1,720,977 research outputs found
Spatial instabilities untie the exclusion-principle constraint on species coexistence
No full textUsing a spatially explicit mathematical model for water-limited
vegetation we show that spatial instabilities of uniform states can lead
to species coexistence under conditions where uniformly distributed
species competitively exclude one another. Coexistence is made possible
when water-rich patches formed by a pattern forming species provide
habitats for a highly dispersive species that is a better competitor in
uniform settings. (C) 2013 Elsevier Ltd. All rights reserved
OSTWALD RIPENING IN DRYLAND VEGETATION
No full textDry land landscapes self-organize to form various patterns of vegetation
patchiness. Two major classes of patterns can be distinguished: regular
patterns with characteristic length scales and scale-free patterns. The
latter form under conditions of global competition over the water
resource. In this paper we show that the asymptotic dynamics of
scale-free vegetation patterns involve patch coarsening similar to
Ostwald ripening in two-phase mixtures. We demonstrate it numerically,
using a spatially explicit model for water-limited vegetation, and
further study it by drawing an analogy to an activator-inhibitor system
that shares many properties with the vegetation system. The ecological
implications of patch coarsening may not be highly significant due to
the long time scales involved. The reported results, however, raise an
interesting pattern formation question associated with the
incompatibility of mechanisms that stabilize vegetation spots and the
condition of global competition
Phenotypic plasticity: A missing element in the theory of vegetation pattern formation
No full textRegular spatial patterns of vegetation are a common sight in drylands. Their formation is a population-level response to water stress that increases water availability for the few via partial plant mortality. At the individual level, plants can also adapt to water stress by changing their phenotype. Phenotypic plasticity of individual plants and spatial patterning of plant populations have extensively been studied independently, but the likely interplay between the two robust mechanisms has remained unexplored. In this paper, we incorporate phenotypic plasticity into a multi-level theory of vegetation pattern formation and use a fascinating ecological phenomenon, the Namibian "fairy circles," to demonstrate the need for such a theory. We show that phenotypic changes in the root structure of plants, coupled with pattern-forming feedback within soil layers, can resolve two puzzles that the current theory fails to explain: observations of multi-scale patterns and the absence of theoretically predicted large-scale stripe and spot patterns along the rainfall gradient. Importantly, we find that multi-level responses to stress unveil a wide variety of more effective stress-relaxation pathways, compared to single-level responses, implying a previously underestimated resilience of dryland ecosystems
Clarifying misunderstandings regarding vegetation self-organisation and spatial patterns of fairy circles in Namibia: a response to recent termite hypotheses
Full text linkAdopting a spatially explicit perspective to study the mysterious fairy circles of Namibia
Full text linkThe mysterious fairy circles' are vegetation-free discs that cover vast
areas along the pro-Namib Desert. Despite 30 yr of research their origin
remains unknown. Here we adopt a novel approach that focuses on analysis
of the spatial patterns of fairy circles obtained from representative
25-ha aerial images of north-west Namibia. We use spatial point pattern
analysis to quantify different features of their spatial structures and
then critically inspect existing hypotheses with respect to their
ability to generate the observed circle patterns. Our working hypothesis
is that fairy circles are a self-organized vegetation pattern. Finally,
we test if an existing partial-differential-equation model, that was
designed to describe vegetation pattern formation, is able to reproduce
the characteristic features of the observed fairy circle patterns. The
model is based on key-processes in arid areas such as plant competition
for water and local resource-biomass feedbacks. The fairy circles showed
at all three study areas the same regular spatial distribution patterns,
characterized by Voronoi cells with mostly six corners, negative
correlations in their size up to a distance of 13 m, and remarkable
homogeneity over large spatial scales. These results cast doubts on
abiotic gas-leakage along geological lines or social insects as causal
agents of their origin. However, our mathematical model was able to
generate spatial patterns that agreed quantitatively in all of these
features with the observed patterns. This supports the hypothesis that
fairy circles are self-organized vegetation patterns that emerge from
