A box model of carrying capacity for suspended mussel aquaculture in Lagune de la Grande-Entrée,Iles-de-la-Madeleine,Québec

An object-oriented model of environment–mussel aquaculture interactions and mussel carrying-capacity within Lagune de la Grande-Entrée (GEL), Iles-de-la-Madeleine, Québec, was constructed to assist in development of sustainable mussel culture in this region. A multiple box ecosystem model for GEL tied to the output of a hydrodynamic model was constructed using Simile software, which has inherent ability to represent spatial elements and specify water exchange between modelled regions. Mussel growth and other field data were used for model validation. Plackett–Burman sensitivity analysis demonstrated that a variety of bioenergetic parameters of zooplankton and phytoplankton submodels were important in model outcomes. Model results demonstrated that mussel aquaculture can be further developed throughout the lagoon. At present culture densities, phytoplankton depletion is minimal, and there is little food limitation of mussel growth. Results indicated that increased stocking density of mussels in the existing farm will lead to decreased mass per individual mussel. Depending on the location of new farm emplacement within the lagoon, implementation of new aquaculture sites either reduced mussel growth in the existing farm due to depletion of phytoplankton, or exhibited minimum negative impact on the existing farm. With development throughout GEL, an excess of phytoplankton was observed during the year in all modelled regions, even at stocking densities as high as 20 mussels m⁻³. Although mussels cultured at this density do not substantially impact the ecosystem, their growth is controlled by the flux of phytoplankton food and abundance of zooplankton competitors. This model provides an effective tool to examine expansion of shellfish farming to new areas, balancing culture location and density.     

Modelling the horizontal spatial structure of planktonic community in Lake Trasimeno (Umbria,Italy) using multivariate geostatistical methods

Spatial heterogeneity of ecological systems has been recognised in recent years as an important ecological feature of an ecosystem, rather than a mere statistical nuisance. However, although considerable interest has been paid to the development of statistical methods for the analysis of spatial environmental data, when in presence of more species or environmental variables common analyses still fail to recognise the necessity of a joint modelling of the whole correlation structure. In this paper, we propose to study the multivariate spatial autocorrelation of a plankton community by making explicit reference to a spatial linear factor model entailing a set of constraints for the spatial structure of the planktonic species. The data set examined come from an intensive 2-day sampling survey performed in July 1991 on Lake Trasimeno (Italy) to investigate the horizontal spatial heterogeneity and distribution of the planktonic community, from small (50 m) to large (1000–10,000 m) scale. The analysis revealed that zooplankton and phytoplankton essentially have different degrees of heterogeneity and different spatial structures which required separate modelling. On the other hand, the similarity of the spatial autocorrelation found within zooplankton and phytoplankton communities, indicates that at the investigated scales of observation the horizontal organisation of both components is not appreciably affected by species-specific behaviours. The analysis of the multivariate spatial patterns emerging from the mapping of the extracted factors suggested an interpretation of the distribution of macrozooplankton and phytoplankton assemblages in terms of planktonic responses to environmental factors of a lake-size scale.     

Nitrogen and plankton dynamics in the lagoon of Venice

The nitrogen cycle in the lagoon of Venice, which is the largest Italian lagoon, was investigated by means of a 3D fully coupled transport – water quality model, which had been validated against a substantial amount of real-world data. Nitrogen fluxes among different ecosystem compartments were computed for each month of a reference year, and for each one of the three sub-basins into which the lagoon is conventionally subdivided. The computation included the loads of nitrogen discharged by the tributaries, the direct inputs from the industrial area and the city of Venice, the atmospheric loads, the fluxes at the three lagoon inlets and the internal fluxes between sediment and water compartments and among the three sub-basins. The results of the analysis show that the lagoon, as a whole, exports nitrogen towards the sea. Approximately 4000 tN/year are recycled by the system, while 4640 tN/year is the net input from the drainage basin and the other sources, thus leading to about 8640 tN/year of dissolved inorganic nitrogen that enter the water compartment. Around half of the this amount is used by primary producers, one fourth is exported towards the sea, and one fourth is transferred into the sediment compartment, or lost to atmosphere. These findings suggest that the exchanges through the inlets play an important role in keeping nitrogen concentration at an acceptable level. A more detailed analysis of the model results shows that the non-homogeneous spatial distribution of tributary discharges and point sources is the main cause of the differences in the ecosystem response and water quality among the three sub-basins. Nutrient poorer sub-basins fix a ration of available inorganic nutrient higher than nutrient rich ones. However, they are more efficient in transferring the biomass to the highest trophic levels. Results also include estimates of fluxes that were not quantified so far (such as grazing and recycling), and a validated model, which could have a practical use, for example for assessing implications of reduction of nutrient loads.     

