Logic, not convention, should dictate methodology: spatial pattern as a case study (original) (raw)
Related papers
Journal of Vegetation Science, 2013
Questions: A species' spatial pattern is the outcome of a series of filters: demographic, disturbance, environmental and functional, all varying over space and time. To evaluate the importance of function as a filter we ask: (1) do a species' functional traits allow prediction of its fine-scale spatial patterning, and (2) how consistent is the fine-scale spatial pattern shown by the same species across multiple sites where the properties of the other filters may vary? Location: Species-rich Mediterranean climate shrublands of southwestern Australia's northern sandplains. Methods: Using fully mapped plots at four sites (each >10 000 individuals, 74-112 species), we characterized individual species spatial patterning using point pattern analyses. We classified species spatial patterns in three ways: (1) whether they departed from a null model controlling for first-order effects (i.e. are they aggregated?), (2) parameterization of Thomas cluster processes (what form does any clustering take?), and (3) their position in a multivariate 'pattern space' (do species show different types of pattern?). We then explored the extent to which a species' functional traits, abundance and/or the site at which it occurred predicted these three facets of its spatial pattern. Results: Although at all sites most species were aggregated, site was consistently important in predicting a species' spatial pattern. Regenerative response to firewhether a species is killed by fire and recruits solely via seeds, or survives and resprouts vegetatively after firewas the functional trait most consistently useful in predicting a species' spatial pattern. Fire-killed species tended to show more aggregated distributions than resprouters. Species present at multiple sites did not show consistency in their spatial patterns across those sites more than expected by chance alone. Conclusions: Although functional traits relating to fire responses and water use predict species spatial distributions at the landscape level, at the fine scales we considered site effects were as important as functional traits in explaining spatial pattern. Within and between site heterogeneity, some of which can be generated by the stochastic properties of fire, may mask the deterministic effects of species functional traits.
Journal of Vegetation Science, 2006
Questions: To what extent can spatial structure and its causes be determined in a highly disturbed environment? What are the main determinants of pattern and are these species-specific? How much do spatial patterns change over generations? Location: Wimmera region of southern Australia. Methods: Broad-leaved weeds were counted in 225 000 contiguous 20-cm square quadrats. A substantial number of these quadrats were recorded again after two and four years. An hierarchical 'adaptive analysis' approach was used to select spatial analytical methods to examine specific aspects of pattern and variation in pattern from year to year. Results: Patterns varied among species and included both dense and sparse patches surrounded by areas of zero density, diffuse gradations of density and clear anisotropy. Patterns in Erodium botrys and Oxalis pes-caprae persisted over years, whereas patterns in Arctotheca calendula were less pronounced and varied over time. Edaphic factors appeared to have only a minor influence over the spatial distribution of the weed community as a whole. In Oxalis pes-caprae, whose patches were hypothesized to have been shaped by cultivation, there was no spread in four years, despite further tillage. Outlying plants of O. pes-caprae failed to establish new patches, even in the year of greatest population increase. Little evidence of localised recruitment events was found. Conclusions: Despite repeated annual disturbances by natural and anthropogenic mechanisms, clear and interpretable spatial structure develops in annual weeds over a range of spatial resolutions. Adaptive analysis is a useful approach to the characterization of such patterns.
Plant Ecology, 2011
At scales from microsites to entire ranges, species' distributions reflect limited adaptation and/or limited dispersal. To what extent are specific distribution patterns and processes similar across scales? We investigated environmental effects-presumed because of adaptation-and independent spatial effects-presumed because of dispersal-on distribution at two scales (landscape patches of approximately 1,300 m 2 , sampled along transects, and 4-m 2 cells, sampled in contiguous grids within populations) and on individual performance (water status, reproduction) in the California annual, Clarkia xantiana ssp. xantiana. Because water limitation helps set this species' regional borders, we expected occupancy and performance at smaller scales to correlate with topographic and soil features affecting water relations. At the patch scale, environmental features associated with reduced water stress (i.e., steep slopes that face north; coarse, soft soils; igneous rather than metasedimentary parent rock) predicted occupancy. Spatial aggregation was not detected, but incomplete occupancy of apparently suitable patches indicated that dispersal limits occupancy. At the scale of small cells, relationships between environmental variables, occupancy, density, and performance varied among populations. Associations sometimes resembled those at the patch scale but sometimes opposed them. Spatial aggregation in cell occupancy and/or density occurred in all populations, implying limited dispersal, whereas spatial aggregation of water potential values in some populations might have arisen from spatially structured unmeasured environmental variables. Limited adaptation to drought and limited patch colonization appear to affect patch occupancy in C. xantiana ssp. xantiana, whereas smaller-scale patterns indicate consistent effects of limited dispersal and somewhat variable environmental effects.
