Stephen D. Murphy

Weed ecology in natural and agricultural systems

Population ecology species interactions community ecology applied weed ecology. (Part contents).

Managing the whole landscape: historical, hybrid, and novel ecosystems

The reality confronting ecosystem managers today is one of heterogeneous, rapidly transforming landscapes, particularly in the areas more affected by urban and agricultural development. A landscape management framework that incorporates all systems, across the spectrum of degrees of alteration, provides a fuller set of options for how and when to intervene, uses limited resources more effectively, and increases the chances of achieving management goals. That many ecosystems have departed so substantially from their historical trajectory that they defy conventional restoration is not in dispute. Acknowledging novel ecosystems need not constitute a threat to existing policy and management approaches. Rather, the development of an integrated approach to management interventions can provide options that are in tune with the current reality of rapid ecosystem change.

Promotion of weed species diversity and reduction of weed seedbanks with conservation tillage and crop rotation

Abstract In a 6-yr study on four farms (36 fields) in Ontario, Canada, we tested the effects of tillage (moldboard, chisel plow, no tillage) and crop rotations (continuous corn, corn-soybean, corn-soybean-winter wheat) on emerged and seedbank weed species diversity and density. Aside from the imposed experimental treatments, all other management was generally consistent among farms. Tillage had the largest effect on weed diversity and density. No tillage promoted the highest weed species diversity, chisel plow was intermediate, and moldboard plow resulted in the lowest species diversity. These results are consistent with ecological succession theory. The increase in weed species diversity resulted from 20 species being associated with no tillage systems, 15 of which were winter annuals, biennials, or perennials. Emerged weed density was affected only by tillage. Over 6 yr, seedbank declined in no-tillage systems from 41,000 to 8,000 seeds m−3. Crop yields were not affected by tillage or crop rotation. In practical terms, reduced tillage in combination with a good crop rotation may reduce weed density and expenditures on weed management. Nomenclature: Glyphosate; corn, Zea mays L. ‘Pioneer 3902’; soybean, Glycine max (L.) Merr. ‘KG 40’; winter wheat Triticum aestivum L.

Simulation of Chenopodium album seedling emergence

Abstract Studies were conducted to develop a model from field and laboratory studies to predict the emergence phenology of Chenopodium album. A mechanistic model to predict the phenology of weed seedling emergence across locations, years, and tillage systems is presented. This was accomplished by the integration of hydrothermal time to describe germination and thermal time to describe shoot elongation. The interaction of soil moisture and temperature in the model was accounted for by the integration of hydrothermal time in algorithms predicting seed germination. Soil temperatures within the weed seed germination zone were predicted by temperature ranges at different depths in the soil. Emergence phenology of C. album seedlings was predicted with greater accuracy under no-till and moldboard plow systems than under a chisel plow system. We attributed this lower accuracy in the chisel plow system to increased heterogeneity in the soil matrix and vertical distribution of the seedbank caused by the chisel plow. The presence or absence of Zea mays did not affect model performance. The use of soil temperature to calculate thermal time was a better predictor of C. album seedling emergence than air temperature. The ability to predict weed seedling emergence phenology is an important component of an integrated weed management strategy. Nomenclature: Chenopodium album L. CHEAL, common lambsquarters.

Assessing tillage disturbance on assemblages of ground beetles (Coleoptera: Carabidae) by using a range of ecological indices

Many ecological studies have used diversity indices to assess the impact of environmental disturbance. In particular, ground beetles have been advocated as a good group for assessing disturbance. Most studies on various organisms have used only one or two indices. For our study of the impact of tillage disturbance on carabid beetles in farm fields in southern Ontario, Canada, we used seven different diversity indices (richness, Shannon–Wiener, Berger–Parker, Q-statistic, Margalef, α and evenness). Few studies have used deviation from diversity abundance models as a measure of disturbance; however, we use three that are applicable to our data (geometric, log-normal and log-series). The indices and models were used to test the null hypothesis that there is no change in diversity with increasing tillage disturbance, and that there is no difference in diversity with different crops or years. We were not able to reject the null hypothesis that there is any diversity difference among farms. We also found that there was no single diversity index or model that was better than any other at detecting disturbance. These results are supplemented by a meta-analysis of 45 published data sets for the same taxon but in different habitats. The meta-analysis supports the conclusions from our field research that diversity indices and models are not useful for detecting the possible effect of disturbance on assemblages of carabid beetles.

