Habitat Geometry Research Articles

Interactions between habitat quality and connectivity affect immigration but not abundance or population growth of the butterfly,Parnassius smintheus

Habitat geometry has been a primary focus in studies of spatially structured systems. Recent studies have indicated that a more comprehensive approach including habitat quality may be needed, however most previous studies have neglected potential interactions between quality and geometry. We investigated the effects of habitat quality for the butterflyParnassius smintheusamong a series of 17 sub‐populations. Specifically, we examined how habitat connectivity and local nectar flower density affect dispersal, and local population abundance and growth. We first determined which flower species were potentially important by examining nectar flower electivity and then quantified nectar flower density in meadows over a five year period (2003–2007). These data along with meadow connectivity were compared to local population statistics derived from mark–recapture over the same time period. The number of immigrants to a meadow increased as meadow connectivity increased, but showed no direct relationship with nectar flower density; however, there was a significant interaction between meadow connectivity and nectar flower density such that meadows with high connectivity and a high density of nectar flowers received the greatest number of immigrants. The number of emigrants from a meadow increased with increasing habitat quality and connectivity, but showed no interactive effect.The abundance of butterflies increased with meadow connectivity, but showed no relationship with habitat quality or any interactive effect. Separate experiments showed that access to nectar flowers significantly increased female reproductive output, but not lifespan. Despite the effects on immigration and reproductive output, local population growth rates also showed no relationship to nectar flower density. Our results indicate that habitat quality can be important for immigration in spatially structured populations; however, effects of habitat quality may not necessarily translate into higher abundance or population growth. Additionally, habitat quality should not be considered independently from habitat isolation, particularly if it directly affects dispersal. Preserving or augmenting habitat quality will do little to bolster immigration or colonization without adequate connectivity.

Genetic structure of peripheral, island-like populations: a case study of Saponaria bellidifolia Sm. (Caryophyllaceae) from the Southeastern Carpathians

Geographically peripheral populations often experience a reduction of genetic diversity and divergence from the core populations. Habitat geometry and quality can induce a local genetic diversity pattern, which overlies the regional variability issued from the range-wide phylogeo- graphy. We evaluated the genetic variation and genetic divergence of Saponaria bellidifolia Sm. on limestone outcrops within peripheral island-like populations from the Southeastern Carpathians, using RAPD markers. We also determined the degree of isolation related to other European populations, using AFLP. The Romanian populations had a decreased overall genetic diversity shared among popula- tions, with lower level in small populations. Potential habitat size had a positive effect on genetic diversity estimates. Fisher's exact tests of genetic differentiation revealed sig- nificant divergences only between the geographically most distant populations. Romanian populations were genetically pauperised as compared to Bulgarian and Italian populations and our results suggest that they might have originated from a recent range expansion from southern glacial refugia.

A MULTILOCUS PERSPECTIVE ON COLONIZATION ACCOMPANIED BY SELECTION AND GENE FLOW

The colonization of novel habitats involves complex interactions between founder events, selection, and ongoing migration, and can lead to diverse evolutionary outcomes from local extinction to adaptation to speciation. Although there have been several studies of the demography of colonization of remote habitats, less is known about the demographic consequences of colonization of novel habitats within a continuous species range. Populations of the Eastern Fence Lizard, Sceloporus undulatus, are continuously distributed across two dramatic transitions in substrate color in southern New Mexico and have undergone rapid adaptation following colonization of these novel environments. Blanched forms inhabit the gypsum sand dunes of White Sands and melanic forms are found on the black basalt rocks of the Carrizozo lava flow. Each of these habitats formed within the last 10,000 years, allowing comparison of genetic signatures of population history for two independent colonizations from the same source population. We present evidence on phenotypic variation in lizard color, environmental variation in substrate color, and sequence variation for mitochondrial DNA and 19 independent nuclear loci. To confirm the influence of natural selection and gene flow in this system, we show that phenotypic variation is best explained by environmental variation and that neutral genetic variation is related to distance between populations, not partitioned by habitat. The historical demography of colonization was inferred using an Approximate Bayesian Computation (ABC) framework that incorporates known geological information and allows for ongoing migration with the source population. The inferences differed somewhat between mtDNA and nuclear markers, but overall provided strong evidence of historical size reductions in both white sand and black lava populations at the time of colonization. Populations in both novel habitats appear to have undergone partial but incomplete recovery from the initial bottleneck. Both ABC analyses and measures of mtDNA sequence diversity also suggested that population reductions were more severe in the black lava compared to the white sands habitat. Differences observed between habitats may be explained by differences in colonization time, habitat geometry, and strength or response to natural selection for substrate matching. Finally, effective population size reductions in this system appear to be more dramatic when colonization is accompanied by a change in selection regime. Our analyses are consistent with a demographic cost of adaptation to novel environments and show that it is possible to infer aspects of the historical demography of local adaptation even in the presence of ongoing gene flow.

