Natural experiments

We have seen that interspecific competition is commonly studied by an experimenter comparing species alone and in combination. Nature, too, often provides information of this sort: the distribution of certain potentially competing species is such that they sometimes occur together (sympatry) and sometimes occur alone (allopatry). These 'natural experiments' can provide additional information about interspecific competition, and especially about evolutionary effects, since the differences between sympatric and allopatric populations are often of long standing. The attractions of natural experiments are first that they pros and cons of natural experiments are natural - they are concerned with organisms living in their natural habitats - and second, that they can be 'carried out' simply by observation - no difficult or impracticable experimental manipulations are required. They have the disadvantage, however, of lacking truly 'experimental' and 'control' populations. Ideally, there should be only one difference between the populations: the presence or absence of a competitor species. In practice, though, populations typically differ in other ways too, simply because they exist in different locations. Natural experiments should therefore always be interpreted cautiously.

Evidence for competition from natural experiments usually comes either from niche expansion in the absence of a competitor (known as competitive release) or simply from a difference in the realized niche of a species between sympatric and allopatric populations. If this difference is accompanied by morphological changes, then the effect is referred to as character displacement. On the other hand, physiological, behavioral and morphological traits are all equally likely to be involved in competitive interactions and to be reflections of a species' realized niche. One difference may be that morphological distinctions are most obviously the result of evolutionary change, but as we shall see, physiological and behavioral 'characters' are also liable to 'competitive displacement'.

One example of natural competitive release is provided by work on two gerbilline rodents living in the coastal sand dunes of Israel (Abramsky & Sellah, 1982). In northern Israel, the protrusion of the Mt Carmel ridge towards the sea separates the narrow coastal strip into two isolated areas, north and south. Meriones tristrami is a gerbil that has colonized Israel from the north. It now occurs, associated with the dunes, throughout the length of the coast, including the areas both north and south of Mt Carmel. Gerbillus allenbyi is another gerbil, also associated with the dunes and feeding on similar seeds to M. tristrami; but this species has colonized Israel from the south and has not crossed the Mt Carmel ridge. To the north of Mt Carmel, where M. tristrami lives alone, it is found on sand as well as other soil types. However, south of Mt Carmel it occupies several soil types but not the coastal sand dunes. Here, only G. allenbyi occurs on dunes.

This appears to be a case of competitive exclusion and competitive release: exclusion of M. tristrami by G. allenbyi from the sand to the south of Mt Carmel; release of M. tristrami to the north. Is this present day competitive exclusion, however, or an evolutionary effect? Abramsky and Sellah set up a number of plots south of Mt Carmel from which G. allenbyi was removed, and they compared the densities of M. tristrami in these plots with those in a number of similar control plots. They monitored the plots for 1 year, but the abundance of M. tristrami remained essentially unchanged. It seems that south of Mt Carmel, M. tristrami has

Herpestes Javanicus
Figure 8.21 Native geographic ranges (I-VII) of Herpestes javanicus (j), H. edwardsii (e) and H. smithii (s). (From Simberloff et al., 2000.)

evolved to select those habitats in which it avoids competition with G. allenbyi, and that even in the absence of G. allenbyi it retains this genetically fixed preference. Note, though, as ever, that this interpretation, because it invokes the ghost of competition past, may be sound and sensible - but it is not established fact.

A case of apparent morphological character displacement comes from work on Indian mongooses. In the western parts of its natural range, the small Indian mongoose (Herpestes javanicus) coexists with one or two slightly larger species in the same genus (H. edwardsii and H. smithii), but these species are absent in the eastern part of its range (Figure 8.21). Simberloff et al. (2000) examined size variation in the upper canine tooth, the animal's principal prey-killing organ (note that female mongooses are smaller than males). In the east where it occurs alone (area VII in Figure 8.21), both males and females have larger canines than in the western areas (III, V, VI) where it coexists with the larger species (Figure 8.22). This is consistent with the view that where similar but larger predators are present, the prey-catching apparatus of H. javanicus has been selected for reduced size. This is likely to reduce the strength of competition with other species in the genus because smaller predators tend to take smaller prey than larger predators. Where H. javanicus occurs in isolation, its canine teeth are much larger.

It is of particular interest that the small Indian mongoose was introduced about a century ago to many islands outside its competitive release and character displacement gerbils in Israel: competitive release morphological character displacement... ... in Indian mongooses . . .

Canine diameter (mm)

native range (often as part of a naive attempt to control introduced rodents). In these places, the larger competitor mongoose species are absent. Within 100-200 generations the small Indian mongoose has increased in size (Figure 8.22), so that the sizes of island individuals are now intermediate between those in the region of origin (where they coexisted with other species and were small) and those in the east where they occur alone. On the islands they show variation consistent with 'ecological release' from competition with larger species.