Nitrogen controls on ecosystem carbon sequestration: a model implementation and application to Saskatchewan,Canada

A plant–soil nitrogen (N) cycling model was developed and incorporated into the Integrated BIosphere Simulator (IBIS) of Foley et al. [Foley, J.A., Prentice, I.C., Ramankutty, N., Levis, S., Pollard, D., Sitch, S., Haxeltine, A., 1996. An integrated biosphere model of land surface process, terrestrial carbon balance and vegetation dynamics. Global Biogeochem. Cycles 10, 603–628]. In the N-model, soil mineral N regulates ecosystem carbon (C) fluxes and ecosystem C:N ratios. Net primary productivity (NPP) is controlled by feedbacks from both leaf C:N and soil mineral N. Leaf C:N determines the foliar and canopy photosynthesis rates, while soil mineral N determines the N availability for plant growth and the efficiency of biomass construction. Nitrogen controls on the decomposition of soil organic matter (SOM) are implemented through N immobilization and mineralization separately. The model allows greater SOM mineralization at lower mineral N, and conversely, allows greater N immobilization at higher mineral N. The model's seasonal and inter-annual behaviours are demonstrated. A regional simulation for Saskatchewan, Canada, was performed for the period 1851–2000 at a 10 km × 10 km resolution. Simulated NPP was compared with high-resolution (1 km × 1 km) NPP estimated from remote sensing data using the boreal ecosystem productivity simulator (BEPS) [Liu, J., Chen, J.M., Cihlar, J., Park, W.M., 1997. A process-based boreal ecosystem productivity simulator using remote sensing inputs. Remote Sens. Environ. 44, 81–87]. The agreement between IBIS and BEPS, particularly in NPP spatial variation, was considerably improved when the N controls were introduced into IBIS.     

Modelling CO2 and energy exchanges in a northern semiarid grassland using the carbon- and nitrogen-coupled Canadian Land Surface Scheme (C-CLASS)

The development of carbon (C) and nitrogen (N) simulations is one of the ongoing efforts in the land surface schemes of climate models. The C- and N-coupled Canadian Land Surface Scheme (C-CLASS) was recently modified to better represent grassland ecosystems. Improvements include revised plant growth and senescence calculations that are driven by the plant C balance between fixation and respiration, and leaf-out and leaf-fall schemes that are regulated by the seasonal dynamics of C and N reserves. These revisions were developed to better simulate the stress-related senescence and regrowth of perennials. The model was tested with observations of surface carbon and energy fluxes, soil temperature and moisture, and plant growth during 3 years of declining precipitation at a northern semiarid grassland near Lethbridge, Alberta, Canada. The R² and standard deviations between the simulated and observed half-hourly fluxes were 0.95 and 22.5 W m⁻² for net radiation, 0.82 and 42.1 W m⁻² for sensible heat, 0.66 and 29.2 W m⁻² for latent heat, and 0.63 and 0.95 μmol C m⁻² s⁻¹ for net CO₂ exchange. The model and observations both showed a strong impact of declining precipitation on annual carbon budgets in this semi-arid grassland. In a wet year (1998, precipitation = 482 mm), the ecosystem acted as a strong C sink (92 g C m⁻² modelled and 109 g C m⁻² measured from June 20th to December 31st). In a near-normal year (1999, precipitation = 341 mm), a smaller C sink was indicated (24 g C m⁻² modelled and 21 g C m⁻² measured). In a dry year (2000, precipitation = 276 mm), the ecosystem acted as a small C source (−18 g C m⁻² modelled and −17 g C m⁻² measured).     

