Distorted plant diversity patterns due to ungulate herbivory could be explained by changes in community assembly processes, but the effects of ungulate herbivory on plant community assembly remain unclear. Here, we examined the role of deer herbivory in the regulation of the assembly processes of a forest floor plant community by assessing species and functional diversity in over-and no-grazing plots (control and exclosure plots, respectively) in Shiretoko National Park in Japan. Compared with the exclosure plot, vegetation coverage was considerably lower, and species richness and diversity were higher in the control plot. Functional traits associated with competitive ability (leaf area and chlorophyll content) were significantly higher in the exclosure plot. The pattern of functional diversity changed from overdispersion to clustering with an increase in local crowdedness. This trait clustering indicates that the local communities that were free from ungulate disturbance gradually became dominated by some competitively superior plant species, which led to low species diversity and biotic homogenization. In contrast, the reduction in vegetation due to overgrazing by deer resulted in an increase in the relative importance of stochastic assembly processes, which enabled the coexistence of various species, including less competitive ones. Our results emphasize that although deer overabundance is of concern, their complete exclusion has a negative consequence from an ecological perspective.Because deer herbivory is an inherent process that affects the biodiversity of plants on the forest floor, the establishment of fences requires careful consideration to ensure the conservation of ecological processes and their associated biodiversity.

Beta(β)-diversity, or site-to-site variation in species composition, generally decreases with increasing latitude, and the underlying processes driving this pattern have been challenging to elucidate because the signals of community assembly processes are scale-dependent. In this meta-analysis, by synthesising the results of 103 studies that were distributed globally and conducted at various spatial scales, we revealed a latitudinal gradient in the detectable assembly processes of vascular plant communities. Variations in plant community composition at low and high latitudes were mainly explained by geographic variables, suggesting that distance decay and dispersal limitations causing spatial aggregation are influential in these regions. In contrast, variation in species composition correlated most strongly with environmental variables at mid-latitudes (20-30°), reflecting the importance of environmental filtering, although this unimodal pattern was not statistically significant. Importantly, our analysis revealed the effects of different spatial scales, such that the correlation with spatial variables was stronger at smaller sampling extents, and environmental variables were more influential at larger sampling extents. We concluded that plant communities are driven by different community assembly processes in distinct biogeographical regions, suggesting that the latitudinal gradient of biodiversity is created by a combination of multiple processes that vary with environmental and species size differences.

Evidence of the indirect effects of increasing global deer populations on other trophic levels is increasing. However, it remains unknown if excluding deer alters ecosystem functional relationships. We investigated how sika deer exclosure after 18 years changed soil conditions, the understory plant community, the traits of a dominant understory plant (Sasa palmata), herbivory by three insect-feeding guilds, and the functional relationships between these properties. Deer absence decreased understory plant diversity, but increased soil organic matter and ammonium concentrations. When deer were absent, S. palmata plants grew taller, with more, larger, and tougher leaves with higher polyphenol concentrations. Deer absence led to higher leaf area consumed by all insect guilds, but lower insect herbivory per plant due to increased resource abundance (i.e., a dilution effect). This indicates that deer presence strengthened insect herbivory per plant, while in deer absence plants compensated losses with growth. Because plant defenses increased in the absence of deer, higher insect abundances in deer absence may have outweighed lower consumption rates. A path model revealed that the functional relationships between the measured properties were similar between deer absence versus presence. Taken together, deer altered the abiotic and biotic environment, thereby changing insect herbivory, which might impact upon nutrient cycling and primary productivity. These results provide evidence that deer can alter interactions between trophic levels, but that functional relationships between certain ecosystem components may remain constant. These findings highlight the need to consider how increasing global deer populations can have cascade effects that might alter ecosystem dynamics.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-017-3895-3) contains supplementary material, which is available to authorized users.

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