Introduction
For decades we have witnessed the rapid decline of biodiversity across
ecosystems due to the synergistic impacts of natural and human-caused
disturbances (Stork 2010; Pimm et al . 2014). The combined effects
of current and future disturbances represent a threat to the stability
of ecosystems that can have significant and broad-scale impacts on
multiple environmental services that are linked to and supported by
biodiversity (Millennium Ecosystem Assessment 2005). The stability of
ecosystems in response to disturbance has intrigued ecologists for
decades (Odum 1950; Holling 1973; McCann 2000) and considerable research
has been devoted to the relationship between diversity and ecosystem
stability. The work of MacArthur (1955) and Elton (1958) pioneered the
diversity-stability hypothesis, which states that diversity promotes
stability. However, later theoretical and applied ecological studies
found evidence of positive, negative, neutral, and dynamic effects of
diversity on ecosystem stability (Ives & Carpenter 2007; Tilmanet al . 2012; Isbell et al . 2015; Pennekamp et al .
2018). These varied findings and lack of consensus correspond to the
multifaceted nature of disturbances, species diversity, and stability,
coupled with the limitations of what can be measured by ecologists
(Donohue et al . 2016).
Ecological stability is a broad concept that can be described using
three main components in response to a disturbance: temporal variability
in a system property, persistence of functions and structure in the face
of a change in external conditions (resistance), and rate of recovery
from disturbance (resilience) (Pimm 1984; McCann 2000; Donohue et
al . 2016). Empirical studies of biodiversity-stability have focused
primarily on the temporal variability of one major measure of ecosystem
function – plant productivity – in order to establish and measure the
integrity of a community or ecosystem (Tilman 1996; Pfisterer & Schmid
2002; Vogel et al . 2012; Isbell et al . 2015; Lepš et
al . 2018). But it is also important to assess the ecological resistance
as a measure of stability, as pressures of climate change become more
frequent, that can help inform policies governing land und use and
biodiversity conservation (Donohue et al . 2016; Pennekampet al . 2018).
We focused our study on the Colorado Plateau, a dryland ecoregion of the
Southwest United States, where increasing aridity and drought severity
are among several expressions of climate change likely to impact these
ecosystems and the human societies that depend on their services (Seageret al . 2007; Munson et al . 2011; Cook et al . 2015;
Hoover et al . 2015). While responses of dryland ecosystems to the
changing climate are still uncertain (Huang et al . 2017), global
drylands could exhibit shifts in the distribution, structure, function,
and composition of ecosystems; however, geographical and latitudinal
variation suggest different outcomes for particular regions (Trenberthet al . 2014; Huang et al . 2017). The cold desert of the
Colorado Plateau in southern Utah is one of several dryland regions
suffering the combined effects of climate change, changes in land use
and invasive species (Munson et al . 2011; Copeland et al .
2017).
One nearly ubiquitous feature of drylands is the presence of a soil
surface biocrust community. In these systems, the soil beneath and
between the canopy of widely spaced vascular plants is generally
dominated by the soil surface community of cyanobacteria, algae, fungi,
lichens, bryophytes, and soil microbes collectively known as a
biological soil crust (biocrust). This community is a major ecosystem
component involved in water balance maintenance, nitrogen fixation, soil
stability, wind and water erosion resistance, and dynamic interactions
with other biota of dryland ecosystems (Belnap & Gardner 1993; Belnap
& Weber 2013; Maestre et al . 2016; Seitz et al . 2017;
Havrilla et al . 2019; Eldridge et al . 2020).
Biocrusts often follow a pattern of successional sequence, in which the
bare ground is colonized by lightly-pigmented, filamentous cyanobacteria
that aggregate the soil particles, making soils suitable for subsequent
colonization by nitrogen-fixing, darkly pigmented cyanobacteria, and
then by mosses and/or lichens (Read et al . 2016; Weber et
al . 2016). Based on the dominance of cyanobacteria versus mosses and/or
lichens, biocrusts are often classified into early and late successional
stages, respectively, which inform the properties of the community (Lanet al . 2012). Despite being well-adapted to harsh arid
conditions, biocrusts and particularly mosses are sensitive to shifts in
the timing and magnitude of precipitation and to rising temperatures
that shorten the hydration periods in which biocrusts are
physiologically active (Reed et al . 2012). Therefore, climate
change is a disturbance that can potentially lead to a decrease of the
late successional constituents, mosses and lichens, i.e., a reversal of
succession (Ferrenberg et al . 2015).
We used a long-term precipitation reduction experiment in the field to
investigate the effects of drought on biocrust community composition and
successional maturity, and to test the diversity-stability hypothesis
using resistance to climate disturbance as one component of ecosystem
stability (Pimm 1984). In this context, we defined resistance as the
ability of a community to maintain compositional and structural
integrity following a disturbance (Sankaran & McNaughton 1999; Connell
& Ghedini 2015). Biocrust composition was measured by species presence
and abundance, while the successional maturity was considered as the
abundances of the functional groups of the community. We hypothesized
that: (1) Precipitation reduction would induce a successional reversal
in biocrust composition, which would be indicated by a decline in moss
and lichen cover and increase in cyanobacterial cover; (2) Higher
initial diversity would confer greater compositional resistance, which
would be indicated by less extreme changes of community composition to
the imposed climate perturbation; and (3) The degree of compositional
resistance observed in the biocrust community would be inversely related
to the degree of the environmental stress experienced. Testing these
hypotheses helped to illuminate the factors controlling community change
under a climate change scenario and to move us closer to being able to
predict the magnitude of changes in dryland communities.