Differences in spatial synchrony and interspecific concordance inform guild‐level population trends for aerial insectivorous birds

Many animal species exhibit spatiotemporal synchrony in population fluctuations, which may provide crucial information about ecological processes driving population change. We examined spatial synchrony and concordance among population trajectories of five aerial insectivorous bird species: chimney swift Chaetura pelagica, purple martin Progne subis, barn swallow Hirundo rustica, tree swallow Tachycineta bicolor, and northern rough‐winged swallow Stelgidopteryx serripennis. Aerial insectivores have undergone severe guild‐wide declines that were considered more prevalent in northeastern North America. Here, we addressed four general questions including spatial synchrony within species, spatial concordance among species, frequency of declining trends among species, and geographic location of declining trends. We used dynamic factor analysis to identify large‐scale common trends underlying stratum‐specific annual indices for each species, representing population trajectories shared by spatially synchronous populations, from 46 yr of North American Breeding Bird Survey data. Indices were derived from Bayesian hierarchical models with continuous autoregressive spatial structures. Stratum‐level spatial concordance among species was assessed using cross‐correlation analysis. Probability of long‐term declining trends was compared among species using Bayesian generalized linear models. Chimney swifts exhibited declining trends throughout North America, with less severe declines through the industrialized Mid‐Atlantic and Great Lakes regions. Northern rough‐winged swallows exhibited declining trends throughout the west. Spatial concordance among species was limited, the proportion of declining trends varied among species, and contrary to previous reports, declining trends were not more prevalent in the northeast. Purple martins, barn swallows, and tree swallows exhibited synchrony across smaller spatial scales. The extensive within‐species synchrony and limited concordance suggest that population trajectories of these aerial insectivores are responding to large‐scale but complex and species‐ and region‐specific environmental conditions (e.g. climate, land use). A single driver of trends for aerial insectivores as a guild appears unlikely.     

Spatial dynamics in the absence of dispersal: acorn production by oaks in central coastal California

We examined spatial patterns and spatial autocorrelation (synchrony) of annual acorn production in three species of oaks (genus Quercus ) over A 288 km transect in central coastal California. Over small (within-site) distances of <4 km, synchrony of acorn production between individual trees wits significant but varied through time and. for coast live oaks Q. agrifolia. differed al two sites 135 km apart. On a larger geographic scale, valley Q. lobata and blue Q. douglasii oaks exhibited significant synchrony in most distance categories between trees and sites up to 135 km apart and. in the case of coast live oaks, up to the maximum extent of the transect. Spatial patterns over this geographic scale also differed among species, with valley and blue oaks, but not coast live oaks, exhibiting distinct declines in synchrony of acorn production with distance. Interspecific synchrony in acorn production was generally lower than that within species but still significant over the entire extent of the survey. Spatial synchrony between sites was to some extent related to the same environmental variables previously found to correlate with annual acorn production within a site, suggesting that the environmental factors determining acorn production locally also influence spatial patterns over larger geographic areas. These results demonstrate that mast-fruiting in oaks occurs not only on a widespread geographic scale but also across species. They also confirm that synchrony over large geographic areas and complex spatial patterns varying in time can occur in systems where dispersal does not occur and thus environmental variability (the Moran effect) alone is likely to be driving spatial dynamics.     

Similarity in seasonal flow regimes,not regional environmental classifications explain synchrony in brown trout population dynamics in France

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Spatially patterned guild structure in larval parasitoids of cyclically outbreaking winter moth populations

1. Wide temporal fluctuations in host abundance are a potential source of instability and stochasticity in the spatiotemporal population dynamics of associated parasitoid species. Within parasitoid guilds (i.e. parasitoids with similar modes of host utilisation), a conceivable outcome is guild organisation according to a lottery model, in which guild members attain local dominance by colonising previously emptied habitats during increasing host density, before other guild members. In the spatial dimension, an expected manifestation of such dynamics is variable guild structure even across homogeneous habitats. 2. We examined the extent of large‐scale spatial patterning of guild characteristics in larval parasitoid wasps associated with cyclically outbreaking populations of the geometrid moth Operophtera brumata in northern Fennoscandia. The study was performed at the onset of the crash‐phase of the geometrid's outbreak cycle, along a 70‐km transect in costal northern Norway, characterised by largely homogeneous environmental conditions, except for a small climatic gradient. 3. There was a distinct large‐scale spatial turnover in dominance among the major parasitoid groups (i.e. guild structure) in O. brumata along the transect, whereas the total prevalence rate of the guild and its diversity showed no consistent variation. Guild structure was unrelated to host density. 4. Although group‐specific responses to a slight spatial climatic gradient cannot be rejected as a causal mechanism, we conclude that our results are consistent with the expectation from large‐scale stochastic extinction‐recolonisation dynamics among functionally equivalent parasitoids relying on a host with strongly cyclic population fluctuations.     

