Population and geographic range dynamics: implications for conservation planning

Continuing downward trends in the population sizes of many species, in the conservation status of threatened species, and in the quality, extent and connectedness of habitats are of increasing concern. Identifying the attributes of declining populations will help predict how biodiversity will be impacted and guide conservation actions. However, the drivers of biodiversity declines have changed over time and average trends in abundance or distributional change hide significant variation among species. While some populations are declining rapidly, the majority remain relatively stable and others are increasing. Here we dissect out some of the changing drivers of population and geographic range change, and identify biological and geographical correlates of winners and losers in two large datasets covering local population sizes of vertebrates since 1970 and the distributions of Galliform birds over the last two centuries. We find weak evidence for ecological and biological traits being predictors of local decline in range or abundance, but stronger evidence for the role of local anthropogenic threats and environmental change. An improved understanding of the dynamics of threat processes and how they may affect different species will help to guide better conservation planning in a continuously changing world.     

Winter bird communities in the northern Negev: species dispersal patterns, habitat use and implications for habitat conservation

Bird habitat conservation may require different management strategies for different seasonal bird assemblages. We studied habitat use by winter birds in forest and scrubland habitat patches in the northern Negev, Israel. Our goal was to assess whether differences in responses to landscape and habitat structure between breeding and non-breeding seasons require changes in future conservation plans that have been suggested for the Negev breeding bird community. We evaluated habitat and area effects on bird abundance and distribution and tested whether species habitat use during winter involves niche shifts. Compared with breeding birds, a larger proportion of winter bird species occupied both scrubland and forest. As in summer, forest bird species responded to habitat structure, whereas scrubland species were associated with both habitat structure and area. Resident birds disperse into habitats in which they were not present during summer. Consequently, for several species, the correlation between bird densities and environmental factors showed a better fit at the landscape rather than at the habitat scale. In addition, rather than niche shift, birds actually extended their niche breadth. Nest site selection may constrain bird distribution into a realized niche, smaller than their fundamental niche. Despite the scale differences in habitat use, the similar species diversity patterns between seasons suggest that both winter and summer birds would benefit from conservation of scrub patches larger than 50 ha, and enrichment of foliage layers within the planted forests.     

Temporally explicit habitat ecology and the coexistence of species

Habitats may have dynamics that exist independently of the population densities of species occupying the habitat. For example, ephemeral habitat patches may disappear regardless of whether a particular species is present or not. Such habitat dynamics are frequently modelled by ignoring age-related variation in patch turnover rates. This can be thought of as a temporally implicit approach. An alternative, temporally explicit approach involves using age-structured models in order to describe variations in habitat dynamics. Simple models of coexistence between competing species show that temporally implicit models may be misleading where there is age-related variation in patch dynamics. Changing the shape of the patch survivorship function but not the average patch survivorship can result in mutual extinction, monocultures or coexistence of an inferior and a superior competitor. An explicit treatment of habitat demography may therefore offer improved predictive models and alternative landscape management strategies.     

Spatial patterns of coexistence of competing species in patchy habitat

The single-species spatially realistic patch occupancy metapopulation model is, in this study, extended to a metacommunity of many competing species. Competition is assumed to reduce the local carrying capacity (effective patch area), which in turn increases local extinction rates and reduces colonization rates because of smaller population sizes. Each species is described by three parameters: pre-competitive abundance (equilibrium incidence of patch occupancy, which reflects the rate of colonization in relation to extinction rate), the spatial range of migration, and competitive ability. The model ignores spatio–temporal correlations caused by interspecific interactions, because in metacommunities of unequal competitors inhabiting heterogeneous landscapes, correlations in the occurrence of species are driven more by patch heterogeneity than by competition. The model allows the calculation of multispecies equilibria in patchy habitats without simulations. In general, the number of coexisting species in the metacommunity increases with decreasing strength of competition, increasing rate of colonization, and decreasing range of migration. Habitat heterogeneity in the form of spatial variation in patch areas tends to facilitate coexistence. Poor competitors may coexist with superior competitors in the patch network if the former have higher colonization rates (competition–colonization trade-off). When migration distances are short, competition leads to spatial pattern formation: Species tend to have restricted spatial distributions in the network, but contrary to intuitive expectations, often the distributions of many species are nested. Having more dispersive species enhances both local and global diversity, whereas more local migration decreases local but increases global diversity.     

