Showing papers by "Ralph Charles Mac Nally published in 2018"

Second rate or a second chance? Assessing biomass and biodiversity recovery in regenerating Amazonian forests.

Abstract: © 2018 The Authors Global Change Biology Published by John Wiley & Sons Ltd Secondary forests (SFs) regenerating on previously deforested land account for large, expanding areas of tropical forest cover Given that tropical forests rank among Earth’s most important reservoirs of carbon and biodiversity, SFs play an increasingly pivotal role in the carbon cycle and as potential habitat for forest biota Nevertheless, their capacity to regain the biotic attributes of undisturbed primary forests (UPFs) remains poorly understood Here, we provide a comprehensive assessment of SF recovery, using extensive tropical biodiversity, biomass, and environmental datasets These data, collected in 59 naturally regenerating SFs and 30 co-located UPFs in the eastern Amazon, cover >1,600 large- and small-stemmed plant, bird, and dung beetles species and a suite of forest structure, landscape context, and topoedaphic predictors After up to 40 years of regeneration, the SFs we surveyed showed a high degree of biodiversity resilience, recovering, on average among taxa, 88% and 85% mean UPF species richness and composition, respectively Across the first 20 years of succession, the period for which we have accurate SF age data, biomass recovered at 12% per year, equivalent to a carbon uptake rate of 225 Mg/ha per year, while, on average, species richness and composition recovered at 26% and 23% per year, respectively For all taxonomic groups, biomass was strongly associated with SF species distributions However, other variables describing habitat complexity—canopy cover and understory stem density—were equally important occurrence predictors for most taxa Species responses to biomass revealed a successional transition at approximately 75 Mg/ha, marking the influx of high-conservation-value forest species Overall, our results show that naturally regenerating SFs can accumulate substantial amounts of carbon and support many forest species However, given that the surveyed SFs failed to return to a typical UPF state, SFs are not substitutes for UPFs

Carbon-focused conservation may fail to protect the most biodiverse tropical forests

Abstract: As one of Earth’s most carbon-dense regions, tropical forests are central to climate change mitigation efforts. Their unparalleled species richness also makes them vital for safeguarding biodiversity. However, because research has not been conducted at management-relevant scales and has often not accounted for forest disturbance, the biodiversity implications of carbon conservation strategies remain poorly understood. We investigated tropical carbon–biodiversity relationships and trade-offs along a forest-disturbance gradient, using detailed and extensive carbon and biodiversity datasets. Biodiversity was positively associated with carbon in secondary and highly disturbed primary forests. Positive carbon–biodiversity relationships dissipated at around 100 MgC ha–1, meaning that in less disturbed forests more carbon did not equal more biodiversity. Simulated carbon conservation schemes therefore failed to protect many species in the most species-rich forests. These biodiversity shortfalls were sensitive to opportunity costs and could be decreased for small carbon penalties. To ensure that the most ecologically valuable forests are protected, biodiversity needs to be incorporated into carbon conservation planning.

Is environmental legislation conserving tropical stream faunas? A large-scale assessment of local, riparian and catchment-scale influences on Amazonian fish.

