153 research outputs found
The effect of cross-boundary management on the trajectory to commonness in biological invasions
The number of alien species introduced and undergoing range expansion in novel environments is steadily increasing, with important consequences for native ecosystems. The efficacy of management planning and decision making to limit such invasions can be improved by understanding how interventions will impact the population dynamics of recently introduced species. To do so, here we expand on a typological framework that enables the classification of populations over time into 10 categories of commonness, and apply it to a spatially discrete metapopulation with heterogeneous abundance across spatial units (patches). We use this framework to assess the effect of cross-boundary management on the capacity of a metapopulation with different demographic and dispersal characteristics, including time lags in population growth, to become common. We demonstrate this framework by simulating a simple theoretical metapopulation model capable of exploring a range of environments, species characteristics, and management actions. Management can vary in the efficacy of propagule interception between patches, and in the synchronisation of the implementation of these measures across patches (i.e. if management is implemented simultaneously across patches). Simulations show that poor interception efficacy that only modestly reduces the number of propagules entering a given spatial unit cannot be compensated for by strong management synchronisation between spatial units. Management synchronisation will nonetheless result in a reduction in rates of spread once a critical threshold of interception efficacy has been met. Finally, time lags in population growth that may result in delayed spread are an important aspect to be considered in management as they can amplify the efficacy of management. Our results demonstrate how a typological framework of categories of commonness can be used to provide practical insights for the management of biological invasions
Does the self-similar species distribution model lead to unrealistic predictions?
J. Harte et al. demonstrated that the power law form of the species–area relationship may be derived from a bisected, self-similar landscape and a community-level probability rule. Harte’s self-similarity model has been widely applied in modeling species distributions. However, R. D. Maddux showed that this self-similarity model generates
biologically unrealistic predictions. We resolve the Harte–Maddux debate by demonstrating that the problems identified by Maddux result from an assumption that the probability of occurrence of a species at one scale is independent of its probability of occurrence at the next. We refer to this as a ‘‘non-heritage assumption.’’ By altering this assumption to one in which
each species in the community has an occupancy status that is partially inherited across scales (a scale-heritage assumption), the predictions of the self-similarity model are neither mathematically inconsistent nor biologically unrealistic. Harte’s self-similarity model remains an important framework for modeling species distributions. Our results illustrate the
importance of considering patterns of species co-occurrence, and the way in which species occupancy patterns change with scale, when modeling species distributions.the DST-NRF Centre of Invasion Biology for financial support
zetadiv:An R package for computing compositional change across multiple sites, assemblages or cases
AbstractSpatial variation in compositional diversity, or species turnover, is necessary for capturing the components of heterogeneity that constitute biodiversity. However, no incidence-based metric of pairwise species turnover can calculate all components of diversity partitioning. Zeta (ζ) diversity, the mean number of species shared by any given number of sites or assemblages, captures all diversity components produced by assemblage partitioning. zetadiv is an R package for analysing and measuring compositional change for occurrence data using zeta diversity. Four types of analyses are performed on bird composition data in Australia: (i) decline in zeta diversity; (ii) distance decay; (iii) multi-site generalised dissimilarity modelling; and (iv) hierarchical scaling. Some analyses, such as the zeta decline, are specific to zeta diversity, whereas others, such as distance decay, are commonly applied to beta diversity, and have been adapted using zeta diversity to differentiate the contribution of common and rare species to compositional change.HighlightsAn R package to analyse compositional change using zeta diversity is presented.Zeta diversity is the mean number of species shared by any number of assemblagesZeta diversity captures all diversity components produced by assemblage partitioningAnalyses relate zeta diversity to space, environment and spatial scaleAnalyses differentiate the contribution of rare and common species to biodiversity</jats:sec
Using scale–area curves to quantify the distribution, abundance and range expansion potential of an invasive species
Aim: Invasive species distribution and abundance data are essential for management decisions on mitigating impacts but is seldom available. Here, we use scale–area curves to assess the distribution, abundance and consequent management implications of an invasive plant (Acacia longifolia) within selected occupancy grid cells, spread across regional ranges and representing the full
national extent. We determine whether occupancy patterns are explained by climatic suitability or range structure and identify areas where A. longifolia can still be regarded as an important invasive based on contiguous occupancy.
Location: South Africa including the Fynbos, Thicket, Savanna and Grassland biomes.
Methods: The quarter degree occupancy of A. longifolia was used to select core, edge, and climatically unsuitable grid cells within different regions of the national range. Cells were surveyed across a linear resolution from 25 km to 2.5 m allowing the first multi-scales description of an invasive species’ space-filling properties. Patterns from grid cells in turn were viewed regionally to describe regional variation in spatial structure.
Results: In regions with contiguous areas of favourable habitat, scale–area curves indicated greater occupancy in core than edge areas, whereas patterns were reversed when suitable areas were more fragmented. Also, at times climatically suitable areas were unoccupied, while unsuitable areas were occupied. Within cells, occupancy was well explained by the presence of fynbos vegetation types, while nationally, contiguous occupancy was almost exclusive to the Fynbos Biome.
