William E. Grant

Importance of Vegetative Cover to Cycles of Microtus Populations

A hypothesis is presented that a site-dependent threshold level of vegetative cover is necessary for a population of Microtus to increase in numbers sufficiently to undergo a multi-year cycle. The hypothesis is supported by results of studies in grassland habitats in the tallgrass prairie of Oklahoma and Minnesota, the mixed grass prairie of South Dakota, and the shortgrass prairie of Colorado. Levels of cover below the threshold sometimes support resident, breeding populations of voles. Levels above the threshold may influence amplitude, duration, and synchrony of Microtus cycles.

Coastal erosion, global sea‐level rise, and the loss of sand dune plant habitats

Much of Americas coastline is threatened by overdevelopment and coastal erosion, driven by global sea-level rise, a problem that is attracting the attention of researchers around the world. Although we have now acknowledged the impending risks, little is known about the response of spatially dependent dune plant communities to the loss or restriction of their habitat. In order to study this development, a spatially explicit model of sand dune plant succession on Galveston Island, Texas, was created, using sea-level rise as the primary mechanism causing local erosion. Simulations of sea-level rise scenarios developed by the Intergovernmental Panel on Climate Change demonstrated that beach erosion constrained plants to a narrow area, resulting in a breakdown of the successional process. The loss of late-succession plants along coastlines, their dependent faunal species, and possible solutions are discussed. This model and example serves as a harbinger of the future for many of the USs sandy beaches and co...

Structure and Productivity of Grassland Small Mammal Communities Related to Grazing-Induced Changes in Vegetative Cover

Effect of grazing on the structure and productivity of small mammal communities in four types of North American grasslands is examined quantitatively using data collected during a 4-year period. Small mammal communities in tallgrass and montane grasslands appear more affected by grazing and the subsequent reduction in vegetative cover (=total above-ground standing crop of plants) than do small mammal communities in shortgrass and bunchgrass grasslands. This trend is evident when small mammals in grazed versus ungrazed habitats are compared with regard to mean community biomass, annual community respiration, production, and consumption, mean species diversity and evenness, proportional species and functional group composition, seasonal dynamics of community biomass, and temporal variation in proportional species composition. These results lend further support to the hypothesis that the general composition of grassland small mammal communities is determined primarily by structural attributes of the habitat.

Simulation of a fire-sensitive ecological threshold: a case study of Ashe juniper on the Edwards Plateau of Texas, USA

A model was developed to represent the establishment of a fire-sensitive woody species from seeds and subsequent survival and growth through five size classes. Simulations accurately represent structural changes associated with increased density and cover of the fire-sensitive Ashe juniper (Juniperus ashei, Buckbolz) and provide substantial evidence for multiple steady states and ecological thresholds. Without fire, Ashe juniper increases and herbaceous biomass decreases at exponential rates until a dense-canopy woodland is formed after approximately 75 years. Maintenance of a grass-dominated community for 1.50 years requires cool-season fires at a return interval of less than 25 years. When initial cool-season fires are delayed or return intervals are increased, herbaceous biomass (fuel) decreases below a threshold and changes from grassland to woodland become irreversible. With warm-season fires, longer return intervals maintain grass dominance, and under extreme warm-season conditions even nearly closed-canopy stands can be opened with catastrophic wildfires.

AN ARTIFICIAL INTELLIGENCE MODELLING APPROACH TO SIMULATING ANIMAL/HABITAT INTERACTIONS

Ecological modellers have begun to recognize the potential of object-oriented programming techniques in structuring models. However, little has been done to take advantage of artificial intelligences (AI) symbolic representations to model the decision-making processes of animals. Here, a generic model of animal-habitat interaction and a specific model of moose-, Alces alces L., forest interactions in Finland are described that are event-driven and behavior-based. Individual level simulation is accomplished through an object-oriented knowledge representation scheme and AI techniques to implement a hierarchical decision-making model of behavior. The habitat is likewise represented in an object-oriented scheme, allowing the simulation of a heterogeneous environment. Other AI techniques for modelling behavior, memory, and actions are discussed including LISP methods, rule-based reasoning, and several search algorithms. Simulations of the moose-forest system show the power of this approach but are not intended to advance the theory of large-herbivore behavior and foraging. AI techniques are found to be most beneficial in (a) studying population processes based on individual level models of behavior, (b) modelling spatial heterogeneity, (c) building event-driven models, (d) providing a conceptual clarity to model construction, and (e) providing a structure equally well suited to simulating resource management.

AI modelling of animal movements in a heterogeneous habitat

Abstract We demonstrate use of object-oriented programming, dynamic linkages, rule-based decission procedures, and several other concepts from the field of artificial intelligence (AI) for modelling animal movements in a heterogeneous habitat. An object-oriented model of a deer that learns about habitat structure, plans movements, and accommodates to changes in a patchy brushland habitat is described and used to simulate effects of patch size on deer movements. Innovative features of this model include: (a) representation of habitat as a network of heterogeneous patches, (b) representation of an individuals knowledge of the environment (memory network) as different from but related to the habitat (habitat network), (c) individuals use of knowledge of the environment to plan paths to goals, and (d) ability for an individual to change its knowledge base when it encounters changes in the environment. Decision rules in the model are hypothetical, but the current application suggests that object-oriented modelling provides a concise yet detailed technology for modelling animal movements.

