Metapopulation dynamics of a diadromous galaxiid fish and potential effects of salmonid aquaculture

1. Direct ecological effects of biological invasions have been widely documented, but indirect genetic effects on native species are poorly known. In many cases, this is because of the lack of information on the genetic structure of species affected by invasions. 2. We used microsatellite DNA loci to estimate the genetic structure and gene flow patterns of Galaxias maculatus, a galaxiid fish endemic to the southern hemisphere, which is increasingly being threatened by salmonid invasions. 3. Analysis of nine diadromous populations of G. maculatus in Chilean Patagonia (an area heavily impacted by farming of non‐native salmonids) indicates that dispersal is mostly a passive process, seemingly driven by wind and currents and resulting in high gene flow and weak population structuring. 4. Gene flow was asymmetrical, with three populations acting as sources and six populations acting as sinks. Sinks had lower habitat quality and had a greater incidence of adults than sources, which consisted mostly of juveniles. 5. Rivers invaded by salmonid escapees experienced significantly higher aquaculture pressure than rivers where salmonid escapees were apparently absent, but no effect on genetic diversity of G. maculatus could be detected. 6. We discuss whether salmonid aquaculture might affect the demography and connectivity of galaxiid metapopulations: indirectly through habitat alteration and directly through escapes of predatory fish.     

Lethal effects of habitat degradation on fishes through changing competitive advantage

Coral bleaching has caused catastrophic changes to coral reef ecosystems around the world with profound ecological, social and economic repercussions. While its occurrence is predicted to increase in the future, we have little understanding of mechanisms that underlie changes in the fish community associated with coral degradation. The present study uses a field-based experiment to examine how the intensity of interference competition between juveniles of two species of damselfish changes as healthy corals degrade through thermal bleaching. The mortality of a damselfish that is a live coral specialist (Pomacentrus moluccensis) increased on bleached and dead coral in the presence of the habitat generalist (Pomacentrus amboinensis). Increased mortality of the specialist was indirectly owing to enhanced aggression by the generalist forcing the specialist higher up and further away from shelter on bleached and dead coral. Evidence from this study stresses the importance of changing interspecific interactions to community dynamics as habitats change.     

Combined effects of predators and parasites on shoaling behavior of fishes

A review of experimental, field and theoretical papers on several topics related to the study of variability of fish shoaling behavior caused by separate impacts of predators and parasites as well as by combined effects of these factors. First, antipredator functions and changes in fish shoal parameters caused by predation risk are briefly discussed. Then, effects of parasites that have the potential to act as a force that can select for either larger or smaller group size, or even for solitary behavior are reviewed. Predation- and parasitism-induced variations in the shoal size and shape, distance between members of a shoal, position of fish within a shoal, effects of habitat complexity, parasite-assortative shoaling are described. Finally, an interplay between the parasitism and predation risks that could influence protective functions of fish shoaling is discussed. It is emphasized that not only the binary “predator-prey” and “host-parasite” systems, but a three-component system “parasite.-host-predator” which embrace both direct and indirect effects have to be studied.     

Both spatiotemporal connectivity and habitat quality limit the immigration of forest plants into wooded corridors

Extensive afforestation of agricultural areas has increased the importance of green corridors as a dispersal network. We tested the effect of spatiotemporal connectivity, edge effect and habitat structural quality of wooded corridors on the long-term immigration success of forest specialist plants relative to the performance of forest generalists. In agricultural landscapes of central and southern Estonia, we sampled 28 historically connected and 52 isolated tree lines and alleys with a minimum age of 50 years, and 93 edges of ancient forests. The regional pool of common forest plants was compiled using species’ frequency data in 91 ancient forests. Both landscape connectivity and habitat quality affected the richness of response groups, but specialists and generalists responded to different drivers. Forest specialists required long-term neighbourhoods of ancient forest and benefited from a direct connection between forest and corridor. Habitat generalists reacted positively to the recently modified structure of the landscape. When a corridor was connected to forest, the dual edge in the corridor did not result in an increased negative edge effect on forest specialist arrival. Even if both specialists and generalists required wide corridors with optimum shade, forest specialists also benefited from mature overstorey and outward overhanging branches, whereas forest generalists used disturbance-created microhabitats. We conclude that only wooded corridors with long-term connectivity to seed source forests and widely branched tree canopies will function as a green infrastructure supporting forest-specific biodiversity.     

