Before-after Control-impact Experiment Research Articles

High site fidelity and reduced survival of a mycophagous mammal after prescribed fire

AbstractFire offers both opportunities and risks for wildlife. Its impact will depend on the fire’s scale, how it alters key resources and how animals move. Understanding how wildlife respond to fire is crucial as climate change is predicted to increase wildfire risk and will likely result in more frequent prescribed fire to reduce wildfire risk. Invasive predators and inappropriate fire regimes in south-eastern Australia threaten the long-nosed potoroo (Potorous tridactylus), a vulnerable marsupial often residing in areas frequently exposed to fire. The cumulative impacts of fire and predation may increase the threat to P. tridactylus after fire, as predators can be more effective in the immediate post-fire environment and P. tridactylus is often dependent on thick ground cover. We present a before-after control-impact experiment describing the influence of prescribed fire on P. tridactylus. We fitted GPS collars to 52 individuals at nine independent sites to test if exposure to prescribed burning reduced their survival or altered their movement behavior. Prescribed fire reduced P. tridactylus survival, yet range size and diffusion (movement) rate remained largely unaffected. With limited fire exposure, P. tridactylus tended to continue using burnt areas whereas activity became restricted to unburnt areas when larger proportions of their home range burnt. Site fidelity was very high - individuals rarely moved their home ranges after fire, regardless of fire exposure. Our results suggest recently burnt areas may be particularly dangerous for P. tridactylus: areas that can be attractive yet confer lower fitness outcomes. P. tridactylus may benefit from smaller fire scars, retention of structurally complex vegetation, and integrating invasive predator control with prescribed burning.

Oil and gas platforms degrade benthic invertebrate diversity and food web structure

Oil and gas exploitation introduces toxic contaminants such as hydrocarbons and heavy metals to the surrounding sediment, resulting in deleterious impacts on marine benthic communities. This study combines benthic monitoring data over a 30-year period in the North Sea with dietary information on >1400 taxa to quantify the effects of active oil and gas platforms on benthic food webs using a multiple before-after control-impact experiment. Contamination from oil and gas platforms caused declines in benthic food web complexity, community abundance, and biodiversity. Fewer trophic interactions and increased connectance indicated that the community became dominated by generalists adapting to alternative resources, leading to simpler but more connected food webs in contaminated environments. Decreased mean body mass, shorter food chains, and the dominance of small detritivores such as Capitella capitata near to structures suggested a disproportionate loss of larger organisms from higher trophic levels. These patterns were associated with concentrations of hydrocarbons and heavy metals that exceed OSPAR's guideline thresholds of sediment toxicity. This study provides new evidence to better quantify and manage the environmental consequences of oil and gas exploitation at sea.

Evaluating bottom-up forcing of a rocky intertidal resource harvest on a high trophic-level consumer group

The importance of ecosystem-based management (EBM) frameworks for resource harvest has increased over the past several decades as ecosystems face numerous anthropogenic stressors. In these frameworks, resource managers must consider the suite of interactions that comprise food webs and how resource harvest drives responses in non-target organisms. In rocky intertidal zones along North Atlantic coastlines, rockweed (Ascophyllum nodosum)—a canopy-forming brown seaweed—has been commercially harvested for centuries, yet most research on the effects of harvest have focused on the responses of the target resource and the macroinvertebrate assemblage. In this study, we used a Before-After Control-Impact experiment to assess the bottom-up effects of commercial rockweed harvest on a high trophic-level consumer group (birds) in Maine (USA). Overall, there was no evidence for strong bottom-up forcing of rockweed harvest on birds' site visitation. There was a small (fewer than two birds) positive effect size of harvest during the one-year post-harvest recovery interval. Several of the trends observed for the full bird assemblage appear to be driven more strongly by the low tide than the high tide bird assemblage. Independent of treatment, site visitation by birds was low in our study (60% of surveys recorded no birds) and highlights questions about the use of rocky intertidal habitats relative to other habitat types in coastal birds' home ranges. To our knowledge, there are no comparable assemblage-level bird studies in this system that can provide context to our results. Further research on coastal birds’ habitat use within home ranges and food-web connections with rockweed-associated macroinvertebrates is needed to more confidently incorporate this high trophic-level consumer group into an EBM framework for a rocky intertidal resource harvest.

Trajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal.

Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressure from local (land use/land cover change) to global (climate change) processes. Woody vegetation is expanding in grassland riparian zones worldwide. Here we report on a decade-long watershed-scale mechanical removal of woody riparian vegetation along 4.5 km of stream channel in a before-after control impact experiment. Prior to this removal, woody plants had expanded into grassy riparian areas, associated with a decline in streamflow, loss of grassy plant species, and other ecosystem-scale impacts. We confirmed some expected responses, including rapid increases in stream nutrients and sediments, disappearance of stream mosses, and decreased organic inputs to streams via riparian leaves. We were surprised that nutrient and sediment increases were transient for 3 years, that there was no recovery of stream discharge, and that areas with woody removal did not shift back to a grassland state, even when reseeded with grassland species. Rapid expansion of shrubs (Cornus drummondii, Prunus americana) in the areas where trees were removed allowed woody vegetation to remain dominant despite repeating the cutting every 2 years. Our results suggest woody expansion can fundamentally alter terrestrial and aquatic habitat connections in grasslands, resulting in inexorable movement toward a new ecosystem state. Human pressures, such as climate change, atmospheric CO2 increases, and elevated atmospheric nitrogen deposition, could continue to push the ecosystem along a trajectory that is difficult to change. Our results suggest that predicting relationships between riparian zones and the streams they border could be difficult in the face of global change in all biomes, even in well-studied sites.

Short-term experimental support for bird diversity retention measures during thinning in European boreal forests

Current levels of forestry expansion in boreal forests threaten to radically reduce biodiversity. For many forest-dwelling species the threat is not simply because the forest is used for biomass production, but rather how it is used. Retention forestry practices aim to limit impacts on biodiversity during the final felling stage of a forestry cycle. However, the efficiency of such methods in retaining biodiversity have rarely been studied for intermediate forestry stages (e.g., forest thinning) with experimental approaches entirely absent from boreal forests. Therefore, we conducted a before-after control-impact experiment in Sweden to investigate the short-term response in occurrence of individual bird species, guilds, and population trend groups (positive, stable, or negative population trend), as well as the response in nest box occupancy and in the reproductive success of Parus major to three different thinning treatments. The three treatments were i) conventional thinning at the plot and stand scale, (ii) understory retention thinning, where at least 250 spruces with live branches below 2 m above ground are retained per hectare, and (iii) complete retention plots, where the forest was not thinned on ∼1 ha plots within conventionally thinned forest stands. We found that conventional forest thinning was likely the cause for observed declines in bird occurrence, with ∼20% of the species showing clear negative responses. Our results indicate that understory retention thinning with double the retention level as previously suggested and combining conventional thinning with complete retention plots, could largely alleviate the short term negative effects of conventional thinning. This was the case for all guilds, population trend groups and individual bird species, except for Poecile montanus, which responded negatively to complete retention. Among the birds occupying the nest boxes, only Cyanistes caeruleus responded to any of the treatments, with higher nest box occupancy upon understory retention thinning. No thinning treatment affected reproductive success in P. major. Our results clearly show that thinning practices that retain the diversity of forest-dwelling birds are possible. Hence, this study provides a crucial puzzle piece towards more sustainable forestry practices in the boreal region.

Nest boxes do not cause a shift in bat community composition in an urbanised landscape

Nest boxes are often used to provide supplementary roosts for cavity-dependent wildlife, but little is known about if they influence faunal community composition. Long-term monitoring of bat boxes in south-eastern Australia indicated that their use was dominated by one generalist species (Chalinolobus gouldii), causing concern that installing bat boxes could cause a shift toward less diverse bat communities. To test this, we conducted a large-scale before-after control-impact experiment at 18 sites, over five years. Sites were either: (1) those with existing bat boxes, (2) those where boxes were added during the study, or (3) controls without boxes. We used echolocation call data from 9035 bat detector nights to compare community composition, diversity, and species’ relative activity between the sites. Chalinolobus gouldii continued to dominate the use of existing boxes, but we found little difference in community composition between sites based on the presence, absence, or addition of boxes. Our study is the first to explore the influence installing artificial hollows has on localized faunal assemblages over spatio-temporal scales relevant to management. We conclude that there is cause for optimism that bat boxes might not have perverse outcomes on local community composition in the short- to medium-term, as we had feared.

