Underdispersion and overdispersion of traits in terrestrial snail communities on islands

Understanding and disentangling different processes underlying the assembly and diversity of communities remains a key challenge in ecology. Species can assemble into communities either randomly or due to deterministic processes. Deterministic assembly leads to species being more similar (underdispersed) or more different (overdispersed) in certain traits than would be expected by chance. However, the relative importance of those processes is not well understood for many organisms, including terrestrial invertebrates. Based on knowledge of a broad range of species traits, we tested for the presence of trait underdispersion (indicating dispersal or environmental filtering) and trait overdispersion (indicating niche partitioning) and their relative importance in explaining land snail community composition on lake islands. The analysis of community assembly was performed using a functional diversity index (Rao's quadratic entropy) in combination with a null model approach. Regression analysis with the effect sizes of the assembly tests and environmental variables gave information on the strength of under‐ and overdispersion along environmental gradients. Additionally, we examined the link between community weighted mean trait values and environmental variables using a CWM‐RDA. We found both trait underdispersion and trait overdispersion, but underdispersion (eight traits) was more frequently detected than overdispersion (two traits). Underdispersion was related to four environmental variables (tree cover, habitat diversity, productivity of ground vegetation, and location on an esker ridge). Our results show clear evidence for underdispersion in traits driven by environmental filtering, but no clear evidence for dispersal filtering. We did not find evidence for overdispersion of traits due to diet or body size, but overdispersion in shell shape may indicate niche differentiation between snail species driven by small‐scale habitat heterogeneity. The use of species traits enabled us to identify key traits involved in snail community assembly and to detect the simultaneous occurrence of trait underdispersion and overdispersion.     

Environmental impact studies on marine communities: Pragmatical considerations

Abstract The measurement of changes in the structure of natural marine communities is widely used for the detection and monitoring of man's impact on the sea. Such studies are almost always a compromise between the scientific ideal and political, financial and logistical constraints. This paper considers a number of these constraints, suggests some possible solutions, and discusses the consequences (in terms of loss of information) of adopting strategies which might be considered suboptimal. Identification of all organisms in a community to species level is a major time and cost constraint. Many recent studies have shown that very little information is lost by working at a taxonomic level higher than species (e.g. family or even phylum). This implies a functional coherence of species within each higher taxon. Indeed, there are theoretical reasons, and some empirical evidence, for supposing that community responses to human perturbations may be more easily detected above the noise of natural variability by working at high taxonomic levels. Many environmental impacts, for example oil spillages from tankers, are accidental and it is unusual to have the luxury of a time-series of data from the site prior to the incident. Good spatial control samples may also be difficult to obtain in a heterogeneous environment. Absolute rather than comparative measures of community degradation are valuable in this context. Methods which fall into this category include comparisons of community structure with some theoretical expectation or with empirical‘training’ data, comparing attributes of community structure that respond differently to perturbation, or identifying highly conservative properties of unperturbed communities that are modified in a predictable way by perturbation. All studies are selective with regard to which components and attributes of the biota should be investigated. The choice frequently depends on local expertise and research interests, rather than on an objective decision about what biological data are most appropriate for the problem in hand. The paper concludes by a discussion of the relative merits of the various alternatives.     

Regional effects as important determinants of local diversity in both marine and terrestrial systems

Community ecologists have struggled to create unified theories across diverse ecosystems, but it has been difficult to acertain whether marine and terrestrial communities differ in the mechanisms responsible for structure and dynamics. One apparent difference between marine and terrestrial ecology is that the influence of regional processes on local populations and communities is better established in the marine literature. We examine three potential explanations: 1) influential early studies emphasized local interactions in terrestrial communities and regional dispersal in marine communities. 2) regional‐scale processes are actually more important in marine than in terrestrial communities. 3) recruitment from a regional species pool is easier to study in marine than terrestrial communities. We conclude that these are interrelated, but that the second and especially the third explanations are more important than the first. We also conclude that in both marine and terrestrial systems, there are ways to improve our understanding of regional influences on local community diversity. In particular, we advocate examining local vs regional diversity relationships at localities within environmentally similar regions that differ in their diversity either because of their sizes or their varying degrees of isolation from a species source.     