EFFECTS OF POCKET GOPHER MOUNDS ON PLANT PRODUCTION IN SHORTGRASS PRAIRIE ECOSYSTEMS

Field measurements are presented which quantify the extent of surface area covered by pocket gopher mounds and the effects of gopher mound microrelief on local plant production and on local rates of infiltration of rainfall and evaporation of soil water. The percentage of total surface area covered was 8% on a heavily grazed grid, 2.5% on a lightly grazed grid, and 6.5% on an ungrazed grid. The relationship between surface area covered in m2 and kg of soil in mounds was linear. Density of above-ground vegetation decreased significantly with increasing distance from the edge of mounds up to a distance of 40 cm. Rates of infiltration of rainfall were significantly higher on mounds than on undisturbed areas, but average volumetric soil water content was not significantly different between mounds and undisturbed areas. Studies of the roles of animals in ecosystems commonly have empha- sized animals as consumers of energy and materials; the consensus of these studies is that, in most terrestrial ecosystems, less than 10% of total ecosystem energy flow passes through vertebrate populations (Golley 1973). The role of animals is not, however, determined solely by trophic relationships (Gross 1969, Chew 1978). Several quantita- tive studies have documented the influence of small mammals on vari- ous aspects of the physical structure of ecosystems, particularly with regard to the effect of their burrowing on soil structure (Zlotin and Khodashova 1974). Pocket gophers (Geomyidae) provide an obvious example. In Texas, Geomys bursarius brought to the surface an esti- mated 7 tons of soil per acre per year (Buechner 1942); whereas, in mountain rangelands, Thomomys spp. may bring to the surface from 5 (Ellison 1946) to 38 (Richens 1966) tons of soil per acre per year.

Fish bioenergetics and growth in aquaculture ponds: II. Effects of interactions among, size, temperature, dissolved oxygen, unionized ammonia and food on growth of individual fish

Abstract In a previous paper, predictions of the individual effects of size, temperature, dissolved oxygen, unionized ammonia and food on fish growth from an organismal model of fish bioenergetics and growth agreed well with available data. Model predictions of the combined effects of size and temperature, size and food, temperature and food, and dissolved oxygen and food on growth also agreed well with available data. Model predictions of six undocumented two-factor interactions were consistent with other known interactions and general principles of fish energetics. The model was also used to predict maximum, optimum, and maintenance rations under controlled environments.

BRAZILIAN FREE‐TAILED BATS AS INSECT PEST REGULATORS IN TRANSGENIC AND CONVENTIONAL COTTON CROPS

During the past 12000 years agricultural systems have transitioned from natural habitats to conventional agricultural regions and recently to large areas of genetically engineered (GE) croplands. This GE revolution occurred for cotton in a span of slightly more than a decade during which a switch occurred in major cotton production areas from growing 100% conventional cotton to an environment in which 95% transgenics are grown. Ecological interactions between GE targeted insects and other insectivorous insects have been investigated. However, the relationships between ecological functions (such as herbivory and ecosystem transport) and agronomic benefits of avian or mammalian insectivores in the transgenic environment generally remain unclear, although the importance of some agricultural pest management services provided by insectivorous species such as the Brazilian free-tailed bat, Tadarida brasiliensis, have been recognized. We developed a dynamic model to predict regional-scale ecological functions in agricultural food webs by using the indicators of insect pest herbivory measured by cotton boll damage and insect emigration from cotton. In the south-central Texas Winter Garden agricultural region we find that the process of insectivory by bats has a considerable impact on both the ecology and valuation of harvest in Bacillus thuringiensis (Bt) transgenic and nontransgenic cotton crops. Predation on agricultural pests by insectivorous bats may enhance the economic value of agricultural systems by reducing the frequency of required spraying and delaying the ultimate need for new pesticides. In the Winter Garden region, the presence of large numbers of insectivorous bats yields a regional summer dispersion of adult pest insects from Bt cotton that is considerably reduced from the moth emigration when bats are absent in either transgenic or non-transgenic crops. This regional decrease of pest numbers impacts insect herbivory on a transcontinental scale. With a few exceptions, we find that the agronomics of both Bt and conventional cotton production is more profitable when large numbers of insectivorous bats are present.

Fish bioenergetics and growth in aquaculture ponds: I. Individual fish model development

Abstract A dynamic model of fish bioenergetics and growth at the organismal level under controlled environments was developed as a tool to study, evaluate, and improve the management of fishpond grow-out system. The model incorporated five key variables, namely body size temperature, dissolved oxygen, unioinized ammonia and amount of food, and 17 growth parameters which defined the species of fish used. Parameters were estimated for channel catfish. Fish growth was more sensitive to changes in food consumption parameters than to changes in metabolic parameters. Within either the food consumption component or the metabolic component, fish growth was more sensitive to changes in temperature parameters than to changes in parameters for body size, dissolved oxygen or unionized ammonia. The model may be reparameterized to represent different species of fish in different environments and provides a common ground for theoretical, laboratory and field studies. It also provides a means by which research activities of several groups can be coordinated toward achievement of a common goal.