (Holmgren et al. 2000). We also
expect the relative importance of herbivory and bioturbation on runways
to vary with accumulated disturbance pressure at different locations of
the colony, being highest in the older colony center. Finally, we
predict that the impacts of perturbation and disturbance on the dominant
native taxon, grasses, may mediate increases in non-native herbs
(Holmgren et al. 2000, del Pozo et al. 2006).

Materials and Methods

\emph{Study site} . The study took place in September and October 2011
at the Estación Experimental Rinconada de Maipú (33°23\selectlanguage{english}' S, 70deg31\selectlanguage{english}' W,
altitude 495 m), a field station of Universidad de Chile, Santiago,
Chile. The field station consists of espinal (\emph{Acacia
caven}savanna) subject to occasional fires (1-2 events per decade) and
grazing by cattle and sheep (\textless{} 1 sheep per hectare over almost
900 ha, C. Araneda pers. comm. 2014), open grasslands dominated
primarily by native perennial grasses, and denser matorral (evergreen
shrubland), dominated by sclerophyllous shrubs and perennial herbs. The
study site includes extensive degu colonies with lawns of hebaceous
species, found essentially in the valleys of a small mountain range
(less than 1000 m in elevation) forming one side of the field station.

\emph{Plot selection} . We set up 13 ``peripheral plots'' and 10
``central plots'' on the degu colony (Figure 2). Central plots were
defined as 10 m x 10 m squares containing \textgreater{} 20 degu
runways. Central plots were separated by at least one burrow system.
Peripheral plots were defined as 10 m x 10 m squares with \textless{} 5
degu runways. The colony center-periphery distance at our field site is
a radius of a minimum of approximately 300 m. Additional selection
criteria for plots included evidence of fresh soil from recent
excavation at burrows in or adjacent to the plot, and little or no
evidence of rabbit droppings or cururo mounds. We considered the
presence of fresh soil from excavations or the presence of fesh
droppings to determine that a burrow was occupied (Ebensperger et al.
2011). There were no rabbit burrows within the degu colony. None of the
plots included cururo (\emph{Spalacopus cyanu} s) mounds. Plots were
oriented so that they were bisected by one or more degu runways forming
a transect across the plot.

\emph{Disturbance accumulation proxies.} Our methods are based on a form
of space for time substitution (Pickett 1989). Following the advice of
Pickett for appropriate design of space for time substitutions, we
selected several space-for-time proxies, drawing on our knowledge of the
local vegetation community change process on degu colonies
(Root-Bernstein et al. 2014). We also draw on long-term studies of
prairie dog (\emph{Cynomys ludovicianus} ) colonies, since this species
has a similar life history, ecological niche, and use of habitat,
compared to degus. Prairie dog colonies expand outwards, so that central
areas are older than peripheral areas (Whicker \& Detling 1988). Since
we have observed similar patterns for degus (see Introduction), our
first proxy for disturbance and perturbation accumulation is the
location, peripheral or central, of the plots examined. Over time, cover
of tall grass in praire dog colonies declines and is replaced by low
herb cover, starting in the colony center and moving outwards to the
periphery (Garrett et al. 1982, Archer et al. 1987). In central Chile,
grasses are also dominant taxa that decline under herbivory (Holmgren et
al. 2000, del Pozo et al. 2006). We therefore expect grass cover to be
negatively correlated with age of colony sections, as well as
potentially mediating the increase in other herb species through
reduction in competition. A third proxy is derived from field
observations at our research site: moss is common on runways and may be
an early colonizer of runways. We thus used moss cover on runways as a
third proxy for accumulated disturbance over time.