Snorkelers impact on fish communities and algae in a temperate marine protected area

Multiple-use marine protected areas (MPAs) are used to manage marine resources, allocate space to different users and reduce conflicts while protecting marine biodiversity. In the Mediterranean, MPA managers are increasingly interested in containing the effects of coastal recreation within underwater trails, but snorkelers impacts on the surrounding ecosystem remain largely unknown. In a Mediterranean MPA, an underwater snorkeling trail was established to concentrate snorkelers and increase their awareness of marine habitats and species. The high level of summer frequentation may have negative impacts on the surrounding environment through trampling on the sessile flora or disturbance to the vagile fauna. We used a before-after-control-impact (BACI) design to analyse these potential human impacts. The structure of macroalgae and fish assemblages were used as indicators. Permutational multivariate analyses of variance (PERMANOVA) were carried out to assess potential temporal and spatial changes of the indicators between the trail and a control location within the adjacent no-take/no-use area. Fish communities and macroalgae were subjected to natural temporal trends but no significant impacts of snorkelers were found. Four reasons could explain the absence of snorkelers impact on the surrounding marine environment: (1) the absence of very fragile organisms within the trail (and the control no-take/no-use area) such as gorgonians or bryozoans; (2) the life cycle of the algae with a natural decreasing trend in summer, corresponding to the trail opening period; (3) only a few snorkelers are practicing apnoea; and (4) the information at the entrance and along the trail may influence the snorkelers’ behaviour.     

Associational plant refuges: convergent patterns in marine and terrestrial communities result from differing mechanisms

Summary An associational plant refuge occurs when a plant that is susceptible to herbivory gains protection from herbivory when it is associated with another plant. In coastal North Carolina, the abundance of the palatable red alga Gracilaria tikvahiae is positively correlated with the abundance of the unpalatable brown alga Sargassum filipendula during times of increased herbivore activity. To see if grazing by the sea urchin Arbacia punctulata could generate this pattern, controlled experiments were conducted in out-door microcosms and in the laboratory. Gracilaria beneath a canopy of Sargassum was eaten significantly less than Gracilaria alone. When Arbacia were excluded, Gracilaria alone grew significantly more than Gracilaria beneath Sargassum, demonstrating that Sargassum is a competitor of Gracilaria. Experiments investigating Sargassum's deterrent role indicated that Sargassum decreased the foraging range of Arbacia and the rate at which it fed on Gracilaria. Additional experiments with plastic Sargassum mimics indicated that the decreased grazing on Gracilaria was not a result of Sargassum morphology, but was probably attributable to some chemical characteristic of Sargassum. The pattern of increased grazing in monocultures (only Gracilaria present) versus polycultures (both Gracilaria and Sargassum present) demonstrated in this study also has been demonstrated for plant-insect interactions in terrestrial communities. In these communities, insect density is higher in monocultures than in polycultures because insects find and immigrate to monocultures more rapidly, and once in a monoculture, they emigrate from them less often than from polycultures. In this study, urchins did not find and immigrate to monocultures more rapidly, nor did they tend to stay in them once they were found; in fact, they emigrated from monocultures of Gracilaria more rapidly than from Gracilaria and Sargassum polycultures. Increased grazing in Gracilaria monocultures resulted from increased rates of movement and feeding of individual herbivores, not from increased herbivore density as has been reported for terrestrial systems.     

On the direct impact of ice on marine benthic communities, a review

Ice has a significant impact on the polar and sub-polar benthos, but relationships between corresponding physical and biological processes are not yet sufficiently understood. Sea ice contributes to a vertical zonation in shallow waters, which also experience other important disturbances. Due to the length of the non-glaciated coastline, sea ice is of greater relevance in the north than in the south. Scouring by icebergs and ridged sea ice causes an increased diversity when different recolonisation stages coexist. Frequently scoured areas do not recover, especially in the Antarctic, due to slow growth rates of the fauna. Iceberg grounding in the Arctic is mainly restricted to the western Eurasian and northeastern American shelf, including Greenland. Around Antarctica, scouring is more evenly distributed. Glacier termini prevent sessile animals from settling in their proximity where only few motile species occur.     

Egg predation in forest bird communities on islands and mainland

Summary To examine if differences in egg predation rates could explain differences in bird community composition, egg predation was studied in two years on small islands in a South Swedish lake and on the nearby mainland using both natural and artificial nests.In plots with similar vegetation, the combined density of ground- and tree-nesting bird species did not differ between the islands and the mainland. Egg predation rates were similar on islands and the mainland for natural Turdus nests in two years, and for artificial Turdus and Phylloscopus nests. Unmarked and unvisited experimental nests suffered similar rate of egg predation as marked and visited nests. Egg predation rates were higher on natural nests when artificial nests were also put out, increasing the total nest density. Initial egg predation rates in artificial nests were also higher than later when nest density had decreased by 75%.The egg predators involved differed for artificial Phylloscopus nests between the islands and the mainland. Small mammals were apparently responsible for 29% of the predation on the mainland, but none on the islands. Artificial Turdus nests near crow nests suffered from a higher egg predation rate than nests further away from crow nests. Daily survival rates of Turdus nests increased from the laying to the incubation and further to the fledging state.Egg predation can not explain differences in bird community composition between islands and mainland in the present case.     

