Henry S. Horn

Measurement of "Overlap" in Comparative Ecological Studies

Objective, empirical measures of overlap between samples of items distributed proportionally into various qualitative categories are presented and reviewed. These indices of overlap, derived from either probability or information theory, should prove useful to the ecologist in comparative studies of diet, habitat preference, seasonal patterns of abundance, faunal lists, or similar data.

The Adaptive Geometry of Trees

Through use of the models Professor Horn has devised, plant ecologists, foresters, and botanists will be able to predict the growth and productivity of a forest, the invading and senile species in a forest, the effect of shade tolerance on forest succession, and similar questions.

Mechanisms of long-distance dispersal of seeds by wind

Long-distance dispersal (LDD) is central to species expansion following climate change, re-colonization of disturbed areas and control of pests. The current paradigm is that the frequency and spatial extent of LDD events are extremely difficult to predict. Here we show that mechanistic models coupling seed release and aerodynamics with turbulent transport processes provide accurate probabilistic descriptions of LDD of seeds by wind. The proposed model reliably predicts the vertical distribution of dispersed seeds of five tree species observed along a 45-m high tower in an eastern US deciduous forest. Simulations show that uplifting above the forest canopy is necessary and sufficient for LDD, hence, they provide the means to define LDD quantitatively rather than arbitrarily. Seed uplifting probability thus sets an upper bound on the probability of long-distance colonization. Uplifted yellow poplar seeds are on average lighter than seeds at the forest floor, but also include the heaviest seeds. Because uplifting probabilities are appreciable (as much as 1–5%), and tree seed crops are commonly massive, some LDD events will establish individuals that can critically affect plant dynamics on large scales.

Competition among Fugitive Species in a Harlequin Environment

We examine the qualitative behavior of differential equations for the proportion of insular patches of each of two kinds of habitat occupied by each of two species with characteristics rates of migration between patches and of local extinction within a habitat. Certain migration and extinction rates result in stable coexistence, even of closely similar species; others lead to competitive exclusion, even when each species is competitively superior in one kind of habitat. For a community of many species in many habitats, we surmise qualitative limits to the subdivision of resources, and alternative stable communities. Our results extend to species that live in successional or ephemeral habitats. We therefore conjecture equilibrial theories for the number of patches of habitat occupied by insular species, fugitive plants and invertebrates, or infesting parasites.

The Adaptive Significance of Colonial Nesting in the Brewer's Blackbird (Euphagus Cyanocephalus)

The adaptive significance of colonial nesting for the Brewers blackbird was studied in a sagebrush desert in eastern Washington. The colonies were small enough (5—30 pairs) to allow analysis of the behavior of individual birds and their reproductive success. The structural simplicity of the study site allowed easy access to the nests and accurate measurement of their spatial distribution. The effects of predation on breeding success were analyzed with respect to colony shape and the spatial distribution of the nests. On the study area the predation pattern favored more widely spaced nests in liner colonies, but more closely spaced nests in round colonies. I examined the role of colonial nesting in aiding the exploitation of a spatially and temporally variable food supply by taking food samples from nestling birds, measuring patterns of food availability, and observing the foraging patterns of the adult birds. An idealized model relates the spatial distribution of nests to the efficiency of individual exp...

Foliage Profile by Vertical Measurements

A technique is described for estimating a foilage profile in a forest by a combination of measurements over random lines of points: (1) counts of leaf contacts on a plumb line below a tripod, (2) sightings on the heights of lowest leaves over grids of points, and (3) the proportions of sky unobscured by leaves.

