# Can Vegetation Removal Successfully Restore Coastal Dune Biodiversity?
## 4 Discussion
### Effect of Treatment on Semi-Fixed Dunes  
Establishing dune dynamics on the wind facing slope can lead to a rejuvenation of the landscape through reactivation of sand, leading to burial of any newly developing vegetation [31]. This intervention enables pioneer species found in mobile dunes to recolonize [4]. Rodents responded well to treatment in semi-fixed dunes with a significant shift in composition detected, achieving 30% of the desired Euclidean ordination distance towards reference, as expected by Hypothesis \(H_{4}\). In addition, though the change was not significant for reptiles or beetles, the response to treatment was consistently in the desired direction towards mobile-dune composition. For annuals the shift was minor, but away from the desired composition, becoming less similar to mobile dunes, and \(H_{4}\) was rejected.  
At the temporal scale, rodents showed significant differences in some years. The change in reptile and beetle composition was more pronounced in the final years of the study, and became significant for beetles in the last year. Similar temporal responses were also detected at the indicator species (IS) level, particularly for \"mobile-dune indicator species\" (MIS). These results indicate some support for Hypothesis \(H_{4}a\), where the composition turnover is due to recolonisation by MIS.  
The IS were selected objectively based first on their affinity to fixed and mobile dunes, and secondly as the species with the lowest and highest weighted PPC. Three MIS (all except the reptile), as well as the rodent \"fixed-dune indicator species\" (FIS) appeared to respond desirably in the direction intended for restoration. Given that the beetle _S. striatus_ was almost entirely absent from control dunes and increased over time in treated dunes, we can infer that its increased abundance was due to colonization rather than competitive release. The small increase detected for rodents and reptiles was not significant when looking at averages across years, but the gerbil _G. pyramidum_ did respond favorably over time, showing significant increases in the final year of the study. This increase may be due to the overall decline in abundance of the subordinate _G. a. allenbyi_ populations, since these two species compete sympatrically at the patch scale [95; 96; 97]. Further investigation is needed, since the trends of the two species were not consistent over time. The annual MIS _C. memphitica_ also had relatively high cover in some years on treated dunes, while remaining much less prevalent in control dunes. This was most likely a response to competitive release following the removal of perennial vegetation. Over time, this species was erratic in both mobile and treated dunes and the temporal trends were not synchronized, making it a poor IS, as expected due to the relatively low IV.  
Given similarities between control and reference dunes in terms of reptile and beetle species richness, there was no expectation for treated dunes to elicit any response. However, control and reference dunes were different for annual plants, so for this taxon, a lack of response can be seen as lack of desired treatment effect. Only rodents increased in richness as expected under Hypothesis \(H_{3}b\) (from one to two species) and \(H_{3}a\) was rejected for all taxa in semi-fixed treated dunes. These results highlight the importance of examining composition rather than univariate measures when studying restoration effects on biodiversity.  
The composition shift in semi-fixed treated dunes may continue towards a mobile state with more species responding positively, but without a press disturbance, we may observe a return to the control state. In the Netherlands, aeolian activity increased after an intervention, up to a climax in the third year, and then started to decline because of vegetation development on bare spots [4]. [PERSON] et al. [40] suggested that follow-up removal of roots is needed for three to five years and it is not clear whether the dunes will remain mobile in the long term.