Edge effects in fire-prone landscapes: Ecological importance and implications for fauna

Contrasting responses of native and introduced mammal communities to fire mosaics in a modified landscape

Planned fire is increasingly recognized as an important tool in conservation, but other factors such as land-use change may hinder the ability of land managers to use fire for the benefit of biodiversity. The mosaic of past fires in native vegetation may interact with the mosaic of other land-cover types in human-modified landscapes, yet the effects of these interactions on mammal communities are unknown. We investigated the responses of ground-dwelling mammal community composition and species richness to interactions between land cover and post-fire vegetation growth-stage mosaics in southern Australia. This fire-prone, human-modified landscape features a fine-scale fire mosaic in native vegetation patches surrounded by pasture, horticulture, and peri-urban environments. We measured the composition of land-cover types and fire mosaics (landscape structure) at multiple scales of up to 1257 ha surrounding 129 study sites, and considered native and introduced species together and separately. Land-cover composition was the primary driver of community composition: native species favored areas with a greater proportion of native heathy woodland, whereas introduced species were associated with landscapes comprising more cleared land. The fire mosaic also influenced community composition and species richness: greater growth-stage diversity was associated with native habitat-specialist communities and fewer introduced species. In areas with more cleared land, native species richness increased when there was a greater proportion of mid-successional vegetation, demonstrating that the effect of fire mosaics on mammal diversity depended on land-cover composition. The positive relationship between introduced species richness and cleared land extent was also stronger in recently burned sites than in other growth stages, suggesting that introduced species are well suited to more modified areas of the landscape. Land managers need to consider the underlying land-cover composition and the potential interactions it may have with fire mosaics and species composition. In this landscape a greater diversity of growth stages may disadvantage introduced species yet an increase in mid-successional vegetation in more modified areas would be likely to benefit native mammal communities. Our study highlights that fire management may need to be tailored depending on the context of land use and the species of interest.

Fire as a driver and mediator of predator-prey interactions

Both fire and predators have strong influences on the population dynamics and behaviour of animals, and the effects of predators may either be strengthened or weakened by fire. However, knowledge of how fire drives or mediates predator–prey interactions is fragmented and has not been synthesised. Here, we review and synthesise knowledge of how fire influences predator and prey behaviour and interactions. We develop a conceptual model based on predator–prey theory and empirical examples to address four key questions: (i) how and why do predators respond to fire; (ii) how and why does prey vulnerability change post‐fire; (iii) what mechanisms do prey use to reduce predation risk post‐fire; and (iv) what are the outcomes of predator–fire interactions for prey populations? We then discuss these findings in the context of wildlife conservation and ecosystem management before outlining priorities for future research. Fire‐induced changes in vegetation structure, resource availability, and animal behaviour influence predator–prey encounter rates, the amount of time prey are vulnerable during an encounter, and the conditional probability of prey death given an encounter. How a predator responds to fire depends on fire characteristics (e.g. season, severity), their hunting behaviour (ambush or pursuit predator), movement behaviour, territoriality, and intra‐guild dynamics. Prey species that rely on habitat structure for avoiding predation often experience increased predation rates and lower survival in recently burnt areas. By contrast, some prey species benefit from the opening up of habitat after fire because it makes it easier to detect predators and to modify their behaviour appropriately. Reduced prey body condition after fire can increase predation risk either through impaired ability to escape predators, or increased need to forage in risky areas due to being energetically stressed. To reduce risk of predation in the post‐fire environment, prey may change their habitat use, increase sheltering behaviour, change their movement behaviour, or use camouflage through cryptic colouring and background matching. Field experiments and population viability modelling show instances where fire either amplifies or does not amplify the impacts of predators on prey populations, and vice versa. In some instances, intense and sustained post‐fire predation may lead to local extinctions of prey populations. Human disruption of fire regimes is impacting faunal communities, with consequences for predator and prey behaviour and population dynamics. Key areas for future research include: capturing data continuously before, during and after fires; teasing out the relative importance of changes in visibility and shelter availability in different contexts; documenting changes in acoustic and olfactory cues for both predators and prey; addressing taxonomic and geographic biases in the literature; and predicting and testing how changes in fire‐regime characteristics reshape predator–prey interactions. Understanding and managing the consequences for predator–prey communities will be critical for effective ecosystem management and species conservation in this era of global change.

