The maintenance and ecological effects of flight

Once flight originated, for its ecological effects to be expressed it had to be maintained. Loss of flight is an interesting phenomenon, for it has occurred very frequently in insects and birds, and not at all in bats (nor probably in pterosaurs). The obvious difference between these two groups of organisms is that the former (insects and birds) retained a functional distinction between the flight apparatus and the legs: they can both walk or run without using their wings. In pterosaurs and bats this is not the case and both groups would be relatively ineffective on the ground, hindering the transition to a terrestrial lifestyle again. In birds and insects, loss of flight could mean a reallocation of energy away from the flight apparatus and increases in reproductive expenditure (Roff 1990, 1994). They lost much of course, and in birds loss of flight is only viable under special circumstances. It has happened mostly in a few taxonomic groups (rails notably) and under special ecological circumstances (notably on islands, see Figure 13.3) (Roff 1994). There are three likely reasons for the latter: first, an absence of land predators that makes escape (and especially nesting off the ground) less important. Second, on islands, high dispersal tendencies might increase the risk of mortality through loss of individuals at sea. Third, a less active metabolism might be very advantageous in surviving long periods of food shortage on islands, where birds cannot simply move elsewhere.

Insects have lost their powers of flight many more times than birds, and they have done so in a variety of ecological circumstances (at least once in nearly all major habitats). There are many flightless island insects, no doubt many for the same reasons as birds, but there are also many flightless insects in other habitats. Loss of flight is very rare in freshwater insects. This is unsurprising given the ephemeral nature of many freshwater habitats, most of which stand a good chance of temporary or permanent drought. Conversely, flightlessness is very common among parasitic insects, particularly of vertebrates (Figure 3.4). In fact only two large radiations of secondarily flightless insects have occurred: among the fleas and the lice, which primarily use mammals and birds as hosts. These do not require flight for dispersal to new hosts. Wing loss may also bring a particular advantage in facilitating movement within the fur and feathers of their hosts. Given that loss of flight is apparently so easy in insects, one may wonder why only two large radiations of flightless insects have occurred. One possibility is that speciation is frequently associated with niche shifts that would favour the reversal of winglessness again. Another part of the answer may be that, in general, flightlessness leads to poorer net rates of cladogenesis. Both extinction risk and speciation probability may be affected and both these subjects will be the focus of later chapters.

The maintenance of flight enabled it to become a major transition. Why did this transition have ecological repercussions? I have argued above that these were manifested in three main ways: first, flight in itself represents an occupation of ecospace (the atmosphere) previously unoccupied, just as terrestrialization does. This requires no special explanation: the innovation

Fig. 3.4 A body louse, Campanulotes compar, 1 mm long, which infests feral pigeons, Columba livia.

Lice and fleas represent some of the few major radiations of insects that have lost their wings. Photo courtesy of Sarah Bush and E. King.

Fig. 3.4 A body louse, Campanulotes compar, 1 mm long, which infests feral pigeons, Columba livia.

Lice and fleas represent some of the few major radiations of insects that have lost their wings. Photo courtesy of Sarah Bush and E. King.

and the ecological change are synonymous. Second, flight probably also increased the species richness of clades that evolved it (de Queiroz 1998). It is interesting to speculate why this might have been. Dispersal is often linked with the speciation process because it can transport organisms to isolated areas where they can differentiate from their ancestors (Chapter 1).Dispersal may also inhibit extinction by allowing areas in which local extinction has occurred to be recolonized. Flight may also have had interesting repercussions on life history (see Chapter 4). Birds, for example, have considerably extended lifespans compared with mammals of the same mass. Bats also have much longer lifespans than other mammals of the same mass. It does not necessarily follow that an increase in lifespan will reduce the extinction risk of species, but it is possible. This theme is investigated in more detail in Chapter 14. Of course, reduction in mortality, such as from predators, may have helped select for powered flight in the first place. Third, flying organisms perform important ecological roles that may have stimulated the evolution of other groups, the most obvious case being plant-pollinator interactions. Families of angiosperm with animal pollination are significantly more species rich than those without animal pollination (Ricklefs and Renner 2000). Insects, bats, and birds are the major pollinating animals and they can also fly. Their dispersal abilities give them much greater potential to visit other flowers and promote outcrossing.

The evolution of flight then, illustrates well the challenges, but also the rewards, of understanding a major transition in ecology.We have to consider origins, and in particular, scenarios of progressive selective advantage in the face of small continuous changes. Those are available for birds and insects, and to a lesser extent for bats and pterosaurs, and the scenarios are diverse. We must also explain the timing of the events and what stimulated them. Invoking high metabolic requirements for flight and periods of high oxygen concentration may explain that, implying that changes in the environment were critical, perhaps stimulated by previous evolutionary novelties.We also have to consider the frequency of loss of the character and its relative effects on speciation and extinction rates. Losses of the character are few in some groups, where the flight mechanism is at odds with terrestrial locomotion, and common in others where it is not. Where it is common, losses do not seem to confer any consistent benefit in terms of increased speciation or reduced extinction rates. Finally, we need to explain why the ecological effects of the transition have been great. Some of these themes may also be found in some of the other transitions, and it will be exciting to discover if a consensus on this emerges in the coming years of the sort that Maynard Smith and Szathmary provided in the previous chapter.

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