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1.
Grazing by large herbivores is a major determinant of vegetation dynamics in many semi-natural ecosystems, including the replacement of heather moorland by rough grassland in the British uplands. Herbivore foraging is influenced by vegetation patterns and, in turn, their grazing drives vegetation dynamics. Although vegetation impacts are local, spatially heterogeneous local impacts can have different consequences as would the same impacts distributed uniformly. We constructed a simulation model of the spatial effects of grazing by sheep on the vegetation dynamics of heather moorland, a vegetation community of international conservation importance in the UK. The model comprised three sub-models to predict (1) annual average heather utilisation, (2) spatial variation in heather utilisation (higher near the edge of grass patches) and (3) competition between heather and grass. Here we compare the predicted heather utilisation and vegetation dynamics of the spatial model, relative to those of a non-spatial model. The spatial model resulted in a reduced loss of heather cover for a given sheep stocking rate. The model demonstrates how spatial interactions between large herbivores and their forage drive vegetation dynamics, leading to changes in community structure and composition. Indeed, omitting spatial effects in grazing models may lead to inaccurate predictions. We have shown that ecosystem modelling, based around an iterative dialogue between developers and experienced researchers, has the potential to make a substantial contribution towards the conservation and management of vulnerable landscapes. Combining modelling with experimental studies will facilitate progress towards understanding long-term vegetation/herbivore dynamics. 相似文献
2.
Grazing by large herbivores is a major determinant of vegetation dynamics in many semi-natural ecosystems, including ghe replacement of heather moorland by rough grassland in the British uplands. Herbivore foraging is influenced by vegetation patterns and, in turn, their grazing drives vegetation dynamics. Although vegetation impacts are local, spatially heterogeneous local impacts can have different conseqences as would the same impact distributed uniformly. We constructed a simulation model of the spatial effects of grazing by sheep on the vegetation dynamics of heather moorland, a vegetation community of international conservation importance in the UK. The model comprised three sub-models to predict (1) annual average heather utilisation, (2) spatial variation in heather utilisation (higher near the edge of grass patches) and (3) competition between heather and grass. Here we compare the predicted heather utilisation and vegetation dynamics of the spatial model, relative to those of a non-spatial model. The spatial model resulted in a reduced loss of heather cover for a given sheep stocking rate. The model demonstrtaes how spatial interactions between large herbivores and their forage drive vegetation dynamics, leading to changes in community structure and composition. Indeed, omitting spatial effects in grazing models may lead to inaccurate predictions. We have shown that ecosystem modelling, based around an iterative dialogue between developers and experienced researchers, has the potential to make a substantial contribution towards the conservation and management of vulnerable landscapes. Combining modelling with experimental studies will facilitate progress towards understanding long-term vegetation/herbivore dynamics. 相似文献
3.
Semi-natural grasslands in Sweden are threatened by land-use change and lack of management with attendant risk to their biodiversity. We present a model to explore the effects of grazing frequency and intensity on plant species persistence, and the relative effects of grassland size and pattern. We used a landscape modelling platform, LAMOS (LAndscape MOdelling Shell), to design a landscape model of vegetation dynamics incorporating the effects of local succession, dispersal and grazing disturbance. Five plant functional groups (PFG), representing various combinations of persistence and dispersal character, light requirements and disturbance responses, were defined to model species dynamics. Based on old cadastral maps three different landscapes were designed representing specific time-layers, i.e., a historical (17th to 18th century), a pre-modern (1940s) and a present-day landscape. Simulations showed that a threshold was crossed when grasslands decreased in area to about 10–30% of the modelled area, and as a consequence the biomass of grassland-specific PFGs was strongly reduced. These competition sensitive groups did not persist in the model even with intense grazing in the present-day landscape, where grasslands occupy 11% of the total area. However, all grassland species would have been able to persist in the historical landscape, where grasslands occupied 59% of the total area, even without grazing. Our results suggest that continuous but low-intensity grazing is more positive for grassland PFGs than discontinuous but highly intensive grazing. This effect was particularly strong when the frequency and/or intensity of grazing dropped below a threshold of 20%. Simulations using three landscape maps designed to explore effects of further fragmentation and habitat loss showed that the spatial pattern of remaining grasslands is important for the persistence of grassland-specific PFG. The model presented here is an advance towards more realistic grazing models to explore the effects of prescribed grazing and landscape fragmentation on the persistence species or plant functional groups.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
4.