Habitat selection by a predator of rodent pests is resilient to wildfire in a vineyard agroecosystem

Conservation of uncultivated habitats can increase the potential for ecosystem services in agroecosystems, but these lands are also susceptible to wildfires in the arid western United States. In Napa Valley, California, abundant rodent pests and an interest in integrated pest management have led wine producers to use nest boxes to attract Barn Owls (Tyto furcata) to winegrape vineyards. The viability of this practice as a method to control rodent pests depends heavily on the amount of hunting effort that Barn Owls expend in vineyards, which is known to be influenced by the amount of uncultivated land cover types surrounding the nest box. Wildfires burned nearly 60,000 ha of mainly urban and uncultivated lands surrounding Napa Valley in 2017, altering Barn Owl habitats. We compared GPS tracking data from 32 Barn Owls nesting in 24 individual nest boxes before and after the fires to analyze their hunting habitat selection. Owls with burned areas available to them after the fires had home ranges that shifted toward the fires, but selection was not strongly associated with burned areas. Though there was some spatial use of burned areas, selection of land cover types was similar for birds before and after the fires and in burned and unburned areas. The strongest selection was for areas closest to the nest box, and most recorded locations were in grassland, though selection indicated that owls used land cover types in proportion to their availability. Overall, habitat selection was resilient to changes caused by wildfires. These results are important for farmers who use nest boxes as a means of rodent control, which may be affected after dramatic disturbance events, especially as wildfires increase in the western United States.

How fire interacts with habitat loss and fragmentation

Biodiversity faces many threats and these can interact to produce outcomes that may not be predicted by considering their effects in isolation. Habitat loss and fragmentation (hereafter 'fragmentation') and altered fire regimes are important threats to biodiversity, but their interactions have not been systematically evaluated across the globe. In this comprehensive synthesis, including 162 papers which provided 274 cases, we offer a framework for understanding how fire interacts with fragmentation. Fire and fragmentation interact in three main ways: (i) fire influences fragmentation (59% of 274 cases), where fire either destroys and fragments habitat or creates and connects habitat; (ii) fragmentation influences fire (25% of cases) where, after habitat is reduced in area and fragmented, fire in the landscape is subsequently altered because people suppress or ignite fires, or there is increased edge flammability or increased obstruction to fire spread; and (iii) where the two do not influence each other, but fire interacts with fragmentation to affect responses like species richness, abundance and extinction risk (16% of cases). Where fire and fragmentation do influence each other, feedback loops are possible that can lead to ecosystem conversion (e.g. forest to grassland). This is a well-documented threat in the tropics but with potential also to be important elsewhere. Fire interacts with fragmentation through scale-specific mechanisms: fire creates edges and drives edge effects; fire alters patch quality; and fire alters landscape-scale connectivity. We found only 12 cases in which studies reported the four essential strata for testing a full interaction, which were fragmented and unfragmented landscapes that both span contrasting fire histories, such as recently burnt and long unburnt vegetation. Simulation and empirical studies show that fire and fragmentation can interact synergistically, multiplicatively, antagonistically or additively. These cases highlight a key reason why understanding interactions is so important: when fire and fragmentation act together they can cause local extinctions, even when their separate effects are neutral. Whether fire-fragmentation interactions benefit or disadvantage species is often determined by the species' preferred successional stage. Adding fire to landscapes generally benefits early-successional plant and animal species, whereas it is detrimental to late-successional species. However, when fire interacts with fragmentation, the direction of effect of fire on a species could be reversed from the effect expected by successional preferences. Adding fire to fragmented landscapes can be detrimental for species that would normally co-exist with fire, because species may no longer be able to disperse to their preferred successional stage. Further, animals may be attracted to particular successional stages leading to unexpected responses to fragmentation, such as higher abundance in more isolated unburnt patches. Growing human populations and increasing resource consumption suggest that fragmentation trends will worsen over coming years. Combined with increasing alteration of fire regimes due to climate change and human-caused ignitions, interactions of fire with fragmentation are likely to become more common. Our new framework paves the way for developing a better understanding of how fire interacts with fragmentation, and for conserving biodiversity in the face of these emerging challenges.