Abstract: 1. Agricultural expansion and intensification are major threats to tropical biodiversity. In addition to the direct removal of native vegetation, agricultural expansion often elicits other human-induced disturbances, many of which are poorly addressed by existing environmental legislation and conservation programmes. This is particularly true for tropical freshwater systems, where there is considerable uncertainty about whether a legislative focus on protecting riparian vegetation is sufficient to conserve stream fauna. 2. To assess the extent to which stream fish are being effectively conserved in agricultural landscapes, we examined the spatial distribution of assemblages in river basins to identify the relative importance of human impacts at instream, riparian and catchment scales, in shaping observed patterns. We used an extensive dataset on the ecological condition of 83 low-order streams distributed in three river basins in the eastern Brazilian Amazon. 3. We collected and identified 24,420 individual fish from 134 species. Multiplicative diversity partitioning revealed high levels of compositional dissimilarity (DS) among stream sites (DS = 0.74 to 0.83) and river basins (DS = 0.82), due mainly to turnover (77.8% to 81.8%) rather than nestedness. The highly heterogeneous fish faunas in small Amazonian streams underscore the vital importance of enacting measures to protect forests on private lands outside of public protected areas. 4. Instream habitat features explained more variability in fish assemblages (15%-19%) than riparian (2%-12%), catchment (4%-13%) or natural covariates (4%-11%). Although grouping species into functional guilds allowed us to explain up to 31% of their abundance (i.e. for nektonic herbivores), individual riparian - and catchment- scale predictor variables that are commonly a focus of environmental legislation explained very little of the observed variation (partial R-2 values mostly 5. Policy implications. Current rates of agricultural intensification and mechanization in tropical landscapes are unprecedented, yet the existing legislative frameworks focusing on protecting riparian vegetation seem insufficient to conserve stream environments and their fish assemblages. To safeguard the species-rich freshwater biota of small Amazonian streams, conservation actions must shift towards managing whole basins and drainage networks, as well as agricultural practices in already-cleared land.

Legacies, lags and long-term trends: Effective flow restoration in a changed and changing world

Abstract: Human impacts on natural ecosystems are pervasive and will play out more severely as human populations and per capita resource use increase. Freshwater ecosystems are critical for human well-being and experience a diverse range of human-induced pressures. Most river systems throughout the world have much-altered flow regimes. The Murray–Darling Basin in southeastern Australian has been the focus of an extensive water reform process to address the over-allocation of water for human uses. This has included many scientific investigations, hydrological modelling and the development of institutional and market structures to reallocate water. Substantial recovery of water has been achieved, which has been used to restore aspects of the natural flow regime. We reviewed recent papers on responses to flow restoration in the Murray–Darling Basin and complemented this with inferences from the global literature. Ecological responses to flow restoration are often inconsistent, site and taxon specific and difficult to detect. By combining ideas from mainstream thinking in restoration ecology with the insights from our review, we propose a conceptual model for understanding responses to flow restoration. This model incorporates key factors that influence the size of ecological responses to restoration, including existing ecological condition, legacy impacts of past change, interactions with other variables, life-history traits of taxa and broad-scale and long-term trends due to climate or land-use change.

Seeing the woods through the saplings: Using wood density to assess the recovery of human-modified Amazonian forests

Abstract: Most of the world's remaining tropical forests have been affected by either selective logging, understorey fires, fragmentation or are regrowing in areas that were previously deforested. Despite the ubiquity of these human‐modified forests, we have a limited knowledge of their potential to recover key traits linked to ecosystem processes and consequent services. Here we present data from 31,095 trees and saplings distributed across 121 plots of undisturbed and disturbed primary forests as well as secondary forests in the eastern Amazon. We examined the post‐disturbance recovery trajectory of an important plant functional trait, wood density. We tested whether human‐modified Amazonian forests are experiencing a rapid or a slow, or even impeded, recovery of this trait, which is associated with the provision of a fundamental ecosystem service—carbon storage. As expected, we found that the plot‐level wood density of trees and saplings in disturbed primary and secondary forests was significantly lower than in undisturbed forests. However, there was no significant difference in the average wood density of saplings between disturbed primary and secondary forests, possibly indicating a process of secondarization. We also found evidence that the recovery of wood density in human‐modified forests is being severely disrupted due to edge effects (in the case of disturbed primary forests) and high liana densities (in the case of both disturbed primary and secondary forests). Surprisingly, these two factors were more important predictors of wood density recovery than the time elapsed since the disturbance event. Synthesis. Plant communities in human‐modified Amazonian forests appear to not be recovering a key functional property—wood density, which in turn may affect their ability to store carbon in the future. If the aim of conservation programs in tropical forests is to maintain existing rates of ecosystem functions, processes and services, then they must concentrate efforts on avoiding anthropogenic disturbance in areas of currently undisturbed forests. It is also vital to prevent further disturbance in human‐modified forests to avoid disrupting even more their recovery.

