The traditional ways to design biological control systems for plant invaders include (in order of decreasing emphasis) introducing, augmenting, or conserving natural enemies. Manipulating consumer–resource relationships in this way (1) emphasizes top‐down control of the invader by consumers rather than bottom‐up control of the invader by limiting resources, and (2) contributes to a rising number of control organisms introduced to North America that is creating complexity, redundancy, and risk. New concepts and methods have started to transform the way biological control organisms are found and developed by (1) combining herbivore and resource limitation of plant population growth and (2) using targeted life‐cycle disruption, which involves identifying plant life‐cycle transitions that are both amenable to manipulation and influential on population growth, and then targeting these for control. To illustrate these developments, we outline an experimental and computational approach for measuring how the processes of disturbance, colonization, and organism interactions (plant competition and herbivory) manifest their influence on weed life cycles and population growth of ragwort Senecio jacobaea, a biennial or short‐lived perennial herb. Manipulating these forces may lead to designs of biological control systems that are parsimonious, potent, and pose minimum risk to non‐target organisms.

The purpose of our study was to estimate the variability in a biological control process on a regional scale, identify its causes, and quantitatively evaluate overall control success. We present evidence of the success of biological control of Senecio jacobaea (ragwort) in western Oregon following introduction of three natural enemies. First, observations from a single site showed that ragwort declined to <1% of its former abundance and has been replaced by a plant community composed predominantly of introduced perennial grasses. Second, a perturbation experiment showed that introduced insects, within one ragwort generation, can depress the density, biomass, and reproduction of ragwort to <1% of populations protected from natural enemies. Third, a 12-yr survey of 42 ragwort populations showed that strong and persistent depression of ragwort recurred at many sites and at different times. Three features of this case history may be useful in the development of ecological theory as an explanation and guide for biological control: (1) the impact of the natural enemies depends on the distribution of individual sizes and ages in the ragwort population; (2) the long-term dynamics of ragwort may be influenced by the presence of large persistent seed bank which is invulnerable to the natural enemies; and (3) the success of biological control of ragwort in western Oregon appears to be independent of variation in environmental conditions. Combining local, short-term experiments and regional long-term observations is a powerful method for demonstrating successful biological control.

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Invasive plants can simplify plant community structure, alter ecosystem processes and undermine the ecosystem services that we derive from biotic diversity. Two invasive plants, purple loosestrife ( Lythrum salicaria ) and reed canary grass ( Phalaris arundinacea ), are becoming the dominant species in many wetlands across temperate North America. We used a horizontal, observational study to estimate per capita effects (PCEs) of purple loosestrife and reed canary grass on plant diversity in 24 wetland communities in the Pacific Northwest, USA. Four measures of diversity were used: the number of species (S), evenness of relative abundance (J), the Shannon-Wiener index (H ′ ) and Simpson's index (D). We show that (1) the PCEs on biotic diversity were similar for both invasive species among the four measures of diversity we examined; (2) the relationship between plant diversity and invasive plant abundance ranges from linear (constant slope) to negative exponential (variable slope), the latter signifying that the PCEs are density-dependent; (3) the PCEs were density-dependent for measures of diversity sensitive to the number of species (S, H ′ , D) but not for the measure that relied solely upon relative abundance (J); and (4) invader abundance was not correlated with other potential influences on biodiversity (hydrology, soils, topography). These results indicate that both species are capable of reducing plant community diversity, and management strategies need to consider the simultaneous control of multiple species if the goal is to maintain diverse plant communities.