Colonization-extinction Dynamics Research Articles

Habitat requirements of deadwood‐dependent invertebrates that occupy tree hollows

ABSTRACTTree hollows support a specialised species‐rich fauna. We review the habitat requirements of saproxylic (= deadwood dependent) invertebrates which occupy tree hollows. We focus on studies quantifying relationships between species occurrence patterns and characteristics of tree hollows, hollow trees, and the surrounding landscape. We also explore the processes influencing species occurrence patterns by reviewing studies on the spatio‐temporal dynamics of populations, including their dispersal and genetic structure. Our literature search in the database Scopus identified 52 relevant publications, all of which were studies from Europe. The dominant taxonomic group studied was beetles. Invertebrates in hollow trees were often more likely to be recorded in trees with characteristics reflecting a large amount of resources or a stable and warm microclimate, such as a large diameter, large amounts of wood mould (= loose material accumulated in the hollows mainly consisting of decaying wood), a high level of sun exposure, and with entrance holes that are large and either at a low or high height, and in dry hollows, with entrances not directed upwards. A stable microclimate is probably a key factor why some species of saproxylic invertebrates are confined to tree hollows. Other factors that are different in comparison to downed dead wood is the fact that hollows at a given height from the ground provide shelter from ground‐living predators, that hollows persist for longer, and that the content of nutrients might be enhanced by the accumulation of dead leaves, insect frass, and remains from dead insects. Several studies have identified a positive relationship between species occupancy per tree and the amount of habitat in the surrounding landscape, with a variation in the spatial scale at which characteristics of the surrounding landscape had the strongest effect over spatial scales from 200 to 3000 m. We found empirical support for the extinction threshold hypothesis, which predicts that the frequency of species presence per tree is greater if a certain number of trees are aggregated into a few large clusters of hollow trees rather than distributed among many small clusters. Observed thresholds in species occurrence patterns can be explained by colonisation–extinction dynamics, with species occupancy per tree influenced by variation in rates of immigration. Consistent with this assumption, field studies suggest that dispersal rate and range can be low for invertebrates occupying tree hollows, although higher in a warmer climate. For one species in which population dynamics has been studied over 25 years (Osmoderma eremita), the observed population dynamics have characteristics of a “habitat‐tracking metapopulation”, as local extinctions from trees occur possibly because those trees become unsuitable as well as due to stochastic processes in small populations. The persistence of invertebrate fauna confined to tree hollows may be improved by prolonging the standing life of existing hollow trees. It is also important to recruit new generations of hollow trees, preferably close to existing larger groups of hollow trees. Thus, the spatio‐temporal dynamics of hollow trees is crucial for the invertebrate fauna that rely upon them.

Hot-spots of epiphytic and epixylic lichens in fragmented temperate forests are underpinned by microhabitat heterogeneity and spatiotemporal habitat continuity

Habitat loss, fragmentation and degradation are major causes of the ongoing decline of epiphytic and epixylic lichen species in temperate forests throughout Europe. We investigated how extant species richness and composition of epiphytic and epixylic lichen communities in ten hot-spots of lichen diversity in the Czech Republic reflected the occurrence and properties of potentially suitable microhabitats and habitats. At each hot-spot, we surveyed a pair of 1-ha square plots, one in (over-)mature managed and the second in unmanaged forest. In total, we recorded 513 epiphytic and epixylic lichen species which represent a substantial part of lichen biota in Central Europe. Species richness and composition of lichen communities were explained by microhabitat heterogeneity, and also by the area of near-natural forest habitats (habitat extent) at the landscape scale. In addition, lichen species richness and number of red-listed species were explained by a categorial variable distinguishing mature managed and unmanaged plots, used as a proxy of temporal continuity of natural succession. This finding illustrates that temporal continuity of natural succession in unmanaged forests likely had an extra stimulus for lichen communities that may not be reflected by observed aspects of forest habitats. Hence, we confirmed indispensable positive effects of (micro)habitat heterogeneity, and spatial and temporal continuity for preserved hot-spots of lichen diversity in Central Europe. Due to generally slow colonization-extinction dynamics of epiphytic and epixylic lichens we call for strengthening microhabitat heterogeneity, and the spatial and temporal continuity of European temperate forests at the landscape scale.

