ISSN:0012-9658    

DOI:10.2307/2265692

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The potential harmful effects of non—indigenous species introduced for biological control remain an important unanswered question, which we addressed by undertaking a literature review. There are few documented instances of damage to non—target organisms or the environment from non—indigenous species released for biological pest control, relative to the number of such releases. However, this fact is not evidence that biological control is safe, because monitoring of non—target species is minimal, particularly in sites and habitats far from the point of release. In fact, the discovery of such impacts usually rests on a remarkable concatenation of events. In addition to trophic and competitive interactions between an individual introduced species and a native one, many effects of introduced species on ecosystems are possible, as are numerous types of indirect interactions. Predicting such impacts is no mean feat, and the difficulty is exacerbated by the fact that introduced species can disperse and evolve. Current regulation of introduced biological—control agents, particularly of entomophages, is insufficient. At the very least, strong consideration should be given to the likely impact of both the pest and its natural enemy on natural ecosystems and their species, and not only on potential costs to agriculture, silvi—culture, and species of immediate commercial value.

ISSN:0012-9658    

DOI:10.2307/2265693

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

The cultivation of tropical Asian rice, which may have originated 9000 yr ago, represents an agricultural ecosystem of unrivaled ecological complexity. We undertook a study of the community ecology of irrigated tropical rice fields on Java, Indonesia, as a supporting study for the Indonesian National Integrated Pest Management Programme, whose purpose is to train farmers to be better agronomists and to employ the principles of integrated pest management (IPM). Two of our study objectives, reported on here, were (1) to explore whether there exist general and consistent patterns of arthropod community dynamics related to natural or intrinsic levels of biological control, and (2) to understand how the existing levels of biological control are affected by insecticide use, as well as by large—scale habitat factors relating to differing patterns for vegetational landscapes, planting times, and the length of dry fallow periods. We performed a series of observational studies and two experimental studies. Abundant and well—distributed populations of generalist predators can be found in most early—season tropical rice fields. We took samples from plants and water surface using a vacuum—suction device, and from the subsurface using a dip net. Our results show that high populations of generalist predators are likely to be supported, in the early season, by feeding on abundant populations of detritus—feeding and plankton—feeding insects, whose populations consistently peak and decline in the first third of the season. We hypothesize that since this abundance of alternative prey gives the predator populations a "head start" on later—developing pest populations, this process should strongly suppress pest populations and generally lend stability to rice ecosystems by decoupling predator populations from a strict dependence on herbivore populations. We experimentally tested our hypothesis of trophic linkages among organic matter, detritivores and plankton feeders, and generalist predators and showed that by increasing organic matter in test plots we could boost populations of detritivores and plankton—feeders, and in turn significantly boost the abundance of generalist predators. These results hold for populations found on the plant, on the water surface, and below the water surface. We also demonstrated the link between early—season natural enemy populations and later—season pest populations by experimentally reducing early—season predator populations with insecticide applications, causing pest populations to resurge later in the season. Overall, these results demonstrate the existence of a mechanism in tropical irrigated rice systems that supports high levels of natural biological control. This mechanism depends on season—long successional processes and interactions among a wide array of species, many of which have hitherto been ignored as important elements in a rice ecosystem. Our results support a management strategy that promotes the conservation of existing natural biological control through a major reduction in insecticide use, and the corresponding increase in habitat heterogeneity.

ISSN:0012-9658    

DOI:10.2307/2265694

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Biological control, as used in terrestrial systems, may hold promise for use against exotic marine species. We first review some marine pests, displaying their diversity, the damage they cause, and possible controls. We then contrast approaches for marine and terrestrial pest control, providing guidelines for adapting terrestrial controls to the marine environment. Although several of the same principles apply in terrestrial and marine environments, marine systems differ with respect to the types of control agents available, the degree of pest—population reduction needed for effective control, the spatial scale over which biological control must operate effectively, the practicality of implementation, and the nature and degree of concern over safety. As an example, we propose a strategy for developing a biological control program against the European green crab, Carcinus maenas, which has had substantial negative impacts where previously introduced (New England, Atlantic Canada, South Africa, south Australia) and which has recently been introduced to central California, and to Tasmania. We conclude that biological control may be possible for some marine pests, but that existing strategies and expectations will require modification.

