Discrete Habitat Patches Research Articles

Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community

Ecological communities are structured by both deterministic, niche-based processes and stochastic processes such as dispersal. A pressing issue in ecology is to determine when and for which organisms each of these types of processes is important in community assembly. The roles of deterministic and stochastic processes have been studied for a variety of communities, but very few researchers have addressed their contribution to insect herbivore community structure. Insect herbivore niches are often described as largely shaped by the antagonistic pressures of predation and host plant defenses. However host plants are frequently discrete patches of habitat, and their spatial arrangement can affect herbivore dispersal patterns. We studied the roles of predation, host plant quality, and host spatial proximity for the assembly of a diverse insect herbivore community on Quercus alba (white oak) across two growing seasons. We examined abundances of feeding guilds to determine if ecologically similar species responded similarly to variation in niches. Most guilds responded similarly to leaf quality, preferring high-nitrogen, low-tannin host plants, particularly late in the growing season, while bird predation had little impact on herbivore abundance. The communities on the high-quality plants tended to be larger and, in some cases, have greater species richness. We analyzed community composition by correlating indices of community similarity with predator presence, leaf quality similarity, and host plant proximity. Birds did not affect community composition. Community similarity was significantly associated with distance between host plants and uncorrelated with leaf quality similarity. Thus although leaf quality significantly affected the total abundance of herbivores on a host plant, in some cases leading to increased species richness, dispersal limitation may weaken this relationship. The species composition of these communities may be driven by stochastic processes rather than variation in host plant characteristics or differential predation by insectivorous birds.

Density-dependent population dynamics and dispersal in heterogeneous metapopulations

1. Metapopulation microcosms were constructed to test the effect of four different types of habitat heterogeneity on the dynamics and dispersal in spatially extended systems; homogeneity, spatial heterogeneity, temporal heterogeneity and spatio-temporal heterogeneity. Resources were distributed across discrete habitat patches in bruchid beetle (Callosobruchus maculatus) metapopulations, and long-term time series were recorded. 2. Mathematical models were fitted to the long-term time series from the experimental systems using a maximum likelihood approach. Models were composed of separate birth, death, emigration and immigration terms all of which incorporated stochasticity drawn from different probability distributions. Models with density-dependent and density-independent birth, death and emigration terms were investigated and, in each case, the model that best described the empirical data was identified. 3. At the local scale, population sizes differed between patches depending on the type of heterogeneity. Larger populations were associated with higher resource availabilities. As a result of this, the variation between local population sizes was greatest when there was spatial heterogeneity in which mean resource abundance varied from patch to patch. Variation in population sizes within patches was largest when there was temporal heterogeneity. 4. Density-dependent processes leading to the regulation of local population dynamics in our experimental systems were strongest in homogeneity or temporal heterogeneity treatments. Associated with this, we found that these systems were best described using mathematical models with density dependence acting on mortality. In contrast, spatial and spatio-temporal time series were adequately described using density-independent population processes. 5. Experimental metapopulations showed varying degrees of density-dependent dispersal. Local net dispersal each week was primarily driven by the local population size and secondarily affected by neighbourhood population density. Mathematical population models illustrated the importance of explicit description of density-dependent dispersal in all systems except the homogeneous metapopulations.

Conservation status and management of the Gove Crow Euploea alcathoe enastri (Lepidoptera: Nymphalidae), a threatened tropical butterfly from the indigenous Aboriginal lands of north-eastern Arnhem Land, Australia