Synchrony in small mammal community dynamics across a forested landscape

Long‐term studies at local scales indicate that fluctuations in abundance among trophically similar species are often temporally synchronized. Complementary studies on synchrony across larger spatial extents are less common, as are studies that investigate the subsequent impacts on community dynamics across the landscape. We investigate the impact of species population fluctuations on concordance in community dynamics for the small mammal fauna of the White Mountain National Forest, USA. Hierarchical open population models, which account for imperfect detection, were used to model abundance of the most common species at 108 sites over a three year period. Most species displayed individualistic responses of abundance to forest type and physiographic characteristics. However, among species, we found marked synchrony in population fluctuations across years, regardless of landscape affinities or trophic level. Across the region, this population synchrony led to high within‐year concordance of community composition and aggregate properties (e.g. richness and diversity) independent of forest type and low among‐year similarity in communities, even for years with similar species richness. Results suggest that extrinsic factors primarily drive abundance fluctuations and subsequently community dynamics, although local community assembly may be modified by species dispersal abilities and biotic interactions. Concordant community dynamics across space and over time may impact the stability of regional food webs and ecosystem functions.     

Population ecology and spatial synchrony in the abundance of leaf gall wasps within and among populations of valley oak (Quercus lobata)

What factors drive population variability through space and time? Here we assess patterns of abundance of seven species of gall wasps in three genera occurring on the leaves of valley oaks (Quercus lobata) at 10 sites throughout this species' statewide range in California, from 2000 to 2006. Our primary goals were to understand the factors driving variability in gall abundance and to assess the extent of spatial synchrony in gall wasp communities at both large and small geographic scales. On the large, statewide scale, there was significant site-to-site variation in gall abundance, driven in all cases primarily by differences in mean maximum seasonal temperatures, and lesser year-to-year variation. In contrast, on the small, local scale, differences were more pronounced from year to year than from tree to tree, and were to some extent correlated with differences in acorn production, suggesting an interaction with the reproductive effort of hosts. Significant spatial synchrony was detected, particularly at the statewide scale, but in no case did synchrony decline significantly with distance, despite sites being up to 741 km apart. Variation in spatial synchrony was correlated with a number of exogenous factors, including seasonal weather conditions, the acorn crop at the statewide scale and soil phosphorus availability at the local scale; however, most variation in spatial synchrony in our analyses remained unexplained.     

Spatial dynamics of Microtus vole populations in continuous and fragmented agricultural landscapes

Small mammal populations often exhibit large-scale spatial synchrony, which is purportedly caused by stochastic weather-related environmental perturbations, predation or dispersal. To elucidate the relative synchronizing effects of environmental perturbations from those of dispersal movements of small mammalian prey or their predators, we investigated the spatial dynamics of Microtus vole populations in two differently structured landscapes which experience similar patterns of weather and climatic conditions. Vole and predator abundances were monitored for three years on 28 agricultural field sites arranged into two 120-km-long transect lines in western Finland. Sites on one transect were interconnected by continuous agricultural farmland (continuous landscape), while sites on the other were isolated from one another to a varying degree by mainly forests (fragmented landscape). Vole populations exhibited large-scale (>120 km) spatial synchrony in fluctuations, which did not differ in degree between the landscapes or decline with increasing distance between trapping sites. However, spatial variation in vole population growth rates was higher in the fragmented than in the continuous landscape. Although vole-eating predators were more numerous in the continuous agricultural landscape than in the fragmented, our results suggest that predators do not exert a great influence on the degree of spatial synchrony of vole population fluctuations, but they may contribute to bringing out-of-phase prey patches towards a regional density level. The spatial dynamics of vole populations were similar in both fragmented and continuous landscapes despite inter-landscape differences in both predator abundance and possibilities of vole dispersal. This implies that the primary source of synchronization lies in a common weather-related environment.     