Interactions among stressors may be weak: Implications for management of freshwater macroinvertebrate communities

Abstract: Aim: Ecological models that do not account for interactions among stressors, if interactions are important, could be inaccurate and lead to inefficient conservation strategies. Conversely, if interactions are not important (i.e., stressors operate largely independently), then actions concentrating on a stressor-by- stressor basis would be warranted. Here, we investigated whether interactions among multiple stressors affected widely used indices of freshwater macroinvertebrate biodiversity, which are sensitive to environmental change at management-relevant scales (i.e., reaches and catchments). Location: State of Victoria, south-eastern Australia. Methods: We used a 7,418-sample dataset for stream macroinvertebrates from 2,165 sites distributed over 237,630 km2 for 20 years. We calculated the interactive effects on stream macroinvertebrates of stressors operating at different scales, namely vegetation loss at the catchment and reach scales and hydrological change and salinization at the local scale. The importance of interactions among multiple stressors was assessed by comparing the cross-validated predictive performance of models with and without multiple stressor interaction terms. Results: Cross-validated models explained 31%–63% of the variation in the macroinvertebrate responses. The most important stressors were catchment vegetation loss (the proportion of remaining native vegetation cover) and salinity. The inclusion of interaction terms did not increase cross-validated predictive performance, which indicates that there was little evidence that interactions among stressors were important for explaining variation in commonly used freshwater macroinvertebrate condition indices. Main conclusions: Interactions among vegetation, salinity and hydrological change stressors may not always be of importance for determining patterns of stream macroinvertebrate biodiversity, so that such interactions may not necessarily be critical considerations for catchment and reach scale management, at least if based on these or comparable condition indices. The mitigation of the impacts of vegetation loss, salinization and hydrological change stressors one-by- one probably is sufficient to guide conservation activities and might be advantageous if socio-political contexts make it difficult to address interactions among stressors.

Assessment of ecosystems: A system for rigorous and rapid mapping of floodplain forest condition for Australia's most important river

Abstract: Methods that provide rapid assessments of changing ecosystems at multiple scales are needed to inform management to address undesirable change. We developed a remote-sensing method in partnership with, and for use by, natural resource managers to predict annually stand condition of floodplain forests along Australia's longest river, the Murray River. A measure of stand condition, which was developed in collaboration with responsible natural resource managers, is a function of plant area index, crown extent, and the percentage live basal area. We surveyed a broad range of spatial and temporal variation in condition, built predictive stand-condition models using satellite-derived variables, and validated predictions with surveys of new sites. A multi-year model using data from two drought years and a year following extensive floods provided better predictions of stand condition than did models based on data for individual years. The model provided good predictions for data collected after the build for 50 sites and for resurveys of build sites in later years (R2 ≥ 0.86). There was limited, temporary improvement in stand condition after the extensive flooding (2010–late 2010) that followed a 13-year (1997–early 2010) drought. Forest condition can be mapped accurately and annually at medium resolution (25 × 25 m) for large areas (100,000s ha) if quantitative ground surveys, satellite imagery, machine learning and future validation are combined. Regular assessments of forest condition can be related to likely causes of change by using regular, rapid assessments, and hence can provide important management information.

High-productivity vegetation is important for lessening bird declines during prolonged drought

Abstract: Locations in which ecological assemblages show high resistance to climate pressures, such as drought, are likely to be important refuges for biota in changing climates. We asked whether environmental characteristics of locations were associated with the capacity of bird assemblages to withstand prolonged drought. We used a multispecies index to quantify trends in bird assemblages during a 13‐year drought at >500 locations (>18,000 surveys) in the Murray–Darling Basin, south‐eastern Australia, using data from the Atlas of Australian Birds. We investigated whether the resistance of bird assemblages was associated with (1) vegetation structure; (2) vegetation productivity (vegetation greenness); (3) landscape context (patch size, landscape vegetation cover); or (4) physical environment (elevation, terrain, topography, availability of surface water). Vegetation productivity, measured by vegetation greenness (Normalized Difference Vegetation Index), was the only potential predictor with strong evidence of an effect, and was positively associated with the index of drought resistance. There was little evidence that variables characterizing landscape context, vegetation structure or the physical environment of sites were associated with drought resistance of bird communities. Synthesis and applications. Bird assemblages in locations with high vegetation greenness are more resistant to severe drought. Prioritizing conservation investments in areas with locally high vegetation productivity is likely to be an effective strategy for increasing the resistance of bird assemblages to extreme drought, especially in areas where mean productivity is relatively low, such as arid and semi‐arid regions. Remotely sensed vegetation greenness may be a promising source of information for identifying drought refuges for birds and possibly other biota.