Main conclusions: Scale–area curves can advance the understanding of biological invasions and invasive plant distributions. Here, we detected potential areas of invasive concern, plus differences in abundance and distribution patterns, and associated correlates, at landscape and national scales. As there was no general relationship between range position or climatic suitability and A. longifolia’s spatial structure, we propose habitat suitability as an alternative explanation which, in turn, suggests limited range expansion potential.Centre of Excellence for Invasion Biolog
A multi-site method to capture turnover in rare to common interactions in bipartite species networks
1. Ecological network structure is maintained by a generalist core of common species. However, rare species contribute substantially to both the species and functional diversity of networks. Capturing changes in species composition and interactions, measured as turnover, is central to understanding the contribution of rare and common species and their interactions. Due to a large contribution of rare interactions, the pairwise metrics used to quantify interaction turnover are, however, sensitive to compositional change in the interactions of, often rare, peripheral specialists rather than common generalists in the network. 2. Here we expand on pairwise interaction turnover using a multi-site metric that enables quantifying turnover in rare to common interactions (in terms of occurrence of interactions). The metric further separates this turnover into interaction turnover due to species turnover and interaction rewiring. 3. We demonstrate the application and value of this method using a host–parasitoid system sampled along gradients of environmental modification. 4. In the study system, both the type and amount of habitat needed to maintain interaction composition depended on the properties of the interactions considered, that is, from rare to common. The analyses further revealed the potential of host switching to prevent or delay species loss, and thereby buffer the system from perturbation. 5. Multi-site interaction turnover provides a comprehensive measure of network change that can, for example, detect ecological thresholds to habitat loss for rare to common interactions. Accurate description of turnover in common, in addition to rare, species and their interactions is particularly relevant for understanding how network structure and function can be maintained
Prioritizing species, pathways, and sites to achieve conservation targets for biological invasion
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Road verges as invasion corridors? A spatial hierarchical test in an arid ecosystem
CITATION: Kalwij, J. M., Milton, S. J. & McGeoch, M. A. 2008. Road verges as invasion corridors? A spatial hierarchical test in an arid ecosystem. Landscape Ecology, 23: 439-451. doi:10.1007/s10980-008-9201-3The original publication is available at https://www.springer.com/journal/10980Disturbed habitats are often swiftly colonized by alien plant species. Human inhabited areas may act as sources from which such aliens disperse, while road verges have been suggested as corridors facilitating their dispersal. We therefore hypothesized that (i) houses and urban areas are propagule sources from which aliens disperse, and that (ii) road verges act as corridors for their dispersal. We sampled presence and cover of aliens in 20 plots (6 x 25 m) per road at 5-km intervals for four roads, nested within three localities around cities (n = 240). Plots consisted of three adjacent nested transects. Houses (n = 3349) were mapped within a 5-km radius from plots using topographical maps. Environmental processes as predictors of alien composition differed across spatial levels. At the broadest scale road-surface type, soil type, and competition from indigenous plants were the strongest predictors of alien composition. Within localities disturbance-related variables such as distance from dwellings and urban areas were associated with alien composition, but their effect differed between localities. Within roads, density and proximity of houses was related with higher alien species richness. Plot distance from urban areas, however, was not a significant predictor of alien richness or cover at any of the spatial levels, refuting the corridor hypothesis. Verges hosted but did not facilitate the spread of alien species. The scale dependence and multiplicity of mechanisms explaining alien plant communities found here highlight the importance of considering regional climatic gradients, landscape context and road-verge properties themselves when managing verges.DST-NRF Center of Excellence for Invasion Biology.Publisher’s versio
Wind and seed: a conceptual model of shape-formation in the cushion plant Azorella Selago
Aims:
The sub-Antarctic cushion plant, Azorella selago, is usually hemispherical when small but frequently crescent-shaped when larger. Spatial variation in wind speed and in air-borne seed and sediment deposition is examined to determine if wind scouring and deposition patterns could contribute to the development of non-hemispherical shapes in cushion plants.
Methods:
Computational fluid dynamic analyses were conducted for hemispherical and crescent-shaped cushion plants parameterizing models with data from A. selago habitats on Marion Island. Numerical data were contextualized with field observations to arrive at a conceptual model for shape development.
Results:
Airflow modelling showed that both wind scouring and seed deposition of the commonly co-occurring grass Agrostis magellanica are greater on the windward side of the plant. By contrast, heavier sediment particles are predominantly deposited on the leeward side of plants, leading to burial of lee-side A. selago stems. This sediment accumulation may initiate the development of the crescent-shape in hemispherical plants by increasing stem mortality on the plant’s leeward edge. Once developed, the crescent-shape is probably self-reinforcing because it generates greater air recirculation (and lower air velocities) which enhances further deposition and establishment of A. magellanica grasses in the lee of the crescent. The conceptual model consists therefore of three stages namely, (1) negligible air recirculation, (2) sediment deposition and grass establishment, and (3) differential cushion growth.
Conclusion:
This conceptual model of plant shape development may explain the occurrence and orientation of crescent-shaped cushion plants and highlights how predicted changes in wind patterns may affect vegetation patterns
Positive plant–plant interactions expand the upper distributional limits of some vascular plant species
Biotic interactions can shape species’ distributions through their impact on species’ realized
niches, potentially constraining or expanding the range of conditions under which species occur. We examine
whether fine-scale plant–plant interactions scale up to shape broad-scale species’ distributions, using
Azorella selago, a widespread cushion plant that facilitates other species, and the rest of the vascular flora of
sub-Antarctic Marion Island as a model system. We compared the upper elevational distributional limit of
each species when growing on vs. away from A. selago to test how the interaction with this cushion plant
species affects species’ ranges. Three out of 19 vascular plant species occurred at higher altitudes in the
presence of A. selago than in the absence of A. selago: Acaena magellanica (+26 m higher), Colobanthus kerguelensis
(+37 m higher), and Lycopodium saururus (+19 m higher). Therefore, A. selago’s fine-scale impacts
scaled up to shape the distribution of a subset of the vascular flora of Marion Island. Plant–plant interactions
thus have the potential to expand species upper distributional limits by increasing the niche space
that a species can occupy, although the influence of these interactions may be strongly species-specific.The South African
National Antarctic Program, the National
Research Foundation (NRF) and the Swiss National Science Foundation.http://www.esajournals.org/loi/ecspam2020Plant Production and Soil Scienc
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