Invasion patterns across multiple scales by Hieracium species over 25 years in tussock grasslands of New Zealand's South Island

Studying patterns of species invasions over time at multiple spatial scales may help us to elucidate important factors driving those patterns and how they change according to temporal or spatial resolution. Here we provide a large, long‐term, landscape‐scale study of the invasion of three Hieracium species using a dataset that encompasses vegetation change on 124 transects over 25 years across the lower eastern South Island of New Zealand. We investigated the relationships between key environmental and ecological factors and the invasion trajectories of H. lepidulum, H. pilosella and H. praealtum, at two spatial scales: (i) among‐transect colonization and (ii) within‐transect changes in frequency and per cent cover. Our results show that the colonization and spread of Hieracium species among and within transects reflect (i) the importance of initial environmental and biological conditions, (ii) that our sampling captured different periods of the invasion trajectories of each of the three species, and (iii) the effects of differences in life histories of the three species.     

Spatio‐temporal covariation in abundance between the cyclic common vole Microtus arvalis and other small mammal prey species

Populations of the common vole Microtus arvalis in mid‐western France show cyclic dynamics with a three‐year period. Studies of cyclic vole populations in Fennoscandia have often found inter‐specific synchrony between the voles and other small mammals which share the voles' predators. Although predators are central to the favoured mechanism to explain Fennoscandian vole cycles and the spatial variation of small mammal populations, their role in vole cycles elsewhere, including France, is less clear. Establishing whether alternative prey species in France cycle in parallel with voles as they do in Fennoscandia is thus an important step towards understanding the generality of predators' influence on cyclic vole populations. We applied spatial and temporal autocorrelation and cross‐correlation methods to French populations of M. arvalis and two sympatric non‐cyclic small mammal species, Apodemus sylvaticus and Crocidura russula. Patterns of time‐lagged cross‐correlation between the abundance of M. arvalis and the other two species suggested synchrony in their dynamics beyond that expected of stochastic environmental variation, and indicated a weak three‐year cycle in A. sylvaticus and C. russula that was in phase with that of M. arvalis. We interpret the synchrony between these species as the effect of shared predators and environmental stochasticity. Abundance within species showed weak spatial autocorrelation in June at scales consistent with dispersal being the mechanism responsible, but a more general lack of spatial structure within and between species was consistent with the strong spatial synchrony at regional scales often found in fluctuations of small mammal abundance.     

Comparing agglomerative clustering and three weed classification frameworks to assess the invasiveness of alien species across spatial scales

To prioritize weed management at the catchment scale, information is required on the species present, their relatively frequency, abundance, and likely spread and impact. The objective of this study was to classify the invasiveness of alien species that have invaded the Upper Burdekin Catchment in Queensland, Australia, at three spatial scales. A combination of three published weed classification frameworks and multivariate techniques were employed to classify species based on their frequency and cover at a range of spatial scales. We surveyed the Upper Burdekin Catchment for alien species, and for each species determined the following distribution indices — site frequency, total cover, transect frequency per site frequency and quadrat frequency per site frequency, cover per quadrat when present, cover per transect when present, and cover per site when present. These indices capture the effect of species abundance and frequency between sites (site frequency and total cover), within sites (transect frequency per site and cover per transect when present), and within transects (quadrat frequency per site frequency and cover per site). They were used to classify the species into seven groups using a hierarchical cluster analysis. The relationship between the indices was explored to determine how effective the small scale, site‐specific indices were at predicting the broader, landscape‐scale patterns. Strong correlations were observed between transect frequency per site and frequency (r² = 0.89) and cover per transect when present and total cover (r² = 0.62). This suggests that if a weed is abundant at the site level, it has the potential to occupy large areas of the catchment. The species groupings derived from the application of the three published weed classification frameworks were compared graphically to the groupings derived from the cluster analysis. One of the frameworks classified species into three groups. The other two frameworks classified species into four groups. There was a high degree of subjectivity in applying the frameworks to the survey data. Some of the data were of no relevance to the classification frameworks and were therefore ignored. We suggest that the weed classification frameworks should be used in conjunction with existing multivariate techniques to ensure that classifications capture important natural variations in observed data that may reflect invasion processes. The combined use of the frameworks and multivariate techniques enabled us to aggregate species into categories appropriate for management.     

