Community Structure and Forest Invasion by an Exotic Herb Over 23 Years

Abstract

We studied the invasion of a New Zealand mountain beech (Nothofagus solandri var. cliffortioides) forest by the exotic perennial herb, Hieracium lepidulum. We used data from 250 randomly located permanent plots (400 m2) established in 1970 that sampled 9000 ha of forest. Frequency of H. lepidulum was 11%, 43%, and 57% in 1970, 1985, and 1993, respectively. For each year of measurement, invasion patterns were related to (a) distance to the forest margin as a measure of dispersal limitation, (b) community structure, (c) stem biomass dynamics indicating disturbance history, and (d) environmental characteristics. In 1970, invaded plots had more species and lower potential solar radiation, and they were closer to the forest margin; however, invaded plots were only weakly predicted by these site variables. H. lepidulum also invaded relatively species‐rich subplots (0.75 m2) showing that community structure was also significant at a microsite scale. Using the same sets of variables, the ability to predict which plots were invaded in any year increased from 1970 to 1993. This supports our hypothesis that in early invasion stages, with dispersal limitation, an invader may occur in only a subset of suitable sites giving a weak relationship with site variables. By 1993, distance to the forest margin was no longer related to which plots were invaded, and invaded plots had more species, occurred at lower elevations on more sheltered topographic positions, and had more fertile soils than uninvaded plots. Even though site variables were not independent (e.g., plots on fertile soils tend to have more species), multiple logistic regression showed that, all else being equal, invaded plots still tended to have more species than those not invaded. Our study therefore questions the hypothesis that, all else being equal, species‐poor habitats are more prone to invasion by exotic species.

Confounding factors in the detection of species responses to habitat fragmentation

“…Similarly, fragments of woodland and shrubland in the predominantly agricultural landscapes of southwest Australia are more likely to be grazed and trampled by livestock than continuous forest (Hobbs, 2001). These effects are likely further amplified by the greater ease with which small fragments and fragment edges are invaded by introduced species (Robinson, Quinn & Stanton, 1995;Wiser et al, 1998 ;Hobbs, 2001 ;Yates, Levia & Williams, 2004). For instance, trampling by cattle compacts soil structure, reducing the regeneration ability of native tree species that are already struggling with competition from introduced weeds (Hobbs, 2001).…”

BIODIVERSITY RESEARCH: Experimental introduction of the alien plant Hieracium lepidulum reveals no significant impact on montane plant communities in New Zealand

“…It was introduced from Europe to New Zealand, where it was first recorded in 1941 (Miller, 2006). It has become invasive in the South Island and has steadily increased in distribution and abundance over the last 60 years (Rose et al, 1995;Duncan et al, 1997Duncan et al, , 2001Wiser et al, 1998;Mark et al, 1999;Wiser & Allen, 2000). This invasion success is thought to be because of competitive advantages which allow H. lepidulum to displace native species and become dominant over time, sometimes forming dense mats to the exclusion of other species (Rose & Frampton, 1999;Radford et al, 2007).…”

“…Along with other members of the genus invasive in New Zealand, notably Hieracium pilosella and Hieracium praeltum, it is thought to compete strongly with native species (Fan & Harris, 1996;Moen & Meurk, 2001;Weigelt et al, 2002;Winkler & Stocklin, 2002;Lamoureaux et al, 2003), leading to declines in the species diversity of plant communities it has invaded (Scott et al, 1990;Wiser & Allen, 2000;Espie, 2001). Unlike those congeners, H. lepidulum is able to establish in shade under forest, shrub or tussock canopies and is able to invade higher rainfall and higher elevation areas (Wiser et al, 1998;Rose & Frampton, 1999). Consequently, this species is of particular conservation concern because of its widespread distribution and potential to impact the extensive and largely intact native communities in montane and subalpine regions throughout the South Island (Wiser & Allen, 2000;Miller, 2006;Radford et al, 2007).…”

“…Our aim is to examine the long-term impacts of an invasive alien plant on the richness, diversity and evenness of native plant communities using intentional introductions to establish an experimental gradient of invader density and abundance. We use the alien plant Hieracium lepidulum (botanical nomenclature throughout this paper follows Webb et al, 1988) as a model, since it is widely regarded as posing a significant threat to the diversity of montane and subalpine native forest and grassland communities in the South Island, New Zealand (Wiser et al, 1998;Rose & Frampton, 1999;Wiser & Allen, 2000;Radford et al, 2007). By establishing an experimental gradient of alien plant densities and assessing impacts over 6 years and at two spatial scales, we undertake one of the most thorough assessments of alien plant impacts on vegetation to date.…”

Size‐related deterioration of semi‐natural grassland fragments in Sweden

“…Invasion from surrounding vegetation may actually increase the total species richness of fragmented habitats, but reduce the fraction of habitat specialists (Harrison, 1999). An increased propensity for small habitat fragments to become invaded by generalist or alien species may be due to several factors: (a) an increased edge-to-area ratio increases the fraction of the habitat fragment that may support edge species (Saunders et al , 1991;Robinson et al , 1995;Planty-Tabacchi et al , 1996); (b) species loss in fragments may increase resource fluctuations, thereby increasing the likelihood that resource availability coincides with influx of propagules ( Wiser et al , 1998;Levine & D'Antonio, 1999;Davis et al ., 2000;Levine, 2000); and (c) an increasing size of source populations of invaders in the surrounding landscape (Saunders et al , 1991). Thus, a reduction of habitat fragment size may promote a 'run-away decline process' of the populations originally inhabiting the fragment.…”