Insurance Hypothesis Research Articles

Household Food Security is Associated with Central Obesity but Not with Overweight and Obesity in Early Adolescence in the City of Colombo in Sri Lanka: A Cross Sectional Study

Background: The prevalence of overnutrition among early adolescents in the city of Colombo, the capital of Sri Lanka is rising steeply and the precise reason for the epidemic remains unclear. According to the insurance hypothesis, proximate driver of obesity could be food insecurity rather than food abundance per se. Aim: This study was conducted to investigate the association between household food security status and overweight, obesity and central obesity in early adolescence in the city of Colombo. Methods: A cross sectional study was conducted using 634 randomly selected adolescents aged 11-13 years in the city of Colombo, in which subjects were recruited using multistage stratified cluster sampling technique. Household food security score was assessed using USDA 18 Item Household Food Security/ Hunger Survey Module, while height, weight and waist circumference were measured. BMI-for-age Z score was calculated using WHO AnthroPlus software and waist: height ratio was calculated. Partial correlation between household food security score and BMI-for-age Z score and waist:height ratio were analyzed, after adjusting for daily energy intake, physical activity level, pubertal stage, birth weight and age. Results: Household food security score was not associated with BMI-for-age Z score (p>0.05). Household food security score was inversely and weakly (r=-0.373) associated with waist:height ratio (p = 0.002) in girls but not in boys. Conclusion: Overweight and obesity are not associated with food security status, however food secure girls tend to have more fat distribution in the abdominal area.

Parental overproduction allows siblicidal bird to adjust brood size to climate-driven prey variation.

Parental overproduction is hypothesized to hedge against uncertainty over food availability and stochastic death of offspring and to improve brood fitness. Understanding the evolution of overproduction requires quantifying its benefits to parents across a wide range of ecological conditions, which has rarely been done. Using a multiple hypotheses approach and 30 years of data, we evaluated the benefits of overproduction in the Blue-footed booby, a seabird that lays up to three eggs asynchronously, resulting in an aggressive brood hierarchy that facilitates the death of last-hatched chicks under low food abundance. Results support the resource-tracking hypothesis, as low prey abundance (estimated from sea surface temperature and chlorophyll-a concentration) led to rapid brood reduction. The insurance hypothesis was supported in broods of three, where last-hatched chicks' survival increased after a sibling's death. Conversely, in broods of two, results suggested that parents abandoned last-hatched chicks following first-hatched chicks' deaths. No direct evidence supported the facilitation hypothesis: the presence of a last-hatched chick during development did not enhance its sibling's fitness in the short or long term. The value of last-hatched offspring to parents, as "extra" or "insurance" varied with indices of food abundance, brood size, and parental age. Ninety percent of overproduction benefits came from enabling parents to capitalize on favorable conditions by fledging additional offspring. Our study provides insight into the forces driving overproduction, explaining the adaptiveness of this apparently wasteful behavior and allowing us to better predict how overproduction's benefits might be modified by ocean warming.

The Impact of Childhood Mortality on Fertility in Rural Tanzania: Evidence From the Ifakara and Rufiji Health and Demographic Surveillance Systems

This manuscript examines the relationship between child mortality and subsequent fertility using longitudinal data on births and childhood deaths occurring among 15,291 Tanzanian mothers between 2000 and 2015. Generalized hazard regression analyses assess the effect of child loss on the hazard of conception, adjusting for child-level, mother-level, and contextual covariates. Results show that time to conception is most reduced if an index child dies during the subsequent birth interval, representing the combined effect of biological and volitional replacement. Deaths occurring during prior birth intervals were associated with accelerated time to conception during future intervals, consistent with hypothesized insurance effects of anticipating future child loss, but this effect is smaller than replacement effects. The analysis reveals that residence in areas of relatively high child mortality is associated with hastened parity progression, again consistent with the insurance hypothesis. Investigation of high-order interactions suggests that insurance effects tend to be greater in low-mortality communities, replacement effects tend to be stronger in high-mortality community contexts, and wealthier families tend to exhibit a weaker insurance response but a stronger replacement response to childhood mortality relative to poorer families.

