Textural Discontinuity Hypothesis Research Articles

The textural discontinuity hypothesis: an exploration at a regional level. Shortened version: exploring Holling's TDH

Dominant physical and biological processes in ecosystems occur at specific scales of space and time. The life‐spans and the life‐spaces (areas used by species over their lifetime) become entrained to operate at similar scales. Because life‐spans and life‐spaces are related to body size, ecosystems display polymodality in body size distributions: Holling's textural discontinuity hypothesis, TDH. Falsification of the TDH requires either changing the frequencies of the dominant processes or changing the species. Both are difficult to achieve for regional‐scale faunas, but the transformation of the terrestrial fauna of New Zealand by humans over the past 800 years provides an opportunity to explore the effect of changing the species. Our analyses of the pre and post first‐contact with humans assemblages show that species body size spectra are polymodal and similar (the spectrum is conservative in shape), both pre‐ and post‐spectra exhibiting three distinct modes, despite significant changes in the taxonomic make‐up of the fauna. Our findings are consistent with the TDH, but not consistent with other known competing explanations. There is also a compelling case that invasions and introductions have been more successful in the body size range that falls between modes. This is also consistent with the TDH, but not necessarily at odds with explanations based on propagule pressure.

Patterns in estuarine macrofauna body size distributions: The role of habitat and disturbance impact

Schwinghamer's (1981) habitat architecture hypothesis for body mass spectra in marine sediments predicts a single macrofauna mode in response to the bulk nature of the sediment. This proposition was examined for intertidal macrofauna from a well-studied estuarine system, using kernel density estimation to define modality and the locations of peaks and troughs. Three sedimentary environments and habitats were examined along a disturbance gradient related to eutrophication. Our results indicate that bimodality is likely to occur within the macrofauna size range, which weakens the habitat architecture model and casts doubts on the mechanisms behind other modes in benthic size spectra. The location of the modes and intervening trough were not conservative and not apparently related to sediment grain size or habitat structure, but somewhat dependent on the presence of particular species: the presence or absence of large numbers of individuals of Hydrobia ulvae and larger-bodied taxa such as Scrobicularia plana and Hediste diversicolor. Alternative competing hypotheses are explored for the observed results, including Warwick's (1984) phylogenetic explanation, but taking into consideration both species composition and disturbance impact, it seems most likely Holling's (1992) textural discontinuity hypothesis, as a measure of resilience, could be a plausible explanation.

Cross-scale Habitat Structure Drives Fish Body Size Distributions on Coral Reefs

Despite a large number of studies focusing on the complexity of coral reef habitats and the characteristics of associated fish assemblages, the relationship between reef structure and fish assemblages remains unclear. The textural discontinuity hypothesis, which proposes that multi-modal body size distributions of organisms are driven by discontinuous habitat structure, provides a theoretical basis that may explain the influence of habitat availability on associated organisms. In this study we use fractal techniques to characterize patterns of cross-scale habitat complexity, and examine how this relates to body-depth abundance distributions of associated fish assemblages over corresponding spatial scales. Our study demonstrates that: (1) Reefs formed from different underlying substrata exhibit distinct patterns of cross-scale habitat complexity; (2) The availability of potential refuges at different scales correlates with patterns in fish body depth distributions, but habitat structure is more strongly related to the relative abundance of fish in the body depth modes, rather than to the number of modes; (3) As reefs change from coral- to algal-dominated states, the complexity of the underlying reef substratum may change, presenting a more homogenous environment to associated assemblages; (4) Individual fish body depth distributions may be multi-modal, however, these distributions are not static characteristics of the fish assemblage and may change to uni-modal forms in response to changing habitat condition. In light of predicted anthropogenic changes, there is a clear need to improve our understanding of the scale of ecological relationships to anticipate future changes and vulnerabilities.