Release thresholds strongly determine the range of seed dispersal by wind

The effect of the seed abscission process on the dispersal distance of seeds has never been studied explicitly and is often ignored in studies that aim to estimate the seed shadows of species. To examine the importance of the abscission process for the seed shadow we used a seed trajectory model that keeps track of the release threshold dynamics of the individual seeds on mother plant. We defined the release threshold as the critical wind speed that induces a mechanical force that is just large enough to release a seed from its mother plant. The model used real wind speed sequences and seed appearance over time on the mother plant.Several calculations were performed to investigate the effect of release thresholds dynamics on seed shadow of two herbaceous species with contrasting terminal velocity values (V_t): Centaurea jacea (V_t = 4.1 m s⁻¹) and Hypochaeris radicata (V_t = 0.49 m s⁻¹).Release thresholds were responsible for a two-fold increase of median dispersal distances in both species. Tails of the seed shadows, the fraction of seeds that travel furthest, were even more sensitive and increased with a factor 4.5 for Centaurea and 7.0 for Hypochaeris. Our work indicates that the abscission process appears to be very important and suggests that dispersal distance of plants is currently severely underestimated, which, in turn, has major consequences for our current understanding of the distribution, metapopulation dynamics and survival of plant species.     

Modelling carbon storage in highly fragmented and human-dominated landscapes: Linking land-cover patterns and ecosystem models

To extend coupled human–environment systems research and include the ecological effects of land-use and land-cover change and policy scenarios, we present an analysis of the effects of forest patch size and shape and landscape pattern on carbon storage estimated by BIOME-BGC. We evaluate the effects of including within-patch and landscape-scale heterogeneity in air temperature on carbon estimates using two modelling experiments. In the first, we combine fieldwork, spatial analysis, and BIOME-BGC at a 15-m resolution to estimate carbon storage in the highly fragmented and human-dominated landscape of Southeastern Michigan, USA. In the second, we perform the same analysis on 12 hypothetical landscapes that differ only in their degree of fragmentation. For each experiment we conduct four air-temperature treatments, three guided by field-based data and one empirically informed by local National Weather Service station data. The three field data sets were measured (1) exterior to a forest patch, (2) from the patch edge inward to 60 m on east-, south-, and west-facing aspects, separately, and (3) interior to that forest patch. Our field-data analysis revealed a decrease in maximum air temperature from the forest patch edge to a depth of 80 m. Within-patch air-temperature values were significantly different (α = 0.01) among transects (c.v. = 13.28) and for all measurement locations (c.v. = 30.58). Results from the first experiment showed that the interior treatment underestimated carbon storage by ～8000 Mg C and the exterior treatment overestimated carbon storage by 30,000 Mg C within Dundee Township, Southeastern Michigan, when compared to a treatment that included within-patch heterogeneity. In the second experiment we found a logarithmic increase in carbon storage with increasing fragmentation (r² = 0.91). While a number of other processes (e.g. altered disturbance frequency or severity) remain to be included in future experiments, this combined field and modelling study clearly demonstrated that the inclusion of within-patch and landscape heterogeneity, and landscape fragmentation, each have a strong effect on forest carbon cycling and storage as simulated by a widely used ecosystem process model.     

Population growth versus population spread of an ant-dispersed neotropical herb with a mixed reproductive strategy

In plants that produce seeds with contrasting genetic background (selfed versus outcrossed), the question arises whether the ecological function of the two types of progeny differ. This paper addresses this issue for the ant-dispersed Calathea micans by introducing a novel application of the Neubert–Caswell model for analysis of wave speed for structured populations. Because dispersal as well as vital rates are structured, the model allows for distinct dispersal kernels for different types of progeny and thus permits comparisons of the sensitivity to changes in demographic and dispersal parameters of in situ population growth rate versus population spread across space. The study site was a lowland, evergreen tropical rain forest at La Selva Biological station, Costa Rica, where the species is commonly found throughout the forest. In C. micans, seeds produced by open flowers (potentially outcrossed) or by closed flowers (selfed) bear oily arils and are dispersed by ants. Five life-history stages were used to characterize the population: seedlings originating from seeds produced by open flowers, seedlings originating from seeds produced by closed flowers, juvenile vegetative plants, reproductive plants without new shoots and reproductive plants with new shoots. Demography varied seasonally. Transitions were estimated from marking and following the fate of plants (N = 400) in a natural population over a dry and a wet season. The population dynamics was described by a 10 × 10 matrix, with five life-history stages and two habitat states. The habitat states cycle repeatedly, dry–wet–dry–wet. To estimate dispersal kernels for each seed type, individual seeds (N = 225 and 306 seeds produced by open and closed flowers, respectively) were color-coded and placed in depots, allowing the ants to redistribute them. Five months later, seedlings with an attached seed coat bearing the intact color-coding, were surveyed around the depots. Radial distances and angles were recorded for each seedling (N = 67 and 81 seedlings arising from open and closed flowers, respectively). The results of the model give an asymptotic growth rate of 1.06 per season and an asymptotic rate of spread of 8.36 cm per season. There is a high correlation (r = 0.99) between elasticity of growth rate and elasticity of rate of spread of the population. Both rates are most sensitive to changes in stasis of juveniles during the dry season. However, most interesting is the analysis that revealed that population spread is more sensitive than in situ population growth to demographic rates of seedlings arising from open flowers. The analysis suggests a new way of thinking about ecological functions of multiple modes of reproduction.     

