S16.2: Niche expansion and exploratory behaviour on islands: Are they linked?

Islands differ greatly in their geographical range, their size, altitude, and other characters, but in general they have in common that they are more or less separated from greater landmasses and so can not easily be reached by terrestrial animals. As a result island communities differ in several aspects from nearby mainland communities.

One phenomenon often observed on islands is a remarkably impoverished species diversity holding only around 50% of species compared to similar mainland areas (Table 1). Despite the low number of species on islands some species can be very abundant. Niche changes occur in 25 to 75 percent of the island species as compared to their mainland relatives (Table 1). These changes may consist of a niche shift, where the whole niche is shifted in one or the other direction so that the mean value of a niche use changes, or of a niche enlargement or narrowing, where the mean value remains the same but the variance increases or decreases (Williamson 1981). Niche changes can concern a variety of environmental conditions. Island species expand into new habitats, use a broader range of altitudes, increase their feeding spectrum, use a wider range of vertical feeding strata, or change their behaviour (Table 2). Several species developed new skills mainly linked to the context of feeding. For example, various island rails from different genera and the Galapagos Mockingbird Nesomimus sp. developed skills to break eggs of other birds (Amadon 1966), the Sharp-beaked-Groundfinch Geospiza difficilis feed on blood by pecking the secondaries of incubating bobbies Sula sp. (Bowman & Billeb 1965). Additionally, tool use behaviour evolved in 44% of all known cases on islands or island like areas (Morse 1980). A well known example is the Woodpecker finch Camarhynchus pallidus, which uses a spine to dislodge prey from crevices (Eibl-Eibesfeldt 1961).

As mentioned earlier, islands are often species impoverished which may be due to limited dispersal abilities of the species (Mac Arthur & Wilson 1963) or to ecologically less differentiated island habitats (Lack 1976). Thus, island species experience less interspecific competition (competitive release, Diamond 1970; Keast 1970; Mac Arthur et al. 1972; Terborgh & Faaborg 1973; Yeaton & Cody 1974; Morse 1977; Baker-Gabb 1986; Keeler-Wolf 1986; Löfgren 1995), which opens the possibility to occupy vacant niches.

Species impoverishment not only influences species interactions at the same trophical level but between different trophical levels as well. Typically, islands carry less predators than the mainland or they are predator free, especially from ground predators (Morse 1971; McLaughlin & Roughgarden 1989; Keast 1991; Daugherty et al. 1993; Blazquez et al. 1997). Flightlessness in birds and insects is a well known phenomenon on islands (Grant 1998) and on many islands animals are less cautious.

Food resources often differ in quality and quantity on islands from those on nearby mainland (Diamond 1975; Stamps & Buechner 1985). For example, insectivorous animals on islands at a lake in Sweden have to cope with a smaller abundance of arthropods than on the surrounding mainland. Additionally, the decrease in abundance is unequally distributed among different size classes (Nilsson & Ebenman 1981). Low abundance in resources facilitates a broader feeding range (Cody 1974; Pulliam 1986). But arthropod abundance of some specific guilds can also be increased as is the case for the foliage arthropods on Tobago island (Keeler-Wolf 1986). Further, seasonality and predictability of resources are important factors in determining niche shape (Cody 1974). Milder climatic conditions, especially on mediterranean islands, provide a more constant and predictable food supply as compared to the mainland (Morse 1977; Blondel et al. 1988) thus facilitating the specialisation on specific food sources. Many investigations show that despite the fact of a broader feeding niche of a particular species on islands, each individual is more specialised than on the mainland (Ebenman & Nilsson 1982; Werner & Sherry 1987). Additionally, the higher population densities often observed on islands are put down to a more stable food supply (Crowell 1962; Mac Arthur et al. 1972; Blondel et al. 1988).

Of course, there are many more factors in which islands differ from the mainland and on each island different constellations of factors will work. Additionally, species react differently to these changes as I will show in the following.

Some species become extremely abundant on islands and show niche expansions. Obviously, these species found optimal conditions on their particular islands. The increase in abundance may be due to higher resource availability because of reduced interspecific competition for the same resources (Crowell 1962; Stamps & Buechner 1985), or to better climatic conditions (Blondel et al. 1988). Several hypotheses exist as to why habitat expansion occurred in these species. One possibility is that suboptimal habitats could be inhabitat because of the lack of competitors (competitive release). For example, on Mona island Catbirds Dumetella carolinensis, Cardinals Richmondena cardinalis, and White-eyed Vireos Vireo griseus expanded their breeding range into more open habitats (Crowell 1962). As Crowell pointed out less competition facilitated these expansions.

