Spatial patterns along the Rallarvägen
As expected, ruderal species were concentrated around points of
introduction with continuous disturbance, with a progressive decline in
ruderal species richness with increasing distance to these introductory
points (Fig. 4 and 5). Disturbance was the strongest driver –
especially for the total ruderal community – since the number of
ruderals decreased significantly with distance to the railroad at
average (9.8 °C) and below average (9.4 °C) MSST. This supports other
studies that illustrate the key role of disturbance for new plant
species introductions (Alexander et al. 2010; Lembrechts et al. 2016b).
However, our findings also showed a climatic response on ruderal species
distributions as the predicted number of both the total ruderals and
non-native ruderals increased with distance to the railroad in transects
where summer soil temperatures (MSST) reached above average values (10.3
°C; Fig. 4). This could indicate a potential role for climate change: if
climate warms, the current climatic limitations might decrease, allowing
for non-natives to expand further away from disturbed sites (Lembrechts
et al. 2018).
Interestingly, average ruderal species abundances were negatively
correlated with the total number of ruderal species per transect (Fig.
5), revealing a gradient from sites that accommodated relatively few and
predominantly common ruderal species to sites that accommodated
relatively many and often rare ruderals. Transects with train stations,
such as Björkliden, Abisko Östra and Riksgränsen, experience continuous
disturbance and appear to be the sites where rare ruderals reside and
are thus (re)introduced (Brandes, 2002). From here they must spread out
through local human-mediated dispersal (e.g., hiking) or through
self-dispersal. In less disturbed transects along the Rallarvägen, only
fewer and more common ruderal species still reside. Those sites were
sometimes not easily accessible, lay further away from train stations,
and were places where significant disturbance (outside of occasional
hiking) only happened once (i.e., during railroad building in the early
20th century) (Rendeková et al. 2019). Exact drivers
for this relationship are difficult to determine from observational data
only, but we can assume a link with the degree of disturbance intensity.
Distance to the E10 highway, railroad, and train station, and
hiking-intensity were all interconnected and sum-up to a certain degree
of disturbance intensity that facilitates species dispersal (Kowarik,
2003; Pauchard et al. 2009).
We propose that the correlation between human impact and ruderal species
distributions demonstrates Horizontal Directional Ecological Filtering
(HDEF; Fig. 5). The term Directional Ecological Filtering (DEF) was
first coined by Alexander et al. (2010) for elevational (climatic)
gradients, but here we demonstrate that the concept is applicable to a
horizontal anthropogenic disturbance intensity gradient as well. The
original DEF hypothesis states that non-native species migrations happen
from anthropogenically disturbed sites in the lowlands to higher
elevations in the mountains, and that their richness declines with
elevation but their elevational range increases with their maximum
elevation (Alexander et al. 2010). Non-native species thus progressively
drop out with increasing elevation. Unlike in the DEF, where climate
harshness is considered the most likely filter, native and non-native
ruderal species originating at the train stations were here thus
progressively filtered out with increasing distance to these
introductory points as the degree of disturbance intensity declined,
showing that such directional filters can still be strong when climatic
gradients are minimal. This led to the conclusion that gradients of
disturbance intensity coinciding with the elevation gradient should not
be ruled out as a critical driver of the DEF either (Pauchard et al.
2009).