\emph{Herbaceous plant data} . Along each runway-transect, we collected
quadrat data at two distances from the runway-transect: over the runway
(``on-runway'') (runways are about 8 cm wide) and 25 cm from the runway
edge (``off-runway''). Each quadrat was a square cardboard frame of 10
cm\textsuperscript{2} which we laid on the ground over the sample and
photographed with digital cameras in autofocus mode at a distance of
approximately 1 m from the ground. Five quadrats of on-runway samples
were recorded every 2 m along the runway-transect at odd-numbered meter
marks. Five quadrats of off-runway samples were recorded at
even-numbered meter marks. Quadrats alternated to the left and right of
the transect.

\emph{Plant identification} . Plant species rarely completely overlapped
each other due to an absolute low abundance in all plots and it was
possible to identify each species from the photographs of the quadrats
by the shape of cotyledons, leaves, and flowers. Names and distributions
are according to Hoffmann (1978), except for mosses and grasses. We
classified mosses as endemic following Larrain (2009), which indicated
that the majority of mosses found in Chile are native to Chile. We were
not able to identify grasses to species, but grasses were assumed to be
mainly native or endemic species (Finot et al. 2011). Whenever herbs
could not be identified by species, they were classed according to
morphospecies according to cotyledon, colors and leaf shapes.

Plants were identified and counted using SamplePoint (in R), which
records the classification into user-defined categories for pixels at
crosshairs arranged in a regular grid over the digital photograph. We
set our grid to 25 crosshairs per photo. Dead plants were not counted.
When plants overlapped, we counted the top (visible) species only.
Overlap occurred only rarely (estimated \textless{} 10\% of all
crosshairs). Cover was calculated as count number.

\emph{Measures of herbivory} . We assessed herbivory by measuring the
amount of rolled oats eaten from dishes. Oats are a favorite food of
degus and are used to trap them (e.g. Ebensperger et al. 2011, Quirici
et al. 2011). During the study season (spring), degus graze diurnally,
alone or in loosely associated foraging groups (Lagos et al. 2009). We
placed small metal dishes 11 cm in diameter in each plot, near the degu
runway-transect. Dishes were filled with 25 g of rolled oats on the
mornings of 3/10/11, 16/10/11 and 23/10/11. Dishes were checked and
weighed with an electronic weight (Acculab GS-200) 24 hours later. The
amount of oats eaten was calculated as the difference between the weight
of oats with which the dish was filled the previous day, and the current
weight. In some cases the dishes of oats gained up to 2 g of water from
dew. The amount eaten is thus precise to within \selectlanguage{english}[?] 2 g. We did not
observe ant activity at the oat dishes. Spillages were noted and not
treated as eaten. Just prior to data collection, another experiment
started in the same research site, which involved baiting Sherman traps
with oats, and some of these traps were close to one of our central
plots. Thus we did not collect foraging data from this plot.

\emph{Measuring bioturbation on runways} . We measured bioturbation on
runways as the amount of degu traffic along the runway-transect in each
plot. We recorded degu traffic using tracking cards (Meserve 1981). We
cut strips of rag paper approximately 32 cm x 7 cm (to fit within a degu
runway). A central square on the paper strip about 7
cm\textsuperscript{2} was colored in with a black hard pastel. Two such
tracking cards were placed end-to-end along the runway-transect in each
plot, and fixed in place with nails. Tracking cards were put in place on
17/10/11 and collected 27/10/11. Collected tracking cards were sprayed
with a fixative for pastels to prevent smudging. Tracking cards were
photographed and the images were analyzed in Adobe Photoshop (r), using
the ``count'' function. We counted discrete toe and palm marks (``foot
marks''). We summed the total number of foot marks for the two tracking
cards for each site to yield a measure of bioturbation along runways. We
observed only one footprint from another species, not identified. We did
not collect bioturbation data for the site omitted from the foraging
data collection.

\emph{Statistical note} . Because space-for-time substitutions involve
interpretive assumptions that are not present in long-term manipulative
experiments, we chose to use the most conservative (least powerful)
statistical approaches available to decrease the likelihood of detecting
ecologically weak effects (Amrhein et al. 2019).

