Gregg Hartvigsen

Use and Analysis of Complex Adaptive Systems in Ecosystem Science: Overview of Special Section

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

Tri-trophic interactions influenced by resource availability: predator effects on plant performance depend on plant resources

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.

Tradeoff between height and relative growth rate in a dominant grass from the Serengeti ecosystem

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.

The vegetation of the Wave Hill natural area, Bronx, New York.

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 Hills 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.

Competition between co-dominant plants of the Serengeti Plains depends on competitor identity, water, and urine.

AbstractKyllinga 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. Sporobolus kentrophyllus was a superior competitor to K. nervosa over the tested treatment levels with respect to growth and reproductive effort. Sporobolus kentrophyllus exhibited rapid growth in response to urine addition, leading to a significant species × urine interaction while reduced growth by K. nervosa in response to low water availability explained the significant species × water interaction and is likely explained by K. nervosas shallow root system. Kyllinga nervosa, however, appears to be more tolerant of low nitrogen conditions based on its similar growth with and without the urine treatment. The effect of intraspecific competition on total biomass was similar for S. kentrophyllus and K. nervosa. Competition resulted in increased size differences (asymmetry) for K. nervosa and for the interspecific competition treatments compared to the size differences observed for plants grown individually (in absence of competition).Total reproductive biomass was reduced most by competition with S. kentrophyllus, irrespective of target species. The water treatment did not influence reproduction while the urine treatment significantly increased reproductive biomass and interacted with target species, competitor species, and yielded a three-way urine × target × competitor species interaction.Results suggest that codominance of these two species in the Serengeti is regulated by water availability, nitrogen input from grazers, and local neighbor identity.

The Analysis of Leaf Shape Using Fractal Geometry

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 moths 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

FRACTALS, AVERAGE DISTANCE AND THE CANTOR SET

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

Applied Population Ecology

\frac{2}{5}

Network structure, and vaccination strategy and effort interact to affect the dynamics of influenza epidemics.

, the average distance between points of the Cantor p-set is

The vegetation of the Wave Hill natural area

\frac{p+1}{p+3}