Limited theoretical and empirical evidence that response diversity determines the resilience of ecosystems to environmental change

AbstractResponse diversity describes the variation in species' ecological responses to environmental change, where higher response diversity represents a greater variety of species–environment responses. The concept complements the insurance hypothesis that biodiversity increases and stabilizes ecosystem functions because population declines of one species with environmental change are compensated for by opposing responses of others. Accordingly, response diversity is a promising, but still largely untested, candidate mechanism explaining the generally positive relationship between biodiversity and ecosystem stability across a range of systems. In the 10 years since, Mori et al. (2013, Biological Reviews, 88, 349–364) reviewed the relationship between response diversity and ecological resilience (a key dimension of stability), the topic has garnered significant attention. Their review has attracted over 600 citations to date and continues to be well cited. However, in a recent review of empirical work on response diversity, we found fewer than 50 studies empirically measuring what the authors called response diversity. Here we discuss possible reasons for the slow uptake of empirical response diversity research, including lack of standardized methods and heavy data requirements, as well as next steps to achieve a robust framework for empirical research on response diversity and stability, such as by leveraging new methods and coordinating research efforts through interdisciplinarity. By identifying a disconnect between interest in response diversity and theoretical and empirical work on the topic, we hope this article will shed light on the challenges studies of response diversity must overcome, in turn catalyzing new avenues of research on response diversity, ecological stability, and ecosystem functioning.

Quantifying the influence of tree species richness on community drought resistance using drone-derived NDVI and ground-based measures of Plant Area Index and leaf chlorophyll in a young tree diversity experiment

Tree diversity may buffer the negative impact of drought events according to the diversity insurance hypothesis. During the extreme pan-European drought of 2018, we tested whether tree species richness modulated drought impacts on communities of a young tree diversity experiment in Freiburg, Germany. We utilized drone-based hyperspectral images to capture early symptoms of drought stress indicated by variation in Normalized Difference Vegetation Index (NDVI), and ground-based measures that are indicative of tree canopy function, including Plant Area Index (PAI) and leaf chlorophyll content (Chl). Measures were taken the fifth growing season after planting on six broadleaved and six conifer species originating from Europe and North America in monocultures, two-, four- and six-species mixtures. NDVI decreased successively in response to the summer drought. In contrast to our expectation, tree species richness did not reduce declines in NDVI at the plot level. NDVI values were generally lower in six-species mixtures, compared to monocultures or two-species mixtures, which may be indicative of antagonistic tree species interactions in mixtures of high diversity. Changes in PAI in response to the drought were similar to changes in NDVI; however, modeled differences in PAI between mixtures were not statistically significant. Chl initially decreased, but recovered within a period of two months following the drought. Remotely sensed NDVI facilitated monitoring of the drought response of our large field experiment and could aid in monitoring canopy health in response to extreme drought events. Our observations suggest that NDVI responses are likely more strongly related to leaf shedding within tree canopies than decreases in Chl. Tree stands with a high abundance of deciduous trees may therefore be especially sensitive in triggering NDVI changes in response to drought during the stages leading up to tree mortality. Future studies of drought-related canopy responses should include monitoring of leaf shedding as well as the Chl of shed leaves.

Food insecurity as a cause of adiposity: evolutionary and mechanistic hypotheses.

Food insecurity (FI) is associated with obesity among women in high-income countries. This seemingly paradoxical association can be explained by the insurance hypothesis, which states that humans possess evolved mechanisms that increase fat storage to buffer against energy shortfall when access to food is unpredictable. The evolutionary logic underlying the insurance hypothesis is well established and experiments on animals confirm that exposure to unpredictable food causes weight gain, but the mechanisms involved are less clear. Drawing on data from humans and other vertebrates, we review a suite of behavioural and physiological mechanisms that could increase fat storage under FI. FI causes short-term hyperphagia, but evidence that it is associated with increased total energy intake is lacking. Experiments on animals suggest that unpredictable food causes increases in retained metabolizable energy and reductions in energy expenditure sufficient to fuel weight gain in the absence of increased food intake. Reducing energy expenditure by diverting energy from somatic maintenance into fat stores should improve short-term survival under FI, but the trade-offs potentially include increased disease risk and accelerated ageing. We conclude that exposure to FI plausibly causes increased adiposity, poor health and shorter lifespan. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