A comparison of methods for the statistical analysis of spatial point patterns in plant ecology
Plant Ecology, 2006
We describe a range of methods for the description and analysis of spatial point patterns in plant ecology. The conceptual basis of the methods is presented, and specific tests are compared, with the goal of providing guidelines concerning their appropriate selection and use. Simulated and real data sets are used to explore the ability of these methods to identify different components of spatial pattern (e.g. departure from randomness, regularity vs. aggregation, scale and strength of pattern). First-order tests suffer from their inability to characterise pattern at distances beyond those at which local interactions (i.e. nearest neighbours) occur. Nevertheless, the tests explored (first-order nearest neighbour, Diggle's G and F) are useful first steps in analysing spatial point patterns, and all seem capable of accurately describing patterns at these (shorter) distances. Among second-order tests, a density-corrected form of the neighbourhood density function (NDF), a non-cumulative analogue of the commonly used Ripley's K-function, most informatively characterised spatial patterns at a range of distances for both univariate and bivariate analyses. Although Ripley's K is more commonly used, it can give very different results to the NDF because of its cumulative nature. A modified form of the K-function suitable for inhomogeneous point patterns is discussed. We also explore the use of local and spatially-explicit methods for point pattern analysis. Local methods are powerful in that they allow variations from global averages to be detected and potentially provide a link to recent spatial ecological theory by taking the 'plant's-eye view'. We conclude by discussing the problems of linking spatial pattern with ecological process using three case studies, and consider some ways that this issue might be addressed.
Geographic variation in dynamics of an annual plant with a seed bank
Journal of Ecology, 2009
Most population dynamics studies are geographically restricted, yet species ranges are large. We performed multiyear roadside surveys of the sunflower, Helianthus annuus, at two locations that differ in precipitation (eastern Kansas, KS; western Nebraska, NE). Our goals were to (i) document if there was geographic variation in dynamics and evaluate the role of habitat variables and the landscape matrix; (ii) determine the likely amount of occupiable habitat and (iii) explore the role of seed banks in dynamics. 2. Geographical variation: Occupancy and mean numbers of plants per occupied roadside unit were generally higher in NE than KS. Sunflower abundance was linearly related to spring precipitation in NE but not in KS. Soil disturbance was associated with increased occupancy and apparent colonization, and reduced apparent extinction. Variation in the landscape adjacent to roads had a larger effect on occupancy in KS than in NE. In KS, smaller populations were more prone to apparent extinction; NE results were more variable. Note that we refer to 'apparent' colonization or extinction because seed banks may persist even when above-ground plants are absent. 3. Occupiable habitat: 25% of the roadside was never occupied by sunflowers in KS, despite surveying for 6 years. An asymptotic limit to occupancy in NE was not apparent, but fewer years were surveyed. 4. Seed banks: Seed banks appear prevalent. The strongest evidence comes from a year following a spring drought in NE, when 100s to 1000s of plants were found in units that lacked plants the year before. 5. Synthesis. We found both geographical similarities (role of soil disturbance, seed banks) and differences (response to rainfall variation, importance of landscape matrix) in sunflower dynamics. Our work suggests that for appropriate species (including many weedy and invasive plants), replicate roadside surveys are an efficient way to evaluate geographic variation in dynamics, the landscape matrix and habitat characteristics across a broad geographic area. Such data help bridge the gap between broad-scale distributional studies and small experimental plot studies, and provide insights on the population dynamics that underlie species ranges.