The Gove Crow butterfly, Euploea alcathoe enastri Fenner, 1991, is restricted to Gove Peninsula of north-eastern Arnhem Land, a remote area of northern Australia. The subspecies has been listed as an Endangered taxon under federal and Northern Territory legislation, and represents one of only a few cases in the Australian Region in which a tropical butterfly has been targeted for species-orientated conservation. However, accurate status evaluation and conservation management have been hampered by lack of detailed information on spatial distribution, critical habitat, and the extent and severity of threatening processes. Surveys carried out during 2006–2008 indicate that the subspecies has a limited geographical range (extent of occurrence approximately 6,700 km2) within which it is recorded from 11 locations or subpopulations embracing a total of 21 sites. Most sites comprise discrete habitat patches that are relatively small in area (<10 ha) within which adults are localised and occur in low abundance (<15 h−1). Of the four major habitat types in which E. alcathoe enastri was detected, only mixed paperbark tall open forest with rainforest elements in the understorey and rainforest edge (i.e. the ecotone between evergreen monsoon vine-forest and eucalypt/paperbark woodland) comprise breeding habitats. These habitat patches were always associated with permanent creeks or perennial groundwater seepages or springs that form swamplands, usually along drainage lines or flood plains in coastal or near coastal lowland areas. Major threats identified at the site level are habitat modification through altered fire regime and habitat disturbance by feral animals (buffalo, pig); potential threats at the landscape level include habitat loss through invasive species (grassy weeds, tramp ants) and global climate change. However, since critical breeding areas are subject to natural disturbance by both fire and flood, and occasionally cyclonic events, an optimal balance in disturbance regime is probably required to sustain breeding populations. Although E. alcathoe enastri is a narrow-range endemic that is ecologically specialised, there is no evidence of decline. Accordingly, the conservation status of the subspecies should be regarded as Near Threatened (‘Conservation Dependent’) under IUCN criteria. Components for an effective long-term conservation management plan of the butterfly and its habitat, which largely depend on the cooperation of traditional landowners and involvement of local indigenous ranger groups, are briefly discussed.

Use of a population viability analysis to evaluate human-induced impacts and mitigation for the endangered Lower Keys marsh rabbit

Rapid development and urbanization of the lower Florida Keys in the last 30 years has fragmented the habitat of the Lower Keys marsh rabbit (Sylvilagus palustris hefneri), hereafter called marsh rabbit, and threatened it with extinction. On the Naval Air Station–Key West (NAS), Boca Chica Key, marsh rabbits exist as a meta-population of discrete habitat patches in a matrix of wetlands and airfield facilities. Airfield safety regulations require NAS to maintain vegetation below a minimum height on runway peripheries (clear zones). We developed a spatially-explicit, stage-structured, stochastic matrix model using the programs RAMAS-Metapop and ArcGIS. Model parameters were estimated using pellet counting (2001–2002), radio tracking (1991–1992 and 2001–2005, n = 75), and published literature. We compared a baseline no-action model to Alternative 1 that simulated impacts from the NAS airfield clearance project with no conservation measures for marsh rabbits and Alternative 2 that also simulated airfield clearance impacts. Alternative 2 included mitigation actions to offset impacts to marsh rabbits in the form of reduced mortality via free-roaming cat (Felis catus) control and creation of marsh rabbit habitat (e.g., salt marsh). Both alternatives increased the extinction risk (probability of extinction) from a baseline of 0.499 to 0.90 and 0.713 for Alternatives 1 and 2, respectively. Although airfield clearance with creation of marsh rabbit habitat (Alternative 2) increased extinction risk from the baseline scenario, reducing marsh rabbit mortality associated with control of free-roaming cats was an effective strategy to decrease this risk. Our research demonstrates the use of population viability analysis as a conservation planning tool for reducing human–wildlife conflicts.

DOES MATE LIMITATION IN SELF-INCOMPATIBLE SPECIES PROMOTE THE EVOLUTION OF SELFING? THE CASE OF LEAVENWORTHIA ALABAMICA

Genetic diversity at the S-locus controlling self-incompatibility (SI) is often high because of negative frequency-dependent selection. In species with highly patchy spatial distributions, genetic drift can overwhelm balancing selection and cause stochastic loss of S-alleles. Natural selection may favor the breakdown of SI in populations with few S-alleles because low S-allele diversity constrains the seed production of self-incompatible plants. We estimated S-allele diversity, effective population sizes, and migration rates in Leavenworthia alabamica, a self-incompatible mustard species restricted to discrete habitat patches in rocky glades. Patterns of polymorphism were investigated at the S-locus and 15 neutral microsatellites in three large and three small populations with 100-fold variation in glade size. Populations on larger glades maintained more S-alleles, but all populations were estimated to harbor at least 20 S-alleles, and mate availabilities typically exceeded 0.80, which is consistent with little mate limitation in nature. Estimates of the effective size (N(e)) in each population ranged from 600 to 1600, and estimated rates of migration (m) ranged from 3 x 10(-4) to nearly 1 x 10(-3). According to theoretical models, there is limited opportunity for genetic drift to reduce S-allele diversity in populations with these attributes. Although pollinators or resources limit seed production in small glades, limited S-allele diversity does not appear to be a factor promoting the incipient breakdown of SI in populations of this species that were studied.

