No abstract

In large parts of North America and Europe, deer overabundance threatens forest plant diversity. (British Columbia, Canada), where Sitka black-tailed deer (Odocoileus hemionus sitkensis) were introduced, we compared islands with no deer, with deer for fewer than 20 years, and with deer for more than 50 years. We sampled invertebrates in three habitat categories: forest edge vegetation below the browse line, forest interior vegetation below the browse line, and forest interior litter. In forest edge vegetation, invertebrate abundance and species density decreased with increasing length of browsing history. In forest interior vegetation, decrease was significant only on islands with more than 50 years of browsing. Insect abundance in the vegetation decreased eightfold and species density sixfold on islands browsed for more than 50 years compared with islands without deer. Primary consumers were most affected. Invertebrates from the litter showed little or no variation related to browsing history. We attributed the difference between vegetation-dwelling and litter-dwelling invertebrates to differences in the effect of browsing on their habitat. In the layer below the browse line deer progressively removed the habitat. The extent of litter habitat was not affected, but its quality changed. We recommend more attention be given to the effect of overabundant ungulates on forest invertebrate conservation with a focus on edge and understory vegetation in addition to litter habitat. Few researchers have examined its effects on invertebrate assemblages. In a natural experiment on Haida GwaiiResumen: La sobreabundancia de venados es una amenaza para la diversidad de plantas de bosques en extensas zonas de Norte América y Europa. Pocos investigadores han examinado sus efectos sobre ensambles de invertebrados. En un experimento natural en Haida Gwaii (Columbia Británica, Canadá), en donde se introdujeron Odocoileus hemionus sitkensis, comparamos islas sin venados, con venados por <20 años y con venados por > 50 años. Muestreamos invertebrados en tres categorías de hábitat: vegetación de borde debajo de la línea de ramoneo, vegetación del interior de bosque debajo de la línea de ramoneo y hojarasca del interior del bosque. En la vegetación del borde del bosque, la abundancia y densidad de especies de invertebrados disminuyeron a mayor historia de ramoneo. En la vegetación del interior del bosque, la disminución sólo fue significativa en islas con más de 50 años de ramoneo. En comparación con islas sin venados, la abundancia de insectos en la vegetación disminuyó ocho veces y la densidad 6 veces en islas con más de 50 años de ramoneo. Los consumidores primarios fueron los más afectados. Los invertebrados de la hojarasca mostraron poca o ninguna variación relacionada con la historia de ramoneo. Atribuimos la diferencia entre invertebrados de la vegetación e invertebrados de la hojarasca a diferencias en el efecto del ramoneo sobre su hábitat. Los venados progresivamente removieron el hábitat en el estrato debajo de la línea de ra...

Debate on the relative importance of competition for resources and trophic interactions in shaping the biological diversity of living communities remains unsettled after almost a century. Recently, dramatic increases in ungulate populations have provided a useful quasi-experiment on the effects of unrestrained ungulates on forest ecology. The islands of Haida Gwaii (Canada) offer a unique situation to investigate the potential of large herbivores to control temperate forest community structure and diversity. Black-tailed deer Odocoileus hemionus Merriam, native to adjacent mainland areas of British Columbia, were introduced in 1878 and spread to all but a few islands. Because deer were not native to the archipelago, islands that still lack deer provide a rare instance of temperate forest vegetation and fauna that developed in the absence of large herbivores. The colonisation of different islands at different times, and the absence of significant predation allow us to assess whether and how a large herbivore can exert ''topdown'' control on vegetation and its associated fauna. We studied plant communities in forest interior and shoreline, on seven small islands of varying browse history. Three islands were untouched by deer, deer had been resident for about 15 years on two, and on another two deer had been present for more than 50 years. Without deer, vegetation in the understorey and/or shrub layer was dense or very dense. Structure and composition varied markedly within and between shoreline and interior communities. Without deer, shoreline communities were dominated by species absent from islands with deer. Where deer had been present for less than 20 years most plant species characteristic of shorelines on islands without deer were already absent or scarce, but in the forest interior species richness was less affected and extensive shrub thickets remained. On islands where deer had been present for[50 years vegetation below the browse line was extremely simplified, converging in both forest interior and shoreline towards an open assemblage of a few deer-tolerant species, basically two coniferous trees. This top down effect on the plant community reflected up the food chain so that understorey invertebrate and shrub-dependent songbird communities became simplified. In contrast, species litter arthropods (especially weevils and millipedes) were highest where deer were present for [50 years. Canopy birds were unaffected by deer presence. In the absence of predators, major climatic stress or other means to control the herbivore, deer browsing created greatly simplified plant and animal communities.