ISSN:0012-9658    

DOI:10.2307/2265695

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

It has been argued that ecological theory has not been useful to the practice of biological control. The purpose of this article is to show how recent theoretical advances may reverse this situation. We first discuss four issues that have arisen in the development of theory for biological control. (1) Recent work has clarified and resolved earlier disagreements concerning the effect of aggregation of the parasitoid to local host density on the stability of parasitoid—host models; this work emphasizes that such aggregation increases the ability of the parasitoid to reduce pest density. (2) There has been disagreement over the conditions under which stability, in the mathematical sense and on a local spatial scale, is an appropriate goal for classical biological control of insect pests, and whether a metapopulation may sometimes provide a more appropriate framework. We also comment on (3) relative size of refuges, which has been proposed as a unifying concept, and (4) density dependence and ratio dependence. We then discuss recent models using a stage—structured approach, particularly those that compare potential biological control agents or agents that have been differentially successful in practice. We argue that this approach has already produced valuable insights into the factors operating in several field situations. It demonstrates the importance of identifying which pest stages are most injurious to the crop, since the natural enemy that wins in competition may not be the most effective at suppressing the crucial pest stage(s): in general, the winning parasitoid reduces the density of the host stage attacked by its competitor below the level at which the latter can maintain a positive growth rate when at low density. A useful criterion is that pest equilibrium density is suppressed most by the parasitoid species that needs the fewest host individuals to allow a female parasitoid to replace herself in the next generation. Caveats are that this may apply only under equilibrium conditions and that the pest stage most suppressed is the one we wish to control. We discuss the connection between successful biological control and evolutionary considerations.

ISSN:0012-9658    

DOI:10.2307/2265696

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Most population models incorporate density dependence in a manner that implies the effects of density set in at increasingly higher rates as population densities decrease to zero. For many populations, it may be more plausible to assume that the effects of density set in at a maximum rate around some characteristic density $K>0$: that is, the per capita rate of increase of the population has an inverted sigmoidal form as a function of population density. In the model discussed here, the rate at which density dependence sets in around $K$ is governed by a parameter $\gamma$, which I refer to as the "abruptness" parameter: the larger the parameter $\gamma$, the weaker the effects of density dependence are for population densities below $K$, the more rapidly density dependence sets in around $K$, and the more severe the effects of density dependence are beyond $K$. Results obtained from an Evolutionary Stable Strategy (ESS) analysis of the abruptness parameter $\gamma$ suggest the hypothesis that populations with a density—independent growth rate of $<$100% from one generation to the next (i.e., a reproductive value of $$100%. Further, if true, this hypothesis suggests that females with high reproductive rates who deposit young on resources and provide no further maternal care should clump their young to reduce the level of abruptness in density dependence. Also, this hypothesis, together with the fact that high levels of abruptness promote oscillations in populations, implies that we are more likely to observe cyclic and chaotic behavior in relatively slower growing populations than previously thought.

ISSN:0012-9658    

DOI:10.2307/2265697

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Models that describe the spread of invading organisms often assume that the dispersal distances of propagules are normally distributed. In contrast, measured dispersal curves are typically leptokurtic, not normal. In this paper, we consider a class of models, integrodifference equations, that directly incorporate detailed dispersal data as well as population growth dynamics. We provide explicit formulas for the speed of invasion for compensatory growth and for different choices of the propagule redistribution kernel and apply these formulas to the spread of D. pseudoobscura. We observe that: (1) the speed of invasion of a spreading population is extremely sensitive to the precise shape of the redistribution kernel and, in particular, to the tail of the distribution; (2) fat—tailed kernels can generate accelerating invasions rather than constant—speed travelling waves; (3) normal redistribution kernels (and by inference, many reaction—diffusion models) may grossly underestimate rates of spread of invading populations in comparison with models that incorporate more realistic leptokurtic distributions; and (4) the relative superiority of different redistribution kernels depends, in general, on the precise magnitude of the net reproductive rate. The addition of an Allee effect to an integrodifference equation may decrease the overall rate of spread. An Allee effect may also introduce a critical range; the population must surpass this spatial threshold in order to invade successfully. Fat—tailed kernels and Allee effects provide alternative explanations for the accelerating rates of spread observed for many invasions.