The Hierarchical Patch Dynamics Paradigm provides a conceptual framework for linking pattern, process and scale in ecosystems,
but there have been few attempts to test this theory because most ecological studies focus on only one spatial scale, or are
limited in their temporal scope. Here I use palaeoecological techniques (analysis of fossil pollen and stable carbon isotopes)
to compare vegetation heterogeneity in an east African savanna at three spatial scales, over hundreds of years. The data show
that patterns of vegetation change are different at the three spatial scales of observation, and suggest that different ecological
processes dominate tree abundance at micro, local and landscape scales. Interactions between plants, disturbance (e.g., by
fire and herbivores), climate and soil type may influence tree density at differing spatial and temporal scales. This hierarchical
explanation of savanna vegetation dynamics could inform future biodiversity conservation and management in savannas.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
Lindsey Gillson 《Landscape Ecology》2005,19(8):883-894
The Hierarchical Patch Dynamics Paradigm provides a conceptual framework for linking pattern, process and scale in ecosystems,
but there have been few attempts to test this theory because most ecological studies focus on only one spatial scale, or are
limited in their temporal scope. Here I use palaeoecological techniques (analysis of fossil pollen and stable carbon isotopes)
to compare vegetation heterogeneity in an east African savanna at three spatial scales, over hundreds of years. The data show
that patterns of vegetation change are different at the three spatial scales of observation, and suggest that different ecological
processes dominate tree abundance at micro, local and landscape scales. Interactions between plants, disturbance (e.g., by
fire and herbivores), climate and soil type may influence tree density at differing spatial and temporal scales. This hierarchical
explanation of savanna vegetation dynamics could inform future biodiversity conservation and management in savannas.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
Small-scale patch structure in North American and South African grasslands responds differently to fire and grazing 总被引：1，自引：1，他引：0
Fire and grazing significantly impact small-scale patch structure and dynamics in savanna grasslands. We assessed small-scale grass-forb associations in long-term fire and grazing experiments in North America (NA) and Southern Africa (SA). Transects of 128 0.25 m2 contiguous quadrats were sampled in areas with different combinations of grazing (ungrazed, single grazer, or multiple grazers) and fire frequency (unburned or annually burned). We predicted that (1) the patch structure of each of the dominant grasses in NA and SA would respond similarly to fire and grazing, and (2) that forb richness would be correlated to grass patch structure. Semi-variance analysis was used to determine patch structure of dominant grasses and forb cover. Community structure responded similarly in NA and SA to fire, grazing, and fire-grazing interactions. Species richness, diversity, and community heterogeneity were significantly higher in unburned-grazed sites. Grazing significantly increased forb cover and decreased cover of the dominant grasses, and the effects of fire on community structure depended on the grazing regime. Contrary to our prediction, we found that small-scale patch structure of the dominant grass species in NA and SA responded differently to grazing and fire. We found strong grass patch structure in unburned-ungrazed grasslands in both sites; however, grazing and fire reduced patch structure in NA but not SA, and in no instance did grass patch structure influence forb community structure. We conclude that fire and grazing have larger impacts on small-scale patch structure in NA than they do in SA even though overall community structure responded similarly on both continents. 相似文献
7.