Landscape geometry determines community response to disturbance

Ecological communities are impacted by anthropogenic changes in both habitat geometry (i.e. amount, shape, fragmentation and connectivity of habitat) and disturbance regime. Although the effect of each of these drivers on diversity is well‐documented, few studies have considered how habitat geometry and disturbance interact to affect diversity. We used a miniature landscape of moss patches to experimentally manipulate both habitat geometry and disturbance frequency on microarthropod communities. Species richness and abundance in local patches declined linearly with disturbance rate in all experimental landscapes, but the speed of this decline (a measure of ecological resilience) depended on the size and connectivity of the surrounding region. Reductions in region size had little effect on community resilience to disturbance until habitat loss resulted in complete loss of connectivity between patches, suggesting a threshold in community response to habitat loss. Beyond this threshold, repeated disturbance resulted in rapid declines in patch species richness and abundance and substantial changes in community composition. These effects of habitat geometry and disturbance on diversity were scale‐dependent. Gamma (regional‐scale) diversity was unaffected by habitat geometry, suggesting experimental reductions in alpha (local‐scale) diversity were offset by increases in beta diversity. There was no effect of body size, abundance, or trophic position in determining species response to disturbance. Taxonomic grouping had a weak effect, with oribatids less affected by drought. We conclude that, in this system, dispersal from the surrounding metacommunity is vital in allowing recovery of local communities from disturbance. When habitat loss and fragmentation disrupt this process, extinctions result. Studies that examine separately the effects of habitat alterations and disturbance on diversity may therefore underestimate their combined effects.

Predicting Abundance Of Desert Riparian Birds: Validation And Calibration Of The Effective Area Model

Reliable prediction of the effects of landscape change on species abundance is critical to land managers who must make frequent, rapid decisions with long-term consequences. However, due to inherent temporal and spatial variability in ecological systems, previous attempts to predict species abundance in novel locations and/or time frames have been largely unsuccessful. The Effective Area Model (EAM) uses change in habitat composition and geometry coupled with response of animals to habitat edges to predict change in species abundance at a landscape scale. Our research goals were to validate EAM abundance predictions in new locations and to develop a calibration framework that enables absolute abundance predictions in novel regions or time frames. For model validation, we compared the EAM to a null model excluding edge effects in terms of accurate prediction of species abundance. The EAM outperformed the null model for 83.3% of species (N=12) for which it was possible to discern a difference when considering 50 validation sites. Likewise, the EAM outperformed the null model when considering subsets of validation sites categorized on the basis of four variables (isolation, presence of water, region, and focal habitat). Additionally, we explored a framework for producing calibrated models to decrease prediction error given inherent temporal and spatial variability in abundance. We calibrated the EAM to new locations using linear regression between observed and predicted abundance with and without additional habitat covariates. We found that model adjustments for unexplained variability in time and space, as well as variability that can be explained by incorporating additional covariates, improved EAM predictions. Calibrated EAM abundance estimates with additional site-level variables explained a significant amount of variability (P < 0.05) in observed abundance for 17 of 20 species, with R2 values >25% for 12 species, >48% for six species, and >60% for four species when considering all predictive models. The calibration framework described in this paper can be used to predict absolute abundance in sites different from those in which data were collected if the target population of sites to which one would like to statistically infer is sampled in a probabilistic way.

