Ecology

Kareiva, Peter

doi: 10.2307/1939376pmid: N/A

Tilman, David

doi: 10.2307/1939377pmid: N/A

All organisms, especially terrestrial plants and other sessile species, interact mainly with their neighbors, but neighborhoods can differ in composition because of dispersal and mortality. There is increasingly strong evidence that the spatial structure created by these forces profoundly influences the dynamics, composition, and biodiversity of communities. Nonspatial models predict that no more consumer species can coexist at equilibrium than there are limiting resources. In contrast, a similar model that includes neighborhood competition and random dispersal among sites predicts stable coexistence of a potentially unlimited number of species on a single resource. Coexistence occurs because species with sufficiently high dispersal rates persist in sites not occupied by superior competitors. Coexistence requires limiting similarity and two—way or three—way interspecific trade—offs among competitive ability, colonization ability, and longevity. This spatial competition hypothesis seems to explain the coexistence of the numerous plant species that compete for a single limiting resource in the grasslands of Cedar Creek Natural History Area. It provides a testable, alternative explanation for other high diversity communities, such as tropical forests. The model can be tested (1) by determining if coexisting species have the requisite trade—offs in colonization, competition, and longevity, (2) by addition of propagules of propagules to determine if local species abundances are limited by dispersal, and (3) by comparisons of the effects on biodiversity of high rates of propagule addition for species that differ in competitive ability.

Holmes, E. E.; Lewis, M. A.; Banks, J. E.; Veit, R. R.

doi: 10.2307/1939378pmid: N/A

Most of the fundamental elements of ecology, ranging from individual behavior to species abundance, diversity, and population dynamics, exhibit spatial variation. Partial differential equation models provide a means of melding organism movement with population processes and have been used extensively to elucidate the effects of spatial variation on populations. While there has been an explosion of theoretical advances in partial differential equation models in the past two decades, this work has been generally neglected in mathematical ecology textbooks. Our goal in this paper is to make this literature accessible to experimentally ecologists. Partial differential equations are used to model a variety of ecological phenomena; here we discuss dispersal, ecological invasions, critical patch size, dispersal—mediated coexistence, and diffusion—driven spatial patterning. These model emphasize that simple organism movement can produce striking large—scale patterns in homogeneous environments, and that in heterogeneous environments, movement of multiple species can change the outcome of competition or predation.

Molofsky, Jane

doi: 10.2307/1939379pmid: N/A

Simple individual—based models of population dynamics and spatial patterns are developed using cellular automata theory. These models assume that individuals within a population are distributed in space and that important interactions among individuals take place over some predefined local scale. Local density dependence in the model produces dynamics that depend on the scale of the dispersal distance. When dispersal occurs over long distances, the population enters a stable limit cycle. However, when colonization is limited to the same local neighborhood where competition is taking place, the spatial and temporal patterns become chaotic. Habitat size also affects the dynamics. Chaotic dynamics become cyclical as habitat size decreased. These results illustrate how a wide array of complex dynamics can arise with simple spatially distributed models and that the resulting dynamics can depend on the habitat size.

Goldwasser, L.; Cook, J.; Silverman, E. D.

doi: 10.2307/1939380pmid: N/A

Many spatial models of ecological processes achieve analytical tractability by neglecting variability among individuals, in their patterns of movement, and among locations, in their favorability for colonization or persistence. We develop a series of simulations, based on standard metapopulation and partial differential equation models, that explicitly track variability among individuals or locations, and compare the results with what would be obtained by ignoring such variability. We find, first, that variability among patches can have a strong influence on the outcome of community dynamics, by promoting the coexistence of species that would otherwise exclude one or the other through competition. Second, variability among individuals in their rates of movement can markedly increase the rate of spread of a population. Construction and analysis of a modified PDE (partial differential equation) model for a population that consists of two classes of individuals, dispersed and nondispersers, confirms the second result, and also shows that the presence of even a few rapidly dispersing individuals can markedly increase the rate of spread of the population. Our simulations and analysis show that the incorporation of variability into ecological thinking is feasible and that it can strengthen our ability to investigate the effects of spatial processes.