Plant spatial pattern has been considered as one of the most important factors influencing forage selection of herbivores in natural grasslands. Previous work has emphasized the effects of spatial distribution patterns of food resource at the scale of whole plant communities. Our objective was to explore whether changes in spatial patterns of food within a patchy site affected forage selection of sheep within and among patches. We conducted a manipulative experiment using three native plant species of different palatability and abundance to artificially create three different quality patches in each treatment. We compared the effects of aggregated and randomly dispersed patterns, within high, medium, and low quality patches respectively, on sheep forage selection. Effects of plant spatial patterns within a patch on sheep forage selection of the patch itself strongly depended on the patch quality. For high quality patches, random dispersion of food resources significantly decreased sheep consumption of the palatable plant within the patch. This effect was reversed in low quality patches, and was not significant in medium quality patches. Changes in plant spatial patterns within high quality patches greatly influenced sheep forage selection of other patches. However, changes in plant spatial patterns within medium or low quality patches significantly influenced foraging responses of sheep only for high quality patches. We therefore conclude that high quality resource sites are the most influential and susceptible foraging areas. Our results highlight the importance of high quality resource sites when considering grazing grassland conservation and management. 相似文献
8.
The development of forage production and utilization gradients around livestock watering points 总被引：1，自引：0，他引：1
Large herbivores can impose spatial patterns on otherwise homogeneous vegetation, but how these patterns change through time is poorly understood. Domestic livestock pastures are model systems for studying how foraging behavior influences the development of coupled grazing and vegetation patterns. We sampled forage production and utilization by cattle along distance-from-water gradients to provide a snapshot of grazing and vegetation patterns, and then evaluated the ability of simulation models to qualitatively reproduce these patterns. In the field, forage production increased with distance from water, as expected, but utilization peaked at intermediate distances from water in two of three study areas. Likewise, simulations based on a variety of foraging strategies produced gradients in forage production and, after forage availability near water declined sufficiently, peaks in utilization at intermediate distances. Distance-from-water gradients thus represent cumulative but not necessarily present day gradients in grazing intensity. The model with a foraging strategy based on time minimization produced slightly more realistic patterns in forage abundance than a model based on energy maximization, although results were sensitive to the value of the threshold for rejecting sites of low forage biomass. However, all models produced implausible thresholds in grazing and forage distribution, suggesting that factors besides resource distribution influence herbivore distributions. Moreover, different foraging rules produced similar vegetation gradients, especially on point water source landscapes, illustrating the difficulty of inferring foraging processes from vegetation patterns. 相似文献
9.
Yunhui Liu Christoph Rothenwöhrer Christoph Scherber Péter Batáry Zoltán Elek Juliane Steckel Stefan Erasmi Teja Tscharntke Catrin Westphal 《Landscape Ecology》2014,29(3):529-540
Current biodiversity conservation policies have so far had limited success because they are mainly targeted to the scale of individual fields with little concern on different responses of organism groups at larger spatial scales. We investigated the relative impacts of multi-scale factors, including local land use intensity, landscape context and region, on functional groups of beetles (Coleoptera). In 2008, beetles were suction-sampled from 95 managed grasslands in three regions, ranging from Southern to Northern Germany. The results showed that region was the most important factor affecting the abundance of herbivores and the abundance and species composition of predators and decomposers. Herbivores were not affected by landscape context and land use intensity. The species composition of the predator communities changed with land use intensity, but only in interaction with landscape context. Interestingly, decomposer abundance was negatively related to land use intensity in low-diversity landscapes, whereas in high-diversity landscapes the relation was positive, possibly due to enhanced spillover effects in complex landscapes. We conclude that (i) management at multiple scales, from local sites to landscapes and regions, is essential for managing biodiversity, (ii) beetle predators and decomposers are more affected than herbivores, supporting the hypothesis that higher trophic levels are more sensitive to environmental change, and (iii) sustaining biological control and decomposition services in managed grassland needs a diverse landscape, while effects of local land use intensity may depend on landscape context. 相似文献
10.
