Summary Dead tree trunks have significant ecosystem functions related to biodiversity and biogeochemical cycles. When lying on the soil surface, they are colonized by an array of invertebrate fauna, but what determines their community composition is still unclear. We apply community assembly theory to colonization of tree logs by invertebrates. During early decomposition, the attached bark is critically important as an environment filter for community assembly through habitat provision. Specifically, we hypothesized that the more dissimilar bark traits were between tree species, the more their faunal community compositions would differ. We tested this hypothesis by investigating the effects of bark traits on the invertebrate communities in the early‐decomposing logs of 11 common, temperate tree species placed in the ‘common garden’ experiment LOGLIFE. Bark traits included bark looseness, fissure index, outer bark thickness, ratio of inner to outer bark thickness, punch resistance, water storage capacity and bark pH. The predominant faunal groups studied were Annelida, Isopoda, Chilopoda, Diplopoda, Diptera and Coleoptera. Our results showed (i) strong interspecific differences in bark traits, (ii) that bark traits related to environmental buffering had profound effects on the abundance of specific invertebrate groups, and (iii) the higher the overall bark trait dissimilarity between tree species, the more dissimilar these tree species were in faunal community composition, and the higher was the joint invertebrate family richness. A suite of bark traits together has fundamental afterlife effects on invertebrate community assembly, strongly filtering the colonizing invertebrates in early‐decomposing logs, driving variation in their community composition and diversity. Our findings indicate that bark trait dissimilarity among tree species in forest stands is likely a better indicator of early‐phase dead trunk fauna diversity than tree species diversity per se. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12676/suppinfo is available for this article.
Productivity is a key driver of ecosystem structure and function, so long‐term studies are critical to understanding ecosystems with high temporal variation in productivity. In some deserts, productivity, driven by moisture availability, varies immensely over time (rainfall) and space (landscape factors). At high productivity, species richness is expected to be driven in opposing directions by abundance (More Individuals Hypothesis – MIH) and competition. While studies investigating the impacts of spatial variation in productivity on community structure are common, the impacts of temporal variability on productivity are poorly understood. We tested how well rainfall predicted the activity, species numbers and assemblage composition of ants and if responses were moderated by landscape position. We also asked whether the number of species (richness per sampling unit and estimated species richness) responded directly to rainfall or was moderated by ant activity or competition from dominant ants. Over a 22‐year period, when annual rainfall fluctuated between 79 mm and 570 mm, we sampled ants using pitfall traps in paired dune and swale habitats in the Simpson Desert, Australia. We used climate records over this period to model changes in ant assemblages. Activity of dominant ants responded primarily to long‐term rainfall, increasing exponentially, while subordinate ants responded to short‐term weather and time. Consistent with the MIH, the number of ant species was best predicted by activity, particularly that of subordinate ants. Activity of dominant ants had a declining positive effect on numbers of species. Landscape position strongly predicted species composition, while long‐term rainfall determined composition at genus level but not species level. Over time, species composition fluctuated, but several genera consistently increased in activity. Productivity moderators such as long‐term rainfall and landscape position are key drivers of ant activity and composition in the study ecosystem, acting indirectly on numbers of species. Numbers of species were explained largely by ant activity, making a strong case for the MIH, but not competition. Longer periods of low rainfall may indirectly reduce species richness in desert ecosystems. However, a trend to increasing richness over time may indicate that conservation management can ameliorate this impact.
Species extinctions alter ecosystem services, and the magnitude of this impact is likely to change across environmental gradients. In Australia, soil‐disturbing mammals that are now considered ecologically extinct are thought to be important ecosystem engineers. Previous studies have demonstrated microsite‐level impacts of reintroduced soil‐disturbing mammals on soil functions, but effects are yet to be tested across larger scales. Further, it is unclear how impacts vary across environmental gradients and if the restoration potential of reintroductions changes with climate. We examined the effects of soil‐disturbing mammal reintroductions across a large rainfall gradient in Australia to test the hypothesis that ecosystem engineering effects on soil function depend on climate. We compared soil labile carbon, available nitrogen and the activity of four enzymes associated with nutrient cycling in three microsite types with and without soil‐disturbing mammals in five sites along a large rainfall gradient (166–870 mm). Soil enzyme activity was greatest in the presence of soil‐disturbing mammals and increased with rainfall, but soil available carbon and nitrogen varied across the gradient and among microsites. Microsite effects were often stronger than any effects of soil‐disturbing mammals, with soil beneath vegetated patches (shrubs and trees) having greater enzyme activity, carbon and nitrogen than bare soils. However, soil‐disturbing mammals homogenised nutrient distributions across microsites. The impacts of soil‐disturbing mammals on soil function previously detected at micro‐scales was detected at a landscape‐scale. However, the overall effects of soil‐disturbing mammals on soil functions varied with productivity (rainfall). The context of soil‐disturbing mammal reintroductions is thus likely to be critical in determining their effectiveness in restoring soil function.
Species loss is often associated with a decline in ecosystem functions. Globally, digging mammals (or ecosystem engineers) are functionally important, altering soil processes at local scales. However, their effects on the process of decomposition are poorly understood, particularly at larger scales, where the environment may moderate the magnitude of effects. We tested the landscape‐scale effects of reintroducing ecologically extinct digging mammals on two aspects of nutrient cycling over a large environmental gradient in Australia, where many digging mammals became extinct or ecologically extinct following the arrival of Europeans. We measured the impacts of digging mammals on soil organic matter content and plant litter decomposition over a 3,000 km transect, where annual average rainfall varied between 166 and 877 mm. We set up paired study plots (n = 8–10) inside and outside five reintroduction reserves. We took soil samples to assess soil organic matter content and set up litter bags to measure plant litter decomposition over 4 and 12 months. We used macroinvertebrate exclusion and macroinvertebrate access treatments to determine the relative importance of macroinvertebrates in decomposition with and without digging mammals. Soil organic matter was greater in reintroduction areas, but the magnitude of the effect was driven by productivity (average annual rainfall as a proxy), with little effect of digging activity at the wettest sites. Short‐term plant matter decomposition was greater in the presence of digging mammals, and their effect was dependent on the amount of rain that fell during the study period. Long‐term litter decomposition increased with annual rainfall, independent of digging mammals. Unexpectedly, macroinvertebrate exclusion increased decomposition rates over 12 months. Reintroduction of digging mammals substantially alters soil processes and organic matter decomposition, but impacts are rainfall‐dependent. Restoring native digging mammals to their historical distribution is likely to reverse degradation of ecosystem processes, but the magnitude of this effect depends on the environment. A free Plain Language Summary can be found within the Supporting Information of this article.
Surface fuel loads are a key driver of forest fires and the target of hazard reduction burns to reduce fire risk. However, the role of biota in decomposition, or feedbacks between fire and decomposer communities are rarely considered. We review the evidence that decomposer organisms play an important role in surface fuel regulation and how this role is affected by fire. First, we outline the contribution of decomposer organisms to the breakdown of surface fuels. Next, we consider the three distinct phases through which fire regulates decomposer communities and how this may affect decomposition and future fire regimes. Finally, we consider interactions between global change and decomposer–fire feedbacks and the implications for fire management. Evidence indicates that decomposer organisms are important in regulating surface fuels and we propose that the biological basis and dynamic nature of fuel load control require greater attention. This includes better understanding of functional redundancy among decomposer organisms, the impacts of global change on the biota that drive decomposition and the factors that limit decomposer persistence and recolonisation following fires. By filling these knowledge gaps, we will be better armed to conserve and manage these functionally critical taxa in fire-prone ecosystems in a changing world.
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