Historical and dispersal processes drive community assembly of multiple aquatic taxa in glacierized catchments in the Qinghai-Tibet plateau

Understanding the relative role of dispersal dynamics and niche constraints is not only a core task in community ecology, but also becomes an important prerequisite for bioassessment. Despite the recent progress in our knowledge of community assembly in space and time, patterns and processes underlying biotic communities in alpine glacierized catchments remain mostly ignored. To fill this knowledge gap, we combined the recently proposed dispersal-niche continuum index (DNCI) with traditional constrained ordinations and idealized patterns of species distributions to unravel community assembly mechanisms of different key groups of primary producers and consumers (i.e., phytoplankton, epiphytic algae, zooplankton, macroinvertebrates, and fishes) in rivers in the Qinghai-Tibet Plateau, the World's Third Pole. We tested whether organismal groups with contrasting body sizes differed in their assembly processes, and discussed their applicability in bioassessment in alpine zones. We found that community structure of alpine river biotas was always predominantly explained in terms of dispersal dynamics and historical biogeography. These patterns are most likely the result of differences in species-specific functional attributes, the stochastic colonization-extinction dynamics driven by multi-year glacier disturbances and the repeated hydrodynamic separation among alpine catchments after the rising of the Qilian mountains. Additionally, we found that the strength of dispersal dynamics and niche constraints was partially mediated by organismal body sizes, with dispersal processes being more influential for microscopic primary producers. Finding that zooplankton and macroinvertebrate communities followed clumped species replacement structures (i.e., Clementsian gradients) supports the notion that environmental filtering also contributes to the structure of high-altitude animal communities in glacierized catchments. In terms of the applied fields, we argue that freshwater bioassessment in glacierized catchments can benefit from incorporating the metacommunity perspective and applying novel approaches to (i) detect the optimal spatial scale for species sorting and (ii) identify and eliminate the species that are sensitive to dispersal-related processes.

A simple genetic method to distinguish mule deer and bighorn sheep fecal pellets and its application to detecting bighorn sheep colonization events in California

Bighorn sheep (Ovis canadensis) habitat frequently is geographically discontinuous and the metapopulation model fits this species well. Consequently, extinction-colonization dynamics are important and need to be monitored. Much of the conservation history of bighorn sheep, however, was based on a theory that natural colonization is not part of the biology of this species. That theory is not supported by a growing body of evidence that natural colonization of vacant habitat does occur in this species. Here I present a simple PCR test that distinguishes bighorn sheep fecal pellets from those of mule deer (Odocoileus hemionus) as a useful tool in documenting bighorn sheep occupancy where mule deer are present. I include examples from California of applications of this method to investigate potential colonization events.

Climate-associated variation in the drivers of benthic macroinvertebrate species-area relationships across shallow freshwater lakes.