Ecological Processes, 2021
Background Ecological processes such as seedling establishment, biotic interactions, and mortality can leave footprints on species spatial structure that can be detectable through spatial point-pattern analysis (SPPA). Being widely used in plant ecology, SPPA is increasingly carried out to describe biotic interactions and interpret pattern-process relationships. However, some aspects are still subjected to a non-negligible debate such as required sample size (in terms of the number of points and plot area), the link between the low number of points and frequently observed random (or independent) patterns, and relating patterns to processes. In this paper, an overview of SPPA is given based on rich and updated literature providing guidance for ecologists (especially beginners) on summary statistics, uni-/bi-/multivariate analysis, unmarked/marked analysis, types of marks, etc. Some ambiguities in SPPA are also discussed. Results SPPA has a long history in plant ecology and is based o...
Plant Ecology (formerly Vegetatio), 2004
Ecologists increasingly use spatial statistics to study vegetation patterns. Mostly, however, these techniques are applied in a purely descriptive fashion without a priori statements on the pattern characteristics expected. We formulated such a priori predictions in a study of spatial pattern in a semi-arid Karoo shrubland, South Africa. Both seed dispersal and root competition have been discussed as processes shaping the spatial structure of this community. If either of the two processes dominates pattern formation, patterns within and between shrub functional groups are expected to show distinct deviations from null models. We predicted the type and scale of these deviations and compared predicted to observed pattern characteristics. As predicted by the seed dispersal hypothesis, small-scale co-occurrence within and between groups of colonisers and successors was increased as compared to complete spatially random arrangement of shrubs. The root competition predictions, however, were not met as shrubs of similar rooting depth co-occurred more frequently than expected under random shrub arrangement. Since the distribution of rooting groups to the given shrub locations also failed to match the root competition predictions, there was little evidence for dominance of root competition in pattern formation. Although other processes may contribute to small-scale plant co-occurrence, the sufficient and most parsimonious explanation for the observed pattern is that its formation was dominated by seed dispersal. To characterise point patterns we applied both cumulative ͑uniand bivariate K-function͒ and local ͑pairand mark-correlation function͒ techniques. Based on our results we recommend that future studies of vegetation patterns include local characteristics as they independently describe a pattern at different scales and can be easily related to processes changing with interplant distance in a predictable fashion.
Journal of Ecology, 2003
1 Many narrowly endemic species are restricted to distinctive edaphic environments. The adaptations that make these species successful in their restricted habitat may incur a cost, and decrease their success in more common habitats. We compared growth, biomass allocation and root morphology of two narrowly endemic Hakea species (Proteaceae) of Mediterranean south-western Australia with those of five more widespread congeners, in a glasshouse study. The rare Hakea species occur in endangered winterwet shrublands that grow on skeletal (0-20 cm deep) soils overlying massive ironstone rock, whereas their common congeners occur nearby on deeper wetland and nonwetland soils. 2 The ironstone endemics differed consistently from their widespread congeners in some important root characteristics. During early development they allocated relatively more biomass to their roots, and had a higher specific root length due to a lower average root diameter and a lower root mass density. Therefore, when compared at the same plant mass, the ironstone endemics had a considerably greater total root length. 3 The ironstone endemics also favoured root growth in deeper layers of the substrate: they invested up to 64% of their root mass in the bottom 10 cm of 40-cm-deep pots, vs. 35% for common species. Only in the ironstone endemics did the extension of the main root axis continue at the same rate after reaching the bottom of the pot. 4 We suggest that the observed differences are the consequence of evolutionary tradeoffs, and represent specializations of the endemic species to increase the chances of getting access to water before the onset of severe summer drought in these habitats. However, while adaptive in a shallow-soil habitat, these traits may reduce success on deeper soils by compromising both below-ground and above-ground competitive abilities.
Models of the development of spatial pattern of an even-aged plant population over time
Ecological Modelling, 1987
. Models of the development of spatial pattern of an even-aged plant population over time. Ecol. Modelling, 39: 45-57. The development of spatial patterns of a single even-aged population in a homogeneous area was studied by means of simulation and analytical models. The simulation model was designed to reflect ecological reality as much as possible, simultaneously keeping a reasonable level of simplicity. Results of simulations were supported by analysis of a simplified and more mathematically tractable model. It was shown that the main factor causing the decrease of aggregation intensity or tendency to regularity in the course of population development is the competition among neighbouring individuals. Random patterns may be a result of changes of initial aggregated pattern caused by competition among neighbours. Hence, an observed random pattern is not evidence for the independence of individuals.