Carry-over effects and habitat quality in migratory populations

Determining the factors that influence migratory population abundance has been constrained by the inability to connect events in different periods of the annual cycle. Carry-over effects are events that occur in one season but influence individual success the following season and recent empirical evidence suggests that they may play an important role in migratory population dynamics. Using a long distance migratory shorebird as an example, I incorporate carry-over effects and changes in the relative amount of habitat quality into a density-dependent equilibrium population model. The model uses the example where the quality of habitat on the wintering grounds (nonbreeding season) influences breeding output the following summer (breeding season). Carry-over effects, however, may be manifested in a number of other ways that could influence population dynamics. In the simulations, population declines occur when habitat is lost on the wintering grounds. However, results show that carry-over effects can magnify these declines when a disproportionate amount of high quality habitat is lost the previous winter. Simulations also show that carry-over effects can have a relative, positive impact on population size when the majority of habitat that is lost in the previous season is low quality. In this case, the carry-over interacts with density-dependence the following season producing an additive and positive effect, buffering the population from severe declines. To predict changes in population size of migratory animals, it will be important to determine (i) which demographic factors in which season produce strong carry-over effects and, (ii) not just the amount, but the relative quality of habitat that is lost. If carry-over effects are significant, they could potentially mitigate 'seasonal compensation effects' from density-dependence, leading to exacerbated population declines.     

The effects of energy input,immigration and habitat size on food web structure: a microcosm experiment

It has been hypothesised that larger habitats should support more complex food webs. We consider three mechanisms which could lead to this pattern. These are increased immigration rates, increased total productivity and spatial effects on the persistence of unstable interactions. Experiments designed to discriminate between these mechanisms were carried out in laboratory aquatic microcosm communities of protista and bacteria, by independently manipulating habitat size, total productivity and immigration rate. Larger habitats supported more complex food webs, with more species, more links per species and longer maximum and mean food chains, even in the absence of differences in total energy input. Increased immigration rate resulted in more complex food webs, but habitats with higher energy input per unit area supported less complex food webs. We conclude that spatial effects on the persistence of unstable interactions, and variation in immigration rates, are plausible mechanisms by which habitat size could affect food web structure. Variation in total productivity with habitat area seems a less likely explanation for variation in food web structure.     

Manipulation of habitat quality: effects on chironomid life history traits

1. To understand how habitat heterogeneity affects organisms, the present authors measured the response of chironomid life-histories to single patch types in a controlled laboratory experiment. The objective was to determine whether the size and type of leaf particles in the substratum affect development rate and survival of larvae, and the fecundity of the resulting adults.
2. Chironomus riparius larvae were raised in sand supplemented with dogwood, Cornus florida, (a rapid decomposer) or American sycamore, Platanus occidentalis, (a refractory decomposer) leaves fully crossed in a factorial design with four leaf particle sizes (0.2, 0.5, 1 and 5 cm²), or in sand only.
3. Development time was shorter in dogwood habitats than sycamore habitats, and shorter in sycamore habitats than in sand habitats. Survival was higher and larval head capsule lengths of instars III and IV were larger in dogwood than in sycamore or sand habitats. Development time was longer and head capsule lengths of instars III and IV were smaller in habitats with 5-cm² leaf particles than in 0.2-, 0.5- and 1-cm² particle size treatments. Female thorax length (a measure of fecundity) did not differ among treatments.
4. The present authors conclude that, for C. riparius confined to a single patch type, fitness is significantly enhanced when the patch contains small particles of dogwood (labile) leaves.     

Differential effects of habitat complexity,predators and competitors on abundance of juvenile and adult coral reef fishes

Greater structural complexity is often associated with greater abundance and diversity, perhaps because high complexity habitats reduce predation and competition. Using 16 spatially isolated live-coral reefs in the Bahamas, I examined how abundance of juvenile (recruit) and adult (non-recruit) fishes was affected by two factors: (1) structural habitat complexity and (2) the presence of predators and interference competitors. Manipulating the abundance of low and high complexity corals created two levels of habitat complexity, which was cross-factored with the presence or absence of resident predators (sea basses and moray eels) plus interference competitors (territorial damselfishes). Over 60 days, predators and competitors greatly reduced recruit abundance regardless of habitat complexity, but did not affect adult abundance. In contrast, increased habitat complexity had a strong positive effect on adult abundance and a weak positive effect on recruit abundance. Differential responses of recruits and adults may be related to the differential effects of habitat complexity on their primary predators. Sedentary recruits are likely most preyed upon by small resident predators that ambush prey, while larger adult fishes that forage widely and use reefs primarily for shelter are likely most preyed upon by large transient predators that chase prey. Increased habitat complexity may have inhibited foraging by transient predators but not resident predators. Results demonstrate the importance of habitat complexity to community dynamics, which is of concern given the accelerated degradation of habitats worldwide.     