Predator-mediated habitat use: some consequences for species interactions

Synopsis Behavioral responses to predators can have a major impact on a fishes' diet and habitat choice. Studies with the bluegill sunfish, Lepomis macrochirus, demonstrate that bluegills undergo pronounced shifts in diet and habitat use as they grow in response to changes in their vulnerability to predators. Other species of fish exhibit similar habitat shifts with body size, presumably also in response to changing predation risks and/or foraging gains. An important but little appreciated consequence of this type of predator-mediated habitat use is that predation risk, by structuring size and/or age-specific resource use, may also indirectly affect species interactions. This paper discusses some of the ways in which behavioral responses to predators may affect intra- and interspecific competition in fish. Observational and experimental studies with sunfish (Centrarchidae) provide most of the examples. These studies suggest that the nonlethal effects of predators may be as important as the actual killing of prey.     

Assembly rules for functional groups of North American shrews: effects of geographic range and habitat partitioning

Trophic cascades exist in numerous terrestrial systems, including many systems with ants as the top predator. Many studies show how behavioral modifications of herbivores are especially important in mediating species interactions and trophic cascades. Although most studies of trophic cascades focus on predator-herbivore-plant links, the trophic cascades concept could be applied to almost any level of trophic interactions. Especially considering the importance of parasites we consider here the interactions between the parasitic phorid fly, Pseudacteon sp. (Diptera: Phoridae), its ant host, Azteca instabilis (F. Smith) (Hymenoptera: Formicidae), and the herbivore, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) in the coffee agroecosystem. We investigated the effects of phorid flies on ant behavior by monitoring ant recruitment to tuna baits over a 30-min period in the presence or absence of phorid flies. To study the indirect effects of phorids on larvae, we placed baits on coffee plants to elevate ant foraging levels to levels near to ant nests, placed larvae near baits, and recorded the effects of ants on larvae either in the presence or absence of phorid flies. We found that phorid fly presence significantly reduced ant ability to recruit to baits through behavioral modifications and also significantly lessened ant ability to attack, carry away, or force herbivores off plants. We conclude there is a behaviorally-modified species-level trophic cascade in the coffee agroecosystem, with potentially important effects in ant and herbivore communities as well as for coffee production.     

Population genetics and the evolution of geographic range limits in an annual plant

Abstract Theoretical models of species' geographic range limits have identified both demographic and evolutionary mechanisms that prevent range expansion. Stable range limits have been paradoxical for evolutionary biologists because they represent locations where populations chronically fail to respond to selection. Distinguishing among the proposed causes of species' range limits requires insight into both current and historical population dynamics. The tools of molecular population genetics provide a window into the stability of range limits, historical demography, and rates of gene flow. Here we evaluate alternative range limit models using a multilocus data set based on DNA sequences and microsatellites along with field demographic data from the annual plant Clarkia xantiana ssp. xantiana. Our data suggest that central and peripheral populations have very large historical and current effective population sizes and that there is little evidence for population size changes or bottlenecks associated with colonization in peripheral populations. Whereas range limit populations appear to have been stable, central populations exhibit a signature of population expansion and have contributed asymmetrically to the genetic diversity of peripheral populations via migration. Overall, our results discount strictly demographic models of range limits and more strongly support evolutionary genetic models of range limits, where adaptation is prevented by a lack of genetic variation or maladaptive gene flow.     