Special representative actions, the insurance value of audits, and investor protection: an empirical study based on the ruling against Kangmei Pharmaceutical Co. Ltd

ABSTRACT The ruling against the Kangmei Pharmaceutical Co., Ltd’s financial fraud was the first special representative action under China’s new Securities Law, serving as a milestone in investor protection in China. Exploiting the natural experiment provided by the ruling, we empirically test the insurance value of audits and find a negative market reaction of the client firms audited by the accounting firms that were still being sued as of the date of Kangmei ruling (i.e., the sued accounting firms). Further, this reaction is weakened when the solvency of the sued accounting firms or the client firms is high, and is intensified when the client firms have high litigation risk. The ruling also eroded investors’ trust in the financial statements for the year 2020 that were audited by the sued accounting firms. Overall, we provide important evidence on the effect of special representative actions from the perspective of audit insurance hypothesis.

Macroalgae maintain growth outside their observed distributions: Implications for biodiversity‐ecosystem functioning at landscape scales

Abstract The spatial insurance hypothesis states that at landscape scales with environmental variability between different places, biodiversity increases ecosystem functioning if species respond asynchronously to environmental variation, and they are highest functioning in places where they dominate the relative abundance. Under this hypothesis, observed species turnover between places in a landscape with environmental variation might suggest that species diversity is an important driver of landscape‐scale ecosystem functioning. However, the spatial insurance implicitly assumes that species found in one place in the landscape will not be able to maintain high functioning in other places. Given this assumption of the spatial insurance hypothesis, we would predict that species' functioning in monoculture should decline if transplanted to a different place on an environmental gradient, away from where they naturally dominate. If this is the case, we would expect that the loss of one species in one place of the environmental gradient could not be compensated through the establishment of another species in that place. We tested this prediction using a model system of marine macroalgae on intertidal rocky shores on the Swedish west coast. We performed a reciprocal transplant experiment with adult individuals of four fucoid seaweeds that dominate the standing stock biomass at different depths on these shores and monitored their relative growth rate over 2 months. Counter to the assumptions made by the spatial insurance hypothesis, growth rates for three of the four species showed limited responses to being transplanted to different depth zones. Spatial insurance may, hence, play a minor role in sustaining landscape ecosystem functioning in this and other systems. Synthesis. The functional consequences of species loss at landscape scales may not be as obvious as observational studies and ecological models suggest. Consequently, natural patterns of species turnover should not be used directly to argue for the role of biodiversity at landscape scales. Instead, how species may or may not be able to compensate for the loss of other species is a critical aspect if we are to understand how changes in biodiversity at the landscape scale affect ecosystem functioning.

Spatial insurance against a heatwave differs between trophic levels in experimental aquatic communities.

Climate change-related heatwaves are major threats to biodiversity and ecosystem functioning. However, our current understanding of the mechanisms governing community resistance to and recovery from extreme temperature events is still rudimentary. The spatial insurance hypothesis postulates that diverse regional species pools can buffer ecosystem functioning against local disturbances through the immigration of better-adapted taxa. Yet, experimental evidence for such predictions from multi-trophic communities and pulse-type disturbances, like heatwaves, is largely missing. We performed an experimental mesocosm study to test whether species dispersal from natural lakes prior to a simulated heatwave could increase the resistance and recovery of plankton communities. As the buffering effect of dispersal may differ among trophic groups, we independently manipulated the dispersal of organisms from lower (phytoplankton) and higher (zooplankton) trophic levels. The experimental heatwave suppressed total community biomass by having a strong negative effect on zooplankton biomass, probably due to a heat-induced increase in metabolic costs, resulting in weaker top-down control on phytoplankton. While zooplankton dispersal did not alleviate the negative heatwave effects on zooplankton biomass, phytoplankton dispersal enhanced biomass recovery at the level of primary producers, providing partial evidence for spatial insurance. The differential responses to dispersal may be linked to the much larger regional species pool of phytoplankton than of zooplankton. Our results suggest high recovery capacity of community biomass independent of dispersal. However, community composition and trophic structure remained altered due to the heatwave, implying longer-lasting changes in ecosystem functioning.