Contrasting effects of habitat quantity and quality on moth communities in fragmented landscapes

Habitat loss is commonly identified as a major threat to the loss of global biodiversity. In this study, we expand on our previous work by addressing the question of how lepidopteran species richness and composition vary among remnants of North American eastern deciduous forest located within agricultural or pastoral landscapes. Specifically, we tested the relative roles of habitat quantity (measured as stand area and percent forest in the greater landscape) and habitat quality (measured as tree species diversity) as determinants of moth species richness. We sampled >19 000 individuals comprising 493 moth species from 21 forest sites in two forested ecoregions. In the unglaciated Western Allegheny Plateau, the species richness of moths with woody host plants diminished as forest stand size and percent forest in the landscape decreased, but the total species richness and abundance of moths were unaffected by stand size, percent forest in the landscape, or tree species diversity. In contrast, the overall species richness and abundance of moths in the glaciated North Central Tillplain were affected primarily by tree species diversity and secondarily by forest size. Higher tree species diversity may reduce species loss from smaller forest stands, suggesting that small, diverse forests can support comparable numbers of species to those in less diverse, large stands. Smaller forests, however, contained a disproportionate number of moth species that possess larvae known to feed on herbaceous vegetation. Thus, although woody plant feeding moths are lost from forests with changes in stand area, new species appear capable of recolonizing smaller fragments from the surrounding habitat matrix. Our study further suggests that when species replacement occurs, local patch size and habitat quality may be more important than landscape context in determining the community structure of forest Lepidoptera.     

Modelling the current distribution of European diadromous fishes: an approach integrating regional anthropogenic pressures

1. Twenty‐eight diadromous fish species occurred in Europe, North Africa and the Middle East in historical times. Their current distributions were assessed in terms of abundance classes (missing, rare, common and abundant) in 196 basins ranging from Morocco to northern Norway and from Greenland to Iran. 2. Their current distributions were modelled using abiotic, biotic, climatic and anthropogenic (regional anthropogenic pressures) variables. Anthropogenic variables were derived from characteristics of large dams (height, distance from the outlet, percentage of main stem river available downstream of dam) and human population density. These data were taken from the EEA Eldred 2.08 (European Lakes, Dams and Reservoirs Database) that deals comprehensively with large European dams and includes all obstacles of this type. To deal with ordinal response variables, we applied proportional odds models. 3. Twenty‐two species‐specific models were successfully built according to the reduction of deviance and the validation process, of which eight included one or more anthropogenic variables. No model could be established for six endemic or highly endangered species such as Acipenser sturio and Coregonus oxyrinchus. 4. Most response curves were easily interpretable since they were related to specific aspects of species’ ecology. Anthropogenic variables related to large dams impacted negatively on the distribution of diadromous fishes through the perturbation of river discharge patterns, the loss of river connectivity and the accessibility to essential habitats, particularly for Salmonid species that spawn in headwater streams. However, one species which can complete its life cycle using only the most downstream part of the basin, Liza ramada, was found to be favoured by the changes in hydrological regime. The bell‐shaped curves obtained from human population density for three diadromous species were connected on one side to a common settlement history of human and animal populations and on the other side to negative impacts of human activities arising at high population density. 5. Our approach can provide the basis for identifying special areas of conservation prior to planning restoration programmes at country or basin scales, as well as for more specialized studies focusing on one species only or at the local scale.     

Combined effects of overwintering temperature and habitat degradation on the survival of boreal wood ant

The survival of insects during diapause may be affected by overwintering temperature and other environmental stress, such as anthropogenic habitat degradation. We experimentally studied the effects of overwintering temperature (+1 and +7°C) and commercial forest clear-cutting on the overwintering survival of the forest-dwelling wood ant Formica aquilonia. We found that both the higher overwintering temperature and clear-cutting lowered the overwintering survival and body fat resources of Formica aquilonia. The survival and body fat resources were highest in lower temperature treatment forest nests and lowest in higher temperature treatment clear-cut nests. The overall survival of ants increased with higher body fat resources. It is possible that both forest clear-cutting and higher winter temperature due to possible climate warming may increase the wintertime mortality of wood ants and other forest-dwelling ants.     