Endemic Population Response to Increasingly Severe Fire: A Cascade of Endangerment for the Mt. Graham Red Squirrel

Drought, past fire suppression, insect invasion, and high-severity fire represent a disturbance cascade characteristic of forests in the western United States. The result is altered forest ecosystems diminished in their function and capacity to support biodiversity. Small habitat specialists are particularly vulnerable to the impacts of disturbances because of their limited movement capacity and high site fidelity. Research suggests that small mammals suffer limited direct mortality from fire but are increasingly vulnerable to local extirpation because of secondary impacts that include habitat loss and reduced food availability, survival, and reproduction. We examine the direct and secondary impacts of increasingly severe fire events on the endangered Mt. Graham red squirrel—a model system to demonstrate how disturbances can threaten the persistence of range-limited species. We document survival, space use, and displacement prior to and following fires and discuss implications for conservation. We suggest that management plans address future threats, including disturbance-related habitat loss.

Performance Evaluation of Machine Learning Methods for Forest Fire Modeling and Prediction

Predicting and mapping fire susceptibility is a top research priority in fire-prone forests worldwide. This study evaluates the abilities of the Bayes Network (BN), Naïve Bayes (NB), Decision Tree (DT), and Multivariate Logistic Regression (MLP) machine learning methods for the prediction and mapping fire susceptibility across the Pu Mat National Park, Nghe An Province, Vietnam. The modeling methodology was formulated based on processing the information from the 57 historical fires and a set of nine spatially explicit explanatory variables, namely elevation, slope degree, aspect, average annual temperate, drought index, river density, land cover, and distance from roads and residential areas. Using the area under the receiver operating characteristic curve (AUC) and seven other performance metrics, the models were validated in terms of their abilities to elucidate the general fire behaviors in the Pu Mat National Park and to predict future fires. Despite a few differences between the AUC values, the BN model with an AUC value of 0.96 was dominant over the other models in predicting future fires. The second best was the DT model (AUC = 0.94), followed by the NB (AUC = 0.939), and MLR (AUC = 0.937) models. Our robust analysis demonstrated that these models are sufficiently robust in response to the training and validation datasets change. Further, the results revealed that moderate to high levels of fire susceptibilities are associated with ~19% of the Pu Mat National Park where human activities are numerous. This study and the resultant susceptibility maps provide a basis for developing more efficient fire-fighting strategies and reorganizing policies in favor of sustainable management of forest resources.

Burning savanna for avian species richness and functional diversity

Prescribed fire is used throughout fire-prone landscapes to conserve biodiversity. Current best practice in managing savanna systems advocates methods based on the assumption that increased fire-mediated landscape heterogeneity (pyrodiversity) will promote biodiversity. However, considerable knowledge gaps remain in our understanding of how savanna wildlife responds to the composition and configuration of pyrodiverse landscapes. The effects of pyrodiversity on functional diversity have rarely been quantified and assessing this relationship at a landscape scale that is commensurate with fire management is important for understanding mechanisms underlying ecosystem resilience. Here, we assess the impact of spatiotemporal variation in a long-term fire regime on avian diversity in North West Province, South Africa. We examined the relationship between (1) species richness, (2) three indices of functional diversity (i.e., functional richness, functional evenness, and functional dispersion) and four measures of pyrodiversity, the spatial extents of fire age classes, and habitat type at the landscape scale. We then used null models to assess differences between observed and expected functional diversity. We found that the proportion of newly burned (<1-yr post-fire), old, unburned (≥10 yr post-fire), and woodland habitat on the landscape predicted species and functional richness. Species richness also increased with the degree of edge contrast between patches of varying fire age, while functional dispersion increased with the degree of patch shape complexity. Lower than expected levels of functional richness suggest that habitat filtering is occurring, resulting in functional redundancy across our study sites. We demonstrate that evaluating functional diversity and redundancy is an important component of conservation planning as they may contribute to previously reported fire resilience. Our findings suggest that it is the type and configuration, rather than the diversity, of fire patches on the landscape that promote avian diversity and conserve ecological functions. A management approach is needed that includes significant coverage of adjacent newly burned and older, unburned savanna habitat; the latter, in particular, is inadequately represented under current burning practices.