Continuum or discrete patch landscape models for savanna birds? Towards a pluralistic approach

Conceptualising landscapes as a mosaic of discrete habitat patches is fundamental to landscape ecology, metapopulation theory and conservation biology. An emerging question in ecology is: when is the discrete patch model more appropriate than alternative and conceptually appealing models such as the continuum model? There is limited empirical testing of the utility of alternative landscape models compared to the discrete patch model for a range of species. In this paper, we constructed three alternative sets of models for testing the effect of landscape structure on diversity and abundance of a suite of woodland birds in a savanna landscape of northern Australia: the null model (only site‐scale habitat variables, landscape context not important), the continuum model, and the discrete patch model. We utilised high‐spatial resolution satellite images to quantify spatial gradients in tree cover density (the continuum model), and to then aggregate the fine‐scale heterogeneity in tree cover into discrete patches of trees, with grass cover forming the “matrix” (the discrete patch‐model). We then evaluated the relative importance of the alternative models using generalised linear models and an information theoretic approach. We found that the importance of the models varied among species, with no single model dominant. Species that move between open grassy areas and woody shelter responded well to the continuum model, reflecting the importance of gradients in density of forage (grasses) and cover (trees), while the discrete model performed best for species that forage in all vegetation strata, and nest predominantly in dense woody vegetation. This finding supports a pluralistic approach, highlighting the need for adopting and testing more than one landscape model in savanna landscapes, and in other landscapes that do not have a well defined patch structure.

A patch perspective on summer habitat use by brown trout Salmo trutta in a high plains stream in Wyoming, USA

Abstract – We quantified the use of habitat patches by brown trout, Salmo trutta, during summer conditions in a plains stream in the western United States. A Global Positioning System was used to map discrete habitat patches (2–420 m2) consisting of macrophytes, wood accumulation, or deep water. Habitat use by brown trout was monitored by radio telemetry. Brown trout used habitat in a nonrandom manner with 99% of all daytime observations and 91% of all nighttime observations occurring in patches that consisted of combinations of deep water, wood accumulations or macrophytes even though such patches constituted only 9.8% of the available habitat. Brown trout used deep water almost exclusively during the day but broadened their habitat use at night. Most fish stayed within a large plunge pool created by a low‐head dam. This pool supplemented the deep‐water habitat that was naturally rare in our study area and illustrates how human modifications can sometimes create habitat patches important for stream fishes.

Spatial and temporal dynamics of habitat selection across canopy gradients generates patterns of species richness and composition in aquatic beetles

Abstract 1. Colonisation is a critical ecological process influencing both population and community level dynamics by connecting spatially discrete habitat patches. How communities respond to both natural and anthropogenic disturbances, furthermore, requires a basic understanding of how any environmental change modifies colonisation rates. For example, disturbance‐induced shifts in the quantity of forest cover surrounding aquatic habitats have been associated with the distribution and abundance of numerous aquatic taxa. However, the mechanisms generating these broad and repeatable field patterns are unclear.2. Such patterns of diversity could result from differential spatial mortality post colonisation, or from colonisation alone if species select sites non‐randomly along canopy coverage gradients. We examined the colonisation/oviposition dynamics of aquatic beetles in experimental ponds placed under both open and closed forest canopies.3. Canopy coverage imposed a substantial behavioural filter on the colonisation and reproduction of aquatic beetles representing multiple trophic levels, and resulted in significantly higher abundance, richness, and oviposition activity in open canopy ponds. These patterns strengthened overtime; although early in the experiment, the most abundant beetle had similar abundance in open and closed ponds. However, its abundance subsequently declined and then most other species heavily colonised open canopy ponds.4. The primary response of many aquatic species to disturbances that generate canopy coverage gradients surrounding aquatic ecosystems is behavioural. The magnitude of the colonisation responses reported here rivals, if not exceeds, those produced by predators, suggesting that aquatic landscapes are behaviourally assessed and partitioned across multiple environmental gradients. The community level structure produced solely by selective colonisation, is predicted to strongly modify how patch area and isolation affect colonisation rates and the degree to which communities are linked by the flux of individuals and species.

