Evidence from morphological patterns

Where niche differentiation is manifested as morphological differentiation, the spacing out of niches can be expected to have its counterpart in regularity in the degree of morphological difference between species belonging to a guild. Specifically, a common feature claimed for animal guilds that appear to segregate strongly along a single-resource dimension is that adjacent species tend to exhibit regular differences in body size or in the size of feeding structures. Hutchinson (1959) was the first to catalog many examples, drawn from vertebrates and invertebrates, of sequences of potential competitors in which average individuals from adjacent species had weight ratios of approximately 2.0 or length ratios of approximately 1.3 (the cube root of 2.0). This 'rule' also seems to hold approximately for guilds as different as coexisting cuckoo-doves (mean body weight ratio of 1.9; Diamond, 1975), bumblebees (mean proboscis length ratio for worker bees of 1.32; Pyke, 1982), weasels (mean canine diameter ratio of between 1.23 and 1.50; Dayan et al., 1989) and even fossil brachiopods (between 1.48 and 1.57 for body outline length, an index of the size of the brachiopod's feeding organ; Hermoyian et al., 2002). Models of competition do not predict specific values for size ratios that might apply across a range of organisms and environments, and whether the apparent regularity is an empirical quirk usually remains to be determined. In the case of the brachiopod community (Figure 19.8), however, Hermoyian et al. (2002) built 100,000 null models that each drew four species at random from the complete strophomenide brachiopod fossil fauna (74 taxa) and calculated size ratios between adjacent species. On the basis of their results, they

Figure 19.8 Distributions of strophomenide body outline length (SOL) of samples of four coexisting species of strophomenide brachiopods collected from a late Ordovician (c. 448-438 million years before present) marine sediment in Indiana, USA. The species shown, from left to right, are Eochonetes clarksvillensis, Leptaena richmondensis, Strophomena planumbona and Rafinesquina alternata. (After Hermoyian et al., 2002.)

Figure 19.8 Distributions of strophomenide body outline length (SOL) of samples of four coexisting species of strophomenide brachiopods collected from a late Ordovician (c. 448-438 million years before present) marine sediment in Indiana, USA. The species shown, from left to right, are Eochonetes clarksvillensis, Leptaena richmondensis, Strophomena planumbona and Rafinesquina alternata. (After Hermoyian et al., 2002.)

rejected the null hypothesis (P < 0.03) that the observed ratios could have arisen from randomly selected taxa, supporting the hypothesis of limiting similarity.

If interspecific competition does in fact shape a community, it will often do so through a process of selective extinction. Species that are too similar will simply fail to persist together. The detailed records of ornithologists from the six main Hawaiian islands during the period 1860-1980 allowed Moulton and Pimm (1986) to estimate, at least to the nearest decade, when each species of passerine bird was introduced and if and when it became extinct. In the records, overall, there were 18 pairs of congeneric species present at the same time on the same island. Of these, six pairs persisted together; in nine cases one species became extinct; and in three cases both species died out (the last category was ignored in the analysis because the outcome is not compatible with pairwise competitive exclusion). In cases where one species became extinct the species pair was morphologically more similar than in cases where both species persisted: the average percentage difference in bill length was 9 and 22%, respectively. This statistically significant result is consistent with the competition hypothesis.

Moulton and Pimm's approach was informative because it invoked historical data, providing a glimpse of the elusive workings of the 'ghost of competition past'. An evolutionary perspective has been even more explicitly incorporated by the use of 'cladistic analysis', which allows us to reconstruct phylogenies (evolutionary trees) based on similarities and differences between species in their DNA molecules and/or in morphological (or other biologically meaningful) characteristics.

... whereas grassland ants do not

Hutchinson's 'rule' about size ratios of coexisting species applied to brachiopods are extinctions more likely for very similar competitors?

The results of such an analysis of the Anolis lizards of Puerto Rico (Figure 19.9a) are consistent with the hypothesis of divergent evolution in body size (Losos, 1992). The two-species stage in evolution (the first, lowermost node in Figure 19.9a) was composed of species with markedly different snout-vent lengths (SVL - a standard index of size for lizards) of approximately 38 and 64 mm (A. occultus and the ancestor of all the remaining types, respectively) whilst sizes at the three-species stage (the next node) were 38, 64 and 127 mm. In Jamaica, on the other hand (Figure 19.9b), no such pattern is observed; the two- and three-species stages were composed of species of similar size (61 and 73 mm, then 57, 61 and 73 mm SVL). However, the phylogenies of the two islands show remarkable consistency when viewed from the point of view of patterns in 'ecomorphs' - each distinct in morphology, ecology and behavior. On both islands, the two-morph stage was composed of a short-legged twig ecomorph, which crawls slowly on narrow supports on the periphery of trees, and a generalist ancestral species. At the three-morph stage, too, both islands possessed the same assembly - a twig ecomorph, one specialized at foraging in the tree crown and a trunk-ground type, the latter being robust and long-legged and using its jumping and running abilities to forage on the ground. At the four-morph stage the patterns were again identical, each having added a trunk-crown type. Only at the five-morph stage was there a difference - the grass-bush morph was the last to evolve on Puerto Rico, but its counterpart has never appeared on Jamaica (Figure 19.10). Note that on each island a morph usually consists of a single species of Anolis, but Puerto Rico has several trunk-ground and grassbush species. This phylogenetic analysis is consistent with the hypothesis that the faunal assembly on both Puerto Rico and Jamaica has occurred via sequential microhabitat partitioning, with morphological differences perhaps being related to differences in microhabitat utilization. Extending this work to further islands, Losos et al. (1998) confirmed that adaptive radiation in similar environments can produce strikingly similar evolutionary outcomes.

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