Results

\emph{Plant community description} . We observed 10 native or endemic
forb species or morphospecies, two taxonomic groups dominated by native
species (mosses and grasses) and 3 non-native species along all of our
transects. There was no significant difference in cover per taxa between
native and non-native forbs (Kruskal-Wallis test, H = 0.092, df = 1, p =
0.756).

\emph{Native and non-native herb distributions} . Natives and
non-natives were distributed differently across the plot locations, with
non-native cover higher in central plots, and native cover higher in
peripheral plots, which was significantly different from the expected
cover distribution (\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} test, \selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} =
177.73, df = 3, p = 2.2 x 10\textsuperscript{-16}). At a finer scale,
natives and non-natives were also distributed differently on or away
from runways: native cover was higher off of runways than on, and higher
than non-natives; non-natives were also higher off runways
(\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} test, \selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} = 232.84, df = 3, p =
2.2 x 10\textsuperscript{-16}). We found an interaction between colony
location (center/ periphery) and runway location (on/ off) on the total
cover of non-native species: the cover of non-natives is higher in
colony center than peripheries, and higher on runways than off runways,
a distribution signficantly different from expected (Figure 3;
\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} test, \selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} = 61.5, df = 3, p = 2.8
x 10\textsuperscript{-13}).

\emph{Space for time proxies} . Mean grass cover on off-runway quadrats,
the proxy for colony age, was not lower in central plots, differing from
our expectation (Student t-test, t = 0.193, df = 15.98, p = 0.84). As
the grass proxy, mean grass cover on off-runway quadrats, decreased, the
ratio of non-native to native herb abundance increased (r = -0.684, df =
19, p = 0.0006). Focusing on moss as a proxy of colony age, we find that
the mean percent cover of moss different significantly across on-off and
center-periphery, and was highest on peripheral runways
(\selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2} test, \selectlanguage{greek}χ\selectlanguage{english}\textsuperscript{2}=38.03, df = 3, p =
2.785 x 10\textsuperscript{-08}).

\emph{Grass distribution as a mediator of competition with other herbs}
. The ratio of grass cover on vs. off runways per plot was not
correlated to the general distribution of other herbs on and off runways
per plot, with grass tending to have relatively greater cover on
runways, unlike other herbs which tended to have relatively greater
cover off runways (Figure 4; r = 0.073, df = 19, p-value = 0.75).

\emph{Herbivory and bioturbation on runways.} The average amount of oats
eaten per day did not differ between peripheral and central plots
(Student t-test, t = 1.31, df = 21, p = 0.203). The number of foot marks
registered on runways also did not differ between peripheral and central
plots (Student t-test, t = 0.54, df = 21, p = 0.592). Foot marks and
total amount of oats (on and off runways) eaten in each plot were highly
correlated (r = 0.99, df = 19, p \textless{} 0.0001). Foot marks
increased as the percent cover of grass on \emph{off-runway} quadrats
increased (r = 0.508, df = 21, p = 0.0134). However, there was only a
weak, non-significant relation between percent grass cover \emph{on
runways} and foot marks (r = 0.262, df = 19, p = 0.251). Biopedturbation
was not correlated with the ratio of non-native to native herbs (r =
-0.189, df = 18, p = 0.43).

Discussion

We found that richness of native non-woody herbaceous taxa (N = 11) was
almost four times that of non-native species (N = 3) on degu colonies.
The number of native or endemic species may have been underestimated as
we were not able to identify mosses or grasses to species level. As
expected, the cover of non-native plants was higher in the center of the
colony, compared to the periphery. At a smaller scale, the runways also
influenced the distribution of native and non-native plants. Non-native
plants had higher cover on runways in central plots than in peripheral
plots.