Experimental evidence indicates variable responses to forest disturbance and thermal refugia by two plethodontid salamanders

Understanding how natural and anthropogenic disturbances affect sensitive species is critical to support conservation programs as land use activities and climatic conditions change. Forest management provides a range of ecosystem services to expanding human populations including production of substantial amounts of woody biomass, thereby reducing area under management and increasing opportunity for conservation designations. However, relatively little experimental information is available to describe how manipulating habitat at multiple scales may affect spatial and temporal variation in populations of climate-sensitive forest taxa. We conducted a Before-After Control-Impact experiment over seven years to evaluate responses of two species of Plethodontid salamanders in operational treatment (harvest, replanting, and herbicide application) and control units, Oregon, USA. In addition, we capitalized on existing variation in downed wood size and accounted for potential behavioral responses to evaluate the thermal refugia hypothesis. For Oregon slender salamanders (Batrachoseps wrighti), mean plot-level occupancy (90% CRI) was increased an estimated 11% (1.11; 0.62–2.1) and estimated mean abundance reduced 16% (0.84; 0.54–1.4) post-harvest. For Ensatina (Ensatina eschscholtzii), mean occupancy was reduced an estimated 80% (0.20; 0.08–0.47) and estimated mean abundance reduced 63% (0.37; 0.22–0.65) post-harvest. We found strong evidence of positive associations for both species with downed wood amount. As predicted, we found ambient temperatures to be higher in harvest than control units. Also, internal downed wood temperatures were warmer in harvested units, suggesting these structures may not have provided adequate thermal refugia. Application of an experimental framework provided evidence about both stand and structure level responses to active management. Habitat structures did not appear to buffer against substantial variation in local climate but evidence was equivocal about importance of these structures to population retention. Monitoring of recruitment and retention of downed wood through subsequent rotations can provide critical information to reduce trade-offs between wood production and conservation of climate sensitive taxa.

Restoration measures emulating natural disturbances alter beetle assemblages in boreal forest

Accelerating declines in biodiversity worldwide have accentuated the need for conservation actions. Unfortunately, the decline is unlikely to be reversed by traditional conservation alone, e.g., green tree retention at clear-felling and setting aside protected areas for free development. Instead the practice of ecological restoration has come to play an ever increasing role. Using a before-after control-impact experiment in boreal forest voluntary set-asides, we evaluated the usefulness of two methods of ecological restoration aimed at promoting biodiversity by emulating natural disturbances through restoration burning and artificial gap creation involving dead wood creation. In burned stands (n = 6) we removed up to 35% of the standing volume prior to burning, and in gap cut stands (n = 10) we removed the cut trees from every second gap; harvested trees covered costs for restoration. We used saproxylic and non-saproxylic beetles as a proxy for biodiversity to evaluate the two restoration methods. We compared species compositions of beetles collected with window traps one year before and one year after treatment. In addition, we compared catches in the treatments with those in untreated reference stands. Before treatment, we found no differences between the three groups of stands in terms of overall abundance, species richness or species composition. After treatment, the overall abundance of beetles was higher in the burned sites compared to the reference stands. Overall species richness was higher in burned stands compared to references and gap-cut stands. Gap-cut stands had higher species richness of cambivores and fire favoured species than reference stands. Species composition differed significantly between all three groups of stands. Among the 96 species that significantly contributed to the differences in species composition, 58 were more common in burned stands and 34 more common in gap-cut stands than in the other two stand groups. Nineteen of the 96 species are considered to be favored by forest fires, and 17 of these fire-favored species were more abundant in burned stands than gap-cut stands and/or references. Based on our results that burning and gap-cutting changed the beetle communities in partly different directions, we propose the use of both methods as complements to traditional conservation efforts in future attempts to improve conditions for biodiversity in managed boreal forest landscapes.

