John E. Banks

Partial Differential Equations in Ecology: Spatial Interactions and Population Dynamics

Most of the fundamental elements of ecology, ranging from individual be- havior 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 experimental ecologists. Partial differential equations are used to model a variety of ecological phenomena; here we discuss dispersal, ecological invasions, critical patch size, dispersal-mediated coexis- tence, and diffusion-driven spatial patterning. These models emphasize that simple organ- ism 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.

Incorporating Ecologically Relevant Measures of Pesticide Effect for Estimating the Compatibility of Pesticides and Biocontrol Agents

Abstract The compatibility of biological control agents with pesticides is a central concern in integrated pest management programs. The most common assessments of compatibility consist of simple comparisons of acute toxicity among pest species and select biocontrol agents. A more sophisticated approach, developed by the International Organisation of Biological Control (IOBC), is based on a tiered hierarchy made up of threshold values for mortality and sublethal effects that is used to determine the compatibility of pesticides and biological control agents. However, this method is unable to capture longer term population dynamics, which is often critical to the success of biological control and pest suppression. In this article, we used the delay in population growth index, a measure of population recovery, to investigate the potential impacts that the threshold values for levels of lethal and sublethal effects developed by the IOBC had on three biocontrol agents: sevenspotted lady beetle, Coccinella septempunctata L.; the aphid parasitoid Diaeretiella rapae (M’Intosh), and Fopius arisanus (Sonan), a parasitoid of tephritid flies. Based on life histories of these economically important natural enemies, we established a delay of 1-generation time interval as sufficient to disrupt biological control success. We found that delays equivalent to 1-generation time interval were caused by mortality as low as 50% or reductions of offspring as low as 58%, both values in line with thresholds developed by the IOBC. However, combinations of mortality and reduction of offspring lower than these values (from 32 to 43% each) over a simulated 4-mo period caused significant population delays. Furthermore, the species used in these simulations reacted differently to the same levels of effect. The parasitoid D. rapae was the most susceptible species, followed by F. arisanus and C. septempunctata. Our results indicate that it is not possible to generalize about potential long-term impacts of pesticides on biocontrol agents because susceptibility is influenced by differences in life history variables. Additionally, populations of biocontrol agents may undergo significant damage when mortality approaches 50% or when there is mortality of ≈30% and a 30% reduction in offspring caused by a sublethal effect. Our results suggest that more ecologically relevant measures of effect such as delays in population growth may advance our knowledge of pesticide impacts on populations of beneficial species.

The scale of landscape fragmentation affects herbivore response to vegetation heterogeneity

Abstract Using alternating bands of weeds and broccoli I experimentally manipulated vegetation composition and the spatial scale at which the landscape was fragmented in a factorial design. This experimental approach allowed me to distinguish the effect of spatial scale from that of simple crop heterogeneity on crop herbivores. The importance of scale depended on which insect species were examined. Cabbage aphids (Brevicoryne brassicae) were influenced by vegetation composition at all tested scales of fragmentation; cabbage butterflies (Pieris rapae) were not affected by scale or by composition and flea beetles (Phyllotreta cruciferae) revealed a striking dependence on scale of fragmentation as well as an interaction between scale and composition. This approach shows the importance of dissecting out the effects of scale from other aspects of landscape manipulation, and emphasizes the challenge of developing a theory that will enable prediction of species–specific responses to scale.

Modelling herbivore movement and colonization: pest management potential of intercropping and trap cropping

1 Using a stochastic simulation model, we explored the effects of agroecosystem diversity on herbivore densities.

Divided culture: integrating agriculture and conservation biology

Production agriculture, with its implied ecosystem simplification, pesticide and fertilizer use, and emphasis on yield, often appears to be at odds with conservation biology. From a farmers perspective, the weight conservation biology places on wildlife may seem overly idealistic and naive, detached from economic and sociopolitical reality. In fact, these endeavors are two sides of the same coin, with a shared heritage in decades of population and community ecological theory and experimentation. Better integration of the two disciplines requires acknowledging their various goals and working to produce mutually beneficial outcomes. The best examples of this type of integrated approach result from careful implementation of sustainable agriculture practices that support biological conservation efforts via habitat amelioration or restructuring. Successful integrated approaches take into account both the environmental and economic costs of different farming schemes and compensate farmers for the costs they in...

“Selective” Pesticides: Are They Less Hazardous to the Environment?

