Biology Reference
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recognizes its dominance as the net result of concurrent top-down (herbivory) and
bottom-up (plant-mediated resource abundance) effects on weed abundance
(Fig. 11.2). Up until the mid 1990s, the magnitude of top-down controls was inad-
equate to reduce seed production to a level where additional seedling mortality
caused by plant competition and resource limitation could prevent dominance by
the species. In addition, the abundance of biological control insects may not have
increased to high enough levels at individual sites to warrant a population decline.
In some areas, these top-down controls are now in place, but the large seed bank
produced by the weed during preinsect control years continues to subsidize knap-
weed densities. Uncertainty exists regarding the intensity and regional extent of the
various species characteristics such as allelopathy, or the microbial community
feedbacks on the plant.
We also note that it is important to recognize the unique interactions between
plants, insects, and soil biota that are represented by
Centaurea
populations in the
invaded range. The
non-coevolved
interactions (e.g., unusually strong top-down
controls imposed by biological control insects) in areas outside the plant's native
range have the potential to neutralize or amplify other non-coevolved interactions
(e.g., unusually strong competitive effects due to plant characteristics; also see
Pearson and Callaway 2005). The non-coevolved competitive interactions have led
to high levels of plant dominance by spotted knapweed in North America, occur-
ring in densities that have never been observed in its home range. However, sustain-
able management activities employing novel combinations of negative biotic and
a
seed production
Flowering
plants
Seed bank
Rosettes
b
c
c
death
death
death
Fig. 11.2
How top-down and bottom-up factors collectively control densities of spotted knap-
weed. The density of the weed per unit area is composed of seeds in the seed bank, immature
plants (seedlings and rosettes), and flowering plants. Each year a portion of the seed bank germi-
nates or dies, rosettes bolt to become flowering plants, and flowering plants produce seed. Key
control points include factors affecting seed production (a), rosette establishment (b), and survival
of rosettes and adult plants (c). Sustainable control procedures involve reducing seed production
by biological (top-down) control by herbivory from multiple insect species, and by decreasing
seedling survivorship by increasing plant competition (bottom-up) activities. This model is simi-
lar to that proposed by Myers and Risley (2000) for diffuse knapweed, except that here, seed
production is reduced to levels where interspecific plant competition affecting seedling survivor-
ship, perhaps mediated by additional herbivory, is capable of controlling densities of the plant
