Not all interactions between individuals produce negative effects. Mutualisms are interactions where both individuals benefit. Generally, a mutualist provides a service to its partner that the partner cannot provide for itself, and in return obtains a reward (Bronstein, 1994b). The types of benefits that mutualists gain include:

• nutrition (each organism supplies different essential nutrients to one another);

• protection (one individual protects the other); and

• transport (one mutualist gains mobility through the actions of the other).

Both organisms do not necessarily gain the same benefit. For example, one mutualist may provide nutrition while the other provides transport.

Types of mutualisms

Obligate mutualisms

In an obligate mutualism, both partners of the association require each other in order to survive. The most extreme examples of an obligate mutualism (often called a 'symbiosis') are lichens, which are associations between fungi and algae. The fungus forms the main body structure of the lichen and the alga provides carbohydrates. Because they no longer can live independently, neither the fungus nor the alga is considered to be a distinct species. Plants and their insect pollinators may be obligate mutual-ists, especially in tropical forests. For example, over 900 species of fig trees (Ficus spp.) are each pollinated by a separate species of aganoid wasp (Janzen, 1979), and similar obligate pollinators associations exist for Yucca spp. and some orchids. Weeds generally do not form obligate mutualisms with pollinators because most weeds are exotics that have not co-adapted with the available fauna.

Obligate mutualisms are beneficial to both mutualists but they can result in limited distribution or extinction. The extinction of one partner will almost certainly lead to the extinction of the other. If one mutualist has a wide potential distribution, its actual distribution could be much smaller if it is limited by its mutualistic relationship. Species that are part of an obligate mutualism generally make poor weeds unless the potential weed and its mutualist invade at the same time, and both species can withstand the environmental conditions of the new habitat. For example, of the 60 species of fig introduced into Florida, the three species that have become weedy only did so after their pollinator wasps were introduced (Nadel et al, 1992; Richardson et al, 2000). Similarly, two species of banyan figs (Ficus spp.) introduced to New Zealand did not set seed until their specific pollinating wasps arrived, apparently through long-distance dispersal, from Australia (Gardner and Early, 1996). This is not to claim that plants that are apparently obligate mutualists can never become weeds. It is possible for the plants to adapt to local fauna or vice-versa, hence the relationship between figs and their pollinator wasps may not be obligate in all cases (Richardson et al., 2000).

Facultative mutualisms

In a facultative mutualism, both species can survive independently, but both benefit when they are found together. Facultative mutualisms are common. For example, mutualistic animals provide plants with 'services' like pollination (Chapter 4) and seed dispersal (Chapter 6). Mutualistic fungi and bacteria provide access to more or different sources of nutrients (Richardson et al., 2000).

Pollen and seed dispersal of weeds is facilitated by the presence of native and non-native animal dispersers. In North America, weeds with simple flowers like ox-eye daisy (Chrysanthemum leucanthemum) and orange hawkweed (Hieracium auranti-acum) are visited by a wide range of insects that include 'domesticated' European honey bees (Apis mellifera) and native bumblebees (Bombus spp.) (Murphy and Aarssen, 1995). This even is true of weeds with more complex flowers like cow vetch (Vicia cracca). Native bumblebees had enough 'experience' with similar native flowers so that pollinating cow vetch was not a difficult task to master (e.g. Murphy and Aarssen, 1995).

Mycorrhizal associations are formed when a fungus infects plant roots. The fungus increases the effective 'root' surface area and therefore increases the supply of nutrients and water. In turn, the plant provides carbohydrates from photosynthesis to the fungus. The fungus can either penetrate the intercellular spaces (ectomycorrizhae) of roots or the cells themselves (endomycor-rhizae). Mycorrhizae are so prevalent and important that most native flora and some weeds will grow only if certain fungal species are present in sufficient densities (Schroft, 1998; van der Heijden et al., 1998; Jordan et al., 2000; Dahlberg, 2001).

Mycorrhizal weeds may be specialists or generalists. When Monterey pines were introduced into New Zealand plantations, their spread was limited until spores from the right species of mycorrhizal fungi from plantation soil had accumulated (Richardson and Higgins, 1998). In contrast, other weeds like Russian thistle (Salsola kali) are generalists; since they can use most mycor-rhizal fungal species, they can invade most habitats. While mycorrhizal weeds often have negative effects on ecosystems (as expected of any weed), they actually can have beneficial effects if the net result of having additional mycorrhizal species is to

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