The use of spatial ecological modelling as a tool for improving the assessment of geographic range size of threatened species

We analysed endemic threatened tree and reptile species of Socotra Island (Yemen), characterised by different ecological requirements and spatial distribution, in order to evaluate the usefulness of spatial ecological modelling in the estimation of species extent of occurrence (EOO) and area of occupancy (AOO). Point occurrences for the entire species range were used to model their spatial distribution by Random Forest (RF) and Generalised Linear Model (GLM). For each species the suitability area (SA) was obtained by applying the 0% omission error criterion on the probability map, and compared or integrated with EOO and AOO area obtained by topological methods such as the minimum convex polygon (MCP), α-hull and 2 km × 2 km grid.RF showed a lower prediction error than GLM. Higher accuracy was achieved for species with higher number of occurrences and narrower ecological niche. SA was always greater than AOO measured with the 2 km × 2 km grid method. SA was greater than EOO, measured by both MCP and α-hull methods, for species with localised distribution, while it was smaller for widely distributed species. EOO-α-hull area was equal or smaller than that calculated by MCP depending on the spatial distribution of species. AOO measured considering the SA within the EOO-MCP was greater than that measured using the standard 2 km × 2 km grid. Conversely, AOO calculated considering the suitable area within the EOO-α-hull showed variable results, being smaller or greater than the 2 km × 2 km grid AOO depending on the ecological niche and spatial distribution of species. According to our results, SEM does not provide an effective alternative to topological methods for the estimate of EOO and AOO. However, it may be considered a useful tool to estimate AOO within the boundaries of EOO measured by the α-hull method, because it reduces some potential sources of inconsistency and bias.     

Endophytic Phomopsis species: host range and implications for diversity estimates

Foliar endophyte assemblages of teak trees growing in dry deciduous and moist deciduous forests of Nilgiri Biosphere Reserve were compared. A species of Phomopsis dominated the endophyte assemblages of teak, irrespective of the location of the host trees. Internal transcribed spacer sequence analysis of 11 different Phomopsis isolates (ten from teak and one from Cassia fistula) showed that they fall into two groups, which are separated by a relatively long branch that is strongly supported. The results showed that this fungus is not host restricted and that it continues to survive as a saprotroph in teak leaf, possibly by exploiting senescent leaves as well as the litter. Although the endophyte assemblage of a teak tree growing about 500 km from the forests was also dominated by a Phomopsis sp., it separated into a different group based on internal transcribed spacer sequence analysis. Our results with an endophytic Phomopsis sp. reinforce the earlier conclusions reached by others for pathogenic Phomopsis sp., i.e., that this fungus is not host specific, and the species concept of Phomopsis needs to be redefined.     

The use of habitat mosaics by terrestrial vertebrate fauna: implications for conservation and management

Many species of vertebrates require multiple habitats to obtain different resources at different stages of their life-cycles. Use of habitat mosaics takes place on a variety of spatial and temporal scales, from a daily requirement for adjacent habitats to seasonal use of geographically separated environments. Mosaics of habitats are also required in some species to allow ontogenetic habitat shifts, while in others each sex may have specific requirements that are met by different habitats. The extent and nature of animal movements are key (but generally poorly known) factors affecting the vulnerability of species to landscape change. The requirement by many species for multiple habitats suggests that their conservation will be most effective in a mosaic environment and that protection of certain high profile habitats alone, such as rainforest, will be insufficient to achieve conservation goals. Management regimes that result in homogenization of habitats should be avoided. Priority should be given to research that identifies the extent to which species can locate habitat mosaics, at different spatial scales and arrangements, in modified environments.     

Impacts of seedling herbivory on plant competition and implications for species coexistence

Background and Aims

Although the causes and consequences of seedling herbivory for plant community composition are well understood, the mechanisms by which herbivores influence plant species recruitment to the established phase remain less clear. The aim was to examine how variation in the intensity of seedling herbivory interacts with growth-defence trade-offs and herbivore feeding preferences to affect plant community development.

Methods

Using 14-d-old seedlings of Trifolium pratense and T. repens, relative growth and susceptibility to herbivory by the snail Helix aspersa was quantified to elucidate putative growth-defence trade-offs for these species. Then mixed assemblages of 14-d-old Trifolium seedlings were exposed to herbivory by zero, two, five or ten snails and determined how variation in the intensity of herbivory affected competitive interactions into the mature phase (as measured by total plant biomass at 120 d old).

