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There is a well known connection between the structural complexity of vegetative stands and ecosystem properties. Developing methods to quantify this structural complexity is an important goal for ecologists. We present an efficient and easily implemented field technique for calculating the shape of forest canopies, and the shape of forest stands as succession occurs, using fractal geometry. Fractal geometry can be used to describe complex, non-Euclidean objects that are common in natural systems. We tested the use of this tool in 22 vegetative and forested plots in Western New York State, USA. We found an asymptotic relationship for fractal dimension (D) as a function of basal area (BA; r2= 0.68). In a randomization test to investigate the robustness of D to different tree canopy shapes, we found that D was sensitive to canopy shape switching, suggesting that the method is able to differentiate among similar forests comprised of species having different shaped crowns. We conclude that the shape is conserved in vegetative areas as they progress from one stage of succession to the next (range of mean D: 2.56 to 2.68 across stages). Furthermore, we conclude that the shape filling properties—i.e., distribution of trunks and limbs in a forested area, measured as mean distance—are also conserved across vegetational chronosequences (F = 1.3189, df = 8, 3, p = 0.3341).

The average distance between points of a fractal is proposed as a natural measure of the way in which the points of a fractal are distributed. The average distance between points of the Cantor Set is found to be [Formula: see text], the average distance between points of the Cantor p-set is [Formula: see text], and the average distance between points of the Fat Cantor p-set is [Formula: see text]. A general formula for computing the average distance between points of a self-similar set satisfying the open set condition is found.

There is growing interest in understanding and controlling the spread of diseases through realistically structured host populations. We investigate how network structures, ranging from circulant, through small-world networks, to random networks, and vaccination strategy and effort interact to influence the proportion of the population infected, the size and timing of the epidemic peak, and the duration of the epidemic. We found these three factors, and their higher-order interactions, significantly influenced epidemic development and extent. Increasing vaccination effort (from 0% to 90%) decreased the number of hosts infected while increasing network randomness worked to increase disease spread. On average, vaccinating hosts based on degree (hubs) resulted in the smallest epidemics while vaccinating hosts with the highest clustering coefficient resulted in the largest epidemics. In a targeted test of five vaccination strategies on a small-world network (probability of rewiring edges=5%) with 10% vaccination effort we found that vaccinating hosts preferentially with high-clustering coefficients (similar to real-world strategies) resulted in twice the number of hosts infected as random vaccinations and nearly a 30-fold higher number of cases than our strategy targeting hubs (highest degree hosts). Our model suggests how vaccinations might be implemented to minimize the extent of an epidemic (e.g., duration and total number infected) as well as the timing and number of hosts infected at a given time over a wide range of structured host networks.

We often begin studying biological systems, such as molecules, organisms, or even aggregations of organisms in groups, by trying to describe their structure. Structure, or more simply, shape, is often used to describe differences between species. Shape also strongly influences function (e.g. the shape of a male moth's antennae greatly influences his ability to detect the pheromones of females that may be miles away [see Vogel 1988]). The shapes of objects and organisms traditionally have been described using Euclidean geometry. This type of geometry is the basis of what we are all familiar with from high school, and leads to simple shapes like lines, squares, circles and cubes. These structures also define our traditional sense of dimensions in space (e.g. a line is one-dimensional, a square is twodimensional, etc.). Organisms, however, rarely fit these simple shapes and, instead, are a very complex combination of these forms or usually something altogether different. Try, for example, to think of the shape of a human as made up of spheres, right prisms and cylinders. The person is likely to look silly. Size also is an important structural component of objects. For organisms, size, by definition, changes during growth and development. Shape, on the other hand, may or may not change. Imagine, for example, what an adult human would look like if she were to retain the same size relationship between head and body from birth to adulthood. Things that do not change shape, or relative sizes of parts as they grow, are referred to as being isometric. When shape does change as a function of size we refer to these objects as having an allometric scaling relationship. We would like to know how to describe the shapes of organisms and how shape changes. The list of Euclidean shapes does not get us very far, even when we try to describe something simple like a leaf (Figure 1). You could describe this leaf as being similar to an oval or square, but that is clearly inadequate if we are interested in details about the leaf. You could attempt a complicated description using many triangles, for instance, but that would be both difficult and of dubious value. With leaves we may think about the amount of area the leaf covers, but this takes the interesting shape of the leaf and converts it into a Euclidean shape. The leaf in Figure 1 might have an area of 9 cm2, but this implies the leaf has dimensions of 3 x 3 cm, which is a square. If I have another 9 cm2 leaf, does it look the same or could it be