Rangeland resilience to droughts: Changes across an intensification gradient

AbstractAimThe intensification of livestock systems generates structural and functional changes in native grassland, which alter their capacity to respond to disturbances. Our objective was to evaluate the resilience to an extraordinary consecutive‐droughts event, across an intensification gradient of livestock systems. Our prediction was that sites with more intensive management (less diverse) will have lower resilience (as resistance and/or recovery) to drought.LocationSouth‐central region of Uruguay.MethodsWe evaluated the resilience (as resistance and recovery) to consecutive droughts, across an intensification gradient of livestock systems. We analyzed the effect of diversity on resilience at two scales: (1) regional — an observational experiment with real commercial farms, where we compared three systems with an increasing level of intensification that determine changes in diversity, (2) patch — we carried out a manipulative experiment with different grazing management intensities determined by the level of biomass removal and fertilization. In both experiments, resistance and recovery were calculated from the Normalized Difference Vegetation Index.ResultsThe most intensified and less diverse management (sown pastures) had significantly lower resistance than native and overseeded grassland. Also, it showed the lowest recovery rate, and its productivity did not reach previous values after the drought's end. The grazing intensification gradient at the plot scale did not show clear differences among treatments both for resistance and recovery. However, resistance of the individual plots showed a positive significant association not only with species richness and diversity, but also with evenness and diversity of functional groups.ConclusionOur research supports the insurance hypothesis by demonstrating that farms with greater biodiversity (less intensive management) have higher resistance to drought. We found that species richness is not enough to account for the effect of diversity on resilience. Our results emphasize the importance of preserving grassland biodiversity to maintain resilient ecosystems in the face of climate change.

Crop Species Production Diversity Enhances Revenue Stability in Low-Income Farm Regions of Mexico

Stabilizing farm revenues is a goal of agricultural policies around the world, especially in vulnerable regions with limited access to crop insurance. One potential pathway to revenue stability follows the agricultural “insurance hypothesis”, which holds that crop diversification has stabilizing effects on productivity that mitigate risks from environmental stressors and market shocks, thereby producing a form of natural insurance against crop loss. While substantial support for the hypothesis exists, most studies testing the hypothesis have occurred at the farm or landscape levels and have controlled for a limited range of socioeconomic and environmental factors. This study tests the insurance hypothesis by examining the effects of crop species production diversity on revenue stability in low-income regions of southern Mexico. Here, rural farms experience chronic vulnerability to climatic shocks and market forces. Using parametric and non-parametric approaches, three groups of models are used to examine the effects of socio-environmental factors and farm structural and functional characteristics on the crop diversity-revenue stability relationship. Additionally factored in the relationship are the effects of cropping portfolios: statistical groupings of different crop species (n = 304) that characterize distinct farming areas (1340 municipalities). Findings support the insurance hypothesis and underscore the importance of crop diversification in the region. However, findings also show that irrigation plays an even stronger role than crop diversification in stabilizing farm revenues. Furthermore, some crop portfolios negatively impact revenue stability, including some portfolios with high crop diversity. In sum, a better understanding of farm contexts—contributing factors and cropping portfolios—is key to designing policies that help stabilize farm revenues through crop diversification.

Forest microclimate and composition mediate long-term trends of breeding bird populations.

Climate change is contributing to biodiversity redistributions and species declines. However, cooler microclimate conditions provided by old-growth forest structures compared with surrounding open or younger forests have been hypothesized to provide thermal refugia for species that are sensitive to climate warming and dampen the negative effects of warming on population trends of animals (i.e., the microclimate buffering hypothesis). In addition to thermal refugia, the compositional and structural diversity of old-growth forest vegetation itself may provide resources to species that are less available in forests with simpler structure (i.e., the insurance hypothesis). We used 8 years of breeding bird abundance data from a forested watershed, accompanied with sub-canopy temperature data, and ground- and LiDAR-based vegetation data to test these hypotheses and identify factors influencing bird population changes from 2011 to 2018. After accounting for imperfect detection, we found that for 5 of 20 bird species analyzed, abundance trends tended to be less negative or neutral at sites with cooler microclimates, which supports the microclimate buffering hypothesis. Negative effects of warming on two species were also reduced in locations with greater forest compositional diversity supporting the insurance hypothesis. We provide the first empirical evidence that complex forest structure and vegetation diversity confer microclimatic advantages to some animal populations in the face of climate change. Conservation of old-growth forests, or their characteristics in managed forests, could help slow the negative effects of climate warming on some breeding bird populations via microclimate buffering and possibly insurance effects.

