Evidence for dispersal evolution

What evidence supports these predictions? It must be admitted that the theoretical richness of dispersal evolution, while matching easily that on sex ratios, is not yet supported by the same wealth of evidence. The reason is primarily the difficulty of measuring the variables: sex ratios simply require counts of males and females, whereas direct measurement of dispersal rates or distances is extremely problematic. Most studies have used some sort of morphological marker for dispersal ability, such as presence or absence of wings in animals, and presence of dispersive seed structures, such as pappi in plants. Nonetheless, many of the theoretical predictions have received empirical support. The prediction with greatest support is undoubtably the effect of temporal heterogeneity.

Southwood (1962) was one of the first researchers to gather evidence in support of the temporal variability hypothesis. He assembled supporting evidence from the existing literature on insect dispersal, including incidence of migratory habits, and catches in airborne insect traps. His paper was not supported by explicit statistics but are consistent with the hypothesis. For example, most species of British dragonfly that migrate also use temporary water bodies like lakes or ponds for breeding habitat. Of the species from rivers, streams, or bogs, none migrate. Roff (1990) did a similarly comprehensive review, this time supported by statistics,using presence or absence of wings as the marker for dispersal. The proportion of species without wings differed significantly among habitats, with woodlands, deserts, and the ocean surface having particularly high levels of winglessness, which he interpreted in favour of the hypothesis. In a survey of North American grasshoppers and crickets (Orthoptera), he categorized species as either being fully winged, wing dimorphic, or fully wingless. The flightless forms were predominantly found in caves, ant nests, alpine areas, and tundra; again habitats that can be interpreted to have low temporal variability.

In a similar study, Denno et al. (1991) surveyed the incidence of the macropterous (fully winged) state in 35 species of planthopping bugs (Delphacidae), and then quantified the persistence of their habitats in relation to the bug lifespan. Again, species have a higher incidence of flight capability in habitats with low persistence times, which happened to be mainly agricultural crops. Within species, it would also make sense for individuals to be able to adjust their dispersal rates according to their individual assessment of local conditions, if such a mechanism exists. Some species indeed display such a plastic dispersive response. For example, the ruderal weed Crepis sancta, which grows in the Mediterranean area of France, produces a greater proportion of seeds with dispersive structures under nutrient depletion (Imbert and Ronce 2001) (Figure 6.3).

Fig. 6.3 Seeds of C. sancta. The seed at the top (a 'peripheral achene') lacks a parachute and is non-dispersive, while the seed at the bottom (a 'central achene') has a parachute and is dispersive. Under nutrient stress, a greater proportion of dispersive seeds is produced. Both seeds are about 4 mm long. Photo courtesy of Eric Imbert.

Fig. 6.3 Seeds of C. sancta. The seed at the top (a 'peripheral achene') lacks a parachute and is non-dispersive, while the seed at the bottom (a 'central achene') has a parachute and is dispersive. Under nutrient stress, a greater proportion of dispersive seeds is produced. Both seeds are about 4 mm long. Photo courtesy of Eric Imbert.

So some evidence supports the notion that variation over time in habitat quality has selected for dispersal ability. What about spatial variability? Here the evidence is more indirect because spatial variability is hard to quantify. However, once again we may be able to use rough markers. Oceanic islands are areas whose suitability unambiguously decreases rapidly in space, as the ocean is reached! As mentioned in Chapter 1, in many bird species, incidence on islands is associated with the development of flightlessness (see Figure 13.3). The phenomenon, however, is not restricted to birds; there are many known instances of plant species loosing their seed dispersal structures once island-locked. Cody and Overton (1996) showed rapid reduction of seed dispersal structures in species from the daisy family (Asteraceae) on islands in Vancouver Sound. The evidence from insects (Roff 1990) is more equivocal, since island communities seem to have approximately the same levels of flightlessness as equivalent mainlands (of similar altitude and latitude). However, given that wingless forms are likely to have been under-represented in the initial colonization of the islands, this may still represent a substantial overall reduction in dispersal ability over time (Grant 1998).

Analogous cases might also come from other island-like habitats, whose persistence is assured in the short term but which become rapidly unsuitable over space. One such habitat may be hollow trees, which can persist for many years but which are rather rare in a woodland landscape overall. The beetle Osmoderma eremita (Figure 6.4) lives in such hollow trees, and only about 15% of the beetles disperse from the tree they were born in each year (Ranius and Hedin 2001). This makes sense given the kind of landscape they live in: the tree they were born in will likely be suitable for a number of generations, and dispersing individuals may have difficulty finding another suitable tree. Therefore it makes sense for most individuals to stay put, just like in many oceanic island organisms. However, hollow trees are not fully permanent habitats; new ones arise and old ones disappear. Thus individuals will also gain some fitness if a small fraction of their offspring is dispersive.

The contributions of inbreeding and competition have been rather scantily tested, but there is a strong relationship between the sex that disperses and the mating system in birds and mammals, as predicted by theory. In a review

Fig. 6.4 The beetle O. eremita (a), and its habitat (b), a hollow tree. The tree hollow contains a circular pitfall trap, into which a beetle is about to fall. Trapping studies have shown that only about 15% of these beetles disperse from their native tree, a finding that is consistent with theory of the evolution of dispersal. Photos courtesy of Thomas Ranius.

Fig. 6.4 The beetle O. eremita (a), and its habitat (b), a hollow tree. The tree hollow contains a circular pitfall trap, into which a beetle is about to fall. Trapping studies have shown that only about 15% of these beetles disperse from their native tree, a finding that is consistent with theory of the evolution of dispersal. Photos courtesy of Thomas Ranius.

of studies of dispersal in birds and mammals, Johnson and Gaines (1990) found that polygynous and promiscuous species show male biased dispersal, while in monogamous species either both species disperse or females only.

Finally, evidence is beginning to suggest that expanding populations do indeed have higher dispersal ability towards the edge of their range. During the expansion of the Lodgepole Pine in North America, the seed morphology shifted towards those with higher dispersal propensity (Cwynar and MacDonald 1987). In several British insects that have recently expanded their range due to climate warming, populations at the edge of the range may have evolved increased dispersal relative to older more established populations. The speckled wood butterfly Pararge aegeria is one such species. Populations at the range margin have more massive thoraxes, which house the flight musculature and is known to be correlated with flight ability. They also have reduced fecundity, suggesting a fecundity trade-off associated with increased dispersal ability (Hughes et al. 2003). There have been no studies on declining populations for comparison, but the studies on expanding populations, while giving comfort for those particular species, give us cause for considerable concern about other species that, despite climate warming, have seen their ranges contract through habitat destruction (Warren et al. 2001). It may be that with increasing isolation they begin to behave like island populations and disperse even less than before.

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