Ecological separation within African ape and dolphin communities
Two closely related, ecologically similar species that share one habitat are presumed to have either diverged from each other in the course of their evolution or else are currently in ecological competition that may lead eventually to the local extinction of one species. This is based on an assumption that the two species are resource-limited. But are African apes and dolphins resource-limited?
Evidence from studies of African apes suggests that they are. Kuroda et al. (1996) found that chimpanzees and gorillas in the Ndoki forest practice mutual avoidance, although they occasionally enter the same food trees. Findings that chimpanzee and gorilla diets converge most when fruit and foliage are most abundant indicate that there is some degree of ecological release during times of food abundance (Tutin and Fernandez, 1993; Stanford and Nkurunungi, 2003). Gorillas are likely able to make use of herbaceous foliage as fall back food when fruit is not available, accounting for the dietary divergence in times of food scarcity.
The food limitation question for dolphins is more complicated due to the difficulties of observing animals in the open ocean and obtaining direct evidence of food prey intake. Similar species co-occuring in the same area are thought to compete for resources unless they occupy different physical locations and/or feed on different prey (Roughgarden, 1976; Pianka, 1978). Sympatric species of odontocetes observed worldwide and living in a restricted region where food is limited were reported to adopt a few similar strategies. These strategies include foraging and feeding at different depths and/or inhabiting shallow and deep waters (Saayman et al., 1972; Norris et al., 1994; Baird and Dill, 1995; Ferretti et al., 1998; Wang et al., 2000; Hobbs, 2004; Bearzi, 2005a; Parra 2006) and divergence in diet (Bigg et al., 1990; Ferretti et al., 1998; Baird, 2000; Das et al., 2000; Hale et al., 2000; Saulitis et al., 2000).
Different dolphin species can be found together in the same microhabitat showing prey resource partitioning and, apparently, no competition for resources (Selzer and Payne, 1988; Gowans and Whitehead, 1995; Bearzi, 2005a,b). In these situations where diets frequently overlap, it appears that sympatric species can differ slightly in prey preferences (Gowans and Whitehead, 1995). Many species of odontocetes, such as common dolphins, are well known to be opportunistic feeders that can vary their diet according to the availability of the most abundant and catchable prey (Evans, 1975, 1994; Klinowska, 1991). A small difference in prey preference may be enough to support the feeding requirements of more than one species, allowing them to co-exist (Hoelzel, 1998).
Similar patterns in the sympatric associations of some dolphin species and African apes are outlined in Table 3. As can be seen, ecological separation exists between sympatric species in both taxa based on microhabitat, ranging patterns, and diet.
In all forests in which both chimpanzees and gorillas have been studied, chimpanzees forage at greater heights than gorillas. Although chimpanzees in Bwindi eat far more tree fruits than gorillas do, gorillas forage in the same fruit trees seasonally and also forage high in trees for epiphytic plants and fungi. Chimpanzees nest at significantly higher forest heights than gorillas do in all months (Stanford and Nkurunungi, 2003). Does competition exist for nesting sites? There is little evidence of this, although two pieces of information are suggestive. First, in the northern section of the study site, the only area of the national park in which gorillas do not occur, chimpanzees nest on the ground at a higher frequency than noted elsewhere. Second, when gorillas nest in trees, they nearly always choose one species, an understory tree that is rarely used as a nest tree by chimpanzees.
Some dolphin species seem to use strategies similar to chimpanzees and gorillas but in an aquatic medium. Foraging at different heights in the forest canopy can be compared with foraging at different depths in the ocean.
A separation of niches based on depth was proposed in the eastern Ionian Sea (Ferretti et al., 1998; Politi et al., 1998). Sympatric species can also display ecological separation utilizing inshore and offshore waters, as observed by Wang et al. (2000) for bottlenose dolphins living in Chinese waters, Dolar (1999) for spinner dolphins (Stenella longirostris) and Fraser’s dolphin (Lagenodelphis hosei) in the Sulu Sea, Bearzi (2005a) for bottlenose dolphins in sympatry with short-beaked common dolphins in California waters, and Baird and Dill (1995) for transient and resident killer whales in British Columbia and Washington State. These killer whales also show different diving patterns (Bigg et al., 1990; Baird 1994, 2000).
Chimpanzees and gorillas can occupy the same home ranges and eat overlapping diets but nevertheless harvest their foods differently in ways that may mitigate competition (Morgan and Sanz, 2006). As Yamagiwa (1996) has pointed out, a chimpanzee community uses its home range on a regular basis, traveling to all parts of a range up to 30 km2, but they canvass it only seasonally, remaining in one small part of the overall range for long periods. Gorillas may be able to forage this way because of their reliance on leafy herbaceous plants, which are more densely and evenly distributed on the landscape than the ripe fruits for which chimpanzees forage (Malenky et al., 1994).