Mechanistic Analytical Models for Long‐Distance Seed Dispersal by Wind

We introduce an analytical model, the Wald analytical long‐distance dispersal (WALD) model, for estimating dispersal kernels of wind‐dispersed seeds and their escape probability from the canopy. The model is based on simplifications to well‐established three‐dimensional Lagrangian stochastic approaches for turbulent scalar transport resulting in a two‐parameter Wald (or inverse Gaussian) distribution. Unlike commonly used phenomenological models, WALD’s parameters can be estimated from the key factors affecting wind dispersal—wind statistics, seed release height, and seed terminal velocity—determined independently of dispersal data. WALD’s asymptotic power‐law tail has an exponent of −3/2, a limiting value verified by a meta‐analysis for a wide variety of measured dispersal kernels and larger than the exponent of the bivariate Student t‐test (2Dt). We tested WALD using three dispersal data sets on forest trees, heathland shrubs, and grassland forbs and compared WALD’s performance with that of other analytical mechanistic models (revised versions of the tilted Gaussian Plume model and the advection‐diffusion equation), revealing fairest agreement between WALD predictions and measurements. Analytical mechanistic models, such as WALD, combine the advantages of simplicity and mechanistic understanding and are valuable tools for modeling large‐scale, long‐term plant population dynamics.

Mechanistic models of seed dispersal by wind

Over the past century, various mechanistic models have been developed to estimate the magnitude of seed dispersal by wind, and to elucidate the relative importance of physical and biological factors affecting this passive transport process. The conceptual development has progressed from ballistic models, through models incorporating vertically variable mean horizontal windspeed and turbulent excursions, to models accounting for discrepancies between airflow and seed motion. Over hourly timescales, accounting for turbulent fluctuations in the vertical velocity component generally leads to a power-law dispersal kernel that is censored by an exponential cutoff far from the seed source. The parameters of this kernel vary with the flow field inside the canopy and the seed terminal velocity. Over the timescale of a dispersal season, with mean wind statistics derived from an “extreme-value” distribution, these distribution-tail effects are compounded by turbulent diffusion to yield seed dispersal distances that are two to three orders of magnitude longer than the corresponding ballistic models. These findings from analytic models engendered explicit simulations of the effects of turbulence on seed dispersal using computationally intensive fluid dynamics tools. This development marks a bifurcation in the approaches to wind dispersal, seeking either finer resolution of the dispersal mechanism at the scale of a single dispersal event, or mechanistically derived analytical dispersal kernels needed to resolve long-term and large-scale processes such as meta-population dynamics and range expansion. Because seed dispersal by wind is molded by processes operating over multiple scales, new insights will require novel theoretical tactics that blend these two approaches while preserving the key interactions across scales.

Long-distance dispersal of tree seeds by wind

Some mechanisms that promote long-distance dispersal of tree seeds by wind are explored. Winged seeds must be lifted above the canopy by updrafts to have a chance of further dispersal in high velocity horizontal winds aloft or in landscape-scale convection cells. Shear-induced turbulent eddies of a scale up to one-third of canopy height provide a lifting mechanism. Preliminary data suggest that all seeds of a given species may be viable candidates for uplift and long-distance dispersal, despite the evidence that slow-falling seeds are dispersed farther under any given wind conditions. Turbulence is argued more often and more extensively to advance long-distance dispersal than to retard it. Seeds may take advantage of ‘Bernoulli sailing’ to move with faster than average winds. Elasticity of branches and trees may play a role in regulating the release of seeds into unusually favorable winds. Dispersal is at least biphasic, and the study of long-distance dispersal calls for mixed models and mixed methods of gathering data.

Overlap in Foods and Foraging of Four Species of Blackbirds in the Potholes of Central Washington

Overlap in foods and foraging of four species of breeding blackbirds in the Potholes of central Washington was estimated from samples of food delivered to the nestings obtained by the pipe—cleaner technique. Prey distribution and availability were sampled with emergence traps for aquatic species and by sweeping with a standard insect net for upland species. Overlap was estimated by the probability than an item of food drawn randomly from the diet of one species would be of the same prey category taken from the same habitat as an item drawn at random from the diet of another species. Because of the diurnal cycle of emergence of aquatic insects, overlaps were calculated for three periods of the day (preemergence, emergence and postemergence) and averaged for the final overlap value. The overall food overlap for three of the species is high for all periods of the day and there is little evidence for differential specialization within habitats. Habitat overlaps, however, are varied, being highest during the emergence period for Redwings and Yellowheads but highest during the afternoon between Redwings and Brewers. The total overlap values are very similar to the theoretical maximum values derived from competition theory by MacArthur and Levins.