For now, though, what other general features emerge from our examples? As with intraspecific competition, a basic distinction can be made between interference and exploitation competition (although elements of both may be found in a single interaction) (see Section 5.1.1). With exploitation, individuals interact with each other indirectly, responding to a resource level that has been depressed by the activity of competitors. The diatom work provides a clear example of this. By contrast, Connell's barnacles provide an equally clear example of interference competition. Balanus, in particular, directly and physically interfered with the occupation by Chthamalus of limited space on the rocky substratum.
Interference, on the other hand, is allelopathy not always as direct as this. Amongst plants, it has often been claimed that interference occurs through the production and release into the environment of chemicals that are toxic to other species but not to the producer (known as allelopathy). There is no doubt that chemicals with such coexisting competitors or the 'ghost of competition past'?...
... or simply evolution?
interference and exploitation properties can be extracted from plants, but establishing a role for them in nature or that they have evolved because of their allelopathic effects, has proved difficult. For example, extracts from more than 100 common agricultural weeds have been reported to have allelopathic potential against crop species (Foy & Inderjit, 2001), but the studies generally involved unnatural laboratory bioassays rather than realistic field experiments. In a similar manner, Vandermeest et al. (2002) showed in the laboratory that an extract from American chestnut leaves (Castanea dentata) suppressed germination of the shrub rosebay rhododendron (Rhododendron maximum). The American chestnut was the most common overstory tree in the USA's eastern deciduous forest until ravaged by chestnut blight (Cryphonectria parasitica). Vandermeest et al. concluded that the expansion of rhododendron thickets throughout the 20th century may have been due as much to the cessation of the chestnut's allelopathic influence as to the more commonly cited invasion of canopy openings following blight, heavy logging and fire. However, their hypothesis cannot be tested. Amongst competing tadpole species, too, water-borne inhibitory products have been implicated as a means of interference (most notably, perhaps, an alga produced in the feces of the common frog, Rana temporaria, inhibiting the natterjack toad, Bufo calamita (Beebee, 1991; Griffiths et al., 1993)), but here again their importance in nature is unclear (Petranka, 1989). Of course, the production by fungi and bacteria of allelopathic chemicals that inhibit the growth of potentially competing microorganisms is widely recognized -and exploited in the selection and production of antibiotics.
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