Community homogenization and the invasiveness of commensal species in Mediterranean afforested landscapes

Abstract

The ecological consequences of homogenization remain relatively unexplored. One example of landscape-homogenizing is the establishment of plantations. We studied the effect of human-made forests by contrasting plant and small-mammal community composition between planted tree stands and adjacent natural habitat in two different Mediterranean habitats in Israel: (1) inland habitat where we focused on pine (Pinus halepensis) and carob (Ceratonia siliqua) stands, and (2) coastal sand dune habitat where we focused on planted acacia (Acacia saligna) stands. We first wanted to verify whether planted trees modify plant species composition, and second, if and how the small-mammal community is affected by the different habitat conditions created in plantations with different canopy cover. We were especially interested in the abundance of the commensal house mouse (Mus musculus). All tree stands underwent biotic homogenization indicated by abundance of house mice coupled with lower diversity of indigenous vegetation and small-mammal abundances and diversities. Habitat structural diversity was positively related with small-mammals diversity and was lower in artificial tree stands in both habitats. Our results suggest that using the abundance of commensal generalist species such as the house mouse relative to other more specialist small-mammals is a good approach to determine ecosystem integrity. Precommercial thinning treatment is a potential management tool to maintain a proportion of native tree species within the canopy of planted tree stands. However, until sufficient data is available for making generalizations, the exact level of thinning necessary to reverse the homogenization processes in man-made plantations and keeping indigenous small-mammal communities diverse and less prone to invasion must be determined empirically.

Conservation implications of competition between generalist and specialist rodents in Mediterranean afforested landscape

“…However, the theory continues with the notion that generalists can coexist with specialists as long as at least one habitat is unused or underused by the specialist (Brown 1996;Morris 1996;Egas et al 2004;Abrams 2006). Indeed, the most generalist species, the commensal house mouse was the most abundant rodent in carob groves within a study area that had almost no maqui layer (thus can be regarded as an extreme case of a homogenized and altered habitat), and was least abundant in natural maqui (Manor et al 2008). The second most abundant species within the carob groves was the habitat generalist Acomys.…”

“…The habitat specialist Apodemus was very rare and was trapped mostly in spring (Manor et al unpublished data). Thus, a sound management protocol would be to increase oak cover to at least 10% to curtail the homogenization process (Manor et al 2008) and keep indigenous small mammal biodiversity high. Moreover, oak cover was correlated with habitat structural diversity that affects small-mammal community diversity (Manor et al unpublished manuscript).…”

Pine Stand Density Influences the Regeneration of Acacia saligna Labill. H.L.Wendl. and Native Woody Species in a Mediterranean Coastal Pine Plantation

“…Conversely, Pistacia showed a much lower regeneration density (mean: 5,700 N ha −1 ), with non-significant variations based on the stand density. Such a pattern was not completely unexpected as Pistacia was found to dominate the understory in high pine cover conditions [30] and its considerable ecological plasticity to solar radiation is recognized [60]. However, saplings of both native species were much less developed than A. saligna, being mostly less than one meter high in Olea (>99%) and Pistacia (>91%).…”

“…Very high stand densities (approximately 1150 pines per hectare) have strong inhibitory effects for shrub species, resulting from a reduction in light, soil water, and nutrient availability [6]. There is a large consensus and much field evidence regarding the negative role played by an excessive pine cover in the renaturalization process of Mediterranean pine plantations, as well as in the richness of the understory [8,13,30]. For this reason, thinning is, generally, performed with the aim of accelerating secondary succession and increasing the overall biodiversity, enhancing the heterogeneity and structural diversity of the forest stand and improving the status of the remaining trees [7,13,52].…”

“…A. saligna may cause a significant shift in native community assemblages, reducing the occurrence of guiding or focal species, whilst favouring the spread of opportunistic and ruderal species, including other invasive species [28,29]. In Mediterranean pine plantations, negative effects caused by A. saligna have been reported on small mammal communities and were attributed to the structural simplification and homogenization of the forest stand [30].…”