Support for a metapopulation structure among mammals

ABSTRACTThe metapopulation metaphor is increasingly used to explain the spatial dynamics of animal populations. However, metapopulation structure is difficult to identify in long‐lived species that are widely distributed in stochastic environments, where they can resist extinctions. The literature on mammals may not provide supporting evidence for classic metapopulation dynamics, which call for the availability of discrete habitat patches, asynchrony in local population dynamics, evidence for extinction and colonization processes, and dispersal between local populations.Empirical evidence for metapopulation structure among mammals may exist when applying more lenient criteria. To meet these criteria, mammals should live in landscapes as discrete local breeding populations, and their demography should be asynchronous.We examined the literature for empirical evidence in support of the classical criteria set byHanski (1999), and for the more lenient subset of criteria proposed byElmhagen &amp; Angerbjörn (2001). We suggest circumstances where metapopulation theory could be important in understanding population processes in mammals of different body sizes.The patchy distribution of large (&gt;100 kg) mammals and dispersal often motivate inferences in support of a metapopulation structure. Published studies seldom address the full suite of classical criteria. However, studies on small mammals are more likely to record classic metapopulation criteria than those on large mammals. The slow turnover rate that is typical for medium‐sized and large mammals apparently makes it difficult to identify a metapopulation structure during studies of short duration.To identify a metapopulation structure, studies should combine the criteria set byHanski (1999) andElmhagen &amp; Angerbjörn (2001). Mammals frequently live in fragmented landscapes, and processes involved in the maintenance of a metapopulation structure should be considered in conservation planning and management.

Rocky intertidal community structure in oceanic islands: scales of spatial variability

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 356:15-24 (2008) - DOI: https://doi.org/10.3354/meps07247 Rocky intertidal community structure in oceanic islands: scales of spatial variability Gustavo M. Martins1,2,3,*, Richard C. Thompson1, Stephen J. Hawkins3,5, Ana I. Neto2, Stuart R. Jenkins3,4 1Marine Biology and Ecology Research Centre, Marine Institute, University of Plymouth, Plymouth PL4 8AA, UK 2Secção Biologia Marinha and CIRN, Departamento Biologia, Universidade dos Açores, 9501-801 Ponta Delgada, Açores, Portugal 3Marine Biological Association, Citadel Hill, Plymouth PL1 2PB, UK 4School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey LL59 5EY, UK 5College of Natural Sciences, University of Bangor, Bangor, Gwynedd LL57 2UW, UK *Email: gustavo.martins@plymouth.ac.uk ABSTRACT: There is a clear bias in the literature on island ecology towards terrestrial rather than marine systems, which have remained comparatively poorly studied. Marine populations are typically open, and local production may have little impact on local recruitment, such that long-distance dispersal is an important determinant of population ecology. Since oceanic islands form discrete patches of habitat surrounded by a structurally different environment, we tested the general hypothesis that processes operating at the scale of islands have a greater influence on these populations than the processes operating at smaller, intra-island scales. A hierarchical design examined the patterns of abundance and distribution of conspicuous taxa at 3 tidal heights at a range of spatial scales, ranging from a few meters to hundreds of kilometres apart in the rocky intertidal of the Azores. Both uni- and multivariate analyses showed that at the largest scale (islands), significant variation was detected in the lower and mid-shore communities, but not on the upper shore. Along the vertical gradient of immersion there was a trend for increasing small-scale patchiness towards the top of the shore. The potential role of local environmental stress gradients and broad-scale oceanographic patterns of recruitment in structuring these assemblages is discussed. This study corroborates the suitability of the analytical tools used here to examine patterns of distribution over a range of spatial scales and its applicability in the field of island marine ecology. KEY WORDS: Hierarchical analysis · Variance component · Spatial scale · Community structure · Rocky intertidal · Fragmented habitats · Oceanic island · Azores · Assemblage biogeography Full text in pdf format PreviousNextCite this article as: Martins GM, Thompson RC, Hawkins SJ, Neto AI, Jenkins SR (2008) Rocky intertidal community structure in oceanic islands: scales of spatial variability. Mar Ecol Prog Ser 356:15-24. https://doi.org/10.3354/meps07247 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 356. Online publication date: March 18, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Offense and defense in landscape‐level invasion control