Observations of herbivore attack on garlic mustard (Alliaria petiolata) in Southwestern Ontario, Canada

We have observed what appears to be the first significant report of herbivory of a population of Alliaria petiolata (M. Bieb.) Cavara & Grande (Garlic Mustard) in Canada. Ceutorhynchus erysimi Fabr., Plutella xylostella L. (Diamondback Moth), and Philaenus spumarius (Meadow Spittlebug) were identified attacking, and successfully proceeding through different stages of development on A. petiolata. From the field observations, we hypothesize that these species have recognized A. petiolata as a host and therefore may be candidates for the management of the invasive species.

Invasive Ecology of Weeds in Agricultural Systems

No community is truly safe from plant invaders. Nor should it be. Succession is the normal process of species invading and replacing other species (Huston, 1994; Johnstone, 1986; Rejmanek, 1989), although this connection between invasion and succession is often ignored (Davis et al., 2001). The problem is that some plant invasions have high ecological and economic costs associated with them (Parker et al., 1999; Pimentel et al., 2000, 2001). Some economic costs of invasion are easily quantifiable (e.g. the cost of weed control, yield loss) whereas other are not (e.g., damage to ecosystems, loss of recreational land, aesthetics). The cost of invasive plants to crop and pasture land in the United States is calculated as well over 34 billion dollars annually (Pimentel et al., 2000), while in India, the cost stands even higher at 38 billion per year (Pimentel et al., 2001). Any ability to predict what species could invade will therefore have economic and ecological benefits.

The Role of Pollen Allelopathy in Weed Ecology1

Pollen allelopathy results when pollen releases toxins that inhibit seed germination, seedling emergence, sporophytic growth, or sexual reproduction. Of the six known pollen-allelopathic species, two are crops (timothy and corn and four are weeds (orange hawkweed, parthenium, yellow hawkweed, and yellow-devil hawkweed). Allelopathic pollen in weeds could pose threats to crops, especially if both are wind pollinated. Even if it is the crop that is pollen-allelopathic, other crops could be threatened, or more likely, weeds might adapt to pollen allelopathy and pose a greater problem. Nonetheless, pollen allelopathy could be a useful approach to biological control because allelochemicals are packaged in a natural targeting system (pollen grains) and are biologically active at low doses (<10 grains/mm2 on stigmas). If it is to be an effective biological control agent, pollen allelopathy must be examined within the wider context of farming systems management and used as one method of varying selection pressures to prevent weeds from adapting to any one particular management technique or suite of techniques. Nomenclature: Corn, Zea mays L. var. chalquiñocónico Hernández; orange hawkweed, Hieracium aurantiacum L. #3 HIEAU; yellow hawkweed, Hieracium pratense Tausch # HIECA; yellow-devil hawkweed, Hieracium floribundum W. et G. # HIEFL; parthenium, Parthenium hysterophorus L. # PTNHY; timothy, Phleum pratense L. # PHLPR. Additional index words: Biological control, farming systems, heterospecific pollen transfer, improper pollen transfer, integrated weed management, pollination, Elytrigia repens, Sonchus oleraceus, AGRRE, SONOL.