For half a century, scientists and the public have been well aware of the risk posed by pesticides to humans and the environment. Worldwide concern about pesticide residues on food and in drinking water has led to legislative efforts to restrict the use of traditional, broad-spectrum pesticides. In the United States, the Food Quality Protection Act (Public Law 104-170), passed by Congress in 1996, effectively mandates a severe reduction in the use of many such pesticides for a wide range of agricultural uses. The principal rationale for restricting the use of many of these chemicals is to protect consumers, especially children, who are judged to be more susceptible to the effects of pesticides (NRC 1993, Goldman 1998).

What Is Ecotoxicology? An Ad-Hoc Grab Bag or an Interdisciplinary Science?

Ecotoxicology is a new discipline that supposedly melds the fields of ecology and toxicology. Yet as today’s scientists grapple with wide-ranging environmental degradation, the field of ecotoxicology often seems an ineffectual, naive and confused science, far from a seamless merger of two well-established and respected disciplines. We set out to examine the current state of ecotoxicology, paying special attention to some of the major simplifications and misunderstandings that underlie its weaknesses. By exposing major areas of needed improvement, we hope to point the way towards giving ecotoxicology a more prominent voice in the analysis of pressing contemporary environmental problems.

DO IMPERFECT TRADE-OFFS AFFECT THE EXTINCTION DEBT PHENOMENON?

A tight trade-off between competitive and colonization abilities is a key assumption of so-called “extinction debt models.” This trade-off implies that species ranked high in competitive ability will be ranked low with respect to dispersal. Although such a strict trade-off is plausible as a general trend, there are many examples in nature of deviations from perfect trade-offs. To determine how robust the “extinction debt” phenomenon is to imperfect trade-off relationships, I reexamined the extinction debt model of Tilman et al. in light of imperfect trade-offs. Numerically solving a system of differential equations, I found that, although the extinction debt phenomenon is not eliminated by the presence of imperfect trade-offs, the loss of species occurs earlier (with less habitat destruction) and more linearly than with perfect trade-offs.

Effects of plot vegetation diversity and spatial scale on Coccinella septempunctata movement in the absence of prey

The influence that vegetation diversity and the spatial scale of that diversity exert on insect behavior has increasingly been explored in the ecological literature, but relatively few experiments have explicitly incorporated both factors in experimental treatments. We conducted a field study designed to explore the effect of both of these factors on insect movement behavior in a broccoli agroecosystem. We caught and released seven‐spotted ladybird beetles (Coccinella septempunctata L.) in plots containing different degrees of vegetation diversity at two different spatial scales in which prey had been removed. Beetle movement was recorded at timed intervals, and move lengths and turning angles were used to generate discrete path maps for each beetle. Observed mean beetle net squared displacements were compared with predicted net squared displacements, and 95% confidence intervals were generated using a bootstrap method described by Turchin (1998 ) [Quantitative Analysis of Movement: Measuring and Modeling Population Redistribution in Animals and Plants. Sinauer Associates Inc., Sunderland, MA.]. Predicted net squared displacements underestimated beetle movement in smaller plots with both low and higher vegetation diversity for the first five move lengths, whereas no significant difference between observed and predicted net squared displacement for beetles in larger plots of either level of vegetation diversity were detected. These findings highlight the need for a better understanding of how natural enemies are influenced by vegetation diversity and the spatial scale of that vegetation in agroecosystems. The implications of these results for biological control are discussed.

Using Semifield Studies to Examine the Effects of Pesticides on Mobile Terrestrial Invertebrates

Many farmers rely on regular pesticide applications to avoid losses from arthropod pests and the diseases they vector. However, widespread and injudicious use of pesticides is detrimental to the environment, poses a health risk, and undermines biocontrol services. Researchers are increasingly required to develop techniques to quantify the trade-offs and risks associated with pesticides. Laboratory studies, though useful for assessing short-term impacts (e.g., mortality), cannot detect longer-term or indirect effects that can potentially be assessed using semifield studies. Here we review the range and scope of studies that have used semifield methods for regulatory testing and risk assessment of pesticides and for understanding the community-level effects of pesticide use in agricultural landscapes. We include studies on target and nontarget species, with an emphasis on quantifying effects when the target species is highly mobile. We suggest improvements in the design and analysis of semifield studies to more effectively assess effects on highly mobile species.