The island species-area relationship (ISAR) describes how species richness increases with increasing area of a given island or island-like habitat, such as freshwater lakes. While the ISAR is one of the most common phenomena observed in ecology, there is variation in both the form of the relationship and its underlying mechanisms. We compiled a global data set of benthic macroinvertebrates from 524 shallow freshwater lakes, ranging from 1 to 293,300 ha in area. We used individual-based rarefaction to determine the degree to which ISAR was influenced by mechanisms other than passive sampling (larger islands passively sample more individuals from the regional pool and, therefore, have more species than smaller islands), which would bias results away from expected relationships between rarefied species richness (and other measures that capture relative abundances) and lake area. We also examined how climate may alter the shape of the ISARs. We found that both rarefied species richness (the number of species standardized by area or number of individuals) and a measure of evenness emphasizing common species exhibit shallow slopes in relationships with lake area, suggesting that the expected ISARs in these lakes most likely result from passive sampling. While there was considerable variation among ISARs across the investigated lakes, we found an overall positive rarefied ISAR for lakes in warm (i.e. tropical/subtropical) regions (n = 195), and in contrast, an overall negative rarefied ISAR in cool (i.e. north temperate) lakes (n = 329). This suggested that mechanisms beyond passive sampling (e.g. colonization-extinction dynamics and/or heterogeneity) were more likely to operate in warm lakes. One possible reason for this difference is that the area-dependent intensity of fish predation, which can lead to flatter ISARs, is weaker in warmer relative to cooler lakes. Our study illustrates the importance of understanding both the pattern and potential processes underlying the ISARs of freshwater lakes in different climatic regions. Furthermore, it provides a baseline for understanding how further changes to the ecosystem (i.e. in lake area or climate) might influence biodiversity patterns.

Natural mega disturbances drive spatial and temporal changes in diversity and genetic structure on the toadfish Aphos porosus

Natural disturbances can modify extinction-colonization dynamics, driving changes in the genetic diversity and structure of marine populations. Along Chilean coast (36°S, 73°W), a strong hypoxic-upwelling event in 2008, and a mega earthquake-tsunami in 2010 caused mass mortality within the Aphos porosus population, which is a vulnerable species with low dispersal potential. We evaluated the effects of these two major disturbances on the diversity and spatial-temporal genetic structure of Aphos porosus in two neighboring areas that were impacted on different levels (High level: Coliumo Bay; Low level: Itata Shelf). Thirteen microsatellites (from 2008 to 2015) amplified in individuals collected from both locations were used to evaluate the effects of the two disturbances. Results showed that after the strong hypoxic-upwelling event and the mega earthquake-tsunami, Aphos porosus populations exhibited lower genetic diversity and less effective population sizes (Ne < 20), as well as asymmetries in migration and spatial-temporal genetic structure. These findings suggest a rise in extinction-recolonization dynamics in local Aphos porosus populations after the disturbances, which led to a loss of local genetic diversity (mainly in Coliumo Bay area impacted the most), and to greater spatial-temporal genetic structure caused by drift and gene flow. Our results suggest that continuous genetic monitoring is needed in order to assess potential risks for Aphos porosus in light of new natural and anthropogenic disturbances.

Historical biogeography and local adaptation explain population genetic structure in a widespread terrestrial orchid.

Historical changes in environmental conditions and colonization-extinction dynamics have a direct impact on the genetic structure of plant populations. However, understanding how past environmental conditions influenced the evolution of species with high gene flow is challenging when signals for genetic isolation and adaptation are swamped by gene flow. We investigated the spatial distribution and genetic structure of the widespread terrestrial orchid Epipactis helleborine to identify glacial refugia, characterize postglacial population dynamics and assess its adaptive potential. Ecological niche modelling was used to locate possible glacial refugia and postglacial recolonization opportunities of E. helleborine. A large single-nucleotide polymorphism (SNP) dataset obtained through genotyping by sequencing was used to define population genetic diversity and structure and to identify sources of postglacial gene flow. Outlier analyses were used to elucidate how adaptation to the local environment contributed to population divergence. The distribution of climatically suitable areas was restricted during the Last Glacial Maximum to the Mediterranean, south-western Europe and small areas in the Alps and Carpathians. Within-population genetic diversity was high in E. helleborine (mean expected heterozygosity, 0.373 ± 0.006; observed heterozygosity, 0.571 ± 0.012; allelic richness, 1.387 ± 0.007). Italy and central Europe are likely to have acted as important genetic sources during postglacial recolonization. Adaptive SNPs were associated with temperature, elevation and precipitation. Forests in the Mediterranean and Carpathians are likely to have acted as glacial refugia for Epipactis helleborine. Postglacial migration northwards and to higher elevations resulted in the dispersal and diversification of E. helleborine in central Europe and Italy, and to geographical isolation and divergent adaptation in Greek and Italian populations. Distinguishing adaptive from neutral genetic diversity allowed us to conclude that E. helleborine has a high adaptive potential to climate change and demonstrates that signals of adaptation and historical isolation can be identified even in species with high gene flow.

