Anthropogenic activities have been related to plant invasions and different mechanisms have been revealed, e.g. increasing propagule pressure (Lonsdale 1999), disturbing native community (Sher and Hyatt 1999), and creating habitat fragmentation
(Deutschewitz et al. 2003). Human mediated propagule pressure (the number and frequency of individuals released into a region to which they are not native), which has been acknowledged as one of the important determinants of plant invasion (Williamson 1996,
Lockwood et al. 2005), might derived from human activities, such as horticulture/agriculture introduction cargo, local transportation, and tourism (Lonsdale 1999). They are often indirectly assessed by population density or percentage of human residence, industrial or agriculture area (Chytry et al. 2008). Many anthropogenic activities have the potential to disturb native plant community, e.g. agricultural activities, roads and buildings construction, and recreational activities (Hill et al. 2005). These disturbances cause by anthropogenic
activities (anthropogenic disturbances, here after) may promote invasion via decreasing competition intensity that exotic species received or increasing the resource availability of exotic species. The latter mechanism known as fluctuating resource hypothesis (Davis et al. 2000). Habitat fragmentation has been shown to facilitate plant invasions by reducing native richness, promote generalist species, and increasing edge effects (Harrison and Bruna 1999, Alpert et al.
2000). Patch and edge density are often utilized to evaluate habitat
fragmentation (Deutschewitz et al. 2003).
Landscape heterogeneity resulted from various kind of
anthropogenic activities has also been related to plant invasions in many cases (Deutschewitz et al. 2003, Kumar et al. 2006). It has been shown that higher habitat heterogeneity may support higher
naturalized species richness as well as native species richness (Deutschewitz et al. 2003). The mechanisms were correlated to the propagule pressure, dispersal, resource availability, and
fragmentation (Kumar et al. 2006). Beside landscape heterogeneity, type of land uses may be correlated to different levels of plant invasions as well (Hobbs and Humphries 1995, Austrheim et al.
1999).
Environmental factors have been shown to be influential on successful plant invasions in terms of the invasibility (Rejmánek et al.
2005). They usually include elevation, temperature, and precipitation (Thuiller et al. 2006, Chytry et al. 2008, Pauchard et al. 2009). The relationships between altitude and invasion were one of the most often addressed links (Pauchard et al. 2009). Studies have revealed that the diversity of naturalized plants is often higher in lower
elevations, probably due to the warm weather (Pauchard and Alaback 2004, Harrison et al. 2006, MacDougall et al. 2006). Harsh
environments and lower anthropogenic activities at higher elevation probably represented habitat limitations for naturalized species.
Habitats with rich precipitation and warm temperature are often more prone to invasions (Dukes and Mooney 1999, Pino et al. 2005).
However, these studies are often conducted in relatively xeric environments; situation may be different in mesic regions.
Since the native communities are often affected by
anthropogenic activities and environmental factors, the relationships between native and naturalized richness may be influenced as well (Stadler et al. 2000, Deutschewitz et al. 2003). Native—exotic richness relationships (NERR) have been discussed widely across different spatial scales, and the varies assumptions regarding the susceptibility of invasions have been proposed (Levine and
D'Antonio 1999). Negative NERR (Fig. 1a) are often addressed in studies with relatively fine scales (< 10m2) indicating a resistance performed by native species to plant invasions (Kennedy et al. 2002).
One the contrary, positive NERR (Fig. 1b) are often demonstrated in studies with relative broad scales (> 1km2); implying the same
species-area relationships performed by native and exotic plants (Stohlgren et al. 2003). One of the explanation is that competitive exclusion operates at fine scale but masked by other covarying
factors (e.g. climate, habitat heterogeneity, substrate…) at broad scale (e. g. Shea and Chesson 2002). These processes may have different importance at different spatial scale. At fine scale, biotic resistance mainly operated and negative NERR demonstrated. At broad scale spatial variances become dominant and positive NERR demonstrated.
The results from most studies were operated by several processes.
Many factors operate the mechanisms of NERR (Table 1). However, no studies focus on the discussion about NERR at the same spatial
scale yet. Here I further propose that native—exotic relationships may change at different anthropogenic factors or environmental factors even at same spatial scale.
a b
FIG. 1. Conceptual diagram of native-exotic richness relationships (NERR) analysis. (a) Negative NERR, indicate rich native community can resist exotic plant invasion. (b) Positive NERR, areas with more native richness also harbor more exotic species.
TABLE 1. Processes and their importance that may result in positive or negative native—exotic richness relationships (NERR) at different spatial scales. Adapted from Fridley et al. (2007).
Process Fine-scale importance Broad-scale
importance Negative NERR
1) Statistical artifact 2) Eltonian biotic resistance 3) Invasional meltdown Positive NERR
4) Neutral processes + spatial variance in community immigration rates
5) Neutral processes + spatial variance in disturbance rates 6) Niche processes: spatial
environmental heterogeneity 7) Niche processes: biotic
acceptance + non-equilibrium conditions
8) Facilitation (generalist)
High
Notes: Neutral processes describe species with identical behaviors where properties of the habitat determine NERRs, while niche processes mandate that species respond differently to the environment.
Taiwan, with an average population density of 636 population per km2, is second only to Bangladesh in countries which area larger than 10,000 km2 (“List of countries and dependencies by population density”, 2009); Thus received high anthropogenic activities.
Although attentions have been paid to plant invasions recently, the effects of anthropogenic activities and environmental factors on plant invasion remains unclear. The main purpose of this study is to
understand the effects of these factors on plant invasion and four main questions were addressed: (1) How is the invasion status in different habitat type? (2) How do anthropogenic activities affect native and naturalized species? (3) How do environmental factors affect native and naturalized species? (4) Does the native—exotic richness relationships change (e.g. from negative to positive) with different site characteristics at same spatial scale?