Dynamics of surface albedo of a boreal forest and its simulation

Surface albedo determines the distribution of solar radiation between the earth's surface and the atmosphere. It affects the global climate directly by altering surface energy balance, and indirectly by controlling ecosystem processes and greenhouse gas exchange. In this study, a land surface albedo model was constructed based on the gap probability approach for ray tracing and the basic optical parameters of ecosystem elements. The model was applied to a boreal deciduous forest and results were compared with field measurements. Results show that seasonal and diurnal albedo dynamics were well simulated by the model. The standard deviation between the simulated and measured reflected radiation was 2.5–5.0 W m⁻² in different seasons. The model also provided an insight into the relationships between surface albedo and radiation components (direct versus diffuse), solar zenith angle, and different wave bands. Model sensitivity analyses show that the surface albedo in winter is very sensitive to the forest wood area index for this boreal aspen forest, suggesting that accurate estimates of wood area index are necessary to improve the accuracy of surface albedo simulation in leafless seasons.     

A process-based model of nitrogen cycling in forest plantations: Part II. Simulating growth and nitrogen mineralisation of Eucalyptus globulus plantations in south-western Australia

We applied a new version of the G’DAY ecosystem model to short-rotation plantations of Eucalyptus globulus growing under a Mediterranean climate in south-western Australia. The new version, that includes modified submodels for biomass production, water balance, litter and soil organic matter (SOM) decomposition, and soil inorganic N balance, was parameterised and applied to three experimental eucalypt sites (Mumballup, Darkan and Northcliffe) of contrasting productivity. With a common base set of parameter values, the model was able to correctly reproduce observed time series of soil water content, canopy leaf area index and stemwood data at the three sites. The model's ability to simulate soil N supply under forest plantations was tested by simulating N mineralisation at each of the three sites over the duration of the experiment (10 years). Simulated annual net N mineralisation in the litter and top 20 cm soil layer ranged from 50 to 170 kg N ha⁻¹ across the sites as a result of differences in rates of litter production, SOM and litter decomposition, and microbial N immobilisation and (re-)mineralisation. Simulations of annual soil N mineralisation were similar to measured rates over a 3-year period, except for an overestimation in 1 year at Mumballup and 2 years at Darkan. Model results indicated the importance of fine root production and turnover for N supply. As plantations age, supply of N to trees increasingly originates from litter decomposition, while the contribution from decomposition of SOM decreases. Although major soil feedbacks associated with litter production, decomposition and N availability are adequately integrated into G’DAY, further work is required in some aspects of the model, including the utility of the C-allocation submodel over a wide range of site conditions and silvicultural treatments.     