Variations in landscape patterns and vegetation cover between 1957 and 1994 in a semiarid Mediterranean ecosystem 总被引：9，自引：2，他引：9
Alodos C.L. Pueyo Y. Barrantes O. Escós J. Giner L. Robles A.B. 《Landscape Ecology》2004,19(5):543-559
The aim of this study was to analyze the main processes that determine changes in landscape patterns and vegetation cover from 1957-1994 to develop a model for land cover dynamics. Land cover and landscape patterns were assessed and compared using aerial photographs taken in 1957, 1985, and 1994. Over this period, tall grass steppe and arid garrigues increased by 6% and 4%, respectively, while crop fields decreased by 15% and tall arid brush remained the same. Over the same period, tall grass steppes and arid garrigues became less fragmented.Changes in land use were triggered by socioeconomic forces, which were constrained by the underlying structure of the physical landscape. The best preserved vegetation (tall arid brushes) was concentrated at higher elevations, with a pronounced slope, not oriented towards the sea, and in volcanic substrate. Communities tended to be better preserved further away from towns and at lower house densities. Tall grass steppe was present on more gradual sea-oriented slope and in calcareous substrate, and increased at higher elevations, although not far from the town but away from high anthropogenic influence. Previous studies have revealed that traditional land uses of this landscape, particularly grazing, favoured the transition from tall arid brush to tall grass steppe. In this study, we analyzed to what extent the underlying structure of the physical landscape imposes limitations to the vulnerability to human activity of the main vegetation types. According to the data on the probability of vegetation transition over the 37-year period, the shift from tall arid brush to tall grass steppe appeared to be favoured by gradual slopes. Tall arid brush recovered from either arid garrigues or tall grass steppes at steeper slopes. Thus, steep terrain had a favourable effect on the formation of brushwood and more gradual terrain favoured tall grass steppe. The prevalent trends were confirmed by a projection of a transition matrix over 100 years.This revised version was published online in May 2005 with corrections to the Cover Date. 相似文献
11.
Moorlands perform a wide variety of roles within modern society. A vital component of these landscapes is the patterning of
vegetation, and management of this requires a thorough understanding of the drivers of vegetation change. Although there has
been a considerable body of research focussed on the processes that are important in patterning contemporary vegetation these
typically lack any significant time-depth. Long-term data, using palaeoecological techniques, offer insights into drivers
of vegetation change that are otherwise unachievable. This paper presents new palaeoecological data from Dartmoor (UK) to
test two hypotheses: (1) that vegetation character of moorland is spatially homogenous through the past 8,000 years; and (2)
that burning has a significant role in the development of open, grass-dominated, vegetation. Four peat cores spanning the
past 8,000 years were subject to pollen and microcharcoal analysis. Thirty-seven radiocarbon age estimates were obtained to
determine age-depth models for the pollen and charcoal stratigraphies. Differences within and between the pollen stratigraphies
have been used as an indirect measure of landscape heterogeneity at a coarse scale. The data reveal periods of time during
which differences in the vegetation (as sensed by pollen) around each site are small, and periods during which differences
between vegetation are large. Periods of time characterised by greater spatial difference, and by inference greater heterogeneity,
correlate with periods characterised by greater human exploitation of the landscape as revealed by archaeological evidence.
Human activities therefore promote greater spatial patterning in the landscape. Fire alone is not an important control on
long-term peatland vegetation development. The results are useful for conservation strategies by demonstrating variability
in spatial diversity of vegetation patterns in the past, and pointing towards opportunities to recreate and maintain diverse
vegetation mosaics. 相似文献
12.
Urban green spaces (UGS) positively impact the population, providing essential ecosystem services and improving public health. Urban vegetation management needs to optimize mowing process costs and reducing impacts on the natural ecosystem. Thus, we implemented a general grass growth model suitable for UGS management in tropical areas, focused on lawns, public parks and squares, roadsides, and around waterways. The model incorporates local edaphoclimatic conditions to simulates the daily dynamics of leaf area index (LAI), biomass, evapotranspiration, and soil water content, going under mowing processes or not, with spatialization capability which might be integrated within geographic information system (GIS) environments. A case study assessing bahiagrass (Paspalum notatum Flüggé) vegetation species in São Carlos, southeastern Brazil, is presented, considering two scenarios to demonstrate the spatial capabilities of the model: (i) UGS as a single area, and (ii) several areas independently. For model validation, vegetation indices calculated based on data from an unmanned aerial vehicle (UAV) and CubeSat imagery (PlanetScope) were used to retrieve LAI time series, calibrated with spectral signatures from leaf ground sampling. For performance analysis, LAI time series from the model and LAI retrieved from both sensors were compared via determination coefficient (R2) and root mean square error (RMSE). Our findings suggest that the proposed model is accurate, and due to its spatialization capability and integration with a GIS, its application may help government administrations to optimize UGS mowing processes. 相似文献
13.