Influence of experimental warming and shading on host–parasitoid synchrony

As the climate warms, many species are showing altered phenology patterns, potentially disrupting synchrony between interacting species. Recent studies have documented disrupted synchrony in plant–herbivore and predator–prey interactions. However, studies investigating climate‐related asynchrony in host–parasitoid interactions and exploring the relative responses of interacting hosts and parasitoids to climate change are lacking. This is an important gap in knowledge given the ubiquity of insect parasitoids and their importance in influencing the abundance and dynamics of their hosts. In the threatened marsh fritillary butterfly Euphydryas aurinia (Lepidoptera: Nymphalidae) and its specialized parasitoid, Cotesia bignellii (Hymenoptera: Braconidae) phenological synchrony (and consequently population fluctuations) are thought to be weather‐dependent. To assess the likely influence of climate and microenvironment change on synchrony between E. aurinia and C. bignellii, we experimentally manipulated the exposure of sensitive‐stage host larvae and parasitoid pupae to temperature (ambient or elevated) and shading (shaded or unshaded) regimes. We also analysed a 20‐year population dynamic dataset from the United Kingdom for E. aurinia to investigate whether population variations could be explained by interannual variations in the thermal and sunshine environment. Development times were affected significantly by the experimental temperature and shading treatments for E. aurinia but not for C. bignellii. However, the contrasting responses were insufficient to significantly affect host availability for parasitoids. In the field, thermal and sunshine conditions did not influence population fluctuations, and population variations across a large (UK‐wide) scale were uncorrelated. Changes to the thermal and sunshine environment of the magnitude investigated in our experiment and within the range experienced by wild E. aurinia populations over the last 20‐years thus seem unlikely to cause breakdown in host–parasitoid synchrony. We suggest that experiments investigating the mechanistic responses of interacting species to environmental change are needed to support the analysis and interpretation of observational data on species' phenology.     

Population differentiation or species formation across the Indian and the Pacific Oceans? An example from the brooding marine hydrozoan Macrorhynchia phoenicea

Assessing population connectivity is necessary to construct effective marine protected areas. This connectivity depends, among other parameters, inherently on species dispersal capacities. Isolation by distance (IBD ) is one of the main modes of differentiation in marine species, above all in species presenting low dispersal abilities. This study reports the genetic structuring in the tropical hydrozoan Macrorhynchia phoenicea α (sensu Postaire et al ., 2016a), a brooding species, from 30 sampling sites in the Western Indian Ocean and the Tropical Southwestern Pacific, using 15 microsatellite loci. At the local scale, genet dispersal relied on asexual propagation at short distance, which was not found at larger scales. Considering one representative per clone, significant positive F _IS values (from ?0.327*** to 0.411***) were found within almost all sites. Gene flow was extremely low at all spatial scales, among sites within islands (<10 km distance) and among islands (100 to >11,000 km distance), with significant pairwise F _ST values (from 0.035*** to 0.645***). A general pattern of IBD was found at the Indo‐Pacific scale, but also within ecoregions in the Western Indian Ocean province. Clustering and network analyses identified each island as a potential independent population, while analysis of molecular variance indicated that population genetic differentiation was significant at small (within island) and intermediate (among islands within province) spatial scales. As shown by this species, a brooding life cycle might be corollary of the high population differentiation found in some coastal marine species, thwarting regular dispersal at distances more than a few kilometers and probably leading to high cryptic diversity, each island housing independent evolutionary lineages.     