Caenorhabditis nematodes colonize ephemeral resource patches in neotropical forests.

Factors shaping the distribution and abundance of species include life‐history traits, population structure, and stochastic colonization–extinction dynamics. Field studies of model species groups help reveal the roles of these factors. Species of Caenorhabditis nematodes are highly divergent at the sequence level but exhibit highly conserved morphology, and many of these species live in sympatry on microbe‐rich patches of rotten material. Here, we use field experiments and large‐scale opportunistic collections to investigate species composition, abundance, and colonization efficiency of Caenorhabditis species in two of the world's best‐studied lowland tropical field sites: Barro Colorado Island in Panamá and La Selva in Sarapiquí, Costa Rica. We observed seven species of Caenorhabditis, four of them known only from these collections. We formally describe two species and place them within the Caenorhabditis phylogeny. While these localities contain species from many parts of the phylogeny, both localities were dominated by globally distributed androdiecious species. We found that Caenorhabditis individuals were able to colonize baits accessible only through phoresy and preferentially colonized baits that were in direct contact with the ground. We estimate the number of colonization events per patch to be low.

A strong decline of the endangered Apollo butterfly over 20 years in the archipelago of southern Finland

Insect groups are declining worldwide; Lepidoptera are among the taxa most affected in terrestrial ecosystems. The main drivers of these declines are a diverse set of factors relating to environmental change including habitat loss, pollution, and climate change. In 2019 and 2020, we surveyed 118 and 90 islands, respectively, in southern Finland’s archipelago for occupancy of Parnassius apollo larvae and counted the abundance of their host plant Hylotelephium telephium. Compared with historical data (1997–2003), the occupancy of Apollo butterfly larvae has decreased remarkably from about 75% to about 20% of islands and abundance declined as well. However, the abundance of their host plant has not changed. Occupancy models showed that the present occupancy probability is not affected by host plant numbers, and shows substantial colonization-extinction dynamics making the population vulnerable to stochastic extinction.Implications for insect conservationOur results show that this Apollo butterfly population is declining, and conservation actions are needed.

Quick recovery of a threatened butterfly in well‐connected patches following an extreme drought

AbstractExtreme weather events are becoming more frequent due to climate change. We therefore need to understand how species respond to these events. In 2018, the worst drought ever recorded hit the island of Gotland in the Baltic Sea, which led to a major decline of the threatened marsh fritillary (Euphydryas aurinia) the succeeding summer.We compared yearly occupancy among 256 habitat patches between 2018 and 2020 and analysed the colonisation–extinction dynamics between 2019 and 2020 in relation to patch area, connectivity, and habitat quality. Moreover, using capture‐mark‐release data in two patch clusters differing in connectivity, we also compared population sizes before and after the drought and analysed ‘population growth rates’. We also compared yearly abundance of host plants (Succisa pratensis) over time.Results show a remarkable recovery of the marsh fritillary. Both patch occupancy in 2020 and the number of individuals in the well‐connected patch cluster were higher than before the drought. In contrast, host plants were fewer and smaller, which taken together suggest that the amount of food resources was roughly half in 2020 compared to the pre‐drought conditions. Moreover, the butterfly population in the less connected patch cluster was eight times smaller compared to the population size before the drought. Local colonisations, extinctions, and population growth rates were explained by connectivity.The ability to quickly recover after extreme droughts is promising in times of climate change. The significance of connectivity for the population dynamics during recovery highlights the importance of maintaining well‐connected patch networks.