The impact of habitat loss and fragmentation on genetic drift and fixation time

In this study, a simple genetic model is integrated with an established method from landscape ecology to investigate the effect of habitat geometry and availability on genetic drift. Previous ecological modelling has identified a sharp threshold in habitat availability for species’ persistence, beyond which the species rapidly becomes extinct. This study demonstrates the existence of a similar threshold for fixation time of selectively neutral genotypes by genetic drift, the location of which is determined by habitat shape and spatial correlation of habitat loss. Time to fixation is greater for habitats if they are long and thin rather than square. Despite reductions in population size due to habitat loss, fixation time remains relatively constant until a pre‐threshold value, beyond which there is often a substantial increase in time to fixation. Further habitat loss results in the percolation threshold being reached and beyond this point the time to fixation decreases very rapidly. This study reveals a complex relationship between habitat availability, habitat geometry and the process of genetic drift. Possible implications of our results for conservation are discussed. Further work is required to improve our understanding of the interaction between evolutionary, ecological and landscape processes.

Habitat geometry, population viscosity and the rate of genetic drift

In all natural populations, individuals located close to one another tend to interact more than those further apart. The extent of population viscosity can have important implications for ecological and evolutionary processes. Here we develop a spatially explicit population model to examine how the rate of genetic drift depends upon both spatial population structure and habitat geometry. The results show that the time to fixation for a new and selectively neutral mutation is dramatically increased in viscous populations. Furthermore, in viscous populations the time to fixation depends critically on habitat geometry. Fixation time for populations of identical size increases markedly as landscape width decreases and length increases. We suggest that similar effects will also be important in metapopulations, with the spatial arrangement of subpopulations and their connectivity likely to determine the rate of drift. We argue that the recent increases in computer power should facilitate major advances in our understanding of evolutionary landscape ecology over the next few years, and suggest that the time is ripe for a unification of spatial population dynamics theory, landscape ecology and population genetics.

A physical habitat model for predicting the effects of flow fluctuations in nursery habitats of the endangered Colorado pikeminnow (Ptychocheilus lucius)

Larval and juvenile Colorado pikeminnow (Ptychocheilus lucius) use shallow, low-velocity, channel-margin areas (backwaters) as nursery habitats. It is hypothesized that within-day flow fluctuations caused by hydropower operations can directly affect the suitability of such habitats by altering water temperature and habitat geometry. Despite the importance of backwaters to juvenile fishes, there is a lack of established approaches for modelling how river management affects these habitats. Here, we describe a physical habitat model that predicts the effects of mainstem flow variation on backwater temperature, geometry and invertebrate availability. We specifically modelled these effects on habitat in a portion of the Green River in Utah below Flaming Gorge Dam. The overall model combines a cell-based model of backwater geometry, a pond-based temperature model and a model of invertebrate production. Results from a series of simulations suggest that the most important biological effects of within-day flow fluctuations are likely to be those associated with the availability of invertebrate prey including (1) minimum wetted area, (2) the proportion of the backwater's volume exchanged with the mainstem, and, to a lesser degree, (3) backwater temperature. Taken together, such effects could have important implications for the growth and survival of juvenile fish when flow fluctuations are sufficiently large. Copyright © 2006 John Wiley & Sons, Ltd.

Local-scale turnover of pond insects: intra-pond habitat quality and inter-pond geometry are both important

The distributions of larvae of seven species of pond insect were recorded from 30 small, adjacent temporary ponds over the course of three years. Incidences were modelled using logistic regression to compare the effectiveness of measures of intra-patch habitat or inter-patch geometry as predictors of distribution. Incidence, extinction and colonisation were modelled separately against systematic environmental variation (e.g. length of dry phase), temporal change (e.g. year) and individual pond characteristics as predictors of presence or absence. Models of incidence created for all species were dominated by negative correlations to the length of preceding summer's dry-phase, positive correlations with length of flood links between ponds and species-specific changes with year. Models of colonisation and extinction events suggested that colonisation and extinction may be driven by different factors. The results suggest that both intra-pond habitat and inter-pond geometry affect the distribution of pond insects. The conservation of pond invertebrates will require strategic policy attentive to both aspects of pond invertebrates' ecology, rather than relying on ad hoc, site by site interventions.