We find at least partial support for our interpretation of peripheral
colony sites as having been formed more recently, and/or occupied more
sporadically over time, compared to colony centers. The grass proxy for
accumulated disturbance did not lend support to the center/ periphery
split, with patches of grass found in both peripheral and central plots.
Moss, as expected, had different distributions across central and
peripheral plots. Although we expected moss to increase in extent over
time on runways, we interpret the result to mean that moss is an early
colonizer of runways created through biopedturbation (as expected), but
reduces in extent as the microhabitat becomes more dry (see below).

The distribution of high and low biopedturbation and herbivory were not
explained by colony position, but were explained by grass cover off
runways. As grass cover in the off-runway quadrats increased, foot
traffic on runways and amount of herbivory on oats increased. Grass
avoidance may thus mediate degu biopedturbation on runways. Grass was
also expected to mediate plant composition change through herbivory on
grass reducing its competition with other herbs. Indeed, as grass cover
decreased off runways, the relative cover of non-native species
increased. Although degus eat both grasses and forbs, including the
non-native forbs we observed in this study (Quirici et al. 2010), the
persistence of grass along runway edges suggests that it is not
opportunistically eaten (unlike the oats). In addition to eating
grasses, degus also gather them to line their nests, which requires long
grass (pers. obs. MR-B): both of these might account for reduction in
grass off runways.

Our interpretation of the temporal process resulting in the observed
spatial pattern of non-native herb distribution is that grass reduction
off runways over time leads to increased non-native populations and the
eventual colonization of the runway edge by non-natives. However, it is
not clear that eventual grass reduction on runways is what allows
non-natives to colonize runway edges in the center of the colony, since
the grass did not reliably disappear in the colony center and was
uncorrelated with other herb distributions relative to runway structure.
Thus, non-native expansion to runway edges may not be mediated by
competition with grass, but by some other mechanism.

Ecosystem engineering effects on the soil may help explain how the
increase in non-natives first seen off runways spreads to runway edges.
Runways are exposed to the sun. Grazing lawns, with vegetation only
\textasciitilde{}2-4 cm high, can be expected to lead to lowered
evapotranspiration and less water being drawn into the soil by plant
roots. These effects may lead to drier soils: the loss of or lack of
moss in central runways may also point to drier soils in colony centers.
Although we did not measure soil moisture in this study, as we did not
anticipate its importance, other researchers have measured soil
characteristics at the same research site. Bauer et al. (2013) show that
soil penetrability, directly related to soil moisture (Ebensperger \&
Bocinovic 2000), is highest at sites with shade cover and lowest at
sites with the most bare earth (see also Ovalle \& Aveda\selectlanguage{ngerman}ño 1984). This
bare earth would mainly correspond to areas of degu biopedturbation.
Ebensperger \& Bocinovic (2000) also show that in colony centers, soil
moisture declines dramatically during the low-precipitation months
(summer and autumn). At the same time, where the grass is less
productive due to being kept short through herbivory lawns, this may
lead directly to a reduced competitive advantage of grass. In fact, the
two most abundant of the three non-native species observed at our field
site, \emph{Camissonia sp.} , and \emph{Erodium cicutarium,} are
considered to be desert-adapted species and poor competitors with grass
(Ehlerginger et al. 1979; Stamp 1984; Holmgren et al. 2000;
Schutzenhofer \& Valone 2006), while only \emph{E. moschatum} , which
was much less abundant, is considered ruderal (IUCN Invasive Species
Database, http://www.iucngisd.org/gisd/species.php?sc=518). If an
important impact of degus is via soil aridity in grazing lawns, this
might also account for Madrigal et al. (2011)'s finding that degu
presence resulted in a numerical dominance of non-native species. Their
research site is more arid than ours, and a further increase in soil
aridity may more strongly favour non-native desert-adapted herbs.