A framework for quantifying deviations from dynamic equilibrium theory

AbstractAimIn an age of increased anthropogenic changes, it is crucially important to understand how and why ecological communities change. Community assembly is governed by colonization and extinction processes, and the simplest model describing it is dynamic equilibrium (DE), which assumes that communities are shaped solely by stochastic colonization and extinction events. Deviations from this model can point to the role of species life histories, niches or human stressors acting on communities. Despite its potential to serve as a null model for community dynamics, there is currently no accepted methodology for identifying and measuring such deviations.InnovationHere we propose a new, easily applicable methodology for testing and quantifying deviations of empirically observed time series of community dynamics from the predictions and assumptions of the classical DE model, in which assemblages of independent species undergo stochastic colonization–extinction dynamics with constant rates. The methodology consists of a novel randomization‐based null model to generate synthetic time series (PARIS) and a set of statistics that quantify and test how different facets of community dynamics deviate from DE. These statistics are designed to test the assumptions of the theory, namely species independence and constancy of colonization and extinction rates, and the predictions of the theory regarding the magnitude of changes in species richness and composition.Main conclusionsTested against simulated data and a case study, the proposed methodology is shown to have good statistical properties: acceptable type I error rates, good statistical power and robustness against errors in the data. We discuss alternative methods and present guidelines for practical use of the methodology, hoping it will enhance the applicability of DE as a reference for studying changes in ecological communities.

Distribution of lacustrine plankton crustaceans and biogeographic patterns of biodiversity in Chile

Abstract The pattern of species richness and its potential causes are fundamental knowledge for an adequate management of biodiversity. In the present study, based on georeferenced lacustrine zooplankton, we evaluated the general species richness pattern and its relationship with hydrographic zonation, assessing four hypotheses: (1) the Rapoport latitudinal effect; (2) geometric restrictions as hard limits to geographical dispersion; (3) nestedness as a measure of the historical dynamics of extinction-colonization; and (4) environmental variables, as a measurement of the habitat as a recent ecological factor. Our results show a heterogeneous species richness pattern, with maxima located between 32-34°S, showing a general decrease towards higher latitudes. However, this pattern does not relate to the Rapoport latitudinal effect or the geometric restrictions. Instead, the pattern is associated with historical extinction-colonization dynamics between the waterbodies and ecological factors such as landscape, water, and energy availability that determine the number of species that these waterbodies can currently support.

Partitioning the colonization and extinction components of beta diversity across disturbance gradients.

Changes in species diversity often result from species losses and gains. The dynamic nature of beta diversity (spatial variation in species composition) that derives from such temporal species turnover, however, has received relatively little attention. Here, we disentangled extinction and colonization components of beta diversity by using the sets of species that went locally extinct and that newly colonized the study sites. We applied this concept of extinction and colonization beta diversity to ground vegetation communities that have been repeatedly surveyed in forests where fire and harvesting were experimentally applied. We first found that fire and harvesting caused no effect on beta diversity 2 yr after the treatments. From this result, we might conclude that they did not alter the ways in which species assemble across space. However, when we analyzed the extinction and colonization beta diversity between pre-treatment and 2 yr after the treatments, both measures were found to be significantly lower in burned sites compared to unburned sites (i.e., the groups of excluded and newly colonized species both showed low beta diversity in the burned sites). These results indicate that the fire excluded similar subsets of species across space, making communities become more heterogeneous, but at the same time induced spatially uniform colonization of new species, causing communities to homogenize. Consequently, the effects of these two processes canceled each other out. The relative importance of extinction and colonization components per se also changed temporally after the treatments. Fire and harvesting showed synergetic negative impacts on extinction beta diversity between pre-treatment and 10 yr after the treatments. Overall, analyses using extinction and colonization beta diversity allowed us to detect nonrandom disassembly and reassembly dynamics in ground vegetation communities. Our results suggest that common practices of analyzing beta diversity at one point in time can mask significant variation driven by disturbance. Acknowledging the extinction-colonization dynamics behind beta diversity is essential for understanding the spatiotemporal organization of biodiversity.