Conservationists, managers, and land planners are faced with the difficult task of balancing many issues regarding humans
impacts on natural systems. Many of these potential impacts arise from local-scale and landscape-scale changes, but such changes
often covary, which makes it difficult to isolate and compare independent effects arising from humans. We partition multi-scale
impacts on riparian forest bird distribution in 105 patches along approximately 500 km of the Madison and Missouri Rivers,
Montana, USA. To do so, we coupled environmental information from local (within-patch), patch, and landscape scales reflecting
potential human impacts from grazing, invasive plant species, habitat loss and fragmentation, and human development with the
distribution of 28 terrestrial breeding bird species in 2004 and 2005. Variation partitioning of the influence of different
spatial scales suggested that local-scale vegetation gradients explained more unique variation in bird distribution than did
information from patch and landscape scales. Partitioning potential human impacts revealed, however, that riparian habitat
loss and fragmentation at the patch and landscape scales explained more unique variation than did local disturbances or landscape-scale
development (i.e., building density in the surrounding landscape). When distribution was correlated with human disturbance,
local-scale disturbance had more consistent impacts than other scales, with species showing consistent negative correlations
with grazing but positive correlations with invasives. We conclude that while local vegetation structure best explains bird
distribution, managers concerned with ongoing human influences in this system need to focus more on mitigating the effects
of large-scale disturbances than on more local land use issues.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
14.
Jin Yao Debra P. C. Peters Kris M. Havstad Robert P. Gibbens Jeffrey E. Herrick 《Landscape Ecology》2006,21(8):1217-1231
Factors with variation at broad (e.g., climate) and fine scales (e.g., soil texture) that influence local processes at the
plant scale (e.g., competition) have often been used to infer controls on spatial patterns and temporal trends in vegetation.
However, these factors can be insufficient to explain spatial and temporal variation in grass cover for arid and semiarid
grasslands during an extreme drought that promotes woody plant encroachment. Transport of materials among patches may also
be important to this variation. We used long-term cover data (1915–2001) combined with recently collected field data and spatial
databases from a site in the northern Chihuahuan Desert to assess temporal trends in cover and the relative importance of
factors at three scales (plant, patch, landscape unit) in explaining spatial variation in grass cover. We examined cover of
five important grass species from two topographic positions before, during, and after the extreme drought of the 1950s. Our
results show that dynamics before, during, and after the drought varied by species rather than by topographic position. Different
factors were related to cover of each species in each time period. Factors at the landscape unit scale (rainfall, stocking
rate) were related to grass cover in the pre- and post-drought periods whereas only the plant-scale factor of soil texture
was significantly related to cover of two upland species during the drought. Patch-scale factors associated with the redistribution
of water (microtopography) were important for different species in the pre- and post-drought period. Another patch-scale factor,
distance from historic shrub populations, was important to the persistence of the dominant grass in uplands (Bouteloua eriopoda) through time. Our results suggest the importance of local processes during the drought, and transport processes before and
after the drought with different relationships for different species. Disentangling the relative importance of factors at
different spatial scales to spatial patterns and long-term trends in grass cover can provide new insights into the key processes
driving these historic patterns, and can be used to improve forecasts of vegetation change in arid and semiarid areas. 相似文献
15.
