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The Millennium Ecosystem Assessment (MEA 2005) identifies the transport of species across biogeographic boundaries as a major threat to biodiversity worldwide. This has been widely accepted by conservationists (cf. Lodge & Shrader-Frechette 2003; Perrings et al. 2005). However, while some studies find that invasive species have strong negative impacts on native biodiversity, comparable to the effects of habitat destruction (Wilcove & Chen 1998; Pimentel et al. 2001; Pauchard & Shea 2006), others fail to document any negative impacts and conclude that plant invasions do not cause species extinctions, at least not on regional or broader scales (Sax et al. 2002; Gurevitch & Padilla 2004; Maskell et al. 2006; Stohlgren et al. 2006). It has thus been argued that direct effects of alien species on the species richness component of biodiversity is not very well documented (Gurevitch & Padilla 2004; but see Powell et al. 2011), and the overall global impact is still rather enigmatic (Rosenzweig 2001; Slobodkin 2001; Barney & Whitlow 2008).
There are several reasons for these ambiguous reports (Davis 2003). First, there is a lack of coherent scaling across studies, which makes direct comparisons of ecological impacts questionable (Shea & Chesson 2002; Gooden et al. 2009). Second, it is often difficult to make clear causal links between the spread of alien plants and the demonstrated negative impact. Third, the concepts and terminology in the emerging field of invasive biology are often ambiguous (Colautti & MacIsaac 2004), for instance, it is important to distinguish between infrequent introduced species that have minimal ecological impacts and species that are invasive due to high rates of population growth, rapid spread and strong negative impacts on native species richness and ecosystem function (Young & Larson 2011).
The potential impact of introduced species will also depend on biogeographic conditions. Alien species may represent a threat to young terrestrial ecosystems, such as forest ecosystems on small islands along biogeographically isolated coasts (Pretto et al. 2010). A number of small islands along the North Atlantic coast of Norway have emerged since 7000 BP due to sea level fluctuations in postglacial times (Kaland 1984; Bondevik et al. 1998). This relatively young archipelago has a very peripheral location at the north-western fringe of the Palaearctic flora region, and it is not certain if these areas are saturated with species from the Eurasian species pool (cf. Svenning & Skov 2007). The flora of Norway, which covers this study area, consists of a high proportion of introduced species, partly because of an almost complete glacial cover during the last glacial period. Between 22% and 50%, depending on the temporal criteria applied (Fremstad & Elven 1997), of the vascular plants in Norway have been introduced. Only a subset (ca. 4%) of these has spread, and only a small proportion of these species has had a potentially significant negative ecological impact (Gederaas et al. 2008). The biogeographical conditions along the North Atlantic coast indicate that this area might be vulnerable to the potential effects of introduced species.
Scots pine (Pinus sylevstris L.) is a potentially dominant canopy tree in large fractions of the boreo-nemoral and North Atlantic parts of the Palaearctic, both on the mainland and the islands. The natural pine forests along the North Atlantic coast of Europe were, however, transformed by humans 1–5 kyr BP. Fire was used to create open heathlands suitable for year-round grazing by domestic animals (Kaland 1986; Loidi et al. 2010). During the last 130 yr, a decreasing farming population, changes in farming practices and public reforestation schemes have resulted in pine forest on many of the islands. Some islands have native Scots pine (P. sylvestris) and others have a mix of mountain dwarf pines (Pinus mugo Torra complex, mainly subsp. mugo and subsp. uncinata). Alongside this reforestation, the traditional land-use regime and open heathlands have persisted. The rugged coast and many islands of western Norway have allowed different forest types to persist alongside each other within the same landscapes. This study is carried out within an archipelago of coastal islands, and compares islands that have been reforested for at least 70 yr with native Scots pine (P. sylvestris) with islands that were planted at least 70 yr ago with introduced mountain dwarf pines from the sub-alpine zone in the Pyrenees, i.e. P. mugo subsp. mugo and P. mugo subsp. uncinata (hereafter P. mugo coll.).
The first step in the study is to analyse the relationship between the size of the islands and their number of species, i.e. the well-documented island species–area relationship (ISAR; Rosenzweig 1993; Whittaker & Fernandez-Palacios 2007; Tjorve & Tjorve 2011). Larger islands have a higher probability of harbouring more species than smaller islands, which is often explained by an increased number of habitats as a function of island size (Whittaker & Fernandez-Palacios 2007). However, some studies have tried to disentangle the ‘size effect’ from the ‘habitat effect’ (Westman 1983; Kallimanis et al. 2008). The number of habitats or ‘entities’ within a landscape is not easy to delimit, and therefore the relative importance of area vs habitat effects is elusive. In this study, we explore these phenomena by contrasting the ISAR of islands reforested with native Scots pine vs islands of the same size range that are planted with introduced pine. We assume that the size of the island correlates with the number of topographically and edaphically defined habitats, but the two tree species have different effects on habitat heterogeneity, as the old-growth Scots pine forests can tolerate the severe wind conditions at the coast and are therefore relatively undisturbed. Within these forests, light conditions appear more uniform due to a closed tree canopy, and on the ground there are extensive moss carpets that cover the microtopographic variation. Thus, we hypothesize that Scots pine forests, which are in a moss-dominated late-successional phase, may counteract the positive effect of island area. Due to proportionally fewer plant habitats on islands with old-growth Scots pine forest, we predict a relatively shallow regression slope of ISAR (i.e. lower z-value) on islands reforested with native Scots pine.
The direct effect of P. mugo coll. on species richness is the overall research question. It is either a passenger species that does not influence the local environment (Chabrerie et al. 2008), or it is more of an ecosystem engineer that directly influences species composition and richness (Jones et al. 1994). The potential biological life span of the introduced pine is much shorter than that of P. sylvestris, and it is easily wind-felled during storms, resulting in higher disturbance and more variable light and temperature conditions on islands reforested with the introduced pine. The higher disturbance frequency may also result in higher colonization success on islands with P. mugo (hereafter P. mugo islands) than islands with P. sylvestris forest (hereafter P. sylvestris islands). These ecological differences predict higher species richness per unit area on P. mugo islands compared with P. sylvestris islands. At the same time, however, many coniferous species are invasive and particularly within the genus Pinus (Richardson & Rejmanek 2004; Richardson 2006), which predicts negative impacts on native diversity. Hence, we test the null hypothesis of no difference in species richness between P. mugo and P. sylvestris islands, and interpret any deviations from this null hypothesis (positive or negative) in light of the invasive species effect and habitat heterogeneity hypotheses outlined above. The clear spatial nature of studies on oceanic islands requires that we take autocorrelation into account (Selmi & Boulinier 2001), and we have explicitly considered both the geographical space and species ordination space in the analyses. The primary aims are to test the following hypotheses
- The increase in number of species as a function of the size of an island is steeper on P. mugo than P. sylvestris islands
- There is no difference in species richness between islands reforested with P. sylvestris and those that are reforested with P. mugo coll.
- The significance of the differences in species richness is independent of the positions of islands in geographical space and species ordination space.
If the two last hypotheses are rejected, we aim to use differences in species composition and ecological attributes among the species unique to one type of island to aid the causal interpretation of the differences. The land-use argument will be reinforced if shade-tolerant forest core species prefer islands with old-growth Scots pine forest, whilst anthropogenic grazing-tolerant species are more frequent on islands that have been reforested by P. mugo coll.