The role of habitat connectivity and landscape geometry in experimental zooplankton metacommunities

Theory predicts that inter‐patch dispersal rates and patterns of patch heterogeneity both have the potential to alter patterns of local and regional species diversity. To test this, we manipulated both rates of habitat connectivity and the geometric arrangement of habitat heterogeneity within regions of experimental zooplankton communities. We found no effects of habitat geometry on any metric of species diversity or composition. Additionally, we found no effect of habitat connectivity rate on local species diversity. We did, however, find that increasing connectivity led to a decrease in regional diversity, as well as an increase in the percent similarity of local communities within regions. Of all of the species in these communities, the relatively large cladoceran Ceriodaphnia reticulata significantly responded to the treatments, and had a higher probability of achieving high densities when connectance was high. As such, we suggest that this species played a large role in driving the increased local community similarity and decreased regional species richness as connectivity increased. These findings are in opposition to previous experimental studies of metacommunities, but support the notion that increased connectance among local patches may decrease regional diversity when patches are heterogeneous.

Avian ultraviolet vision and frequency-dependent seed preferences

It is well established that ultraviolet sensitivity plays an important role in the visually guided behaviour of birds. From a foraging perspective, evidence now exists that ultraviolet wavelengths are used by birds when foraging for insects, berries, seeds and mammals. Here, we present the results of two laboratory experiments that test the effect of removing (i) ultraviolet wavelengths and (ii) wavebands in the human-visible region on the frequency-dependent seed preferences of zebra finches (Taeniopygia guttata). Although the seeds and backgrounds used in our experiments reflected mainly at long wavelengths, we found that removal of ultraviolet wavelengths significantly changed the strength and direction of frequency dependence compared with full-spectrum illumination. We also found that the removal of ultraviolet wavelengths (300-400 nm) did not affect the strength of frequency dependence compared with the removal of short wavelengths (approximately 400-500 nm), medium wavelengths (approximately 500-600 nm) or long wavelengths (approximately 600-700 nm). Since frequency-dependent selection has direct consequences for the stability of prey populations and the spectral quality of ambient light is known to vary considerably with climate, time of day and local habitat geometry, our results suggest that ultraviolet wavelengths might play an important role in the dynamics of plant populations. However, we urge caution about overestimating the importance of ultraviolet wavelengths compared with wavelengths in the human-visible spectrum.

CENTRE-EDGE DIFFERENCES IN BEHAVIOUR, TERRITORY SIZE AND FITNESS IN CLUSTERS OF TERRITORIAL DAMSELFISH: PATTERNS, CAUSES, AND CONSEQUENCES

AbstractVariation in behaviour and fitness with spatial position in a group (centre vs edge), as well as the causes and consequences, was studied in the permanently territorial threespot damselfish, Stegastes planifrons. These fish have individual territories, clusters of which occur on coral patch reefs. Fitness correlates varied with group position. Although, survival and age did not differ between fish resident in the centre or edge, central fish were larger and grew faster than edge fish. Central males also received more clutches, eggs/clutch, and cumulative number of eggs to defend than did edge males. The higher fitness in centre positions was then correlated with differences in behaviour by group position. Territories defended were smaller in the centre of groups. Aggressive interactions with conspecifics were more frequent for central fish, but interactions with heterospecifics, and overall interactions, occurred at much lower rates for fish in central positions. Centre fish lost less of the algal food in their territories to intruders. Feeding and courtship rates, and forays from the territory, did not differ by position. These behavioural observations suggest that the positional differences in fitness measures and territory size are due to lower energy costs of territory defence for central fish, permitting the greater investment in growth and reproduction. Centre-edge differences in behaviour and territory size could be due to a selfish-herdlike group position effect (i.e. position relative to neighbours determines rates of behaviour), the microhabitat variation that was found between these positions, or a combination of both factors. To test among these alternatives, fish peripheral to a central treatment individual were removed experimentally, thus altering that individual's relative position from the centre to the edge of a group, without changing the microhabitat features of its territory. Territory size and behaviour measures of position-altered fish changed in ways consistent with the interpretation that group position has a much greater effect on behaviour and territoriality than does microhabitat. Group position may thus influence the fitness measures that differ by spatial position. These results suggest that habitat geometry and/or fragmentation may affect individual territory size, and hence maximum population density, as well as per capita reproductive output, by altering the relative number of edge individuals in groups. Does S. planifrons recognize and compete for residence in the better central positions? If there is competition, then there should be (1) slower reoccupation, (2) lower rates of intraspecific aggression, and (3) fish resident in the centre should not compete for open edge space in newly opened edge territories, relative to newly cleared centre territories. Also, (4) colonization to completely cleared areas should preferentially be to the centre positions. The first three predictions were supported by a paired removal experiment, while the fourth prediction was partially supported in another experiment. Competition thus appears to be an important factor determining the local distribution of threespot damselfish on patch reefs.