Biological invasions are multi‐stage processes comprising chance demographic events, species interactions, and dispersal. Despite this complexity, simple models can increase understanding of the invasion process. We model the spread of aquatic invasive species through a network of lakes to evaluate the effectiveness of two intervention strategies. The first, which we call offense, contains the invader at sources; the second, which we call defense, protects uninvaded destinations. Deterministic models reveal the effects of these intervention strategies on spread rates. Practical applications involve finite collections of uninvaded lakes, however, and we therefore also present a stochastic model to describe how these strategies affect expected times to important invasion milestones. When the goal is to reduce overall spread rates, both approaches agree that offense is better early in invasions, but that defense is better after 1/2 the lakes are invaded. When the goal is to protect areas of high conservation value, however, defensive site protection always provides lower per site introduction rates. Although we focus on lakes, our results are quite general, and could be applied to any discrete habitat patches including, for example, fragmented terrestrial habitats.

What you see is where you go? Modeling dispersal in mountainous landscapes

Inter-patch connectivity can be strongly influenced by topography and matrix heterogeneity, particularly when dealing with species with high cognitive abilities. To estimate dispersal in such systems, simulation models need to incorporate a behavioral component of matrix effects to result in more realistic connectivity measures. Inter-patch dispersal is important for the persistence of capercaillie (Tetrao urogallus) in central Europe, where this endangered grouse species lives in patchy populations embedded in a mountainous landscape. We simulated capercaillie movements with an individual-based, spatially explicit dispersal model (IBM) and compared the resulting connectivity measure with distance and an expert estimation. We used a landscape comprising discrete habitat patches, temporary habitat, non-habitat forests, and non-habitat open land. First, we assumed that dispersing individuals have perfect knowledge of habitat cells within the perceptual range (null model). Then, we included constraints to perception and accessibility, i.e., mountain chains, open area and valleys (three sub-models). In a full model, all sub-models were included at once. Correlations between the different connectivity measures were high (Spearman’s ρ > 0.7) and connectivity based on the full IBM was closer to expert estimation than distance. For selected cases, simple distance differed strongly from the full IBM measure and the expert estimation. Connectivity based on the IBM was strongly sensitive to the size of perceptual range with higher sensitivity for the null model compared to the full model that included context dependent perceptual ranges. Our heuristic approach is adequate for simulating movements of species with high cognitive abilities in strongly structured landscapes that influence perception and permeability.

Annual Survival and Turnover Rates of an Afrotropical Robin in a Fragmented Forest

Whether or not subdivided populations persist in fragmented landscapes primarily depends on how well individuals can survive within discrete habitat patches. Using data from six capture–recapture sessions, survival probabilities of the white-starred robin were estimated in seven indigenous forest patches in the highly fragmented Taita Hills forests, SE Kenya. We found no significant differences in survival probability either among fragment-size categories (large 135 ha, medium 95 ha, and small 2–8 ha) or between adult and first-year birds. However, males had a higher probability of survival from one year to the next than females. Turnover rates of adult birds were higher for females than males, but also higher in the medium and small patches than in the large one within each sex. That survival probability was similar among fragments, but turnover rates differed denoted that different processes caused extirpation from the patches. We suggest that mortality associated with dispersal was probably a more important cause of extirpation than within-patch mortality in the largest habitat, which had the lowest turnover rates. Conversely, high within-patch mortality, for instance due to predation during incubation, could have been more important in the smaller, more disturbed habitats. These results lend support to the proposition that avian conservation efforts should be focussed both at the landscape level to improve connectivity between fragments and reduce mortality during dispersal, and at the patch level to exclude other mortality sources such as nest predation.