Mammal invaders on islands: impact, control and control impact

The invasion of ecosystems by exotic species is currently viewed as one of the most important sources of biodiversity loss. The largest part of this loss occurs on islands, where indigenous species have often evolved in the absence of strong competition, herbivory, parasitism or predation. As a result, introduced species thrive in those optimal insular ecosystems affecting their plant food, competitors or animal prey. As islands are characterised by a high rate of endemism, the impacted populations often correspond to local subspecies or even unique species. One of the most important taxa concerning biological invasions on islands is mammals. A small number of mammal species is responsible for most of the damage to invaded insular ecosystems: rats, cats, goats, rabbits, pigs and a few others. The effect of alien invasive species may be simple or very complex, especially since a large array of invasive species, mammals and others, can be present simultaneously and interact among themselves as well as with the indigenous species. In most cases, introduced species generally have a strong impact and they often are responsible for the impoverishment of the local flora and fauna. The best response to these effects is almost always to control the alien population, either by regularly reducing their numbers, or better still, by eradicating the population as a whole from the island. Several types of methods are currently used: physical (trapping, shooting), chemical (poisoning) and biological (e.g. directed use of diseases). Each has its own set of advantages and disadvantages, depending on the mammal species targeted. The best strategy is almost always to combine several methods. Whatever the strategy used, its long-term success is critically dependent on solid support from several different areas, including financial support, staff commitment, and public support, to name only a few. In many cases, the elimination of the alien invasive species is followed by a rapid and often spectacular recovery of the impacted local populations. However, in other cases, the removal of the alien is not sufficient for the damaged ecosystem to revert to its former state, and complementary actions, such as species re-introduction, are required. A third situation may be widespread: the sudden removal of the alien species may generate a further disequilibrium, resulting in further or greater damage to the ecosystem. Given the numerous and complex population interactions among island species, it is difficult to predict the outcome of the removal of key species, such as a top predator. This justifies careful pre-control study and preparation prior to initiating the eradication of an alien species, in order to avoid an ecological catastrophe. In addition, long-term monitoring ofthe post-eradication ecosystem is crucial to assess success and prevent reinvasion.     

Mammalian species richness on islands on the Sunda Shelf,Southeast Asia

Summary A rich mammalian fauna is found on islands that lie on the Sunda Shelf, a continental shelf extending from Vietnam to Borneo and Java that was periodically exposed as dry land during the Pleistocene. The correlation between log of island area and number of species is high (r ²=0.94); the slope of the curve is moderate (z=0.235). Distance from small islands to source areas (=Borneo, Sumatra, and the Malay Peninsula) does not appear to affect species richness, nor does depth of water to source area (a measure of isolation time). A species-area curve for forest reserves of varying sizes on the Malay Peninsula has a low slope (z=0.104); comparison of the mainlaind and island curves indicates that decreasing island area is strongly correlated with increased extinction. Extinction has left reduced but ecologically balanced sets of species on all islands, except that carnivores are under-represented on all but the largest islands. Initial body size and rarity appear to play a significant role in determining the probability of extinction of individual species.     

Interspecific variation in life history relates to antipredator decisions by marine mesopredators on temperate reefs

As upper-level predatory fishes become overfished, mesopredators rise to become the new 'top' predators of over-exploited marine communities. To gain insight into ensuing mechanisms that might alter indirect species interactions, we examined how behavioural responses to an upper-level predatory fish might differ between mesopredator species with different life histories. In rocky reefs of the northeast Pacific Ocean, adult lingcod (Ophiodon elongatus) are upper-level predators that use a sit-and-wait hunting mode. Reef mesopredators that are prey to adult lingcod include kelp greenling (Hexagrammos decagrammus), younger lingcod, copper rockfish (Sebastes caurinus) and quillback rockfish (S. maliger). Across these mesopredators species, longevity and age at maturity increases and, consequently, the annual proportion of lifetime reproductive output decreases in the order just listed. Therefore, we hypothesized that the level of risk taken to acquire resources would vary interspecifically in that same order. During field experiments we manipulated predation risk with a model adult lingcod and used fixed video cameras to quantify interactions between mesopredators and tethered prey (Pandalus shrimps). We predicted that the probabilities of inspecting and attacking tethered prey would rank from highest to lowest and the timing of these behaviours would rank from earliest to latest as follows: kelp greenling, lingcod, copper rockfish, and quillback rockfish. We also predicted that responses to the model lingcod, such as avoidance of interactions with tethered prey, would rank from weakest to strongest in the same order. Results were consistent with our predictions suggesting that, despite occupying similar trophic levels, longer-lived mesopredators with late maturity have stronger antipredator responses and therefore experience lower foraging rates in the presence of predators than mesopredators with faster life histories. The corollary is that the fishery removal of top predators, which relaxes predation risk, could potentially lead to stronger increases in foraging rates for mesopredators with slower life histories.     

