Introduced browsing mammals in New Zealand natural forests: Aboveground and belowground consequences

Forest dwelling browsing mammals, notably feral goats and deer, have been i ntroduced to New Zealand over the past 220 years; prior to this such mammal s were absent from New Zealand. The New Zealand forested landscape, therefo re, presents an almost unique opportunity to determine the impacts of intro duction of an entire functional group of alien animals to a habitat from wh ich that group was previously absent. We sampled 30 long-term fenced exclos ure plots in indigenous forests throughout New Zealand to evaluate communit y- and ecosystem-level impacts of introduced browsing mammals, emphasizing the decomposer subsystem. Browsing mammals often significantly altered plant community composition, r educing palatable broad-leaved species and promoting other less palatable t ypes. Vegetation density in the browse layer was also usually reduced. Alth ough there were some small but statistically significant effects of browsin g on some measures of soil quality across the 30 locations, there were no c onsistent effects on components of the soil microfood web (comprising micro flora and nematodes, and spanning three consumer trophic levels); while the re were clear multitrophic effects of browsing on, this food web for severa l locations, comparable numbers of locations showed stimulation and inhibit ion of biomasses or populations of food web components. In contrast, all mi croarthropod and macrofaunal groups were consistently adversely affected by browsing, irrespective of trophic position. Across the 30 locations, the m agnitude of response of the dominant soil biotic groups to browsing mammals (and hence their resistance to browsers) was not correlated with the magni tude of vegetation response to browsing but was often strongly related to a range of other variables, including macroclimatic, soil nutrient, and tree stand properties. There were often strong significant effects of browsing mammals on species composition of the plant community, species composition of leaf litter in t he litter layer, and composition of various litter-dwelling faunal groups. Across the 30 locations, the magnitude of browsing mammal effects on faunal community composition was often correlated with browser effects on litter layer leaf species composition but never with browser effects on plant comm unity composition. Browsing mammals usually reduced browse layer plant dive rsity and often also altered habitat diversity in the litter layer and dive rsity of various soil faunal groups. Across the 30 locations, the magnitude of browser effects on diversity of only one faunal group, humus-dwelling n ematodes, was correlated with browser effects on plant diversity. However, browser effects on diversity of diplopods and gastropods were correlated wi th browser effects on habitat diversity of the litter layer. Reasons for th e lack of unidirectional relationships across locations between effects of browsers on vegetation community attributes and on soil invertebrate commun ity attributes are discussed. Browsing mammals generally did not have strong effects on C mineralization but did significantly influence soil C and N storage on an areal basis for several locations. However the direction of these effects was idiosyncratic and presumably reflects different mechanisms by which browsers affect soil processes. While our study did not support hypotheses predicting consisten t negative effects of browsing mammals on the decomposer subsystem through promotion of plant species with poorer litter quality, our results still sh ow that the introduction of these mammals to New Zealand has caused far-ran ging effects at both the community and ecosystem levels of resolution, with particularly adverse effects for indigenous plant communities and populati ons of most groups of litter-dwelling mesofauna and macrofauna.