Effects of grain size on patterns and drivers of species
richness
The differences in drivers between small and large grains provide
insight into the manner in which the plant communities on Marion Island
assemble.
Higher elevations supported lower native plant richness at both large
and small grain. This suggests that at both grains, elevation restricts
the number of species that can coexist, and that as altitude decreases,
more species coexist at both grains (Figure 5a). Because richness
increases for both grain sizes with decreasing elevation, higher values
of native Δ9-1 at low than high elevations is probably
due to the higher overall number of species at low elevations (Gremmen
and Smith, 2008). High altitude environments, characterised by colder
temperatures, support a limited range of plant species (Figure A3), and
therefore have limited potential for turnover.
Northness increased species richness at the large but not small grain in
our study. Native Δ9-1 was higher in warmer north-facing
plots than cooler south-facing plots. This suggests that, while greater
northness creates warmer and more sunny environments that may benefit
plants, and thus increasing the local species pool, it only leads to
increased species coexistence at large grains (Figure 5b). Therefore,
higher sun exposure does not lead to more species coexisting within 1
m2, possibly due to the influence of other limiting
factors such as competition for nutrients (e.g., Cramer et al., 2022)
restricting the number of species that can coexist per unit area.
However, at higher northness, more species accumulate within the 9
m2 plot, indicating species turnover between small and
large grains.
Native richness increased with TWI at small, but not large, grains; and
native Δ9-1 decreased with increasing TWI. Therefore,
increasing TWI allowed more species to co-exist, but only at small
grain. At large grain, higher TWI did not result in higher richness,
suggesting that the species that are added at smaller grain in high TWI
environments already occur at the large grain. As a result, when TWI is
low, increasing the grain size increase species richness, indicating
local turnover within the 9 m2 grain. However, when
TWI is high, this trend is not observed as more species already coexist
at the smaller grain; increasing grain size does not introduce new
species (Figure 5c). Thus, in drier plots richness may be limited by
competitive exclusion at small grain size, but the effects of
competitive exclusion do not act (or are diluted) at the large grain
size, allowing for more species to coexist and exploit a broader range
of available resources.
Finally, native Δ9-1 was higher (i.e., more species at
large than at small grain) in the presence of A. selago than in
its absence. This may be because A. selago increases the
complexity of the plot as conditions on and at the edge of the cushion
differ to those in the adjacent open habitat, but this effect is only
obvious at large grain. Alternatively, at small grains, the cushion
plant’s expansive growth form, which can cover most of or an entire
plot, may limit other plants through competition, but only at small
grain (Nyakatya and McGeoch, 2008). Another possible explanation could
simply be that the environment that is suitable for A. selago is
also suitable for other species, and these species thus co-exist at
large grains, but less so at small grains where A. selagooutcompetes other plants with its cushion growth form.
Possibly due to most alien species in this study being in their lag
phase with the potential for increased spread, the effect of grain size
on environmental drivers of alien richness is less pronounced. Given the
unfavourable conditions for alien species at high elevations, resulting
in lower alien richness, alien Δ9-1 was also lower
there. In addition, grey-bedded ash geology significantly increased
alien Δ9-1, but only five plots had this geology.
Therefore, the effect of geology on alien Δ9-1 is
considered negligible in this system.