Molecular-Level Variation Affects Population Growth in a Butterfly Metapopulation

The dynamics of natural populations are thought to be dominated by demographic and environmental processes with little influence of intraspecific genetic variation and natural selection, apart from inbreeding depression possibly reducing population growth in small populations. Here we analyse hundreds of well-characterised local populations in a large metapopulation of the Glanville fritillary butterfly (Melitaea cinxia), which persists in a balance between stochastic local extinctions and recolonisations in a network of 4,000 discrete habitat patches. We show that the allelic composition of the glycolytic enzyme phosphoglucose isomerase (Pgi) has a significant effect on the growth of local populations, consistent with previously reported effects of allelic variation on flight metabolic performance and fecundity in the Glanville fritillary and Colias butterflies. The strength and the sign of the molecular effect on population growth are sensitive to the ecological context (the area and spatial connectivity of the habitat patches), which affects genotype-specific gene flow and the influence of migration on the dynamics of local populations. The biological significance of the results for Pgi is underscored by lack of any association between population growth and allelic variation at six other loci typed in the same material. In demonstrating, to our knowledge for the first time, that molecular variation in a candidate gene affects population growth, this study challenges the perception that differential performance of individual genotypes, leading to differential fitness, is irrelevant to population dynamics. These results also demonstrate that the spatial configuration of habitat and spatial dynamics of populations contribute to maintenance of Pgi polymorphism in this species.

Biodiversity as both a cause and consequence of resource availability: a study of reciprocal causality in a predator–prey system

1. One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '... is species diversity the cause or the consequence of resources that limit community biomass?' 2. Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. 3. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. 4. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey.

Why is group size correlated with the size of the host sea anemone in the false clown anemonefish?

When social groups monopolize discrete habitat patches, group size may be positively correlated with patch size. The correlation can be a direct consequence of limited resources. Alternatively, it can be an indirect consequence of patch-size effects on a dominant group member. We asked which of these two mechanisms was responsible for a positive correlation between the group size of false clown anemonefish (Amphiprion ocellaris Cuvier in Cuvier and Valenciennes, 1830) and that of the host sea anemone Stichodactyla gigantea (Forskål, 1775). We argue that some false clown anemonefish groups may have reached the carrying capacities of their hosts, but that the group size : patch size correlation in the population as a whole is best interpreted as an indirect consequence of a positive relationship between anemone size and the length of the dominant group member. The dominant's length in turn limits group size because dominant group members inhibit the growth of their subordinates. Thus, a correlation between group size and patch size need not imply resource limitation of subordinate group members.

A Patch Hath Smaller Patches: Delineating Ecological Neighborhoods for Parasites

Use of the host individual as a boundary for parasite populations and communities provides an unambiguous spatial unit that is useful for pattern description, but this framework precludes consideration of the host landscape and within-host population dynamics. Recognizing host individuals as spatially and temporally complex landscapes requires modified concepts of parasite populations and communities. An outline of the currently accepted hierarchies of parasite populations and communities is provided on the basis of ecological neighborhoods that are delineated by discrete habitat patches or functional dynamics (or both), as opposed to host individuals. This parasite-based framework accommodates consideration of both within- and among-host dynamics and facilitates investigation into the mechanisms by which these 2 levels of investigation interact.