positive biomass-water feedbacks involving water transport by extended
root systems and soil-water diffusion. Future research should search for
mechanisms that explain how the different hypotheses can generate the
patterns observed here and test the ability of self-organization to
match the birth- and death dynamics of fairy circles and their regional
patterns in the density and size with respect to environmental
gradients
Periodic versus scale-free patterns in dryland vegetation
No full textTwo major forms of vegetation patterns have been observed in drylands:
nearly periodic patterns with characteristic length scales, and
amorphous, scale-free patterns with wide patch-size distributions. The
emergence of scale-free patterns has been attributed to global
competition over a limiting resource, but the physical and ecological
origin of this phenomenon is not understood. Using a spatially explicit
mathematical model for vegetation dynamics in water-limited systems, we
unravel a general mechanism for global competition: fast spatial
distribution of the water resource relative to processes that exploit or
absorb it. We study two possible realizations of this mechanism and
identify physical and ecological conditions for scale-free patterns. We
conclude by discussing the implications of this study for interpreting
signals of imminent desertification
Emerged or imposed: a theory on the role of physical templates and self-organisation for vegetation patchiness
No full textIn this article, we develop a unifying framework for the understanding
of spatial vegetation patterns in heterogeneous landscapes. While much
recent research has focused on self-organised vegetation the prevailing
view is still that biological patchiness is mostly due to top-down
control by the physical landscape template, disturbances or predators.
We suggest that vegetation patchiness in real landscapes is controlled
both by the physical template and by self-organisation simultaneously,
and introduce a conceptual model for the relative roles of the two
mechanisms. The model considers four factors that control whether
vegetation patchiness is emerged or imposed: soil patch size, plant
size, resource input and resource availability. The last three factors
determine the plant-patch size, and the plant-to-soil patch size ratio
determines the impact of self-organisation, which becomes important when
this ratio is sufficiently small. A field study and numerical
simulations of a mathematical model support the conceptual model and
give further insight by providing examples of self-organised and
template-controlled vegetation patterns co-occurring in the same
landscape. We conclude that real landscapes are generally mixtures of
template-induced and self-organised patchiness. Patchiness variability
increases due to sourcesink resource relations, and decreases for
species of larger patch sizes
Discovery of fairy circles in Australia supports self-organization theory
No full textVegetation gap patterns in arid grasslands, such as the “fairy circles” of Namibia, are one of nature’s greatest mysteries and subject to a lively debate on their origin. They are characterized by small-scale hexagonal ordering of circular bare-soil gaps that persists uniformly in the landscape scale to form a homogeneous distribution. Pattern-formation theory predicts that such highly ordered gap patterns should be found also in other water-limited systems across the globe, even if the mechanisms of their formation are different. Here we report that so far unknown fairy circles with the same spatial structure exist 10,000 km away from Namibia in the remote outback of Australia. Combining fieldwork, remote sensing, spatial pattern analysis, and process-based mathematical modeling, we demonstrate that these patterns emerge by self-organization, with no correlation with termite activity; the driving mechanism is a positive biomass–water feedback associated with water runoff and biomass-dependent infiltration rates. The remarkable match between the patterns of Australian and Namibian fairy circles and model results indicate that both patterns emerge from a nonuniform stationary instability, supporting a central universality principle of pattern-formation theory. Applied to the context of dryland vegetation, this principle predicts that different systems that go through the same instability type will show similar vegetation patterns even if the feedback mechanisms and resulting soil–water distributions are different, as we indeed found by comparing the Australian and the Namibian fairy-circle ecosystems. These results suggest that biomass–water feedbacks and resultant vegetation gap patterns are likely more common in remote drylands than is currently known
Reply to Walsh et al.: Hexagonal patterns of Australian fairy circles develop without correlation to termitaria
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