Regional patterns of nectar availability in subtropical eastern Australia

Abstract: There are few detailed data for short-term (≤ monthly) fluctuations in flowering and nectar availability at relatively large spatial scales. Such information is critical for understanding the governors of variation in flowering and for the management of floral resources assisting the persistence of nectar consumers in landscapes. To obtain monthly measurements of patterns of nectar availability in a 314,400 ha region, and to relate these patterns to potential environmental predictors. Flowering was measured at 83 sites in natural vegetation and in eight domestic gardens in subtropical, eastern Australia. A nectar-availability index was developed was based on nectarivore visitation rates and plant-specific flowering patterns. Spatial–temporal patterns were related to environmental variables using boosted regression trees. The large between-year variation was due mostly to irregular flowering by several eucalypt species. There was a ‘lean season’ in the austral spring (August–September). Coastal vegetation was an important source of nectar for much of the year, including the lean season. Gardens produced prolific nectar throughout the year, peaking in August–October. Nectar availability was most closely associated with primary productivity over the previous 12 months, average annual solar radiation, topographic wetness, and rainfall over the previous 6 months, although some relationships seemed counter-intuitive. There were large differences in nectar availabilities among broad vegetation types (especially rainforests vs. sclerophyllous forests), which partially accounted for the unintuitive results.

Catchment land use predicts benthic vegetation in small estuaries

Abstract: Many estuaries are becoming increasingly eutrophic from human activities within their catchments Nutrient loads often are used to assess risk of eutrophication to estuaries, but such data are expensive and time consuming to obtain We compared the percent of fertilized land within a catchment, dissolved inorganic nitrogen loads, catchment to estuary area ratio and flushing time as predictors of the proportion of macroalgae to total vegetation within 14 estuaries in south-eastern Australia The percent of fertilized land within the catchment was the best predictor of the proportion of macroalgae within the estuaries studied There was a transition to a dominance of macroalgae once the proportion of fertilized land in the catchment exceeded 24%, highlighting the sensitivity of estuaries to catchment land use

Linking species richness and size diversity in birds and fishes

Abstract: The rapid development of mechanistic, trait-based models has resulted in increasingly reliable predictions of the functional diversity of individuals in populations and communities. However, a focus on individuals’ traits differs from the prevailing focus on species in much of community ecology. We sought to identify correlative links between species richness and size diversity, focusing on size diversity as one component of functional diversity. These links could be used to extend individual, size-based models to predict patterns of species richness. We used the distribution of the sizes of individuals in a community – the individual–size distribution (ISD) – as a measure of size diversity, and constructed Bayesian regression models with species richness as the response variable and ISDs as the predictor variables. We used two methods to include ISDs in our analyses. First, we summarized the ISD with five common diversity indices and used these indices as predictor variables in our analyses. Second, we used functional data analysis to include the entire ISD (a continuous function) as a predictor variable in our analyses. Analyses of diversity indices identified consistent, positive associations between species richness and size diversity. Analyses of entire ISDs revealed that these associations were driven by numbers of small- and medium-sized individuals. In general, a combination of diversity indices predicted species richness as well as or better than continuous ISDs. However, models with ISDs as predictor variables were less sensitive to technical details of model fitting (e.g. discretization method) than those based on diversity indices, and the use of ISDs avoids the arbitrary selection of one or several diversity indices. Our use of functional data analysis allows any trait distribution to be included as a variable in statistical analyses, and has the potential to reveal new diversity patterns in ecology.