Forest canopy phenology is an important constraint on annual water and carbon budgets, and responds to regional interannual
climate variation. In steep terrain, there are complex spatial variations in phenology due to topographic influences on microclimate,
community composition, and available soil moisture. In this study, we investigate spatial patterns of phenology in humid temperate
forest as a function of topography. Moderate-resolution imaging spectro-radiometer (MODIS) vegetation indices are used to
derive local patterns of topography-mediated vegetation phenology using a simple post-processing analysis and a non-linear
model fitting. Elevation has the most explanatory power for all phenological variables with a strong linear relationship with
mid-day of greenup period, following temperatures lapse rates. However, all other phenological variables show quadratic associations
with elevation, reflecting an interaction between topoclimatic patterns of temperature and water availability. Radiation proxies
also have significant explanatory power for all phenological variables. Though hillslope position cannot be adequately resolved
at the MODIS spatial resolution (250 m) to discern impacts of local drainage conditions, extended periods of greenup/senescence
are found to occur in wet years. These findings are strongly supported by previous field measurements at different topographic
positions within the study area. The capability of detecting topography-mediated local phenology offers the potential to detect
vegetation responses to climate change in mountainous terrain. In addition, the large, local variability of meteorological
and edaphic conditions in steep terrain provides a unique opportunity to develop an understanding of canopy response to the
interaction of climate and landscape conditions. 相似文献
16.
Florian Kohler François Gillet Stéphanie Reust Helene H. Wagner Fawziah Gadallah Jean-Michel Gobat Alexandre Buttler 《Landscape Ecology》2006,21(2):281-295
Management-oriented models of cattle habitat use often treat grazing pressure as a single variable summarizing all cattle activities. This paper addresses the following questions: How does the spatial pattern of cattle effects vary between cattle activities in a highly heterogeneous landscape? Do these patterns change over the grazing season as forage availability decreases? What are the respective roles of natural and management-introduced structures? We estimated the intensity of herbage removal, dung deposition and trampling after each of three grazing periods on a grid of 25 m ×25 m cells covering an entire paddock in the Swiss Jura Mountains. We found no significant positive correlations between cattle effects. Spatial patterns weakened through the season for grazing and trampling, whereas dunging patterns changed little between grazing periods. Redundancy analysis showed that different cattle effects were correlated with different environmental variables and that the importance of management-introduced variables was highest for herbage removal. Autocorrelograms and partial redundancy analyses using principal coordinates of neighbour matrices suggested that dunging patterns were more coarse-grained than the others. Systematic differences in the spatial and seasonal patterns of cattle effects may result in complex interactions with vegetation involving feedback effects through nutrient shift, with strong implications for ecosystem management. In heterogeneous environments, such as pasture-woodland landscapes, spatially explicit models of vegetation dynamics need to model cattle effects separately. 相似文献
17.
Spatial vegetation patterns as early signs of desertification: a case study of a desert steppe in Inner Mongolia,China 总被引：1，自引：0，他引：1
Proper assessment and early detection of land degradation and desertification is extremely important in arid and semi-arid
ecosystems. Recent research has proposed to use the characteristics of spatial vegetation patterns, such as parameters derived
from power-law modeling of vegetation patches, for detecting the early signs of desertification. However, contradictory results
have been reported regarding the suitability of those proposed indicators. We used an experiment with multiple grazing intensities
as an analog of a desertification gradient and evaluated the performance of two predictors of desertification: percent plant
cover and a transition from a patch-area distribution characterized by a power law to another portrayed by a truncated power
law, in a desert steppe in Inner Mongolia, China. We found that spatial metrics, such as the largest patch index and coefficient
of variation of mean patch area had negative linear relationships with grazing intensity, suggesting that vegetation patches
became more fragmented and homogeneous under higher grazing pressure. Using a binning-based method to analyze our dataset,
we found that the patch-area relationship deviated from a power-law to a truncated power-law model with increasing grazing
pressure, while the truncated power law was a better fit than the power law for all plots when binning was not used. These
results suggest that the selection of methodology is crucial in using power-law models to detect changes in vegetation patterns.
Plant cover was significantly correlated with stocking rate and all spatial metrics evaluated; however, the relationship between
cover and vegetation spatial pattern still deserves a thorough examination, especially in other types of ecosystems, before
using cover as a universal early sign of desertification. Our results highlight a strong connection between the vegetation
spatial pattern and the desertification associated with heavy grazing and suggest that future studies should incorporate information
about vegetation spatial pattern in monitoring desertification processes. 相似文献
18.