Evaluation of information indices as indicators of environmental stress in terrestrial soils

Information indices from Ecosystem Network Analysis (ENA) can be used to quantify the development of an ecosystem in terms of its size and organization. There are two types of indices, i.e. absolute indices that describe both the size and organization of ecosystem (Total System Throughput (TST)—system size, Ascendancy (A)—size of organized flows and Development Capacity (C)—upper limit for A, Overhead (L)—size of unorganized flows) and relative indices that describe only the organization (Average Mutual Information (AMI = A:TST), Flow Diversity (H = C:TST), Relative Overhead (RL = L:TST)).It is theorized that environmental stress impair the ecosystem development and that the effect of stress can be quantified with the ENA information indices. Here we applied ENA on a case of environmental stress in a terrestrial ecosystem, i.e. soils that have endured long-term exposure to elevated copper concentration and altered pH.The absolute indices showed an unexpected pattern of response to pollution, suggesting that ecosystems in polluted soils are more active and better organized than these in unpolluted soils. The relative indices, alternatively, responded to pollution as predicted by theory, i.e. with decrease of stress (pollution level) the level of specialization increased (increase of AMI) and losses of energy, e.g. due to respiration, decreased (decrease of Overhead). The diversity and evenness of flows showed hump-backed relationship with stress. Less polluted soils appeared to be less vulnerable to external disturbances and more efficient in processing energy (higher Relative Ascendancy (RA = A:C)) than polluted soils. The relative information indices were rigid to changes in values of assumed parameters. The relative indices, opposite to absolute indices, appeared to be useful as indicators of environmental stress on the ecosystem level.     

Modelling the components of the vertical attenuation of ultraviolet radiation in a wetland lake ecosystem

The extinction of solar UV (290–400 nm) radiation in aquatic ecosystems is a complex phenomena. In this paper, we examine and model the attenuation of UV radiation in a shallow lake ecosystem. In particular we focus our analysis on the specific role of the fractions of dissolved and particulate matter in the water column in the attenuation of radiation. This analysis is aimed by representing the spatial distribution of each fraction making it possible to evidence the spatial variation in habitat quality. In situ and laboratory measurement are used to elaborate a UV attenuation model. The attenuation model distinguishes between the contribution of particulate and dissolved matter in the attenuation of the solar UV flux. In the studied wetland lake (Laguna Iberá, sub-tropical latitude, Argentina) the importance of dissolved matter is dominated in the UVB solar spectrum (290–320 nm) but the effects of the particulate fraction are not negligible, in particular in UVA (320–400 nm). The spatial representation of model results demonstrate the non homogeneous nature between attenuation of the two fractions. Local and global environmental change can have important impacts on dissolved and particulate matter concentrations, which cam have ecological consequences in relation to the high flux of incoming UV radiation. The model developed to examine the relative attenuation of the dissolved and particulate fractions and is a useful instrument to identify the role that these fractions have on the optical characteristics of aquatic ecosystems.     

Catastrophic-like shifts in shallow Turkish lakes: a modeling approach

A generalized logistic model (GLM) was developed to determine occurrence of submerged macrophytes in shallow Lake Eymir, and the model was tested independently on the upstream shallow Lake Mogan using the data collected fortnightly from both lakes during 1997–2002. The independent variables included concentrations of chlorophyll-a (chl-a), suspended solids (SS) and total phosphorus (TP), Secchi disc transparency and z-scores of water levels. The dependent variable was the binary index of submerged plant occurrence. We used bootstrapping to determine the maximum number of epochs to train the model and to execute training when the corrected average cross entropy (c-index) leveled off. The model predicted that SS concentration, z-scores of water levels and TP concentration were the most important variables for determining occurrence of submerged plants. Sensitivity analyses showed that the probability of submerged plant occurrence followed a strong hysterisis response to varying water levels and the concentrations of SS and TP, with the break points being ±50 cm, 12–17 mg l⁻¹ and 200–300 μg l⁻¹, respectively. This observed sensitivity was in accordance with the alternative stable states hypothesis of shallow lakes. For occurrence of submerged plants, chlorophyll-a concentration and Secchi disc transparency had low significance. This was in concert with both relevances of input variables and the field results. The model gave a good definition of the system since the c-index and corrected c-index on the training data were high (0.970 and 0.963, respectively). Testing the model on Lake Mogan produced a c-index of 0.815 with around 80% of the cases being correctly classified. This showed that the model had a high ability to generalize over a spatially independent test set; therefore, it had a great reliability as well. In addition, the predictive power of the model was indeed very high. Consequently, the model captured the relationships between the input and output variables successfully and consistently with alternative stable states hypothesis.     