Kyllinga nervosa (Steud.) and Sporobolus kentrophyllus (K. Schum.) are co-dominant plants of the Serengeti short-grass plains, Tanzania. The plains are characterized by seasonal and sporadic rainfall and currently support in excess of 1.5 million migratory ungulates. The interactive effect of simulated bovine urine and water availability were tested on the competitive interactions of these species in the laboratory.

Ecologicalsystemsarecomplexassemblagesofinter-acting organisms embedded in an abiotic environ-ment.Complexityarisesfrominterspecificandintra-specific interactions among individuals or agents,interactions across trophic levels, and the interac-tions of organisms with the abiotic environmentover space and time. In addition, interactions canrange from strong and direct to weak and diffuseand are modified by both positive and negativefeedback with the environment. In our effort tounderstand pattern formation at the community orecosystem level, we are confronted with the daunt-ing array of processes that function across differentspatial and temporal scales. We are thus forced toaddress the question of how these different levels oforganization can be integrated, or how mechanismsand patterns at one level of organization can beunderstood in terms of processes operating at adifferent level of organization.The goal of this

An individual-based model of plant–herbivore interactions was developed to test the potentially interactive effects of explicit space and coevolution on population and community dynamics. Individual plants and herbivores resided in cells on a lattice and carried linked interaction genes. Interaction strength between individual plants and herbivores depended on concordance between these genes (gene-for-gene coevolution). Mating and dispersal among individuals were controlled spatially within variably sized neighbourhoods. Without evolution we observed high-frequency plant–herbivore oscillations (blue spectra) with small individual neighbourhoods, and stochastic fluctuations (white spectra) with large neighbourhoods. Evolution resulted in decreased interaction strength, decreased herbivore-induced plant mortality, increased population sizes, and longer-term fluctuations (reddened spectra). Small herbivore neighbourhoods led to herbivore extinction only with evolution. To explore the increased population size response to evolution we ran simulations without evolution while tuning plant–herbivore interaction strength from high to none. We found that herbivore populations were maximized at intermediate levels of interaction strength that coincided with the interaction strength achieved when the system tuned itself through evolution. Overall, our model shows that the small-scale details of phenotypically variable individual-level interactions, leading to evolutionary dynamics, affect large-scale population and community dynamics.

Carrying capacity is the maximum number, density, or biomass of a population that a specific area can support. This likely varies over time and depends on environmental factors, resources, and the presence of predators, disease agents, and competitors over time. The concept of carrying capacity has been explicitly recognized for approximately 150 years, and its use has waxed and waned during this time. Currently, the use of carrying capacity to describe any particular population requires caution, although the concept remains intuitive and invokes questions that challenge our fundamental understanding of what factors regulated populations over time and space.

We determined the relationship between plant height and whole-plant relative growth rate (g g-1 day-1) for ten genotypes of Sporobolus kentrophyllus collected from an intensively grazed site on the Serengeti Plains, Tanzania. Plants were grown for 7 weeks in a greenhouse in Syracuse, N.Y., and harvested weekly. Plants that received simulated bovine urine showed a negative relationship between plant height and growth rate, suggesting a genetic tradeoff between competitive ability if ungrazed (height) and ability to recover from grazing (growth rate). There was no height-growth rate relationship under nitrogen addition rates similar to field mineralization rates. In addition, faster-growing, shorter plants tended to have relatively higher above-ground growth rates than slower-growing, taller plants. These results suggest that natural selection has maintained a gradient of morphologies within this species ranging from short, rapidly growing genotypes adapted to intense grazing conditions to tall, slow-growing, grazer-susceptible genotypes that are superior light competitors in absence of herbivory.

