Abstract
In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.
Highlights
Plant species associations are a fundamental aspect of plant community ecology [1,2,3]
Study Area and Data Collection The study was conducted in Cabo de Gata-Nıjar Natural Park, which lies along the Mediterranean coast in Southeastern Spain (36u 469 N, 2u 099 W)
Grazing carrying capacity for this plant community is 0.39–0.57 ind?ha21, so we considered G2 as low grazing intensity, G3 as intermediate grazing intensity and G4 as high grazing intensity [34]
Summary
Plant species associations are a fundamental aspect of plant community ecology [1,2,3]. Analyses of plant species associations provide information about environmental heterogeneity, biotic interactions and patterns of seed dispersal [4,5,6,7]. This information is of particular interest in semi-arid plant communities where vegetation often occurs in patches. Vegetation patches are composed of shrubs that can act as shelter against harsh environmental conditions. These shrubs are called ‘nurse plants’, for they appear to provide microhabitats that enhance survival for other plant species [2,8]. Theoretical models based on empirical studies suggest that these positive interactions between plant species are one of the main drivers for the formation of these patches [8,9,10]
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