Patterns of genetic differentiation in Thamnophis and Taricha from the Pacific Northwest

Aim The co‐evolutionary interaction between the common garter snake, Thamnophis sirtalis, and the rough‐skinned newt, Taricha granulosa, takes place throughout much of the Pacific Northwest (North America). The biogeography of the Pacific Northwest has been heavily influenced by the last Pleistocene glaciation, which reached a maximum as late as 14,000 yr bp . We researched: (1) what type of population structure is present for garter snakes and newts, (2) whether the population structure of these species is consistent with a Pleistocene glaciation hypothesis, and (3) how population structure and migration possibly affect co‐evolution between these species. Location The Pacific Northwest of North America, specifically northern California, Oregon and Washington in the USA. Methods We sampled approximately 20 populations for each species from three different transects. Using microsatellite markers and tissue samples from both species, we quantified the population structure for both species. Individual‐based assignment tests were used to estimate contemporary migration rates. Results Both Th. sirtalis and Ta. granulosa exhibited little genetic differentiation among our study sites, even among those separated by large distances. Significant population structure was detected on multiple geographic scales. Differences in population structure were observed among transects and between garter snake and newt transects. Contemporary migration rate estimates indicate high levels of genetic exchange between populations. Main conclusions Prior to this study, little was known about the fine‐scale population structure of either species in this region. Patterns of population structure for garter snakes and newts reflect a shared biogeographical history affected by the Pleistocene glaciation in the Pacific Northwest. Both species apparently migrate frequently between populations, thus potentially retarding the process of adaptive co‐evolution. We find that populations from a northern coastal transect (Washington) are most likely to be locally adapted.     

Population Fluctuations and Synchrony of Grassland Butterflies in Relation to Species Traits

Population fluctuations and synchrony influence population persistence; species with larger fluctuations and more synchronised population fluctuations face higher extinction risks. Here, we analyse the effect of diet specialisation, mobility, length of the flight period, and distance to the northern edge of the species’ distribution in relation to between-year population fluctuations and synchrony of butterfly species. All butterfly species associated with grasslands were surveyed over five successive years at 19 grassland sites in a forest-dominated landscape (50 km²) in southern Sweden. At both the local and regional level, we found larger population fluctuations in species with longer flight periods. Population fluctuations were more synchronous among localities in diet specialists. Species with a long flight period might move more to track nectar resources compared to species with shorter flight period, and if nectar sources vary widely between years and localities it may explain that population fluctuations increase with increasing flight length. Diet generalists can use different resources (in this case host plants) at different localities and this can explain the lower synchrony in population fluctuations among generalist species. Higher degree of synchrony is one possible explanation for the higher extinction risks that have been observed for more specialised species. Therefore, diet specialists are more often threatened and require more conservation efforts than generalists.     

Effects of rain forest logging on species richness and assemblage composition of small mammals in Southeast Asia

Aim The effects of logging and habitat degradation on the richness and abundance of small mammals in Asian rain forests are largely unknown. This work compares the species richness, dominance and evenness of small non‐volant mammals between logged and unlogged forests, and assesses whether assemblage variability (β‐diversity) is similar between forest types. Location Southeast Asia, northern Borneo (Sabah, Malaysia), Sunda‐shelf. Methods We surveyed species‐rich assemblages of small non‐volant mammals in three unlogged and three logged forests for 2 years. At each forest site, we sampled a permanently marked transect and two additional sites in three trapping sessions. All analyses were performed at both levels to include the effects of local abundances and point estimates, separately from the relative abundances of species on a more regional scale. Results We trapped a total of 1218 individuals of 28 species. Eleven common species accounted for 95% of all captures. Species richness and diversity were significantly higher in unlogged forest (27 species) than in logged forest (17 species). This was mainly attributable to the smaller number of rarely recorded species in logged forest (five compared with 16 in unlogged forest, with a total of fewer than 10 captures). However, all common species were present in both logged and unlogged forests, and our analyses revealed similar patterns of dominance, evenness and fluctuations in abundance. Hence overall assemblage composition in multivariate space did not differ greatly between forest types. Assemblages of Muridae and Tupaiidae showed similar population fluctuations in space and time, indicating that the ecology of these taxa may be partially driven by the same environmental factors. Main conclusions Although species were distributed patchily within sites, analyses at local and regional scales revealed similar patterns in diversity and assemblage variability, suggesting that effects of forest modification did not differ extensively locally and regionally, but had a profound effect on rare species. Our results emphasize the importance and conservation value of logged forest stands that are able to hold a large proportion of the small mammals also found in unlogged forests. Rare and more specialized species are more vulnerable to forest degradation than commonly caught species, resulting in the complete loss, or a decrease in numbers, of certain groups, such as arboreal small mammals and Viverridae.     