Another possibility is that the population size of these species increased first in the optimal habitat. Because of the island situation, increasing intraspecific competition and restricted dispersal (fence effect, Krebs et al. 1969) lead to a 'spill over' into suboptimal habitats (spill over hypothesis, Crowell 1983; Blondel et al. 1988). This is the case in the Corsican island (Blondel et al. 1988) where some forest species show heavily increased population densities in the forest and a marked habitat expansion into matorral scrubland habitats.

A third explanation is given by Martin (1992) who reanalysed the data from Corsica from an autecologically point of view. Niche expansion might be possible because of local adaptations to the environmental conditions under geographical isolation. Therefore, the Corsican forest birds adjusted their breeding biology to the later onset of spring on the island. This delay in breeding facilitated them to be successful in matorral habitat as well.

Other species may not find such optimal conditions on islands. They do not increase in abundance but niche expansions occur in different dimensions caused by several reasons. In many of these cases reduced interspecific competition are thought to be the reason for niche expansion. On several small islands north east of New Guinea species expanded into higher altitudes normally occupied by other species (Diamond 1970). Another reason why niche expansion occurred in these species is that the habitat types differ between island and mainland. Wrens on St. Kilda and Fair Isle Troglodytes t. hirtensis and fridariensis show a remarkable shift in breeding habitat from vegetated areas on the mainland to treeless rocky sea cliffs on the islands (Williamson 1981). Resource scarcity or changed resources may be another factor causing niche expansion. Examples are the expansion of feeding niches in Galapagos birds which were sometimes accompanied by new feeding techniques as already mentioned.

For completeness there are also species showing no changes on islands at all, whereas others decrease their niche in one dimension, for example the habitat type, because they can use a wider range of resources exploited by other species on the mainland (Blondel et al. 1988).

Second, new resources or behavioural patterns can be achieved by observational learning. Werner & Sherry (1987) stated that individual feeding specialisation in Cocos finches Pinaraloxias inornata may be achieved by observational learning from other group members or even other species. As a whole this species show an extremely broad food spectrum.

Third, exploratory behaviour can play an important role in acquiring broader niches. New habitats or resources can be discovered through exploration. Do island species exhibit pronounced exploratory behaviour?

There are only few investigations concerning exploratory behaviour of island and mainland species. Haemig (1989) found that Scrub jays Aphelocoma coerulescens on Santa Cruz island approached a noise caused by rubbing on a ball more closely and stayed for a longer time at the source of noise than their Californian relatives. Thus, the island birds seem to be more explorative than the mainland birds. A second investigation was carried out with parrots. In this study, exploratory behaviour of 61 parrot species Psittacidae was investigated in captivity and correlated to their ecological requirements in the wild with respect to habitat use, food spectrum, origin from island or mainland, and other factors (Mettke 1995). Among others the variable 'island-mainland' showed an influence on exploratory behaviour (Table 3). Island species investigated a new object, a wooden ring, in their familiar aviary earlier than mainland species. Thus, the former reacted very quickly to changes in their environment which enable them to adapt their behaviour to new situations early in time. Again, island species were more explorative than mainland species.

There are several factors on islands which favour exploratory behaviour. First, predation pressure is reduced on islands. To be more explorative is therefore not correlated with increased costs of predation. Second, temporary food shortages can occur on islands. Discovering new resources in time through exploration helps to survive such bottlenecks (Haemig 1989). Third, less competing species increase the chance that newly discovered resources can successfully be exploited. And fourth, as already mentioned, resources on islands are often different from those on the mainland. Explorative species are able to find new resources earlier than less explorative ones. This might be important in island colonisation. Thus, the specific island conditions favour exploratory behaviour.

Is exploratory behaviour on islands linked with niche expansion on islands, in other words, do species with a strong niche expansion show more exploratory behaviour? The answer is still unknown. But in the genus Chalcopsitta (Psittacidae) the mainland species’ show a smaller food niche than the island species (C. cardinalis) and the former are less explorative than the latter. Also, the more curious Santa Cruz island Scrub jay investigated by Haemig (1989) occupies a broader range of habitats than their mainland relatives (Atwood 1980). Additionally, van Lawick-Goodall (1976) and Westergaard (1992) showed in chimpanzees and baboons that object exploration can lead to later tool-use behaviour which evolved more often on islands than on the mainland (Morse 1980).

For the future it will be interesting to focus more on the mechanisms of niche change. The investigation of exploratory behaviour may shed some light upon why niche expansion occurs in some species but not in others.

I would like to thank Prof. Dr. H. Winkler for financial support.

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