Local and Regional Diversity in a Patchy Landscape: Native, Alien, and Endemic Herbs on Serpentine

Metacommunity models predict that communities in patchy environments will show different patterns of diversity than communities in continuous habitats. The species richness of herbs was measured in 24 small patches (0.5–3 ha) of serpentine soil and 24 equivalently spaced sets of sampling sites on four large areas (>5 km2) of serpentine soil in the North Coast Ranges of California. Patchy and continuous sites were compared with respect to local diversity (species richness within 5 × 50 m transects), regional diversity (species richness at all 24 sites combined), and among-site differentiation (average proportion of species unshared between any one site and the other 23 sites of its kind). For all herb species together, regional diversity and average local diversity were higher on small patches, mainly because of a significantly higher number of alien species. For the subset (14%) of herb species that were endemic to serpentine, regional diversity was similar on patchy and continuous sites, but it was partitioned differently; local diversity was lower, and among-site differentiation was higher on small patches. Herb diversity also showed significant gradients with soil calcium levels, and these effects were not independent of site type (patchy or continuous). Soil calcium was correlated positively with the number of alien species on small patches, and negatively with the number of serpentine-endemic species on continuous sites. Also, total herb diversity was negatively correlated with elevation on continuous sites. However, patchy and continuous sites did not differ in their average calcium levels or elevation. These results agree with theory predicting that, relative to communities in widespread habitats, local communities in patchy habitats will contain more habitat generalist and fewer habitat specialist species. However, the results also suggest that the effects of habitat geometry interact with those of habitat quality in this system.

Bank voles in linear habitats show restricted gene flow as revealed by mitochondrial DNA (mtDNA).

Genetic structure of bank vole populations in linear river bank habitat in southeast Norway was determined from analyses of DNA sequences for the mitochondrial D-loop. Animals were sampled at sites separated by 1 km, along two forested river banks separated by approximately 100 m of open water. Twenty-six distinct haplotypes were found among 120 voles. The voles showed significant deviation from panmixis on both sides of the river. Animals from the same site or from sites 1 km apart were more likely to share haplotypes than animals 2 km apart or more. Common haplotypes were widespread on both river banks, and had a wider distribution than relatively rare haplotypes. Some rare haplotypes were found on both banks, but most were restricted to a single bank. The results suggest that short-term gene flow may be restricted for female bank voles in linear habitats. Female territorial behaviour may vary with habitat geometry. In the linear habitat described here, females defend only two territorial borders and may effectively limit female dispersal. Results were compared to a previous study of bank voles from this region in a two-dimensional habitat. Gene flow in the linear habitat was much more restricted than gene flow in the two-dimensional habitat. Probable mechanisms underlying this difference are discussed.

Area-dependent migration by ringlet butterflies generates a mixture of patchy population and metapopulation attributes.

Interpretation of spatially structured population systems is critically dependent on levels of migration between habitat patches. If there is considerable movement, with each individual visiting several patches, there is one "patchy population"; if there is intermediate movement, with most individuals staying within their natal patch, there is a metapopulation; and if (virtually) no movement occurs, then the populations are separate (Harrison 1991, 1994). These population types actually represent points along a continuum of much to no mobility in relation to patch structure. Therefore, interpretation of the effects of spatial structure on the dynamics of a population system must be accompanied by information on mobility. We use empirical data on movements by ringlet butterflies, Aphantopus hyperantus, to investigate two key issues that need to be resolved in spatially-structured population systems. First, do local habitat patches contain largely independent local populations (the unit of a metapopulation), or merely aggregations of adult butterflies (as in patchy populations)? Second, what are the effects of patch area on migration in and out of the patches, since patch area varies considerably within most real population systems, and because human landscape modification usually results in changes in habitat patch sizes? Mark-release-recapture (MRR) data from two spatially structured study systems showed that 63% and 79% of recaptures remained in the same patch, and thus it seems reasonable to call both systems metapopulations, with some capacity for separate local dynamics to take place in different local patches. Per capita immigration and emigration rates declined with increasing patch area, while the resident fraction increased. Actual numbers of emigrants either stayed the same or increased with area. The effect of patch area on movement of individuals in the system are exactly what we would have expected if A. hyperantus were responding to habitat geometry. Large patches acted as local populations (metapopulation units) and small patches simply as locations with aggregations (units of patchy populations), all within 0.5 km2. Perhaps not unusually, our study system appears to contain a mixture of metapopulation and patchy-population attributes.

