Upland and lowland landscapes

Upland and lowland landscapes

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Upland and lowland landscapes are currently under heavy pressure from human disturbances^[@CR1]^ and management regimes^[@CR2]^. Habitat fragmentation is particularly an issue as it can lower species diversity^[@CR3]^ and affect connectivity, leading to ecological and evolutionary consequences^[@CR4],[@CR5]^. Habitat fragmentation can additionally lower bird species richness in tropical regions^[@CR6]^, for example by decreasing the total number of different habitats and patch sizes^[@CR7]^. Moreover, studies show that the quality of habitats as well as their connectivity are important for the species diversity of a given area^[@CR8],[@CR9]^.

A comparison of species richness in degraded and undisturbed areas in German Wadden Sea coast revealed that at the community level, species richness is not different between protected and unprotected habitats^[@CR10]^. It has been speculated, however, that disturbances in the form of eutrophication or habitat fragmentation can hamper a shift towards more generalist species, and could have a negative effect on species richness^[@CR10]^. The response of bird communities to the effects of eutrophication is less studied than that of mammals^[@CR11]^. In particular, the role of fragmentation, which is often co-occurring in eutrophicated coastal areas, is so far poorly understood.

At the population level, a higher degree of habitat fragmentation typically leads to lower population size^[@CR12],[@CR13]^, higher susceptibility to nest predation^[@CR14]^ and higher predator encounter^[@CR15]^. Moreover, habitat fragmentation can lower the patch occupancy of a species, leading to lower occupancy rates of the patches in which the species is present^[@CR16]^. The extent to which habitat fragmentation has an impact on community or population level is mostly unknown, due to difficulties in disentangling the effects of these two factors^[@CR17]^. Therefore, we aim to (i) determine how fragmentation affects species richness and population density in birds at the community level and (ii) analyse how this information can be used to identify potentially vulnerable species in coastal areas.

Results {#Sec2}


Patch occupancy and richness {#Sec3}


After excluding six patches from the analysis due to short visibility and four due to a relatively high proportion of saltmarsh (Supplementary Table ,[S1](#MOESM1){ref-type="media"}), we found that 20 out of 28 landscapes had at least one bird species present in both low- and high-fragmented areas. Five out of these 28 landscapes did not have any bird species present in both landscape types. This difference was not significant (Chi-square test, *χ*^2^ = 0.581, *df* = 27, *P* = 0.80). Only the proportion of bird species shared between the landscapes was significantly affected by landscape fragmentation (GLMM: *χ*^2^ = 58.181, *df* = 27, *P* = 0.000, Supplementary Table ,[S2](#MOESM1){ref-type="media"}). The probability of a bird species occurring in two or more habitats, however, did not significantly differ between the low- and high-fragmented landscapes (*χ*^2^ = 1.904, *df* = 1, *P* = 0.165).

Average bird density (birds per 1 km^2^ landscape) did not differ between landscapes (*F*~1,18~ = 1.217, *P* = 0.281, *r* = −0.353, *n* = 20), however, it was significantly affected by fragmentation type (*F*~1,18~ = 14.577, *P* = 0.001, *r* = −0.684, *n* = 20) (Fig. ,[1](#Fig1){ref-type="fig"}). Generally, landscape fragmentation lowered bird density, but the effect was only significant for high-fragmentation landscapes.Figure 1Bird density (bird·km^−2^·year^−1^) in five landscapes in the German Wadden Sea coast. Dots represent the mean density, error bars are the standard error. The fitted line represents the predicted value for the low-fragmentation category. The dotted lines mark the estimated marginal mean for the low- and high-fragmentation landscapes.

The effects of fragmentation on the probability of the patch having at least one bird were significantly different between species groups (GLMM: *χ*^2^ = 6.304, *df* = 4, *P* = 0.095, Supplementary Table ,[S3](#MOESM1){ref-type="media"}). The probability of a patch containing birds was only significant for ungulates (*χ*^2^ = 5.202, *df* = 4, *P* = 0.250) but not for species groups representing other taxa.Aunque la probabilidad de que un parche que contenga un pájaro fuera solo fue significativa para una pequeña proporción de especies en el área estudiada, los ungulados parecen ser particularmente vulnerables a los procesos de fragmentación, ya que la probabilidad de al menos un ave ungulada presente en un parche fue significativamente menor en en El panorama de alta fragmentación (*χ*^ 2^ = 4.129,*df*= 1,*p*=

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