Predictions and retrodictions of the hierarchical representation of habitat in heterogeneous environments

Interaction between habitat and species is central in ecology. Habitat structure may be conceived as being hierarchical, where larger, more diverse, portions or categories contain smaller, more homogeneous portions. When this conceptualization is combined with the observation that species have different abilities to relate to portions of the habitat that differ in their characteristics, a number of known patterns can be derived and new patterns hypothesized. We propose a quantitative form of this habitat–species relationship by considering species abundance to be a function of habitat specialization, habitat fragmentation, amount of habitat, and adult body mass. The model reproduces and explains patterns such as variation in rank–abundance curves, greater variation and extinction probabilities of habitat specialists, discontinuities in traits (abundance, ecological range, pattern of variation, body size) among species sharing a community or area, and triangular distribution of body sizes, among others. The model has affinities to Holling's textural discontinuity hypothesis and metacommunity theory but differs from both by offering a more general perspective. In support of the model, we illustrate its general potential to capture and explain several empirical observations that historically have been treated independently.

Patterns, Processes, and the Textural Discontinuity Hypothesis

Patterns, Processes, and the Textural Discontinuity Hypothesis

Testing Holling’s textural‐discontinuity hypothesis

Testing Holling’s textural‐discontinuity hypothesis

The role of landscape texture in conservation biogeography: a case study on birds in south‐eastern Australia

ABSTRACTThe binary classification of landscapes into suitable vs. unsuitable areas underlies several prominent theories in conservation biogeography. However, a binary classification is not always appropriate. The textural discontinuity hypothesis provides an alternative theoretical framework to examine the geographical distribution of species, and does not rely on a binary classification scheme. The texture of a given landscape is the combination of its vertical structural complexity and horizontal spatial grain. The textural discontinuity hypothesis states that biophysical features in the environment are scaled in a discontinuous way, and that discontinuities in the body size distribution of animals mirror these biophysical discontinuities. As a result of this relationship, a complex landscape texture should be associated with small‐bodied animals, whereas a simple landscape texture should be associated with larger‐bodied animals. We examined this hypothesis for birds in five landscapes in south‐eastern Australia that represented a gradient from simple to complex landscape texture. In landscapes with a complex texture, the number of detections of small birds was higher than expected, and the number of detections of larger‐bodied birds was lower than expected. The opposite pattern was found in landscapes with a simple texture. The pattern remained significant when only bird species found in each of the five landscapes were considered, which demonstrated that the association of landscape texture with body size was not an artefact of landscapes differing in their species pools. Understanding the effects of landscape texture on species distribution patterns may be a promising research frontier for conservation biogeography. We hypothesize that the active management of landscape texture may be used to attract or deter animals of certain body sizes. Consistent with other theories, the textural discontinuity hypothesis therefore suggests that managing entire landscapes, rather than only predefined patches, is an important conservation strategy.

Evaluating Discontinuities in Complex Systems: Toward Quantitative Measures of Resilience

The textural discontinuity hypothesis (TDH) is based on the observation that animal body mass distributions exhibit discontinuities that may reflect the texture of the landscape available for exploitation. This idea has been extended to other complex systems, hinting that the identification and quantification of discontinuities in the distributions of appropriate variables may provide clues to emergent system properties such as resilience. We propose a discontinuity index, based on the vector norm of the full assemblage of observed discontinuities, as a means to quantify and compare this characteristic among systems. We also evaluate four methods to identify the number and location of the most prominent discontinuities. Although results of the four methods are similar, they are not identical, and we conclude that this problem is best addressed with a consistent operationally defined approach in an adaptive inference framework.