Gastropod shell size and morphology influence conspecific interactions in an encrusting hydroid

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 275:153-162 (2004) - doi:10.3354/meps275153 Gastropod shell size and morphology influence conspecific interactions in an encrusting hydroid David L. Ferrell* Department of Biological Science, Florida State University, Tallahassee, Florida 32306-1100, USA *Email: ferrell@bio.fsu.edu ABSTRACT: Discrete patches of suitable habitat and the consequent aggregation of organisms with similar habitat requirements often intensify competition, particularly among conspecifics. Habitat differences may generate interactions between smaller or larger individuals at variable stages of development in many fungi, plants, and colonial animals. Size-dependent competitive outcomes and effects on sexual reproductive allocation indicate that differences among discrete habitats have potentially profound ecological and evolutionary implications. In the northern Gulf of Mexico, the colonial hydroid Hydractinia [GM] colonizes hermit crab-occupied gastropod shells (= microhabitats), which vary greatly in size and morphology. Here, the relationship between the gastropod assemblages available for H. [GM] colonization and the frequency of conspecific encounters and sexual status of those colonies involved is documented. Field-collected shells bore 1 to 3 colonies, and the number of colonies per shell increased with shell size. In contrast to previous studies, conspecific encounters were not limited to juvenile colonies. Sexually mature H. [GM] colonies generally were not distributed differently among shells with and without conspecifics. Moreover, mature colonies predominated conspecific encounters on large shell species exhibiting certain morphology. Inanimate structures, such as dock pilings and rocky surfaces, provide additional large surfaces for Hydractinia colonization in some areas. These results suggest that previous Hydractinia spp. studies represent only a subset of the diversity of ecologically relevant possibilities with respect to available substrata. Also, the characteristics of a given microhabitat (e.g. shell) affect competitive outcomes and sexual reproductive characters of its constituent competitors. KEY WORDS: Gulf of Mexico · Hydractinia · Sexual maturity · Size-dependent Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 275. Online publication date: July 14, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Is the matrix really inhospitable? Vole runway distribution in an experimentally fragmented landscape

Habitat fragmentation is a common feature of modern landscapes, with significant impacts on the population densities of and space use by animals. A frequent model system for studying these effects is that of voles (Microtus spp.) and other rodents in experimentally fragmented grasslands. Design of these study sites is motivated by landscape theory, and often assumes that the matrix between discrete experimental habitat patches is inhospitable to focal species. Our census of M. ochrogaster runways in an experimentally fragmented landscape in eastern Kansas indicates that voles make substantial use of the mowed matrix, and that such use increases with degree of habitat fragmentation. Census season (winter, summer) was also an important predictor of observed runway distributions. Our results suggest that voles may have home ranges that include multiple small fragments and also the intervening matrix, leading to overestimates of population density. While the assumption that the matrix is inhospitable in experimentally fragmented habitats may be justified in many cases, studies using the model system of Microtus spp. or ecologically similar rodents in grasslands should incorporate methods that account for use of the matrix.

Context-dependent reproductive site choice in a Neotropical frog

In organisms whose offspring develop in discrete habitat patches such as pools, studies have frequently shown that adults avoid sites based on a single risk factor facing offspring. However, natural reproductive sites often vary in multiple risk factors in both space and time. In this study, I used choice tests among field mesocosms to determine whether adults of a Neotropical anuran, Edalorhina perezi, select the safest pools for offspring based on two biotic risks of different magnitude: insect predators and conspecific competitors. I also investigated whether adult site-choice was context dependent (i.e., whether it varied by sex or by season or whether it was based on local pool quality). I found that both sexes avoided pools containing predatory insects, but only females significantly avoided those containing conspecific tadpoles. When offered two risky options, both sexes favored pools with competitors over those with predators. Site-choice behavior also varied depending on the temporal and spatial context. Female sensitivity to insect predators decreased late in the season. In addition, both sexes exhibited dampened reproductive activity when only risky sites were available locally. This study emphasizes that social and environmental factors simultaneously impact reproductive site choice. Whether a site is accepted ultimately depends not only on the assessment of mortality risks to offspring but also on the sex and spatiotemporal context of the decision maker. Key words: amphibians, calling sites, choice experiments, Edalorhina perezi, insect predators, oviposition sites, Peru, pool mesocosms, reproductive site choice, tadpole competitors, tadpole mortality risk. [Behav Ecol 14:626–633 (2003)]