Water quality profits by the submerged macrophyte community consisting of multi-functional species-rich groups

The re-establishment of submerged macrophytes facilitates the formation of a clear-water state in shallow eutrophic lakes. But most restorations of submerged macrophytes are often unstable and cannot maintain a stable clear-water state, probably because the species and functional diversity have not been fully taken into account. In this study, we try to explore submerged macrophyte communities and water quality changes under different submerged macrophyte combinations through mesocosm experiments. We hypothesized that communities with high species and functional diversity would be more conducive to improving water quality. The results showed that the mean community biomass of single-species and 8-species were higher than 5-species. And the stability and mean relative growth rate of the 8-species community were higher than the 5-species community. With the same configuration of three functional groups, the 8-species community was more stable and had better water quality than the 5-species community. The path analysis revealed that different functional groups of submerged macrophytes play different roles. The erect and canopy-producing submerged macrophytes were conducive to reducing total suspended solids (TSS) concentrations in the water column during community construction. In contrast, bottom-dwelling submerged macrophytes were conducive to reducing total nitrogen, total phosphorus, and TSS concentrations during the stage of disturbances. Our results also suggested that canopy-producing groups may have a competitive advantage for light over bottom-dwelling species. Based on the above results and biodiversity insurance hypothesis, we conclude that the community consisting of multi-functional species-rich groups is conducive to building stable submerged macrophyte communities and obtaining a stable clear-water state. Our findings will improve water quality management and pollution control for eutrophic shallow lakes.

Tree diversity depending on environmental gradients promotes biomass stability via species asynchrony in China’s forest ecosystems

There is mounting evidence that biodiversity promotes ecological stability in changing environments. However, understanding diversity–stability relationships and their underlying mechanisms across large-scale tree diversity and natural environmental gradients are still controversial and largely lacking. We used thirty-nine 0.12 ha long-term permanent forest plots spanning China’s various forest types to test the effects of multiple abiotic (climate, soil, age and topography) and biotic factors (taxonomic and structural diversity, functional diversity and community-mean traits, and species asynchrony) on biomass stability and its components (mean biomass and biomass variability) over time. We used multiple analytical methods to identify the best explanatory variables and complicated causal relationships for community biomass stability. Our results showed that species richness increased biomass stability by promoting species asynchrony. Structural and functional diversity had a weaker effect on biomass stability. Forest age and structural diversity increased mean biomass and biomass variability significantly and simultaneously. Communities dominated by tree species with high wood density had high biomass stability. Soil nitrogen enhanced biomass stability directly and indirectly through its effects on mean biomass. Soil nitrogen to phosphorus ratio increased biomass stability via increasing species asynchrony. Precipitation indirectly increased biomass stability by affecting tree diversity. Moreover, the direct and indirect effects of soil nutrients on biomass stability were greater than that of climate variables. Our results suggest that species asynchrony is the main mechanism proposed to explain the stabilizing effect of diversity on community biomass, supporting two mechanisms, namely, the biodiversity insurance hypothesis and complementary dynamics. Soil and climate factors also play an important role in shaping diversity–stability relationships. Our results provide a new insight into how tree diversity affects ecosystem stability across diverse community types and large-scale environmental gradients.

What plant–pollinator network structure tells us about the mechanisms underlying the bidirectional biodiversity productivity relationship?