Greenhouse gas emissions from boreal inland waters unchanged after forest harvesting

Abstract. Forestry practices often result in an increased export of carbon and nitrogen to downstream aquatic systems. Although these losses affect the greenhouse gas (GHG) budget of managed forests, it is unknown if they modify GHG emissions of recipient aquatic systems. To assess this question, air–water fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were quantified for humic lakes and their inlet streams in four boreal catchments using a before-after control-impact experiment. Two catchments were treated with forest clear-cuts followed by site preparation (18 % and 44 % of the catchment area). GHG fluxes and hydrological and physicochemical water characteristics were measured at multiple locations in lakes and streams at high temporal resolution throughout the summer season over a 4-year period. Both lakes and streams evaded all GHGs. The treatment did not significantly change GHG fluxes in streams or lakes within 3 years after the treatment, despite significant increases of CO2 and CH4 concentrations in hillslope groundwater. Our results highlight that GHGs leaching from forest clear-cuts may be buffered in the riparian zone–stream continuum, likely acting as effective biogeochemical processors and wind shelters to prevent additional GHG evasion via downstream inland waters. These findings are representative of low productive forests located in relatively flat landscapes where forestry practices cause only a limited initial impact on catchment hydrology and biogeochemistry.

Life in the fast lane: learning from the rare multi-year recaptures of brown lemmings in the High Arctic

Inter-annual recaptures of Arctic lemmings are extremely rare because their life expectancy is very short, typically less than one year. On Bylot Island, Nunavut, Canada, we live-trapped in summer, marked and released brown lemmings (Lemmus trimucronatus, Kerr 1792) between 2004 and 2016 and we performed a large-scale, before-after control-impact experiment from 2014 to 2016 to study the effects of predator reduction on their population dynamics. Although inter-annual recaptures of marked lemmings were rare, our long-term study and predator reduction allowed us to capture 21 (1.4%; n = 1523) individuals over two consecutive years and one over three consecutive years. The inter-annual recapture rate was much higher in the predator-reduction grid (5.7%; n = 193) than in the other grids (0.7%; n = 425) during the experiment. Average distance moved between inter-annual recaptures was small (74 m). Our data thus demonstrate that lemmings are physiologically capable of living up to 24 months in the High Arctic, that predation is a major factor affecting lemming survival, including over winter, and that they show high site fidelity among years.

Cautions on using the Before-After-Control-Impact design in environmental effects monitoring programs

Often the Before-After-Control-Impact (BACI) design is suggested as being a statistically powerful experimental design in environmental impact studies. If the timing and location of the impact are known and adequate pre-data are collected, the BACI design is considered optimal to help isolate the effect of the development from natural variability. This paper presents 9 years of results from a long-term BACI experiment tested using a range of statistical models and post-impact monitoring designs. To explore suboptimal designs that are often utilized in environmental effects monitoring, the same data were also explored assuming either no control system was available (Before-After only), or that no pre-impact data were available (Control-Impact only). The results of the BACI design were robust to the statistical model used, and the BACI design was able to detect effects from the impact that the two suboptimal designs failed to detect. However, the BACI design demonstrated different conclusions depending on the number and configuration of post-impact years included in the analysis. Our results reinforce the idea that caution should be employed when using, or interpreting results from, a BACI design in an environmental impact study, but demonstrate that a well-designed BACI remains one of the best models for environmental effects monitoring programs.

Nestling food of three hole-nesting passerine species and experimental increase in their densities in Mediterranean oak woodlands

We studied nestling food of three species breeding in Mediterranean oak woodlands that collect food in different niches of trees: blue tit (Cyanistes caeruleus; foliage-gleaner), great tit (Parus major; bark–foliage-gleaner) and nuthatch (Sitta europaea; bark-gleaner) in 2013–2014. Species-specific functions were compared using size and composition of preyed caterpillars, and nest-boxes were used in a before-after control-impact (BACI) design to test increase in breeding densities by providing nest-boxes. Our results demonstrate a high importance of caterpillars in the nestling diet of the three passerine species and suggest their complementary predation on early and late instars of the same Lepidoptera species. According to results of our BACI experiment, species’ breeding density increased by providing nest-boxes, with blue tit showing the highest difference in percentage change between manipulated and control plots (38.2, 26.8 and 14.3 % for blue tit, great tit and nuthatch, respectively). Overall, we highlight the combined functions of different tree-foraging guilds in caterpillar predation and propose nest-box provision as a management method to prevent defoliator outbreaks in Mediterranean oak woodlands.