ISSN:0012-9658    

DOI:10.2307/2265698

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Alien plants invade many ecosystems worldwide and often have substantial negative effects on ecosystem structure and functioning. Our ability to quantitatively predict these impacts is, in part, limited by the absence of suitable plant—spread models and by inadequate parameter estimates for such models. This paper explores the effects of model, plant, and environmental attributes on predicted rates and patterns of spread of alien pine trees (Pinus spp.) in South African fynbos (a mediterranean—type shrubland). A factorial experimental design was used to: (1) compare the predictions of a simple reaction—diffusion model and a spatially explicit, individual—based simulation model; (2) investigate the sensitivity of predicted rates and patterns of spread to parameter values; and (3) quantify the effects of the simulation model's spatial grain on its predictions. The results show that the spatial simulation model places greater emphasis on interactions among ecological processes than does the reaction—diffusion model. This ensures that the predictions of the two models differ substantially for some factor combinations. The most important factor in the model is dispersal ability. Fire frequency, fecundity, and age of reproductive maturity are less important, while adult mortality has little effect on the model's predictions. The simulation model's predictions are sensitive to the model's spatial grain. This suggests that simulation models that use matrices as a spatial framework should ensure that the spatial grain of the model is compatible with the spatial processes being modeled. We conclude that parameter estimation and model development must be integrated procedures. This will ensure that the model's structure is compatible with the biological processes being modeled. Failure to do so may result in spurious predictions.

ISSN:0012-9658    

DOI:10.2307/2265699

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Forest models based on the gap dynamics hypothesis (gap models)have gained an important role in forest ecology and have grown rather complex in the last 20 yr. They have been applied extensively to study the impacts of climatic change on ecosystems although they originally were not built for this purpose. The objectives of this study were (1) to develop a new forest gap model, ForClim, that includes only a minimum number of ecological assumptions but robust parameterizations of the effects of climate on plant population dynamics; (2) to test the realism of ForClim as compared to its predecessor model, FORECE; and (3) to examine the behavior of FORECE and ForClim systematically along climate gradients in Europe. ForClim is composed of three modular submodels: ForClim—P for plant population dynamics, ForClim—S for soil carbon/nitrogen turnover, and ForClim—E for providing reliable parameterizations of the abiotic environment. For the core model, ForClim—P, it was found that only four factors are sufficient to model tree growth, another four factors are required to model tree establishment, and only two factors are required to model tree mortality. The behavior of ForClim was tested at a large number of sites in the European Alps. The model yields tree species compositions that conform to field data and are very similar to those of the predecessor model. Based on this evaluation alone, it would not be possible to favor one of the models over the other. The behavior of both models then was examined systematically in a parameter space spanned by the annual mean temperature and the annual precipitation sum. From this exercise it became evident that both the pattern of aboveground biomass and the realized niches of the dominating tree species are simulated realistically by ForClim. Extremely steep gradients are characteristic of FORECE, and many ecotones are simulated to occur in the wrong places in FORECE. Thus, some of the current forest gap models can be simplified without reducing the realism of their behavior, and models other than FORECE should be scrutinized in this respect as well. The present study also suggests that the evaluation of model behavior at scattered sites is insufficient to show their validity for simulating forest dynamics along climate gradients. Further rigorous model comparisons and validation studies are required to increase the reliability of this promising class of models.

ISSN:0012-9658    

DOI:10.2307/2265700

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY

摘要:

Kernel density estimators are becoming more widely used, particularly as home range estimators. Despite extensive interest in their theoretical properties, little empirical research has been done to investigate their performance as home range estimators. We used computer simulations to compare the area and shape of kernel density estimates to the true area and shape of multimodal two—dimensional distributions. The fixed kernel gave area estimates with very little bias when least squares cross validation was used to select the smoothing parameter. The cross—validated fixed kernel also gave surface estimates with the lowest error. The adaptive kernel overestimated the area of the distribution and had higher error associated with its surface estimate.

ISSN:0012-9658    

DOI:10.2307/2265701

出版商:Ecological Society of America    

年代:1996

数据来源: WILEY