Ward, David; Saltz, David

doi: 10.2307/1939381pmid: N/A

We examined the foraging strategies of dorcas gazelles (Gazella dorcas) at different spatial scales. When in sand dunes in the Negev desert of Israel, the gazelles fed on a single plant species, the madonna lily (Pancratium sickenbergeri). In summer, when all live plant material is subterranean, the gazelles dug holes in the sand to remove the stem and bulb. After the winter rains, only the tips of the leaves that had emerged above ground were eaten. We examined search patterns and patch choice on a large scale using both null hypotheses generated by a random walk model and “rules of thumb” that may approximate on optimal foraging strategy. At a smaller scale, we examined the selection of parts of individual lilies, again making predictions about "rules of thumb" that the gazelles should use. Gazelle search paths deviated considerably from a random walk. Gazelle made shorter move lengths in areas of high plant density. Bouts of concentrated feeding in small areas were interspersed with long moves to new forging areas, suggesting that these animals were repeatedly sampling their environment. Congruent with optimal foraging predictions, the gazelles selected plants with more and larger leaves than randomly available, and concentrated their foraging activity in areas of highest lily densities. In summer, gazelles altered the depth of digs in response to differences in sand compaction. There was a surprising negative correlation between the size of the plant and the amount eaten. This selection of small plants appears to be due to the increased probability of reaching the bulb, which contains most of the plant's volume. Summer foraging by gazelles had a negative impact on the lilies. There is strong selection on the lilies to grow to a sufficient depth that damage of the bulb by herbivory is minimized. Conversely, the gazelles must select plants with bulbs close to the surface to maximize energy intake and minimize the energy cost of digging.

Ohsaki, Naota; Sato, Yoshibumi

doi: 10.2307/1939382pmid: N/A

This report assesses the role of specialist parasitoids in providing a major selection pressure for food plant preference in herbivorous insects. Three Pieris butterflies, P. rapae crucivora, P. melete, and P. napi japonica, use different sets of cruciferous larval food plants. P. rapae is oligophagous and uses ephemeral plants. P. melete is polyphagous and uses persistent plants, as well as all of the ephemeral plants used by P. rapae. On the other hand, P. napi is locally monophagous, using persistent Arabis. We assessed the intrinsic suitability of these crucifers by measuring survival rates, development times, and pupal mass of larvae growing on them at a constant temperature. All of the food plants of P. rapae, and P. melete are suitable for larvae of the three Pieris species. On the other hand, food plants of P. napi are the least suitable for all three species. Pieris species. On the other hand, food plants of P. napi are the least suitable for all three species. Pieris larvae have two specialist parasitoids, the braconid wasp Cotesia glomerata (formerly Apanteles glomeratus) and the tachinid fly Epicampocera succincta. In newly established habitats, P. rapae can avoid both parasitoids. In long—lasting habitats, however, P. rapae is heavily parasitized by both parasitoids. P. melete and P. napi, by contrast, live only in long—lasting habitats, where the parasitic pressure is potentially high. However, P. melete can partially avoid parasitism by killing the eggs of C. glomerata by encapsulation, through parasitized by E. succincta. On the other hand, P. napi seems to have evolved behavioral avoidance of parasitoids by specializing on Arabis plants. The different food plant preferences of the three Pieris species can be interpreted as resulting from differences in the balance of a trade—off between parasitoid avoidance and the intrinsic quality of potential food plants of Pieris species.

Dussourd, David E.; Denno, Robert F.