Predator responses to fire: A global systematic review and meta-analysis

Knowledge of how disturbances such as fire shape habitat structure and composition, and affect animal interactions, is fundamental to ecology and ecosystem management. Predators also exert strong effects on ecological communities, through top-down regulation of prey and competitors, which can result in trophic cascades. Despite their ubiquity, ecological importance and potential to interact with fire, our general understanding of how predators respond to fire remains poor, hampering ecosystem management. To address this important knowledge gap, we conducted a systematic review and meta-analysis of the effects of fire on terrestrial, vertebrate predators world-wide. We found 160 studies spanning 1978-2018. There were 36 studies with sufficient information for meta-analysis, from which we extracted 96 effect sizes (Hedges' g) for 67 predator species relating to changes in abundance indices, occupancy or resource selection in burned and unburned areas, or before and after fire. Studies spanned geographic locations, taxonomic families and study designs, but most were located in North America and Oceania (59% and 24%, respectively), and largely focussed on felids (24%) and canids (25%). Half (50%) of the studies reported responses to wildfire, and nearly one third concerned prescribed (management) fires. There were no clear, general responses of predators to fire, nor relationships with geographic area, biome or life-history traits (e.g. body mass, hunting strategy and diet). Responses varied considerably between species. Analysis of species for which at least three effect sizes had been reported in the literature revealed that red foxes Vulpes vulpes mostly responded positively to fire (e.g. higher abundance in burned compared to unburned areas) and eastern racers Coluber constrictor negatively, with variances overlapping zero only slightly for both species. Our systematic review and meta-analysis revealed strong variation in predator responses to fire, and major geographic and taxonomic knowledge gaps. Varied responses of predator species to fire likely depend on ecosystem context. Consistent reporting of ongoing monitoring and management experiments is required to improve understanding of the mechanisms driving predator responses to fire, and any broader effects (e.g. trophic interactions). The divergent responses of species in our study suggest that adaptive, context-specific management of predator-fire relationships is required.

Fire as a key driver of Earth's biodiversity

Many terrestrial ecosystems are fire prone, such that their composition and structure are largely due to their fire regime. Regions subject to regular fire have exceptionally high levels of species richness and endemism, and fire has been proposed as a major driver of their diversity, within the context of climate, resource availability and environmental heterogeneity. However, current fire-management practices rarely take into account the ecological and evolutionary roles of fire in maintaining biodiversity. Here, we focus on the mechanisms that enable fire to act as a major ecological and evolutionary force that promotes and maintains biodiversity over numerous spatiotemporal scales. From an ecological perspective, the vegetation, topography and local weather conditions during a fire generate a landscape with spatial and temporal variation in fire-related patches (pyrodiversity), and these produce the biotic and environmental heterogeneity that drives biodiversity across local and regional scales. There have been few empirical tests of the proposition that 'pyrodiversity begets biodiversity' but we show that biodiversity should peak at moderately high levels of pyrodiversity. Overall species richness is greatest immediately after fire and declines monotonically over time, with postfire successional pathways dictated by animal habitat preferences and varying lifespans among resident plants. Theory and data support the 'intermediate disturbance hypothesis' when mean patch species diversity is correlated with mean fire intervals. Postfire persistence, recruitment and immigration allow species with different life histories to coexist. From an evolutionary perspective, fire drives population turnover and diversification by promoting a wide range of adaptive responses to particular fire regimes. Among 39 comparisons, the number of species in 26 fire-prone lineages is much higher than that in their non-fire-prone sister lineages. Fire and its byproducts may have direct mutagenic effects, producing novel genotypes that can lead to trait innovation and even speciation. A paradigm shift aimed at restoring biodiversity-maintaining fire regimes across broad landscapes is required among the fire research and management communities. This will require ecologists and other professionals to spread the burgeoning fire-science knowledge beyond scientific publications to the broader public, politicians and media.