Utilizing satellite imagery and GLOBE student data to model soil dynamics

General Purpose Atmosphere Plant Soil Simulator (GAPS), a menu-driven soil-vegetation-atmosphere transfer (SVAT) model, was used to simulate soil water dynamics from 1998 through 2001 for Greenville, PA, USA. GLOBE student data collected by students from Reynolds Junior and Senior High School, coupled with normalized difference vegetation index (NDVI) data derived from SPOT4 vegetation imagery, were used to parameterize and validate the model. Data from the National Weather Service Cooperative (NWSC) was used to evaluate the GLOBE dataset. Overall, there was a high index of agreement (d > 0.80) between field measurements and simulated soil water values from both datasets (GLOBE and NWSC). Simulations using the GLOBE climate data outperformed the NWSC data for the 1999, 2000, and 2001 growing seasons. In addition, the GLOBE simulations showed that NDVI could be utilized to predict transpiration periods (QI, QII, and QIII) for northern latitudes >35° with a distinct winter period. In phenological terms, QI reflects the onset of the growing season when vegetation is greening up (NDVI < 0.60) and transpiration is beginning (<2 mm/day) and QII reflects the end of the growing seasons when vegetation is greening down and transpiration is decreasing. QIII reflects the height of the growing season when transpiration rates average between 2 and 5 mm per day and NDVI is at its maximum (>0.60). Results of this study demonstrate that GLOBE student data, coupled with remotely sensed data, can provide an important source of input and validation information for capacitance SVAT models such as GAPS.     

Optimization of exergy and implications of body sizes of phytoplankton and zooplankton in an aquatic ecosystem model

Size appears to be an important parameter in ecological processes. All physiological processes vary with body size ranging from small microorganisms to higher mammals. In this model, five state variables — phosphorus, detritus, phytoplankton, zooplankton and fish are considered. We study the implications of body sizes of phytoplankton and zooplankton for total system dynamics by optimizing exergy as a goal function for system performance indicator. The rates of different sub-processes of phytoplankton and zooplankton are calculated, by means of allometric relationships of their body sizes. We run the model with different combinations of body sizes of phytoplankton and zooplankton and observe the overall biomass of phytoplankton, zooplankton and fish. The highest exergy values in different combinations of phytoplankton and zooplankton size indicate the maximum biomass of fish with relative proportions of phytoplankton and zooplankton. We also test the effect of phosphorus input conditions corresponding to oligotrophic, mesotrophic, eutrophic system on its dynamics. The average exergy to be maximized over phytoplankton and zooplankton size was computed when the system reached a steady state. Since this state is often a limit cycle, and the exergy copies this behaviour, we averaged the exergy computed for 365 days (duration of 1 year) in the stable period of the run. In mesotrophic condition, maximum fish biomass with relative proportional ratio of phytoplankton, zooplankton is recorded for phytoplankton size class 3.12 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume). In oligotrophic condition the highest average exergy is obtained in between phytoplankton size 1.48 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume), whereas in eutrophic condition the result shows the highest exergy in the combination of phytoplankton size 5.25 (log V μm³ volume) and zooplankton size 4 (log V μm³ volume).     

Modeled carbon sequestration variation in a linked erosion–deposition system

Unraveling the consequences of hydrologic transport on carbon (C) storage will help identify feedbacks between land management alternatives, climate change, and soil-vegetation-atmospheric-transfers (SVATs) of C. There is a need for theoretically driven models of erosion and deposition that includes transport induced mineralization to better understand the controls on SVATs of C. Here we present a model developed using a systems-dynamic approach that coupled C-SVATs at a 2-day resolution with a discrete event erosion–deposition model occurring with a prescribed return interval. Five possible mass-balance transformations of C occurring between the two patches were explicitly modeled: net primary production (NPP), decomposition, erosion, transport induced mineralization, and deposition. The net C-SVAT, NPP minus decomposition, exhibited three stable points of no net C flux. Starting with arbitrary initial C pool in each patch above the bifurcation point, the model approached a quasi-steady state, which included both the short-term and longer term consequences of erosion; in the baseline simulation 5080 g C m⁻² was stored prior to erosion and 100 years of low intensity erosion 4840 g C m⁻² SOC remained. Low intensity erosion also generated spatial heterogeneity; from an initial homogeneous distribution to 40% of the C stored in the eroded patch and 60% of the C stored in the deposition patch. Erosion reduction resulted in a corresponding increase in total soil C content that was positively related to the magnitude of erosion reduction. In conjunction with providing a modeling framework for reducing the uncertainty in C-SVAT, this model is a prototype of a growing theory of ecosystem processes within spatially explicit landscapes, a meta-ecosystem model.     