Small odontocetes with overlapping diet in the Gully (Scotian Shelf) occupy the same home ranges in their daily activities but in a slightly different way (Gowans and Whitehead, 1995). Resident and transient killer whales also use the same habitat in British Columbia and Washington State but with different travel routes, sometimes related to the bottom topography (Morton, 1990; Felleman et al., 1991; Gowans and Whitehead, 1995; Baird, 2000). Dietary separation of day and/or during different seasons was observed for Atlantic white-sided dolphins and short-beaked common dolphins in the Gully (Gowans and Whitehead, 1995) and for spotted dolphins and spinner dolphins in the eastern tropical Pacific (Perrin et al., 1973; Norris and Dohl, 1980; Norris et al., 1994; Scott and Catta-nach, 1998).
Chimpanzees and gorillas have different diets in the wild, although the degree of difference varies among study sites (Fig. 1b). At all sites where they have been studied, chimpanzees are ripe fruit specialists, traveling long distances in search of new fruit sources. Gorillas, based on Fossey’s early work, were thought to be obligate folivores. More recent fieldwork on other gorilla populations living at lower elevations has revealed gorillas to be opportunistic, feeding heavily on fruits when available while using foliage as a fallback food during times of food scarcity (Tutin, 1996). In Bwindi, gorilla and chimpanzee diets converge during periods of heavy fruiting, and the two species share many of the same preferred food species. During times of scarcity, chimpanzees scatter into smaller social units to forage further afield for fruits while gorillas turn to fallback foods, primarily herbaceous groundcover.
Sympatric species of dolphins show different diets with a degree of dissimilarities among study areas. Hale et al. (2000) report different preferences in prey for sympatric species of bottlenose dolphins for various areas around the world, and Das et al. (2000) give an account of different diet for striped dolphins and short-beaked common dolphins in association with albacore tuna in the north-east Atlantic (Bay of Biscay).
The most striking example of diet divergence is known for resident and transient killer whales (Bigg et al., 1990) and individual populations of this species also specialize in catching specific types of prey (Fig. 1a, Felleman et al., 1991). Like gorillas, many species of odonto-cetes are opportunistic feeders, able to change their diet based on food availability (Klinowska, 1991). Transient killer whales in Prince William Sound feed almost evenly on harbour seals (Phoca vitulina) and Dall’s porpoises (Phocoenoides dalli), while off British Columbia and south-eastern Alaska, harbour seals are their most favored prey (Saulitis et al., 2000). Off southern Vancouver Island, Baird (1994) reported that transient killer whales killed harbour seals almost exclusively. Attacks by transient killer whales on Dall’s porpoises appeared to be more energetically expensive and less successful. These differences in transient killer whales’ preferential preys in Prince William Sound in comparison with those of British Columbia, Washington and Alaska may been explained by the fact that the former location has a lower number of pinnipeds than the latter locations.
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Direct competition and aggressive behaviour between sympatric species of the family Delphinidae has occasionally ever been observed (Shane, 1995; Ross and Wilson, 1996; Baird, 1998; Patterson et al., 1998; Frantzis and Herzing, 2002; Herzing et al., 2003; Psara-kos et al., 2003).
Ross and Wilson (1996) witnessed four violent dolphin-porpoise interactions in the Moray Firth, Scotland, but these authors did not discuss possible reasons for these interactions. In the same study area, Patterson et al. (1998) recorded aggressive behaviour by sympatric bottlenose dolphins toward harbour porpoises, suggesting that infanticide may be a factor responsible for this type of behaviour. Baird (1998) also reported aggressive behaviour by a Pacific white-sided dolphin on a neonatal harbour porpoise in Washington State. His study showed that aggression was more the result of an object-oriented play than aggressive behaviour displayed by one species competing for food, mate, or space. In the western edge of Great Bahama Bank, Herzing et al. (2003) observed interspecific interactions between Atlantic spotted dolphins (Stenella frontalis) and bottlenose dolphins, with male spotted dolphins displaying dominant mounting behaviour towards bottlenose dolphin males. In Hawaiian waters, Psarakos et al. (2003) also observed aggressive behaviour between sympatric spinner and spotted dolphins. This type of interaction was accompanied by interspecific copulation. It is clear that by contrasting these occasional examples of interactions, the majority of the investigations conducted worldwide to date show that dolphins under limited food resources conditions tend, whenever possible, to avoid direct competition by using behavioural, dietary and physiological habitat specializations (Table 3).
Researchers have long questioned whether gorillas and chimpanzees in sympatry engage in contest food competition. Kuroda et al. (1996) inferred mutual avoidance from the infrequency with which the two species met in the forest despite similar densities and ranging patterns. Although data from Bwindi do not provide systematic evidence of contest food competition, Stanford (pers. obs.) has witnessed one obvious incident, in which a party of nine chimpanzees occupied a fruiting tree, displaying at a gorilla group that attempted to ascend the same tree. The observation that chimpanzees were ecologically dominant, at least in this one encounter, accords with other recent studies of sympatric primates, in which two species assume role of ecological subordinate and dominant (Houle, 2004). Stanford and Nkurunungi (2003) also recorded gorillas and chimpanzees nesting in adjacent trees on the same night, and the home ranges of the gorilla study group and the chimpanzee study community occupied roughly the same area of forest. Rather than direct competition, most sympatric chimpanzee-gorilla associations may partition food resources by having different key food preferences, and through the seasonality of their diets (Stanford and Nkurunungi, 2003). In this way African ape sympatry would resemble sympatric dolphins.