doi: 10.2307/1939383pmid: N/A

To test if secretory canals limit the host ranges of polyphagous moth larvae, we measured the growth of Trichoplusia ni (cabbage looper) and Spodoptera ornithogalli (yellow—striped armyworm) on plants in nine families, including species with and without secretory canals. Larvae were reared to the final instar on excised leaves of the test plant, weighed, then enclosed in the field on either detached leaves with depressurized canals or intact leaves with undamaged canals. After 24 h, the larvae were reweighed and the leaves were examined for evidence of trenching. For comparison, nine additional species of generalist caterpillars were tested on Lactuca serriola (with latex canals) using the same procedure; five of these species were also tested with Petroselinum crispum, which has resin canals. Trichoplusia larvae gained mass on detached and intact leaves of all plant species tested. Larvae cut trenches in plants with canal—borne secretions (Apiaceae–resin, Asteraceae: Cichorieae–latex, Cucurbitaceae–phloem sap), and thereby deactivated the canal systems. Plant species lacking exudates were not trenched. Spodoptera larva failed to cut trenches and performed poorly (with one exception, Daucus carota) on intact leaves of plants with exudates, the same species that T. ni trenched. Detached leaves supported rapid growth by S. ornithogalli larvae, documenting the suitability of these plants in the absence of functional canals. On the latex—bearing Lactuca serriola. S. ornithogalli also grew rapidly on leaves distal to trenches cut by T. ni. This laboratory experiment confirmed that armyworms could feed on plants bearing secretory canals if the larvae were capable of trenching. Performance of the nine other generalist species was likewise correlated with behavior; only species adept at trenching grew rapidly on intact L. serriola and P. crispum. Our results, corroborated across both plant and insect taxa, indicate that the effectiveness of canal—borne exudates as a defense is dependent upon the behavioral capabilities of the herbivore. Generalists lacking trenching behavior are unable to compensate through biochemical or physiological mechanisms. Their host ranges are indeed limited by canalicular defenses due to their inability to deactivate the canals by trenching.

Naeem, Shahid; Hawkins, Bradford A.

doi: 10.2307/1939384pmid: N/A

Parasitoid communities show patterns in the distribution and abundance of species that fit predictions of niche partitioning models. These models treat community resources as unit volumes and conceptually treat niche partitioning, or resource division, as breakage of this unit into fractions that correlate with proportional densities of the species controlling that fraction of the resource. Unlike many other communities, parasitoid communities are particularly appropriate for applying these models. We examine four types of niche partitioning models employing three types of breakage algorithms. Of these, the overall best—fit model was unidimensional—niche model using a breakage algorithm in which each subsequent break occurred on the smaller or larger fraction at random. Successful fit of these empirically derived, abundance distributions of parasitoid communities to this theoretically derived, expected distribution supports the hypothesis that these communities are minimally structured by a hierarchical process of random niche partitioning along a single resource axis.

Dudt, Jan Frederic; Shure, Donald J.

doi: 10.2307/1939385pmid: N/A

We examined the changes in leaf phenolic chemistry and insect herbivory from saplings of two temperate deciduous species, Liriodendron tulipifera (tulip poplar) and Cornus florida (dogwood), planted in five microenvironments in Gilmer County, Georgia, USA. The experimental design permitted comparisons between saplings grown in an open field, under shade cloth within the field, on the edge between field and forest, in forest understory, and within canopy gaps established within the forest. Half of the trees in each microenvironment were fertilized. Leaves from each tree were sampled at the end of the growing season and 1989 and 1990 and analyzed for toughness, percent dry mass, total phenolics, hydrolyzable tannins, condensed tannins, and insect herbivory (percent leaf area damaged). The shade—tolerant dogwood saplings contained higher levels of total phenolics and hydrolyzable tannins than the shade—intolerant tulip poplar saplings. Dogwood generally had lower levels of herbivory. These results support earlier studies suggesting that slow—growing, shade—tolerant species tend to have higher levels of phenolics and experienced lower levels of herbivory than fast growing, shade—intolerant species. However, dogwood leaves contained lower levels of condensed tannins and were as tough as tulip poplar leaves. Sunlight availability had a significant positive influence on levels of phenolics in both species. Leaf phenolics generally increased with greater insolation from forest to field and when sunlight was greater within field for forest habitats. However, the levels of tannins in dogwood saplings only dropped significantly in the deep shade of the forest. The similar levels of dogwood phenolics in most microenvironments are indicative of the relatively high photosynthetic efficiency of this species in reduced light environments. Overall, these results are consistent with carbon/nutrient balance theory that predicts trade—offs in the allocation of photosynthate from defense to growth as light declines. Levels of insect herbivory and total phenolics were inversely related for dogwood. However, the relationship with tannins was less apparent. Herbivory on tulip poplar was unrelated to changes in phenolics, possibly reflecting the greater chemical diversity of that species. Fertilization increased the biomass of both species, but had no apparent influence on levels of leaf phenolics or insect hervivory. The lack of a fertilization effect was unexpected in light of previous suggestions that fertilization results in reduced phenolics and increased herbivory.