Key Results

In the absence of herbivory, communities were dominated by T. pratense; a result expected on the basis that it yielded larger and presumably more competitive seedlings. However, when seedlings were exposed to herbivory, the balance of competition shifted. At low levels of herbivory (two snails), both Trifolium species contributed equally to total plant biomass. More intense herbivory (five snails) resulted in almost total mortality of T. pratense and dominance of the mature community by T. repens. The most intense herbivory (ten snails) effectively removed all seedlings from the experimental community.

Conclusions

The study illustrates a mechanism whereby spatio-temporal fluctuations in seedling herbivory, when coupled with species-specific variation in competitive ability and sensitivity to herbivore attack, can differentially influence plant recruitment into the mature phase. This mechanism may be a key element in our attempts to understand plant species coexistence, since fluctuations in plant recruitment are fundamental to the many theories that view coexistence as a consequence of a spatio-temporal lottery for dominance over regeneration micro-sites.Key words: Growth-defence trade-off, lottery models, plant–animal interactions, plant size variability, seedling acceptability, seedling defence, spatio-temporal niches, Trifolium pratense, Trifolium repens     

Spatiotemporal population distributions and their implications for species coexistence in a variable environment

A population experiences environmental variation both directly, through effects on life history parameters such as fecundity, and indirectly, through effects on the population distributions of competitors and thus on the distribution of competition. Which spatial and temporal scales of environmental variation most influence the coexistence of two species thus depends in part on the degree to which the resident population responds to different scales of variation. In this paper, I calculate an approximation for a spatiotemporal population distribution as the result of a filter function convolved with the environmental variation. I find that there is no straightforward connection between spatial or temporal scales inherent to an organism's life history, such as mean lifetime or dispersal distance, and the population's sensitivity to variation at different scales. Rather, life history traits interact sensitively with the way environmental variation affects the organism. I comment on the implications for variation-mediated coexistence.     

Eukaryotic microbes, species recognition and the geographic limits of species: examples from the kingdom Fungi

The claim that eukaryotic micro-organisms have global geographic ranges, constituting a significant departure from the situation with macro-organisms, has been supported by studies of morphological species from protistan kingdoms. Here, we examine this claim by reviewing examples from another kingdom of eukaryotic microbes, the Fungi. We show that inferred geographic range of a fungal species depends upon the method of species recognition. While some fungal species defined by morphology show global geographic ranges, when fungal species are defined by phylogenetic species recognition they are typically shown to harbour several to many endemic species. We advance two non-exclusive reasons to explain the perceived difference between the size of geographic ranges of microscopic and macroscopic eukaryotic species when morphological methods of species recognition are used. These reasons are that microbial organisms generally have fewer morphological characters, and that the rate of morphological change will be slower for organisms with less elaborate development and fewer cells. Both of these reasons result in fewer discriminatory morphological differences between recently diverged lineages. The rate of genetic change, moreover, is similar for both large and small organisms, which helps to explain why phylogenetic species of large and small organisms show a more similar distribution of geographic ranges. As a consequence of the different rates in fungi of genetic and morphological changes, genetic isolation precedes a recognizable morphological change. The final step in speciation, reproductive isolation, also follows genetic isolation and may precede morphological change.     

Home range sizes in the Wood mouse Apodemus sylvaticus: habitat, sex and seasonal differences

This paper investigates home range sizes in the Wood mouse Apodemus sylvaticus in two habitats of differing productivity; deciduous woodland and sand-dunes. The ranges were measured by radio-telemetry. The home ranges in the sand-dunes were approximately circular whilst those in the woodland were much more complex in shape. At all seasons, the ranges in the sand-dunes were much larger than those in the woodland. Male ranges were larger than female ranges, especially so in the breeding season. In the non-breeding season, sexual differences in range sizes could be accounted for by the greater energy requirements of the larger males. During the breeding season, males expanded their ranges, probably in an attempt to find receptive females.     