Biochemical and physiological aspects of pollen allelopathy

The exudation of allelochemicals from pollen of one species and the inhibitory effect on pollen germination and tube elongation, stigma and style receptivity, ovule development, and seed set in other species are well studied for only a limited number of species (Hieracium aurantiacum, Hieracium x floribundum, Hieracium pratense, Parthenium hysterophorus,Phleum pratense, and Zea mays var. chalquinoconico). With so few pollen allelopathic species studied, it is not surprising that the biochemistry and physiology of pollen allelopathy is not well established. The pioneering work on Zea mays var. chalquinoconico indicated that phenylacetic acid (PAA) is the pollen allelochemical and that it uncouples mitochondria oxidative phosphorylation (between coenzyme Q and cytochrome c), altering membrane permeability and halting mitosis. Since PAA is typically a broad-acting growth hormone, it seems likely that a more specialized PAA derivative is the actual allelochemical. Compounds similar to the flavonoid isorhamnetin 3,4-diglucoside in Phleum pratense could, in theory, uncouple 6-phosphogluconate dehydrogenase and prevent pollen tube germination. Ironically, the most extensive work on the biochemical and physiological mechanisms of pollen allelopathy has been completed on species in which the effect is less established (Artemisia vulgaris, Betula verrucosa, Gaillardia spp., Ledum spp., Urtica dioica). More than 20 pollen allelochemicals have been identified. These may act as free radicals to alter membrane permeability of heterospecific pollen via oxidation-induced changes to the extant enzymes or glycoproteins on the cell walls and membranes. Allelopathic compounds also may enter heterospecific pollen and create a cascade whereby cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) turn off the enzymes involved in pollen tube germination. Nonetheless, there is much more work needed to elucidate all aspects of the physiology and biochemistry of pollen allelopathy. Given that the ecological relevance of pollen allelopathy is established, such work is likely to accelerate over the next five years.

Keep ecological restoration open and flexible

To the Editor — The stakes are high as investment commitments to ecological restoration intensify1. The ‘international standards’ published by the Society for Ecological Restoration (SER)2 and the adoption of a narrow definition of restoration by the United Nations Convention to Combat Desertification (UNCCD)3 are indications that restoration is moving away from the open and flexible approach that we believe is needed to address responsible intervention in times of rapid change. Restoration science originated in the idea that ecosystems could be returned to a defined pre-disturbance condition. This served as a proxy for improving biodiversity, structural attributes and functional traits, as well as for eliminating degradation. Now, three types of change are forcing restoration as a science and practice to reassess appropriate interventions in ecosystems. First, rapid environmental, ecological and cultural changes — ranging from climate change, to species invasions, to the rise of ecosystem-services valuation — are shifting the scope of traditional restoration. Second, innovation at the intersection of ecology, design and engineering is producing new types of ecosystem, such as green infrastructure, agroecosystems and naturalized landscapes. Third, investment in restoration has increased dramatically through major international agreements and conventions. Restoration is positioned to become a go-to approach for addressing future environmental challenges by embracing a wide array of practices. This status must bring with it a signature commitment to ecosystem integrity, a disciplined approach to understanding the legacies and probable trajectories of ecosystems, and an appreciation of food security and human well-being. This may falter, however, if an overly narrow version of restoration is adopted. There is evidence of this narrow approach emerging. For example, the Land Degradation Neutrality (LDN) framework of the UNCCD has adopted a relatively restrictive definition of restoration, emphasizing “pre-existing biotic integrity, in terms of species composition and community structure... ”3. We argue that this characterization cedes too much to large-scale plantation forestry, revegetation, reclamation and rehabilitation approaches, which include little ambition for securing the ability of an ecosystem to support and maintain ecological processes and a diversity of organisms. Although these practices are useful in certain settings, they usually prioritize the realization of defined human expectations over ecological function and legacies. The restoration component of the LDN framework is based on SER’s international standards2, which focus on ‘substantial recovery’ of ‘appropriate local native reference ecosystems’. Under conditions of significant change, this stipulation will increasingly narrow the scope of restoration. We urge professional organizations and governmental agencies, at all levels, to adopt restoration advice that embeds the capacity for practitioners and scientists to respond to ongoing and future change. Policy and operational guidance based on principles, guidelines and best practices suggest a path forward4. Broadly agreed principles would allow many more practices to be conducted within a restoration envelope, and still be anchored by strong ecological commitments and clear operational effectiveness5. We appreciate that the development of standards2 — comparative measures, norms or prescriptions — can be a useful element of restoration advice in ecosystems that are well-known or smaller in scale. But the fundamental changes described above contravene a one-size-fits-all approach. For standards to function well at any level there needs to be a much better-resolved scientific understanding of ecosystems, their natural dynamics and how they respond to change. As international and national organizations refine their policy advice, we encourage an open, flexible approach. ❐