The effects of seafloor habitat complexity on survival of juvenile fishes: Species-specific interactions with structural refuge

Marine fishes are often associated with structurally complex microhabitats that are believed to provide a refuge from predation. However, the effects of habitat complexity on predator foraging success can be strongly modified by predator and prey behaviors. We conducted a series of laboratory experiments to evaluate the effects of sea floor habitat complexity on juvenile fish survivorship using multiple predator (striped searobin and summer flounder) and prey (winter flounder, scup, and black sea bass) species to identify potentially important species-habitat interactions. Three habitats of varying complexity (bare sand, shell, and sponge) common to coastal marine environments were simulated in large aquaria (2.4 m diameter, 2400 L volume). Prey survivorship increased significantly with greater habitat complexity for each species combination tested. However, examination of multiple prey and predator species across habitats revealed important effects of predator × habitat and prey × habitat interactions on prey survival, which appeared to be related to species-specific predator and prey behavior in complex habitats. Significant species × habitat interactions imply that the impact of reduced seafloor habitat complexity may be more severe for some species than others. Our results indicate that the general effects of seafloor habitat complexity on juvenile fish survivorship may be broadly applicable, but that the interaction of particular habitats with search tactics of predators as well as habitat affinities and avoidance responses of prey can produce differences among species that contribute to variable mortality.     

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 butterfly Parnassius smintheus among 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.     

Local species immigration,extinction, and turnover of butterflies in relation to habitat area and habitat isolation

Temporal dynamics of insect communities in terrestrial habitat fragments have been rarely studied. Here it was tested whether immigration, extinction, and turnover of butterfly species change with area and isolation of 31 calcareous grasslands. The area ranged from 0.03 to 5.14 ha, the isolation index from 2,100 to 86,000 (edge-to-edge distance 55–1,894 m). In both study years (1996, 2000), the total number of individuals (16,466, 15,101) and species (60, 54) sampled across all sites were similar and number of species increased with area in both years indicating an equilibrium. Rates of extinction (38% for habitat specialists vs. 20% for generalists) and turnover (51% vs. 35%) were higher, and rates of immigration (11% vs. 30%) were lower for habitat specialists than for generalists. Extinction and turnover rates decreased with increasing fragment size for both specialist (n =25 species) and generalist (n =36) butterflies, but specialists showed a significantly steeper decrease with increasing fragment size than generalists. Immigration rates increased with area. As a result, species number of habitat specialists declined in small habitats but not in large habitats between 1996 and 2000. No significant impact of habitat isolation on the butterfly community was found. The data suggest that large habitat fragments are of special importance for the conservation of the specialized, most endangered butterfly species. Habitat isolation appears to be less important, as butterflies can cope with the habitat mosaic in our study region.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Combining energetic profitability and cover effects to evaluate salmonid habitat quality

Recent studies have demonstrated that the energetic profitability (net energy intake potential; NEI potential) of a habitat, which is calculated as the gross energy gain from foraging minus the energy expenditure from swimming at a focal point, may be a useful tool for predicting the salmonid biomass. The effectiveness of the NEI potential should be tested in various systems. Even if the NEI potential is validated, its predictive accuracy and transferability could be limited if the cover habitat, which is known to be an important factor for determining salmonid abundance, is not considered. We tested whether the NEI potential is effective for predicting the salmonid biomass even in a stream with abundant cover and whether combining the NEI potential and cover effects can improve the predictability of fish biomass using a generalized linear model. Our results demonstrated that the NEI potential could generally predict the fish biomass (percent deviance explained?=?79.9 %), and the model that incorporated both the NEI potential and the cover ratio improved the predictive accuracy (percent deviance explained?=?88.5 %). These results suggest that energetic profitability can be an effective indicator for assessing habitat quality and is relatively transferable to other systems. Furthermore, when cover effects are considered, the habitat quality is more accurately represented; thus, combining the energetic profitability and the cover effects might improve the transferability of the assessment across habitats.