Synchrony of butterfly populations across species' geographic ranges

Understanding the mechanisms by which global climate change and habitat loss impact upon biodiversity is essential in order to mitigate any negative impacts. One such impact may be changes to population synchrony (defined as correlated fluctuations in the density of separate populations). It is well established that synchrony depends on both dispersal ability and correlated environmental conditions, for example shared climate. However, what is not clear is whether differences in habitat or position within a species' range also mediate synchrony. Since synchronous metapopulations are thought to be more extinction‐prone, establishing the drivers of synchrony has clear conservation implications. Using three butterfly species (Maniola jurtina, Pyronia tithonus and Aphantopus hyperantus) we investigated the effects of habitat similarity and range position on population synchrony, after accounting for the effects of distance and climate. Range position was present in all minimum adequate models, though non‐significant using Mantel randomization tests in one case. We show that M. jurtina and P. tithonus synchrony is not consistent across species' ranges, with marginal populations showing more synchronous dynamics. Increased climatic constraints on marginal populations, leading to a narrower range of suitable microhabitats may be responsible for this, which is supported by the result that habitat similarity between sites was also positively correlated with population synchrony. As the landscape becomes increasingly homogeneous, overall population synchrony may be expected to rise. We conclude that habitat modification and climate change have the capacity to drive changes in population synchrony that could make species more vulnerable to extinction.     

Stability and synchrony across ecological hierarchies in heterogeneous metacommunities: linking theory to data

Understanding stability across ecological hierarchies is critical for landscape management in a changing world. Recent studies showed that synchrony among lower‐level components is key to scaling temporal stability across two hierarchical levels, whether spatial or organizational. But an extended framework that integrates both spatial scale and organizational level simultaneously is required to clarify the sources of ecosystem stability at large scales. However, such an extension is far from trivial when taking into account the spatial heterogeneities in real‐world ecosystems. In this paper, we develop a partitioning framework that bridges variability and synchrony measures across spatial scales and organizational levels in heterogeneous metacommunities. In this framework, metacommunity variability is expressed as the product of local‐scale population variability and two synchrony indices that capture the temporal coherence across species and space, respectively. We develop an R function ‘var.partition’ and apply it to five types of desert plant communities to illustrate our framework and test how diversity shapes synchrony and variability at different hierarchical levels. As the observation scale increased from local populations to metacommunities, the temporal variability of plant productivity was reduced mainly by factors that decreased species synchrony. Species synchrony decreased from local to regional scales, and spatial synchrony decreased from species to community levels. Local and regional species diversity were key factors that reduced species synchrony at the two scales. Moreover, beta diversity contributed to decreasing spatial synchrony among communities. We conclude that our new framework offers a valuable toolbox for future empirical studies to disentangle the mechanisms and pathways by which ecological factors influence stability at large scales.     

Do smaller islands host younger populations? A case study on metapopulations of three carabid species

Aims To test a new concept for island ecology and metapopulation studies: if population persistence is a result of colonization and extinction processes, one would expect remarkable variability in the age of neighbouring populations – there would be both old and young (recently established) populations. Location The lake Mamry archipelago, Poland. Methods Forty trap transects with a total of 114 Barber traps were operating from June to September in 1997 and in 1998. The traps (0.5‐L plastic beakers, mouth diameter 120 mm, with a 20 × 20 cm wooden roof) were placed by applying a stratified sampling design into 13 habitat types on the 16 islands and two mainland sites. The frequency of macropterous individuals in wing‐dimorphic populations of three carabid species (Pterostichus melanarius, P. anthracinus and Carabus granulatus) was used as an indicator of population ‘age’. Results The frequency of macropterous individuals in the populations varied from 0% to 100% for P. melanarius (18.5% on average), from 0% to 91% for P. anthracinus (12.6%) and from 0% to 29% for C. granulatus (8.9%). Populations hosted more long‐winged individuals (and were therefore interpreted as being younger) on smaller islands, compared with those inhabiting large islands and the mainland sites. The results also revealed that the viability of the populations of the autumn‐breeding P. melanarius might be more affected by population size than that of the two studied spring breeders. Island connectivity did not have a significant effect on the frequency of macropterous individuals in the studied populations. Main conclusions The inverse relationship between the proportion of macropterous individuals and island size contrasts with the accepted theory of dispersal, which assumes that there is selection against dispersal on small islands. A regression analysis for population age and habitat characteristics reveals the extinction probability of a given population. I suggest that investigations based on variability in population age can help in studies of colonization–extinction processes that would otherwise face logistic and methodological obstacles.     