Long‐term effects of colonization–extinction dynamics of generalist versus specialist wood‐decaying fungi

Abstract Long‐term metapopulation persistence is mediated by the dynamics of colonization and extinction. To understand and predict future species occupancy in changing landscapes, we must account for the dynamic rates that shape the occupancy and disentangle their dependence on environmental conditions. Specialist and generalist species may differ in their dynamics in systematic ways. Using an extensive, large‐scale repeat survey dataset for wood‐decaying fungi, we fitted dynamic metapopulation models for ten species, ranging from generalists to specialists with differing resource requirements. We first estimated base rates of colonization and extinction and tested their relationships to species' degree of specialization. We then tested for effects of local and landscape scale variables on the colonization and extinction rates. Finally, using the fitted models and a scenario of future forest development we projected future colonization–extinction dynamics over 100 years to test for differences in species occupancies between production stands and set‐aside stands. Our study revealed a striking pattern of decreasing colonization rates with increasing degree of specialization across species, along with concomitantly increasing extinction rates. Strong sensitivity of colonization probabilities to local habitat quality (dead wood volume and stand age) in specialist but not generalist species constrained the regional occupancy of specialists, especially in production forest. We found evidence for short‐range dispersal limitation in two specialists, but no effect of our measure for landscape‐scale connectivity on colonization rates. Simulations of future metapopulation dynamics resulted in decreasing and lower final occupancy in production forest than in set‐asides, especially of specialist species. In set‐aside stands, however, specialists increased over time to relatively high occupancies. Synthesis. Colonization–extinction dynamics of wood‐decaying fungi varied with species' degree of specialization. Increasing specialization was associated with increasing sensitivity of colonization rates to habitat conditions, and partly local connectivity, as well as with increasing extinction rates. Low landscape‐scale occupancies of specialist as compared to generalist wood‐decaying fungi are thus maintained by a combination of lower colonization rates, particularly in production forest, and higher extinction rates.

Do ecological specialization and functional traits explain the abundance–frequency relationship? Arable weeds as a case study

AbstractAimThe abundance–frequency relationship (AFR) is among the most‐investigated patterns in biogeography, yet the relative contributions of niche‐based processes related to ecological strategies, and of neutral processes related to spatial colonization–extinction dynamics, remains uncertain. Here, we tested the influences of ecological specialization and functional traits on local abundance and regional frequency, to determine the contribution of niche‐based processes.LocationFrance and the UK.TaxonVascular plants.MethodsWe used two arable weed surveys covering 1,544 fields in Western Europe (France, UK), along with functional traits related to resource acquisition, resource requirements, flowering phenology and dispersal. We quantified specialization both to arable habitat and to individual crop types, and performed phylogenetic path analyses to test competing models accounting for direct and indirect relationships between traits, specialization, abundance and frequency. We performed the analyses for all species in each country, as well as for a subset of the most abundant species.ResultsLocal abundance of weeds increased with their regional frequency, but the relationship became negative or null when considering only the most abundant weeds. Specialization to arable habitat and to individual crop type either had a similar or opposite effect on regional frequency and local abundance explaining these positive and negative relationships, respectively. Regional frequency was not directly explained by any trait but indirectly by resource requirement traits conferring specialization to the arable habitat. Conversely, high local abundance was directly related to low seed mass, high SLA, early and short flowering.Main conclusionsDirect/indirect effects of functional traits on local abundance/regional frequency, respectively, supports a significant role of niche‐based processes in AFR. Neutral spillover dynamics could further explain a direct linkage of abundance and frequency. Similar causal paths and consistent influences of traits on specialization and abundance in the two studied regions suggest genericity of these findings.