Optimal photosynthetic characteristics of individual plants in vegetation stands and implications for species coexistence

AIMS: This paper reviews the way optimization theory has been used in canopy models to analyse the adaptive significance of photosynthesis-related plant characteristics and their consequences for the structure and species composition of vegetation stands. SCOPE: In most studies simple optimization has been used with trait values optimal when they lead to maximum whole-stand photosynthesis. This approach is subject to the condition that the optimum for one individual is independent of the characteristics of its neighbours. This seems unlikely in vegetation stands where neighbour plants strongly influence each other's light climate. Not surprisingly, there are consistent deviations between predicted plant traits and real values: plants tend to be taller, distribute nitrogen more evenly among their leaves and produce more leaf area which is projected more horizontally than predicted by models. CONCLUSIONS: By applying game theory to individual plant-based canopy models, other studies have shown that optimal vegetation stands with maximum whole-stand photosynthesis are not evolutionarily stable. They can be successfully invaded by mutants that are taller, project their leaves more horizontally or that produce greater than optimal leaf areas. While these individual-based models can successfully predict the canopy structure of vegetation stands, they are invariably determined at unique optimal trait values. They do not allow for the co-existence of more than one species with different characteristics. Canopy models can contribute to our understanding of species coexistence through (a) simultaneous analysis of the various traits that determine light capture and photosynthesis and the trade-offs between them, and (b) consideration of trade-offs associated with specialization to different positions in the niche space defined by temporal and spatial heterogeneity of resources.     

Diet and habitat use of frillneck lizards in a seasonal tropical environment

A population of frillneck lizards, Chlamydosaurus kingii, was monitored by mark-recapture and telemetry over a 2 year period in Kakadu National Park. The aims of the study were to document changes in diet, growth, condition and habitat use between the wet and dry seasons of northern Australia, in light of recent research examining seasonal variation in the physiology of this species. Frillneck lizards feed on a diverse range of invertebrates in both seasons, even though there is a substantial reduction in food avail-ability in the dry season. Harvester termites from the genus Drepanotermes constitute a major component of the diet, and the prevalence of termites in the diet of sedentary foragers in a tropical environment is unusual. Adult male body condition remained relatively stable throughout the year, but females experienced considerable variation. These differences are attributed to different reproductive roles of the sexes. Growth in C. kingii was restricted to the wet season, when food availability was high, and growth was minimal in the dry season when food availability was low. The method used in catching lizards was an important factor in determining seasonal habitat use. Telemetered lizards selected a significantly different distribution of tree species than was randomly available, and they selected significantly larger tree species during the dry season. Lizards spotted along roadsides showed little seasonal variation in the selection of tree species or tree sizes. The results suggest a comprehensive change in the ecology of this species, in response to an annual cycle of low food and moisture availability, followed by a period with few resource restrictions.     

Plant species coexistence at local scale in temperate swamp forest: test of habitat heterogeneity hypothesis

It has been suggested that a heterogeneous environment enhances species richness and allows for the coexistence of species. However, there is increasing evidence that environmental heterogeneity can have no effect or even a negative effect on plant species richness and plant coexistence at a local scale. We examined whether plant species richness increases with local heterogeneity in the water table depth, microtopography, pH and light availability in a swamp forest community at three local spatial scales (grain: 0.6, 1.2 and 11.4 m). We also used the variance partitioning approach to assess the relative contributions of niche-based and other spatial processes to species occurrence. We found that heterogeneity in microtopography and light availability positively correlated with species richness, in accordance with the habitat heterogeneity hypothesis. However, we recorded different heterogeneity-diversity relationships for particular functional species groups. An increase in the richness of bryophytes and woody plant species was generally related to habitat heterogeneity at all measured spatial scales, whereas a low impact on herbaceous species richness was recorded only at the 11.4 m scale. The distribution of herbaceous plants was primarily explained by other spatial processes, such as dispersal, in contrast to the occurrence of bryophytes, which was better explained by environmental factors. Our results suggest that both niche-based and other spatial processes are important determinants of the plant composition and species turnover at local spatial scales in swamp forests.