The Biodiversity – Ecosystem Functioning (B–EF) relationship remains a topic of ongoing debate with most studies focusing on primary productivity, and documenting that this relationship takes many forms. It remains unclear if biodiversity drives productivity or productivity shapes biodiversity or the relationship is bidirectional. B-EF studies explore almost exclusively the relationship between species richness and ecosystem functioning, while the role of biotic interactions, a key component of ecosystem functioning, has been neglected. Here, using data of 80 local plant–pollinator networks on 20 Aegean islands, and of gross primary productivity (GPP) from the MODIS satellite, we explored the bidirectional relationship between interaction network structure (nestedness and specialization), species richness (plants and pollinators) and mean and inter-annual variability of GPP. We found that nestedness and specialisation of plant–pollinator networks is driven by mean GPP. However, specialisation alone was a significant predictor of mean GPP, implying that networks tend to be more specialised in low-productivity areas. Pollinator species richness exerted a strong effect on mean GPP with the remaining factors playing a minor role, while the effect of mean GPP on pollinator species richness was weaker. Furthermore, the nestedness of plant–pollinator networks drives inter-annual variability of GPP with more nested networks displaying less variability, which is in accordance with the predictions of the insurance hypothesis. Plant and pollinator species richness were also associated with inter-annual variability of GPP.

Functional redundancy dampens precipitation change impacts on species‐rich invertebrate communities across the Neotropics

Abstract Animal community responses to extreme climate events can be predicted from the functional traits represented within communities. However, it is unclear whether geographic variation in the response of functional community structure to climate change is primarily driven by physiological matching to local conditions (local adaptation hypothesis) or by differences between species pools in functional redundancy (insurance hypothesis). We conducted a coordinated experiment to understand how aquatic invertebrate traits mediate the responses of multitrophic communities to changes in the quantity and evenness of rainfall in 180 natural freshwater microcosms (tank bromeliads) distributed across six sites from 18°N in the Caribbean to 29°S in South America. At each site, we manipulated the mean and dispersion of the daily amount of rainfall that entered tank bromeliads over a 2‐month period. Manipulations covered a response surface representing 50% to 200% of the dispersion of daily rainfall crossed with 10% to 300% of the mean amounts of rainfall. The response of functional community structure to precipitation regimes differed across sites. These geographic differences were not consistent with the local adaptation hypothesis, as responses did not correlate with the current amplitude in precipitation. Geographic differences in community responses were consistent with the insurance hypothesis: sites with the lowest functional redundancy in their species pools had the strongest response to a gradient in hydrological variability induced by uneven precipitation. In such sites, an increase in the hydrologic variability induced a shift from communities with both pelagic and benthic traits using both green and brown energy channels to strictly benthic, brown energy communities. Our results predict uneven impacts of precipitation change on community structure and energy channels within communities across Neotropical regions. This geographic variation is due more to differences in the size and redundancy of species pools than to local adaptation. Strategies for climate change adaptation should thus seek to identify and preserve functionally unique species and their habitats. Read the free Plain Language Summary for this article on the Journal blog.

Biodiversity influences the effects of oil disturbance on coastal ecosystems.

Biodiversity can enhance the response of ecosystems to disturbance. However, whether diversity can reduce the ecological effect of human‐induced novel and extreme disturbances is unclear. In April 2010, the Deepwater Horizon (DwH) platform exploded, allowing an uncontrolled release of crude oil into the northern Gulf of Mexico. Initial surveys following the spill found that ecological impacts on coastal ecosystems varied greatly across habitat‐type and trophic group; however, to date, few studies have tested the influence of local biodiversity on these responses. We used a meta‐analytic approach to synthesize the results of 5 mesocosm studies that included 10 independent oil experiments and 5 independent oil + dispersant experiments. We tested whether biodiversity increased the resistance and/or resilience of coastal ecosystems to oil disturbance and whether a biodiversity effect depended on the type of diversity present (taxonomic or genetic) and/or the response type measured (population, community, or ecosystem level). We found that diversity can influence the effects of oiling, but the direction and magnitude of this diversity effect varied. Diversity reduced the negative impact of oiling for within‐trophic‐level responses and tended to be stronger for taxonomic than genetic diversity. Further, diversity effects were largely driven by the presence of highly resistant or quick to recover taxa and genotypes, consistent with the insurance hypothesis. However, we found no effect of diversity on the response to the combination of oil and dispersant exposure. We conclude that areas of low biodiversity may be particularly vulnerable to future oil disturbances and provide insight into the benefit of incorporating multiple measures of diversity in restoration projects and management decisions.