Integrating Disturbance, Seasonality, Multi-Year Temporal Dynamics, and Dormancy Into the Dynamics and Conservation of Metacommunities

The metacommunity framework has rapidly become a dominant concept used by ecologists to understand community assembly. By emphasizing extinction-colonization dynamics, dispersal, and species’ niche requirements in determining community structure, metacommunity theory unifies local and regional processes as integral to species distributions across landscapes. Metacommunity structure has traditionally been treated as static. However, habitat characteristics and community composition can shift through time because of factors like seasonal dynamics, ecosystem disturbance, multi-year climate variation (e.g., El Niño Southern Oscillation), and production and emergence of dormant propagules. In most systems, the relevance of such temporal variation for the structure and persistence of metacommunities is an open question that is of potential importance for conservation and management. We evaluate and synthesize the theory and concepts relevant to four major forms of temporal dynamics that are pertinent to metacommunities: disturbance, seasonality, multi-year climate variation, and dormancy. For each type of dynamic we review the theoretical underpinnings and empirical evidence to evaluate how the dynamic drives temporal variation in metacommunity structure. We also consider how major forms of anthropogenic change further influence these patterns. Our survey highlights that seasonal climatic differences can modify the distribution and availability of resources and connectivity, with consequences for species’ use of the landscape and species interactions. Disturbance and multi-year climate cycles can increase the importance of dispersal, but implications for environmental filtering and species interactions remain unresolved. Dormant life stages serve to anchor habitat patches and species pools in space and highlight the importance of dispersal-dormancy tradeoffs and environmental variation for metacommunity structure. Temporal variability in biotic and abiotic conditions associated with these drivers can modify the relative strengths of dispersal, environmental filtering, and species interactions, three biological processes that drive metacommunity structure. Inclusion of spatiotemporal drivers creates patterns of species diversity that differ from traditional metacommunity ideas. We use these insights to highlight research needs, suggest a reconceptualization of metacommunities as undergoing continuous change, and discuss the implications of temporal dynamism for the conservation and management of metacommunities.

Extinction-colonization dynamics upon a survival-dispersal trade-off

Dispersal is a key mechanism shaping observed ecological and evolutionary processes. Owing to its inherent costs, dispersal incur trade-offs with many life-history traits. Here we propose and investigate a metapopulation model that follows an extinction-colonization dynamics. The problem is investigated within an evolutionary perspective in which individuals are constrained by a trade-off relationship between survival and dispersal. The dependence of the evolved levels of dispersal on local extinction rates, mortality, and other selective pressures, as well as on the shape of the trade-off relationship and its strength is investigated. The influence of dispersal costs on the evolutionary outcome is dependent on the shape of the trade-off relationship: while for strong trade-offs increased dispersal mortality leads to reduced levels of dispersal, higher mobility is found when the trade-off is weak. Among the main findings is the existence of a critical trade-off strength beyond that the metapopulation is no longer viable, i.e., it is doomed to extinction. These features are not qualitatively changed as local adaptation is taken into account.

Mid-domain effect for food chain length in a colonization\u2013extinction model

The mid-domain effect states that in a spatially bounded domain species richness tends to decrease from the center towards the boundary, thus producing a peak or plateau of species richness in the middle of the domain even in the absence of any environmental gradient. This effect has been frequently used to describe geographic richness gradients of trophically similar species, but how it scales across different trophic levels is poorly understood. Here, we study the role of geometric constraints for the formation of spatial gradients in trophically structured metacommunities. We model colonization–extinction dynamics of a simple food chain on a network of habitat patches embedded in a one- or two-dimensional domain. In a spatially homogeneous or well-mixed system, we find that the food chain length increases with the square root of the ratio of colonization and extinction rates. In a spatially bounded domain, we find that the patch occupancy decreases towards the edge of the domain for all species of the food web, but this spatial gradient varies with the trophic level. As a consequence, the average food chain length peaks in the center and declines towards the boundaries of the domain, thereby extending the notion of a mid-domain effect from species richness to food chain length. This trophic mid-domain effect already arises in a one-dimensional domain, but it is most pronounced at the headlands in a two-dimensional domain. As the mid-domain effect for food chain length is caused solely by spatial boundaries and requires no other environmental heterogeneity, it can be considered a null expectation for geographic patterns in spatially extended food webs.