Cross-scale Structure and Scale Breaks in Ecosystems and Other Complex Systems

The five articles in this special feature extend the discovery of regular patterns of deviation from scaling laws and from continuous distributions of attributes in ecosystems and other complex systems. These patterns suggest that these systems organize over discrete ranges of scale and that organization abruptly shifts with changes in scale. If this is so, scaling laws (for example, see West 1997, 1999; Zipf 1949) serve only as the baseline from which to measure those departures, and those departures indicate “scale breaks” (transitions) between scales of structure in complex systems. Patterns in the deviations from a scaling-law baseline may provide hints of the processes that cause the emergence of the scaling relationships themselves. At minimum, the investigation of departures from scaling laws gives us a clue into the nature of structure and process in ecological systems. Ecosystems may be structured by relatively few key processes, each operating at specific temporal and spatial scales (Carpenter and Leavitt 1991; Levin 1992). The distinct temporal frequencies and spatial scale characterizing these processes creates landscape structures with scale-specific pattern (Burrough 1981; O’Neill and others 1991; Milne and others 1992). This may in turn entrain attributes of animals residing on the landscape (Holling 1992) because different-sized animals living upon the same landscape perceive their environment at different scales (Milne and others 1989; Holling 1992; Peterson and others 1998) (Figure 1). Holling (1992) suggested that this entrainment reflects adaptations to the discontinuous pattern of resource distribution acting through animal community assembly and evolution, both by sorting species and by providing a specific set of evolutionary opportunities and constraints. On the animal community level, this should be expressed by a discontinuous distribution of species body masses (Holling 1992). Within a system, aggregations of species body masses are separated by discrete breaks (or discontinuities) separating different ranges of scale. Animals within a particular body mass aggregation perceive and exploit the environment at the same range of scale (Peterson and others 1998) (Figure 2). Holling’s proposition that ecosystem structure entrains attributes of animals—such as body mass distributions (the textural discontinuity hypothesis)—was received with some skepticism, but also with interest. It is now generally accepted that many attributes of ecosystems are discontinuously distributed, but mechanisms other than entrainment by ecological structure (Brown 1995) have been proposed. However, most of the disagreements with Holling’s proposition have been technical, focusing on the methods used to detect discontinuities (Manly 1996; Siemann and Brown 1999). New evidence for a link between landscape structure at different scales and body mass distributions has provided support for the textural discontinuity hypothesis. Discontinuous body mass patterns have been documented in many systems (see for example, Holling 1992; Restrepo and others 1997; Lambert and Holling 1998; Allen and others 1999; RafReceived 25 January 2002; accepted 28 January 2002. *Corresponding author; e-mail: allencr@clemson.edu Ecosystems (2002) 5: 315–318 DOI: 10.1007/s10021-001-0075-3 ECOSYSTEMS

Gaps in Mammalian Body Size Distributions Reexamined

Holling suggested that discontinuities in the body size distributions among species of animals are a universal feature of terrestrial biomes. We compared the magnitudes of body size gaps of mammal communities of North America and Australia to those generated by a simple random null model. In most biomes, no gaps were significantly larger than random, so discontinuities in body size distributions are the exception, not the rule. We also made intra- and intercontinental comparisons of size distributions to test two alternative hypotheses: (1) Holling’s Textural-Discontinuity Hypothesis, that body size distributions reflect structural characteristics of the habitat; and (2) the Core-Taxa Hypothesis, that body sizes reflect the distributions of widespread taxa. We found that the gaps in body size were similar in structurally dissimilar but adjacent biomes that shared the same or closely related species. We conclude that body size distributions of biomes are not highly discontinuous, and their structure reflects taxonomic constraints on body size.

GAPS IN MAMMALIAN BODY SIZE DISTRIBUTIONS REEXAMINED

Holling suggested that discontinuities in the body size distributions among species of animals are a universal feature of terrestrial biomes. We compared the magnitudes of body size gaps of mammal communities of North America and Australia to those generated by a simple random null model. In most biomes, no gaps were significantly larger than random, so discontinuities in body size distributions are the exception, not the rule. We also made intra- and intercontinental comparisons of size distributions to test two alternative hypotheses: (1) Holling’s Textural-Discontinuity Hypothesis, that body size distributions reflect structural characteristics of the habitat; and (2) the Core-Taxa Hypothesis, that body sizes reflect the distributions of widespread taxa. We found that the gaps in body size were similar in structurally dissimilar but adjacent biomes that shared the same or closely related species. We conclude that body size distributions of biomes are not highly discontinuous, and their structure reflec...