Randomized intervention analysis for detecting non-random change and management impact: Dragonfly examples

The quasi-experimental approach of before–after control–impact (BACI) sampling can help decide when changes are due to human activities rather than natural variability. Detailed arguments for and against BACI designs and analytic methods are widespread in the literature, but far less attention has been paid to the mechanics of analyzing a BACI experiment. This paper demonstrates randomized intervention analysis with user-friendly software, where observations are paired in time before and after intervention. We provide examples using dragonfly count data in vegetation removal experiments.

Impact of habitat management on grey partridge populations: assessing wildlife cover using a multisite BACI experiment

Summary The grey partridge is a species of conservation concern, in common with many farmland birds. Its widespread decline in western Europe has been attributed to agricultural intensification. Attempts to restore populations have concentrated upon habitat management. In France, a mosaic of strips planted with maize‐ or kale‐based mixtures is widely used to benefit the grey partridge on intensively cultivated farmlands. The rationale for planting these summer‐to‐winter cover strips is to increase the overwinter survival rate and hence breeding density. Although this policy is costly to apply, there is little information on its effectiveness. We tested effectiveness of this management scheme as a practical way of improving or restoring partridge populations. We present data from a 6‐year before–after control–impact (BACI) experiment replicated across four study sites. We monitored partridge populations to estimate breeding density, reproductive success and overwinter mortality. During the course of the study, no partridge population increase occurred on managed areas compared with control areas. Wildlife cover strips did not improve reproductive success but were associated with higher overwinter ‘apparent mortality’ rates. Some field data suggested that there was a predation risk at cover strip–field edges. Cover strips may concentrate a number of species within small isolated areas and may act, under some circumstances, as an ecological trap for prey species such as the grey partridge. Due to these complex and unforeseen interactions, this habitat management measure proved unsuitable for partridge restoration. Synthesis and applications. This study has demonstrated the necessity of field experiments at a farm‐scale to test the effectiveness of habitat management schemes. A priori assumptions based on smaller scale studies, even if they are supported by some field evidence, can be misleading because they fail to capture the emergent properties of larger scale systems. This study is a specific illustration of how the BACI approach is a powerful tool for addressing wildlife management problems at large spatial scales.

Short‐term effects of canopy openness on insect herbivores in a rain forest in Guyana

Summary A before/after–control/impact experiment (BACI) was used to examine the short‐term effects of canopy openness on insect herbivores foraging on the seedlings of five rain forest species in an unlogged forest in Guyana. During the first and second study years, insects were collected monthly from 250 sites, representing 9750 seedlings spread over a plot of 1 km2. At the onset of the second collecting year, half of the parent trees at the sites were felled, creating an average canopy openness of 26%. Comparisons of the control and impact sites before and after the felling showed that overall insect abundance, particularly of psyllids, and species richness increased at the impact sites, whereas evenness decreased markedly. These responses were consistent with the prediction that intermediate disturbance promotes high diversity in tropical systems. Twenty‐nine per cent of insect species analysed showed a significant response to felling, with some populations doubling or halving at the impact sites after felling. The abundance of most species increased, although responses varied even between congeners. A canonical correspondence analysis at the impact sites after felling indicated that most species responded to an increase in leaf production by seedlings, not to canopy openness per se. Species declining at the impact sites were more likely to feed indiscriminately on young and mature foliage and probably reacted to changes in microclimate. The study illustrates the difficulty in identifying higher taxa, umbrella or indicator species that adequately integrate the responses of insect herbivores to forest disturbance. Because the integrity of insect communities in the understorey of this forest depends more on minimizing damage to plant resources than on canopy openness, we found some support for the conservation value of reduced impact logging in tropical wet forests.