Functional redundancy of Amazonian dung beetles confers community‐level resistance to primary forest disturbance

AbstractTropical forest biodiversity is being threatened by human activities, and species losses during forest disturbance can compromise important ecosystem functions and services. We assessed how species losses due to tropical forest disturbance affect community functional structure, using Amazonian dung beetles as a model group. We collected empirical data from 106 forest transects and used simulated extinction scenarios to determine how species loss influences community structure at regional and local scales. Although functional and taxonomic community metrics were largely unaffected by primary forest disturbance, they differed markedly between primary and secondary forests. However, our extinction scenarios demonstrated scale‐dependence of species losses, whereby functional structure only eroded with species extinction at the local scale. Hence, we extend the spatial insurance hypothesis by demonstrating that landscape‐scale functional redundancy offsets the impact of local species losses and confers community‐level resistance to primary forest disturbance.Abstract in portuguese is available with online material.

A stochastic model of epilithic algal succession and patch dynamics in streams

AbstractExplaining spatiotemporal variability in metacommunities is challenging because conducting empirical studies that link mechanisms across scales is difficult. We developed a stochastic model of epilithic algal succession in streams to better understand the underlying interactions that drive patch dynamics at three levels of spatial hierarchy: patch, channel habitat, and reach. The state of an algal patch is specified by one of nine community types, which are defined by growth form. Ninety‐two empirical data sets of epilithic succession were used to estimate Markov transition probabilities among patch types as functions of inputs for light, nutrients, and current. Two additional probability matrices determine the effects of herbivory and disturbance based on user inputs. Testing model predictions of patch composition against independent empirical data indicated good correspondence based on resemblance metrics. Early in succession, most patches were occupied by prostrate and erect diatoms. These persisted under low light and nutrient conditions. Motile and stalked diatom patches characterized later stages when resources were moderate. Succession proceeded to mostly filamentous green algal patches under high resource levels. Sensitivity simulations indicated light most affected succession. Herbivory and disturbance increased the probability of prostrate diatom patches. We then used the model to examine how changes in abiotic and biotic parameters affect patch dynamics at different scales. Changes in nutrients or light affected patch diversity differently at the habitat vs. reach scale. Interactions with local resource levels within a channel habitat determined its alpha diversity. The proportion of channel habitat types within a reach, and their collective response to changes, determined beta diversity. Patch diversity within channel habitats generated by herbivory followed the predation hypothesis only when nutrients were low. At the reach scale, herbivory always increased patch similarity among channel habitats, lowering beta diversity. Diversity within channel habitats followed the intermediate disturbance hypothesis for a variety of habitats. Effects of disturbance on beta diversity at the reach scale depended on channel habitat heterogeneity. The stability of patches at the reach scale was highest when disturbance created high patch diversity, supporting the insurance hypothesis. The ideas generated can improve stream bioassessment methods and provide insight into mechanisms underlying patch dynamics in other ecosystems.

Strong influence of climatic extremes on diversity of benthic algae and cyanobacteria in a lowland intermittent stream

AbstractFreshwater ecosystems are threatened by the global change‐induced extreme climatic events worldwide. The unpredictable changes in water supply create strongly disturbed environments and ultimately result in diversity changes. Here, we studied the formation of benthic algal and cyanobacterial assemblages under intermediately disturbed (IDC) and highly disturbed conditions (HDC) in a small lowland intermittent stream. We addressed our hypotheses to the Intermediate Disturbance Theory supposing that intermediate frequencies or intensities of disturbances maximise diversity independently of its level. We expected a larger influence of extremes in water supply on functional than on taxonomic and phylogenetic diversity. Our results only partially proved our first hypothesis highlighting the (i) importance of short‐time but intense disturbing effect on biofilm formation and trait composition under IDC period and (ii) the instability within the assemblages caused by large and opposing influences during HDC. Although extreme weather events caused trait extremes (supporting second hypothesis), they did not result in a decrease in functional richness (rejecting the second hypothesis). These findings in accordance with the insurance hypothesis clearly stress that a system with high functional redundancy can keep its functionality even under drastic hydrological conditions and the accompanying loss of species.