Niche partitioning and the effect of interspecific competition on microhabitat use by two sympatric galaxiid stream fishes

1. Numerous interacting abiotic and biotic factors influence niche use and assemblage structure of freshwater fishes, but the strength of each factor changes with spatial scale. Few studies have examined the role of interspecific competition in structuring stream fish assemblages across spatial scales. We used field and laboratory approaches to examine microhabitat partitioning and the effect of interspecific competition on microhabitat use in two sympatric stream fishes (Galaxias‘southern’ and Galaxias gollumoides) at large (among streams and among sites within streams) and small (within artificial stream channels) spatial scales. 2. Diurnal microhabitat partitioning and interspecific competition at large spatial scales were analysed among three sympatry streams (streams with allotopic and syntopic sites; three separate catchments) and four allopatry streams (streams with only allotopic sites; two separate catchments). Electro‐fishing was used to sample habitat use of fishes at 30 random points within each site by quantifying four variables for each individual: water velocity, depth, distance to nearest cover and substratum size. Habitat availability was then quantified for each site by measuring those variables at each of 50 random points. Diet and stable isotope partitioning was analysed from syntopic sites only. Diel cycles of microhabitat use and interspecific competition at small spatial scales were examined by monitoring water velocity use over 48 h in artificial stream channels for three treatments: (i) allopatric G. ‘southern’ (10 G. ‘southern’); (ii) allopatric G. gollumoides (10 G. gollumoides) and (iii) sympatry (five individuals of each species). 3. One hundred and ninety‐four G. ‘southern’ and 239 G. gollumoides were sampled across all seven streams, and habitat availability between the two species was similar among all sites. Galaxias‘southern’ utilised faster water velocities than G. gollumoides in both the field and in channel experiments. Both species utilised faster water velocities in channels at night than during the day. Diet differences were observed and were supported by isotopic differences (two of three sites). No interspecific differences were observed for the other three microhabitat variables in the field, and multivariate habitat selection did not differ between species. Interspecific competition had no effect on microhabitat use of either species against any variable either in the field (large scale) or in channels (small scale). 4. The results suggest that niche partitioning occurs along a subset of microhabitat variables (water velocity use and diet). Interspecific competition does not appear to be a major biotic factor controlling microhabitat use by these sympatric taxa at any spatial scale. The results further suggest that stream fish assemblages are not primarily structured by biotic factors, reinforcing other studies de‐emphasising interspecific competition.     

Spatial and temporal synchrony in reptile population dynamics in variable environments

Resources are seldom distributed equally across space, but many species exhibit spatially synchronous population dynamics. Such synchrony suggests the operation of large-scale external drivers, such as rainfall or wildfire, or the influence of oasis sites that provide water, shelter, or other resources. However, testing the generality of these factors is not easy, especially in variable environments. Using a long-term dataset (13–22 years) from a large (8000 km²) study region in arid Central Australia, we tested firstly for regional synchrony in annual rainfall and the dynamics of six reptile species across nine widely separated sites. For species that showed synchronous spatial dynamics, we then used multivariate follow a multivariate auto-regressive state–space (MARSS) models to predict that regional rainfall would be positively associated with their populations. For asynchronous species, we used MARSS models to explore four other possible population structures: (1) populations were asynchronous, (2) differed between oasis and non-oasis sites, (3) differed between burnt and unburnt sites, or (4) differed between three sub-regions with different rainfall gradients. Only one species showed evidence of spatial population synchrony and our results provide little evidence that rainfall synchronizes reptile populations. The oasis or the wildfire hypotheses were the best-fitting models for the other five species. Thus, our six study species appear generally to be structured in space into one or two populations across the study region. Our findings suggest that for arid-dwelling reptile populations, spatial and temporal dynamics are structured by abiotic events, but individual responses to covariates at smaller spatial scales are complex and poorly understood.