A three-trophic level estuarine model: Synergism of two mechanistic simulations

Few numerical simulations have attempted to include a high degree of biological detail for several trophic levels. Typically, in planktonic ecosystem models, if the dynamics of nutrients, phytoplankton and herbivorous zooplankton are formulated with ecological complexity, then carnivores are ignored, forced or modeled in an extremely simplified manner. Extensive mechanistic detail for important carnivores is difficult to represent because reliable and relevant ecological data are rarely available for appropriate species and local populations. Further, the wide temporal and spatial differences between life histories of lower plankton and carnivores may be technically difficult to model.In Narragansett Bay, Rhode Island, the ctenophore Mnemiopsis leidyi is an important carnivore to which these objections do not apply. A detailed carbon-based simulation model of this population of ctenophores was developed independently from an ecosystem model of Narragansett Bay which included detailed interactions between phytoplankton, primarily herbivorous zooplankton and nutrients. The interfacing of these two models without changing any of the formulations or values of the coefficients provided a test of the commonly used practice of forcing certain components. Both models were originally constructed with the biomass of a critical compartment forced according to observed data; in the plankton model, ctenophores were forced, and in the ctenophore model, zooplankton were forced.Predicted biomasses for zooplankton and ctenophores in the combined model were similar to the results of the two parent models, but improved relative to the actual field observations. From the findings it appears that the strategy of forcing is valid provided the forced patterns are appropriate and reasonable.     

Quantifying spatial structure of volumetric neutral models

Neutral models in landscape ecology that have been used as a framework to analyze actual landscapes have been largely planar. However, the natural world is greater than two dimensions; hence, many ecological structures, e.g., forest canopies or coral reefs, are better represented by topographies or tomographies. Because pattern and process or structure and function are intertwined, it becomes necessary to develop methods to quantify these complex architectures. With the advent of remote sensing technologies such as lidars and sonars, that permit structural mapping of some of these systems, volumetric data are becoming more prevalent. In this study, we developed a suite of binary voxel-based neutral models that possessed random, anisotropic, and hierarchical properties. We then evaluated the extent to which fractal-derived measurements, i.e., lacunarity, the simple fractal dimension, and multifractal spectra, were able to discern among the constructed model types at two different densities (p = 0.02 and p = 0.05). Multifractal analysis, where spectra were defined by three parameters, was shown to be the most sensitive to the differences among the neutral structures. Lacunarity, defined by a single parameter, was shown to be fairly useful in discerning the structures. The simple fractal dimension was found to have limited capability. To more fully assess the ability of these and additional pattern recognition methods, better representations of natural morphologies need to be developed and analyzed.     

LANDIS and forest landscape models

This paper provides contextual documentation of the LANDIS model development to provide a framework for the other papers in this special issue. The LANDIS model of forest landscape disturbance and succession was developed since the early 1990s as a research and management tool that optimizes the possible landscape extent (100 s ha to 1000 s km²), while providing mechanistic detail adequate for a broad range of potential problems. LANDIS is a raster model, and operates on landscapes mapped as cells, containing tree species age classes. Spatial processes, such as seed dispersal, and disturbances such as fire, wind, and harvesting can occur. LANDIS development benefited from the modelling and research progress of the 1960s to the1980s, including the growth of landscape ecology during the 1980s. In the past decade the model has been used by colleagues across North America, as well as in Europe and China. This has been useful to those not able to undertake the cost and effort of developing their own model, and it has provided a growing diverse set of test landscapes for the model. These areas include temperate, southern, and boreal forests of eastern North America, to montane and boreal western forests, coastal California forest and shrub systems, boreal Finnish forests, and montane forests in Switzerland and northeastern China. The LANDIS model continues to be refined and developed. Papers in this special issue document recent work. Future goals include integration within a larger land use change model, and applications to landscape and regional global change projection based on newly incorporated biomass and carbon dynamics.