Commensalism benefits the symbiont without significantly affecting the host. This is a relatively rare type of interaction because few hosts can be considered to be completely unaffected by their symbionts. Epiphytes, plants that benefit by using their hosts for aerial support but gain their resources from the atmosphere, and cattle egrets, which eat insects flushed by grazing cattle, are well-known examples of commensalism. However, epiphytes may capture and provide nutrients to the host (a benefit) and increase the likelihood that overweight branches will break during high winds (a detriment). Some interactions involving insects may be largely commensal.
Phoretic or vector interactions (see Fig. 2.15) benefit the hitchhiker or pathogen, especially when both partners have the same destination, and may have little or no effect on the host. However, hosts can become overburdened when the symbionts are numerous, inhibiting dispersal, resource acquisition, or escape. In some cases, the phoretic partners may be mutualists, with predaceous hitchhikers reducing competition or parasitism for their host at their destination
(Kinn 1980). Examples of commensalism often may be seen to exemplify other interaction types as additional information becomes available.
A number of insect and other arthropod species function as nest commensals in ant or termite colonies. Such species are called myrmecophiles or termi-tophiles, respectively. These symbionts gain shelter, and often detrital food, from their host colonies with little, if any, effect on their hosts. This relationship is distinguished from interactions involving species that intercept host food (through trophallaxis) and, therefore, function as colony parasites. Some vertebrate species also are commensals of termite castles in the tropics. These termite nests may reach several meters in height and diameter and provide critical shelter for reptile, bird, and mammal species in tropical savannas (see Chapter 14).
Bark beetle galleries provide habitat and resources for a variety of invertebrate and microbial commensals, most of which have little or no effect on the bark beetles (e.g., Stephen et al. 1993). Many of the invertebrate species are fun-givores or detritivores that depend on penetration of the bark by bark beetles to exploit resources provided by the microbial decay of wood (Fig. 8.8).
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