| Effect of Disturbance on Alien and Native Plant Species The three groups of sites that were revealed by the site
PCA corresponded to sites that had high disturbance, sites with high species
richness, and sites with high conservatism scores. This suggests that
disturbance may be an important factor contributing to reduction in conservative
species abundance and native species richness. The PCA done on quadrat data
produced two groups: sites with no aliens, and sites with aliens. Quadrats that
contained aliens tended to be found in sites with a high disturbance loading in
the PCA by site. Therefore, sites with high disturbance tended to be more likely
to contain aliens. Conversely, sites with no aliens tended to have higher
species richness and higher MCC per quadrat. Alien presence is the dominant
variable, rather than the number of alien species or their total cover.
It seems, therefore, that sites that were disturbed had
fewer conservative species, as well as fewer native species in general and had
higher frequency of quadrats containing alien species. On the other hand, sites
that were of high quality in respect to mean conservatism and greater numbers of
native species were less disturbed and tended to have very few quadrats
containing alien species.
Several authors have indicated that disturbance favours
invasion (Burke and Grime 1996, Prieur-Richard and Lavorel 2000, Wiser et al.
1998, Anderson 1999, Pyle 1995). Few, however, have dealt with the dynamics of
conservative species in these situations. The reduction in conservative species
frequency as a result of disturbance or as a result of alien species presence or
both is an important mechanism in the overall process of invasion. Figure 32
shows a possible multiple regression model for the interaction of the frequency
of alien species occurrence throughout a site and the effect of MCC per quadrat,
which is negative, and disturbance index. This particular model explains 79% of
the variation at P<0.005. |
| Site Differences There was significant difference in mean native quadrat
richness between the sites. FAN2, the most disturbed site, had the fewest native
species. Futhermore, FAN2 had the lowest MCC per quadrat of all the sites and
the second highest mean Garlic Mustard cover by quadrat. Garlic Mustard cover
differed significantly between the groups, with KOM1, FAN2, FAN1, and RES1, the
sites that were heavily affected by disturbance, having the highest values for
Garlic Mustard cover.
Site KOM2 differed from the overall pattern in that it had
high mean native richness but also had the most Garlic Mustard cover per quadrat
and the highest alien richness per quadrat. This site had great disturbance and
exhibited the same trends as other sites that had great disturbance, except that
it had high native quadrat richness, yet low mean native species cover. Perhaps
this site was in the initial stages of invasion, with native cover being reduced
by disturbance, yet with some remnant of native species richness. The decrease
in native species cover may allow an invader such as Garlic Mustard, which had
high cover in this site, to establish itself.
Another factor in KOM2's unusual position may be its close
proximity to a large and relatively continuous tract of highly diverse forest,
which may provide a rich supply of seed influx from a variety of nearby native
species.
Alien species cover increased and native species cover
decreased with increased overall site disturbance. This trend was strong. Burke
and Grime (1996) also found that the cover of introduced species responded very
positively to disturbance. They stated that in the absence of disturbance, alien
species typically had less than 25% of the total cover, but in highly disturbed
plots this increased to 40%. Pyle (1995) also found that exotic species
abundance increased in disturbed habitats. Since the presence of aliens cannot
be a causal factor in disturbance, the disturbance is probably a determining
factor in whether aliens are present or not. Whether disturbance is responsible
for the decline in native richness or the presence of aliens or both is unclear.
However, sites that had high native diversity tended to be less disturbed even
when aliens were present. In addition, alien species cover was quite low in such
sites.
Additional Factors
The reduced alien-to-native ratio in the presence of
increasing litter cover may be explained if lack of litter allows seeds of alien
species greater access to germination sites in the mineral soil and greater
success under these conditions. Litter cover is often reduced in disturbed
sites. Conversely, the cover of native species increased with an increase in
litter cover. Litter may help provide optimal conditions for pre-adapted native
species, with established populations, to continue growth.
Alien cover increased as tree basal area increased. This
was particularly evident in sites with numerous large trees. Garlic Mustard, in
particular, often formed dense colonies at the base of very large trees. This
has been observed by Nuzzo (1991) as well. The implications of these results are
particularly important in the conservation of ecologically significant, mature
forest communities. When combined with disturbance, these appear to be under
particular threat of Garlic Mustard invasion. If possible, efforts should be
made to limit public access to sites with large trees to reduce the damage done
to fragile native understory species. Regular monitoring of such sites and
removal of vigorous invaders would reduce the ecological degradation of these
ecosystems.
Mean native species richness was lower in quadrats with
aliens present. The abundance of highly invasive aliens is the most important
variable I have tested in reducing native species richness. Disturbance appears
to be an important factor in the spread of a particularly invasive species
throughout a site. The type of exotic species appears to be very important in
its effect on the native community. Exotic, yet weakly invasive species appear
to be able to coexist within highly diverse native species communities in low
numbers without altering the native composition or affecting the native species
richness. Disturbance seemed to reduce the cover of native species, allowing
Garlic Mustard to spread. The percent cover of the invader did not seem to have
an effect on native species richness or cover unless the conditions for that
invader are ideal and it heavily colonizes an area. The structure of a site is
altered by high frequency of alien species and/or external disturbance. The
occurrence of conservative plant species appeared to decline to a minimum, fixed
value in the presence of a highly invasive plant species. This value did not
change with the abundance of aliens.
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