10 páginas,3 tablas, 3 figuras.

Organisms are usually both enemies and victims of ecological interactions. Even individuals within ‘top predator’ species often suffer from cannibalism (an ecological ‘self-loop’), parasites, or disease agents. Predators capture, kill and consume their victims. Parasites either live on or within a victim and may kill their host directly or, more typically, reduce the host's growth, survival and/or reproductive potential. Parasitoids function like predators but the act of killing the host is done by offspring which, generally, hatch from an egg on or near a host and then enter the victim and consumes it from inside. Disease agents vary in their effects on victims from hardly noticeable to causing death. Finally, herbivory is that act of an animal consuming part, or the entirety, of a plant. The effect of herbivory on plants may be unquantifiable to resulting in the death of the plant. Key Concepts: Predators capture, kill and consume their prey. Parasitoids lay eggs on or near victims. These eggs hatch and the larvae enter and consume their victims from within. Parasites live on or within their victims and usually reduce the victim's ability to grow, survive and/or reproduce. Disease agents generally reduce victim growth, survival and/or reproductive rates. Herbivory is the act by an animal of consuming plant material which may reduce plant growth, survival and/or reproductive success. Researchers continue to debate the relative importance of top-down effects from enemies on victims and the effects of bottom-up effects of resources in population dynamics. Mathematical and computer models can help us to understand and predict these complex species interactions. Keywords: predation; herbivory; plant–animal interactions; host–pathogen dynamics; models

Evolution has led to an incredible diversification of life on Earth. This article summarizes our current understanding of the evolution of biodiversity and discusses how the evolution of biodiversity influences communities, both past and present. The influence of evolution on integrated communities of diverse taxa and the likely changes that will take place in the near future certainly rank among the great frontiers facing students of biodiversity as we watch the profound test of the resilience of Earth's ecosystems to the enterprise of human activities.

We tested the interactive effects of top-down (mite herbivores and predators) and bottom-up (fertilization) factors on cottonwood sapling performance, measured as leaf area, number of leaves, and height growth rate. Herbivorous spider mites in the absence of predators reduced plant performance on both fertilized and unfertilized plants. Plants responded to fertilization with increased growth only in the absence of herbivores. Without fertilization, plant performance with herbivores and predators present exceeded that of control plants with no herbivore damage. With fertilization, plant performance was intermediate between plants with herbivores and plants without herbivores or predators. By four weeks predators had effectively removed herbivorous mites on fertilized and unfertilized plants. Predator populations were unaffected by fertilization but significantly reduced herbivore populations more on fertilized than unfertilized plants. The results suggest that plant performance depends on the complex interaction between top-down and bottom-up factors.

YOST, S. E., S. ANTENEN AND G. HARTVIGSEN (Forest Project, Wave Hill, Bronx, NY 10471). The vegetation of the Wave Hill natural area, Bronx, New York. Bull. Torrey Bot. Club 118: 312-325. 1991.-A vegetation survey of the Wave Hill natural area, an urban woodland in the Bronx, NYC, was conducted in 1987. A permanent grid of 10 x 10 meter quadrats was established throughout the three ha natural area. Importance values were calculated in 238 quadrats. The most important arborescent species were Robinia pseudoacacia, Quercus rubra, and Acer platanoides. The most important non-arborescent species were Ampelopsis brevipedunculata, Lonicera maackii, and Alliaria petiolata. Four vegetation associations were recognized: oak-maple, black locust, black birch, and open areas. The high percentage (48%) and importance of non-native species is related to Wave Hill's urban location and land-use history. Ampelopsis brevipedunculata, the most important of the non-arborescent species, is a vine which appears to inhibit the growth of other species in open areas. A list of the 276 species of vascular plants identified is included in an appendix.