A further example concerns populations of the originally marine three-spined stickleback, Gasterosteus aculeatus, living in freshwater lakes in British Columbia, Canada, having apparently been left behind either following uplifting of the land after deglaciation, around 12,500 years ago, or after the subsequent rise and fall of sea levels around 11,000 years ago (Schluter & McPhail, 1992, 1993). As a result of this 'double invasion', some lakes now support two species of G. aculeatus (although they have not, as yet, been given their own specific names), whilst others support only one. Wherever there are two species, one is always 'limnetic', the other 'benthic'. The first concentrates its feeding on plankton in the open water and has correspondingly long (and closely spaced) gill rakers that seive the plankton from the stream of ingested water. The second, with much shorter gill rakers, concentrates on larger prey that it consumes largely from vegetation or sediments (Figure 8.23b). Wherever there is only one species in a lake, however, this species exploits both food

Figure 8.22 Maximum diameter (mm) of the upper canine for Herpestes javanicus in its native range (data only for areas III, V, VI and VII from Figure 8.21) and introduced range. Black symbols represent mean female size and colored symbols represent mean male size. (From Simberloff et al., 2000.)

resources and is morphologically intermediate (Figure 8.23a). Presumably, either ecological character displacement has evolved since the second invasion, and this has promoted the coexistence of the species pairs, or it was a necessary prerequisite for the second invasion to be successful. Genetic evidence, based on

(a) One species

i i

(b) Two species

i i i

oooo o 1 1 1

Ln mean gill raker length (mm)

Ln mean gill raker length (mm)

Figure 8.23 Character displacement in three-spined sticklebacks (Gasterosteus aculeatus). In small lakes in coastal British Columbia supporting two stickleback species (b), the gill rakers of the benthic species (•) are significantly shorter than those of the limnetic species (o), whilst those species of sticklebacks that occupy comparable lakes alone (a) are intermediate in length. Lengths of gill rakers have been adjusted to take account of species differences in overall size. (After Schluter & McPhail, 1993.)

... and in three-spined sticklebacks in Canada analyses of mitochondrial DNA of several species pairs, supports the idea of repeated patterns of adaptive radiation within individual lakes (Rundle et al., 2000).

If character displacement has ultimately been caused by competition, then the effects of competition should decline with the degree of displacement. Brook sticklebacks (Culaea inconstans) that are sympatric in Canadian lakes with ninespine sticklebacks (Pungitius pungitius) possess significantly shorter gill rakers, longer jaws and deeper bodies than allopatric brook sticklebacks. Gray and Robinson (2002) view allopatric brook sticklebacks as pre-displacement phenotypes and sympatric brook sticklebacks as postdisplacement phenotypes. When each phenotype was separately placed in enclosures in the presence of ninespine sticklebacks, the allopatric (predisplacement) brook sticklebacks grew significantly less well than their sympatric (postdisplacement) counterparts (Figure 8.24). This is consistent with the hypothesis that competition is reduced when divergence between competing species occurs.

Two final, plausible examples of mud snails: a classic character displacement are provided example of character by work on mud snails in Finland displacement? (Hydrobia ulvae and H. ventrosa) and giant rhinoceros beetles in Southeast Asia (Chalcosoma caucasus and C. atlas). When the two mud snail species live apart, their sizes are more or less identical; but when they coexist they are always different in size (Figure 8.25a)

Figure 8.24 Means (with standard error) of group-median growth (natural log of the final mass of fish in each enclosure divided by the initial mass of the group) for sympatric brook sticklebacks representing postdisplacement phenotypes (dark orange bar) and allopatric brook sticklebacks representing predisplacement phenotypes (light bar), both reared in the presence of ninespine sticklebacks. In competition with ninespine sticklebacks, growth was significantly greater for postdisplacement versus predisplacement phenotypes (P = 0.012). (After Gray & Robinson, 2002.)

Figure 8.24 Means (with standard error) of group-median growth (natural log of the final mass of fish in each enclosure divided by the initial mass of the group) for sympatric brook sticklebacks representing postdisplacement phenotypes (dark orange bar) and allopatric brook sticklebacks representing predisplacement phenotypes (light bar), both reared in the presence of ninespine sticklebacks. In competition with ninespine sticklebacks, growth was significantly greater for postdisplacement versus predisplacement phenotypes (P = 0.012). (After Gray & Robinson, 2002.)

(Saloniemi, 1993) and they tend to consume different food particle sizes (Fenchel 1975). The beetles display a similar morphological pattern (Figure 8.25b) (Kawano, 2002). These data, therefore, strongly suggest character displacement, allowing coexistence. However, even an apparently exemplary example such as that of the mud snails is open to serious question (Saloniemi, 1993). In Finland, the sympatric and allopatric habitats were not identical: H. ulvae and H. ventrosa coexisted in sheltered water bodies rarely affected by tidal action, H. ulvae was found alone in relatively exposed tidal mudflats and salt marshes, and H. ventrosa was found alone in nontidal lagoons and pools. Moreover, H. ulvae naturally grows larger in less tidal habitats, and H. ventrosa may grow less well in this habitat. This alone could account for the size differences between sympatry and allopatry in these species. This emphasizes the major problem with natural experiments such as those that seem to demonstrate character displacement: sympatric and allopatric populations can occur in different environmental conditions over which the observer has no control. Sometimes it will be these environmental differences, rather than competition, that have led to the character displacement.

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