Extreme weather affects colonization–extinction dynamics and the persistence of a threatened butterfly

Abstract Extreme weather events can be expected to increase in frequency in the future. Our knowledge on how this may affect species persistence is, however, very limited. For reliable projections of future persistence we need to understand how extreme weather affects species' population dynamics. We analysed the effect of extreme droughts on the host plant Succisa pratensis, colonization–extinction dynamics, and future persistence of the threatened marsh fritillary Euphydryas aurinia. Specifically, we studied a metapopulation inhabiting a network of 256 patches on Gotland (Sweden), where the summer of 2018 was the driest ever recorded. We analysed how the frequency and leaf size of host plants changed between 2017 and 2019, based on 6,833 records in 0.5‐m2 sample plots. Using turnover data on the butterfly from 2018 to 2019 we modelled local extinction and colonization probabilities. Moreover, we projected future population dynamics with an increasing frequency of extreme years under three different management strategies that regulate the grazing regime. Our results show a substantial decrease in both frequency (46%) and size (20%) of host plants due to the drought, which taken together may constitute a 57% loss of food resources. The butterfly occupancy decreased by over 30% between 2018 and 2019 (from 0.36 to 0.27). The extinction probability increased with increasing ‘effective area’ of the patch (taking quality reduction due to grazing into account), and the colonization probability increased with increasing connectivity and ground moisture. Projections of future dynamics showed an increasing risk of metapopulation extinction with increasing frequency of years with extreme droughts. The risk, however, clearly differed between management strategies. Less grazing in years with droughts decreased the extinction risk considerably. Synthesis and applications. Extreme weather events can have profound negative impacts on butterflies and their host plants. For the marsh fritillary, an increased frequency of extreme droughts can lead to extinction of the entire metapopulation, even in a large and seemingly viable metapopulation. Increased grazing, due to fodder deficiency in dry years, may lead to cascading negative effects, while active management that reduce grazing in years with droughts can almost completely mitigate these effects.

Occupancy versus colonization-extinction models for projecting population trends at different spatial scales.

Understanding spatiotemporal population trends and their drivers is a key aim in population ecology. We further need to be able to predict how the dynamics and sizes of populations are affected in the long term by changing landscapes and climate. However, predictions of future population trends are sensitive to a range of modeling assumptions. Deadwood‐dependent fungi are an excellent system for testing the performance of different predictive models of sessile species as these species have different rarity and spatial population dynamics, the populations are structured at different spatial scales, and they utilize distinct substrates. We tested how the projected large‐scale occupancies of species with differing landscape‐scale occupancies are affected over the coming century by different modeling assumptions. We compared projections based on occupancy models against colonization–extinction models, conducting the modeling at alternative spatial scales and using fine‐ or coarse‐resolution deadwood data. We also tested effects of key explanatory variables on species occurrence and colonization–extinction dynamics. The hierarchical Bayesian models applied were fitted to an extensive repeated survey of deadwood and fungi at 174 patches. We projected higher occurrence probabilities and more positive trends using the occupancy models compared to the colonization–extinction models, with greater difference for the species with lower occupancy, colonization rate, and colonization:extinction ratio than for the species with higher estimates of these statistics. The magnitude of future increase in occupancy depended strongly on the spatial modeling scale and resource resolution. We encourage using colonization–extinction models over occupancy models, modeling the process at the finest resource‐unit resolution that is utilizable by the species, and conducting projections for the same spatial scale and resource resolution at which the model fitting is conducted. Further, the models applied should include key variables driving the metapopulation dynamics, such as the availability of suitable resource units, habitat quality, and spatial connectivity.