In summary, contrary to the implication of the IDH, non-trophic
disturbance (biopedturbation) and perturbation (herbivory), appear to
have different impacts that interact with one another and with specific
plant traits to create ecosystem engineering that favours non-native
herbs on degu colonies (see Figure 5). We found the least evidence for
impacts of disturbance via biopedturbation. Biopedturbation clearly
creates the runways, which appears to intially favour mosses. However,
ongoing current biopedturbation disturbance effects could not clearly be
separated from grazing effects as they were highly correlated. Further,
only the least-abundant non-native herb found is ruderal. Perturbation
in the form of grazing effects could be interpreted from the reduction
in grass, relative to other herbs, off runways, associated with
increasing non-native plants. We did not anticipate that ecosystem
engineering affecting soil properties (soil aridity) might explain the
expansion of non-native species, since the majority of literature on
non-native herbs in central Chile focuses on their adaptations to
herbivory rather than aridity, and disturbance/ perturbation literatures
focus on the mechanisms of competition and biomass loss, without linking
to ecosystem engineering. However, pre-existing measurements of soil
hardness and moisture at the same degu colony, and the desert
adaptations of the most abundant non-native species, provide evidence
for the hypothesis that ecosystem engineering of the soil conditions
contributes to the spread of non-native herbaceous species on degu
colonies.

In general terms, these results further expand the notion of pulse/
disturbance developed by Jentsch \& White (2019). Not only are pulse/
disturbance events multi-factorial, they have multi-temporal impacts
that develop over time in ways that do not simply reduce competition or
increase biomass loss (they may not do this at all), but also may
accumulate or interact to form what we call ecosystem engineering. Wilby
et al. (2001) come to a similar conclusion in a study of desert
porcupines. However, the vast majority of other studies either do not
mention both concepts together, or contrast the supposedly independent
and indeed \emph{opposite} processes of disturbance and ecosystem
engineering (e.g. Soissons et al. 2019). This conception of ecosystem
engineering as opposite to disturbance can partly be understood from the
perspective where pulses are also opposite to, rather than dimensions
of, disturbance. The lack of integration of ecosytem engineering ecology
with disturbance ecology also simply reflects the different, and
non-integrated literatures from plant ecology, animal ecology, and
plant-animal interaction ecology (see Root-Bernstein 2013). Further
developing the conceptual link between pulse/ disturbances and ecosystem
engineering offers an interesting direction for development of
theoretical alternatives to the IDH.

Data Accessibility Statement

Data will be uploaded to Dryad upon manuscript acceptance.

Acknowledgements

Thanks to Jacqueline Ho and Carolyn Bauer for transport to the research
site, and Luis Ebensperger for comments and feedback on drafts of this
paper. We also thank the administration of Estación Experimental
Rinconada de Maipú for permission to conduct the study.

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Ecology, 85(8), 2071-2081.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 1. Left, a degu runway recently formed by pushing the grass down
or aside. Right, more established degu runway in a degu grazing lawn.
Both photos are taken during 2013 in early spring. Photos \selectlanguage{ngerman}© MR-B.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 2. The field site with the central and peripheral plots marked.
The letters highlighted in green are the ten central plots, and the
numbers highlighted in yellow are the thirteen peripheral plots. The
white line represents an access road and the black line represents a dry
ditch.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Figure 3. The distribution of non-native herbs across colony areas and
on and off runways, expressed as a percent of the total cover available
in each category.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 4. Each point represents a plot, and its ratio of grass on the
runway to off the runway, plotted against its ratio of all other herbs
(forbs) on the runway to off the runway. The shaded areas indicate where
there are more grasses or more other herbs (forbs) off runways than on
runways, the unshaded area has fewer grasses AND forbs off runways than
on runways. Where the blue and yellow shading overlap and make green,
there is more grass on the runways than off, AND more of all other herbs
(forbs) on the runways than off: thus there are more herbs of all kinds
on the runways than off. Note that the majority of the plots are in the
yellow section, in which grass is relatively more abundant on runways
and other herbs are relatively less abundant on runways.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Figure 5. Diagram of the mechanisms of disturbance and their
accumulation over time into an ecosystem engineering effect, which
favours non-native dry-adapted species.

\selectlanguage{english}
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