Edge effects

There was no significant effect of plot location (edge vs. centre) on abundance of any of the detritivore groups investigated in this study. The only evidence for response to edge habitat found in this study was an increase in abundance of adventive Isopoda at edge habitat and this was not found to be significant. However, if an alpha value of 0.1 had been accepted, Porcellionidae would have been significantly more abundant in edge habitat.

Although invertebrates exhibit a variety of responses to edge effects (Didham, 1997), some previous New Zealand studies also found that edge effects had little impact on the abundance of forest floor invertebrates (Norton, 2002). Out of 13 orders of invertebrates sampled by Bolger et al. (2000) in California, all orders except the native Diptera and non-ant Hymenoptera (which increased in abundance towards the edge of the forest), and Collembola (which increased in abundance toward the forest centre) showed no response to the edge. The abundance of terrestrial Isopoda (dominated by adventive species) was unaffected by proximity to the edge of scrub remnant patches (Bolger et al., 2000). A common response of invertebrates is that there will be an increase in abundance and diversity at forest edge. This is due to an influx of species from human-modified areas outside of the forest into the disturbed forest edge (Didham, 1997). One study even described the invertebrate community in a native woodland forest edge as more similar to that in the surrounding fields than to that in internal forest habitat (Bedford & Usher, 1994). An Australian study on Coleoptera identified an edge effect which penetrated 100 m into fragments and caused an increase in the occurrences of detritivores and fungivores at the edge (this

121 may have been in response to an increase in litter and dead wood on the forest floor, and also an increase in fungal spores) (Davies et al., 2001). It has been noted that there are a range of responses of invertebrates to forest edge, and it is site- and taxon- dependent (Didham, 1997). Higher trophic levels are often considered to be more sensitive to fragmentation and edge effects (Bolger et al., 2000). Detritivores in general, being a low trophic level, may not be sensitive to the effects of fragmentation including edge effects; this has been demonstrated for detritivorous Coleoptera (Didham et al., 1998).

While plot location did not significantly affect the abundance of detritivores in this study, it did influence the prediction that any collected Diplopoda, Isopoda, and possibly Amphipoda would be adventive. For Isopoda and Amphipoda (if the alpha value is increased to 0.1), the probability of an individual being an adventive was higher at the edge of a forest. For Isopoda, edge habitat was a more influential predictor of origin (native or adventive) than forest type. In contrast, the probability of encountering an adventive individual of Diplopoda was lower in edge habitats; this is due to high overall abundance of adventive Diplopoda in centre plots.

Edge habitats could promote the invasion of adventive species into what remains of the native habitat (Norton, 2002; Hickerson et al., 2005). As previously mentioned, there is often an influx of species from human-modified areas outside of the forest into the edge habitat (Didham, 1997). Evidence of this was provided by a USA study which found that an adventive species of Chilopoda was more abundant in the edge habitat and a native species was more abundant in the forest centre. Although the adventive species was present at the majority of interior sites, the native

122 Chilopoda was rarely found at edge sites (Hickerson et al., 2005). Invasion of Argentine ants (Linepithema humile) into native scrub provides another example of higher abundance of adventive species in edge habitat. L. humile was most abundant along edges of scrub habitat with decreased densities found with distance from the edge and in larger unfragmented areas L. humile was only found at the edge. Native ant diversity was negatively correlated with the presence of L. humile (Suarez et al., 1998) and the displacement of native ant species from the exterior of shrub fragments was classified as an edge effect (Holway & Suarez, 2006). In my study there was some evidence that adventive Isopoda species more easily invade, or prefer, edge habitats; adventive and native Diplopoda co-occurred throughout forests.

The uncertainty that surrounds edge effects makes testing for them a difficult task. Firstly, there could be one or more unidentified factors operating on the abundance of detritivores, and response to edge may be undetectable due to an interaction between two or more confounding variables that obscure or neutralize each other (Murcia, 1995; Didham, 1997). For example, Bolger et al. (2000) showed Diptera and Coleoptera tend to increase in abundance closer to the edge, but this effect was cancelled out due to their aversion to argentine ants, which had higher abundance towards edges of forests. Secondly, scale is of crucial importance when studying fragmentation effects (including edge effects), as it can influence perceptions of patterns and processes (Murcia, 1995; Didham, 1997), yet there is still much uncertainty surrounding the way edge effects should be measured (Murcia, 1995). It is possible, that what my study deems to be ‘edge’ or ‘centre’ habitat might not be the edge or centre habitat that detritivores are responding to. This may especially be the case within smaller urban forests, where it is possible that the entire forest should be

123 considered an edge habitat. Bolger et al. (2000) considered fragments (usually long and thin in shape) of a size of less than 9 ha to be all edge. If forest fragments of less than 9 ha should in fact be considered all edge, then forests of less than this size (the Esplanade, Bledisloe Park, McCraes Bush) should not have contained a centre plot. It is also difficult to assume the distance which edge effects would penetrate into forests, because of the inconsistencies that have been found in previous studies (Murcia, 1995). Therefore, my study doesn’t necessarily conclude that Isopoda, Diplopoda, and Amphipoda are not affected by edge effects, even though only limited response to edge was detected.