Movement rules for herbivores in spatially heterogeneous environments: responses to small scale pattern 总被引：4，自引：0，他引：4
John E. Gross Colleen Zank N. Thompson Hobbs Donald E. Spalinger 《Landscape Ecology》1995,10(4):209-217
Foraging herbivores respond to the spatial pattern of resources at a variety of scales. At small scales of space and time, existing models capture the essence of the feeding process and successfully predict intake rates. Models that operate over larger scales have not exhibited a similar success, in part because we have a limited understanding of the rules used by animals to make decisions in spatially complex environments, or of the consequences of departing from these rules. To evaluate the rules that large herbivores use when navigating between forages, we examined movements of bighorn sheep foraging on apparent prey (alfalfa plants) in hand-constructed patches of plants. Observations of movements and path lengths were compared to simulations that used a variety of different rules-of-thumb to determine a search path. Rules used in simulations ranged from a random walk with various detection distances, to more complicated rules that solved a variant of the travelling salesman problem. Simulations of a random walk yielded movement lengths that exceeded observations by a factor of 3 for long detection distances, and by 30-fold for short detection distances. Observed move distances were most closely approximated by simulations based on a nearest-neighbor ruleover 75 % of all moves by bighorn sheep were to the closest available plant. Movement rules based on random walks are clearly inappropriate for many herbivores that typically consume visually apparent plants, and we suggest the use of a nearest-neighbor rule for modelling foraging by large herbivores. 相似文献
19.
In studies of vegetation dynamics, data points describing the changes are often sparse, because changes were not recognized
in early stages or investigations were part of different projects. The snapshots at hand often leave the nature of the dynamics
unrevealed and only give a rough estimation of the directions of changes. Extrapolation of the dynamics with traditional cellular
automaton modeling is also complicated in such cases, because rules often cannot be deduced from field data for each interaction.
We developed a Bayesian MCMC method, using a discrete time stochastic cellular automaton model to reconstruct vegetation dynamics
between vegetation maps available and provide estimation of vegetation pattern in years not surveyed. Spread capability of
each vegetation type was characterized by a lateral spread parameter and another for establishment from species pool. The
method was applied to a series of three vegetation maps depicting vegetation change at a grassland site following abandonment
of grazing in north-eastern Hungary. The Markov chain explored the missing data space (missing maps) as well as the parameter
space. Transitions by lateral expansion had a greater importance than the appearance of new vegetation types without spatial
constraints at our site. We estimated the trajectory of change for each vegetation type, which bore a considerable non-linear
element in most cases. To our best knowledge, this is the first work that tries to estimate vegetation transition parameters
in a stochastic cellular automaton based on field measurements and provides a tool to reconstruct past dynamics from observed
pattern. 相似文献
20.
Context
Fire and controlled grazing have been widely adopted as management interventions to counteract woody shrub proliferation in many arid and semiarid grassland systems. The actual intensity of grazing and fire, along with the timing of the interventions, however, are difficult to determine in practice.
ObjectivesThis study aims to establish model simulations to access the long-term landscape changes under different land management scenarios.
MethodsWe developed a cellular automata model to evaluate landscape dynamics in response to scenarios of grazing, fire, time of intervention, and initial coverage of grasses and shrubs.
ResultsWith current grazing intensity and fire suppression, the landscape may shift to a shrub-dominated landscape in 100–150 years. An appropriate combination of grazing and fire management could help maintain over 50% of grass cover and reduce the shrub cover to less than 2%, keeping the landscape highly reversible. Even using 1% grazing intensity and periodic fire once a year, the management tools should be implemented in 60 years, otherwise, they may lose effectiveness and the vegetation transition to grasslands would become impossible.
ConclusionsThis study highlighted that the reintroduction of fire not only directly removes shrubs but also reallocates soil water and resources among different microsites, which may accelerate grass recovery and suppress shrub regrowth, potentially reversing the shrub invasion process. The combined grazing and fire management plans should be carried out before a threshold time depending on the chosen management tools.
相似文献