Habitat geometry of benthic substrata: effects on arrival and settlement of mobile epifauna

The effect of substratum complexity on the early stages of colonization by mobile epifauna was assessed through a comparative study based on the architecture of artificial substrata. We conducted field observations over 4 years, on six types of small plastic substrata placed in the low intertidal zone of an exposed rocky shore, for varied immersion periods (1, 2, 4 and 12 wk). The use of artificial substrata allowed us to manipulate independently structural and spatial features of the habitat, such as total area, amount of folds, intercepting area, total volume, and interstitial volume. The invertebrate fauna colonizing over 300 sample units was recorded, and their densities compared as a function of substrata type and immersion time. Microcrustaceans predominated during the initial stages in all substrata. In this category, harpacticoid copepods and amphipods were the most abundant taxa. The effect of the original substratum complexity seemed to be restricted to the early stages of colonization, since after 12 wk of immersion the original geometry was greatly modified by fouling organisms, particularly ascidians and epiphytic algae. The geometric characteristic that most influenced epifaunal composition and density was the substratum folding, a one-dimensional measure that evaluates the amount of filaments and folds in the substratum's surface. Folding was correlated with high faunal densities and high initial colonization rates, and proved to be a better density predictor than total substratum area, or volume. This correlation was especially well defined for amphipods.

Effects of habitat geometry on territorial defence costs in a damselfish

Effects of habitat geometry on territorial defence costs in a damselfish

Insular Biogeographic Theory and Diffusion Models in Population Dynamics

We examine a class of mathematical models describing the effects of habitat size and geometry on community structure. We deduce spatial effects at the community level from mechanistic models for the population dynamics and dispersal of individual populations together with some minimal hypotheses about the distribution of growth and dispersal rates among those populations. We use the models to deduce species-area curves, and in that sense they provide an alternative to the dynamic equilibrium theory of island biogeography introduced by MacArthur and Wilson. Since the models can explicitly incorporate various hypotheses about population dynamics and the nature and strength of interactions between species they permit a detailed analysis of how these hypotheses should affect community structure. We illustrate that point by contrasting the implications of different hypotheses in the context of refuge design.

Helpers liberate female fairy-wrens from constraints on extra-pair mate choice

In cooperatively breeding superb fairy-wrens (Malurus cyaneus), all males contribute to the feeding and defence of young. Despite the expectation that such paternal care should be directed only to relatives, DNA fingerprinting revealed that most offspring (76%, 138 out of 181) were sired by extra-group males that contributed no care, and that almost all broods (95%, 38 out of 40) contained young sired by extra-group fathers. This is the highest known incidence of cuckoldry. This remarkable mating system is produced by female control of fertilization and consistent preference for certain extra-group male genotypes. This choice leads to the production of sons that also gain extra-group fertilizations. One constraint on the extra-pair mate choice of females is the level of parental assistance received from males. Males living in pairs contribute relatively more parental care and are more likely to gain paternity in one of their broods (85%, 11 out of 13) than dominant males in multi-male cooperative groups (30%, 8 out of 27). In groups helpers compensate for the lower parental assistance of dominant males, so the total feeding rate is similar between pairs and groups. This suggests that females allow males in pairs more paternity to ensure their assistance with parental care. Helpers provide an alternative source of paternal investment, and allow females to express unrestricted mate choice. Mating options for females in other species with female control of fertilization may also reflect a trade-off between acquiring the genes of high-quality males for their offspring and parental care of those offspring.

Calculating and Miscalculating Density: The Role of Habitat Geometry

The density of a population is a quantitative variable with spatial attributes: density is calculated on the basis of the geometric configuration of a selected area on a two-dimensional plane. This means that a density estimate implies a decision as to what is an ecologically realistic configuration for a study area, but errors in determining the appropriate value for A in the D = N/A equation may have serious consequences for the value of D. I illustrate problems created by such decisions with examples taken from breeding bird distributions in fragmented habitats.