Topographic and altitudinal patterns in plant communities on European arctic islands

 Vegetation on Bear Island, Jan Mayen and Spitsbergen was investigated along altitudinal and topographic gradients in order to describe the main patterns in plant community distributions and compare them with those on the mainland. In a numerical classification the communities were distinctly differentiated; however, physiognomically similar Racomitrium and Sanionia communities dominated in most habitats on Bear Island and Jan Mayen. On Spitsbergen, moss-dominated communities prevailed in depression sites. High-altitude sites were occupied either by moss-dominated communities or by variable assemblages of fragmented moss cover and scattered vascular plants. Dwarf shrub and grass heaths that were common on the mainland did not occur on grazer-free Bear Island and Jan Mayen, and were confined to the lowest altitudes on Spitsbergen. The lack of grazers on Bear Island and Jan Mayen accounts in part for the differences in vegetation between the mainland and the islands. Received: 13 December, 1995/Accepted: 13 March, 1996     

Community composition has greater impact on the functioning of marine phytoplankton communities than ocean acidification

Ecosystem functioning is simultaneously affected by changes in community composition and environmental change such as increasing atmospheric carbon dioxide (CO₂) and subsequent ocean acidification. However, it largely remains uncertain how the effects of these factors compare to each other. Addressing this question, we experimentally tested the hypothesis that initial community composition and elevated CO₂ are equally important to the regulation of phytoplankton biomass. We full‐factorially exposed three compositionally different marine phytoplankton communities to two different CO₂ levels and examined the effects and relative importance (ω²) of the two factors and their interaction on phytoplankton biomass at bloom peak. The results showed that initial community composition had a significantly greater impact than elevated CO₂ on phytoplankton biomass, which varied largely among communities. We suggest that the different initial ratios between cyanobacteria, diatoms, and dinoflagellates might be the key for the varying competitive and thus functional outcome among communities. Furthermore, the results showed that depending on initial community composition elevated CO₂ selected for larger sized diatoms, which led to increased total phytoplankton biomass. This study highlights the relevance of initial community composition, which strongly drives the functional outcome, when assessing impacts of climate change on ecosystem functioning. In particular, the increase in phytoplankton biomass driven by the gain of larger sized diatoms in response to elevated CO₂ potentially has strong implications for nutrient cycling and carbon export in future oceans.     

Antibiotic activity of bonellin and hematoporphyrin on marine and terrestrial bacteria

Bonellin, a chlorin extracted fromBonellia viridis (Echiura), and hemaatoporphyrin exhibit a strong antibiotic and bactericidal activity on marine and terrestrial bacteria. This action is enhanced by light. Oxygen consumption and motility of bacteria are also inhibited, while no chemotactic effects are observed. The drugs induce lysis onBacillus subtilis protoplasts, but they are ineffective onMicrococcus lysodeikticus protoplasts.The results are discussed and compared with those obtained with eukariotic cells. Attention is focused on the ecological role of bonellin in the defense mechanism ofBonellia viridis.     

How lichens impact on terrestrial community and ecosystem properties

Lichens occur in most terrestrial ecosystems; they are often present as minor contributors, but in some forests, drylands and tundras they can make up most of the ground layer biomass. As such, lichens dominate approximately 8% of the Earth's land surface. Despite their potential importance in driving ecosystem biogeochemistry, the influence of lichens on community processes and ecosystem functioning have attracted relatively little attention. Here, we review the role of lichens in terrestrial ecosystems and draw attention to the important, but often overlooked role of lichens as determinants of ecological processes. We start by assessing characteristics that vary among lichens and that may be important in determining their ecological role; these include their growth form, the types of photobionts that they contain, their key functional traits, their water‐holding capacity, their colour, and the levels of secondary compounds in their thalli. We then assess how these differences among lichens influence their impacts on ecosystem and community processes. As such, we consider the consequences of these differences for determining the impacts of lichens on ecosystem nutrient inputs and fluxes, on the loss of mass and nutrients during lichen thallus decomposition, and on the role of lichenivorous invertebrates in moderating decomposition. We then consider how differences among lichens impact on their interactions with consumer organisms that utilize lichen thalli, and that range in size from microfauna (for which the primary role of lichens is habitat provision) to large mammals (for which lichens are primarily a food source). We then address how differences among lichens impact on plants, through for example increasing nutrient inputs and availability during primary succession, and serving as a filter for plant seedling establishment. Finally we identify areas in need of further work for better understanding the role of lichens in terrestrial ecosystems. These include understanding how the high intraspecific trait variation that characterizes many lichens impacts on community assembly processes and ecosystem functioning, how multiple species mixtures of lichens affect the key community‐ and ecosystem‐level processes that they drive, the extent to which lichens in early succession influence vascular plant succession and ecosystem development in the longer term, and how global change drivers may impact on ecosystem functioning through altering the functional composition of lichen communities.     

Variation between freshwater and terrestrial fungal communities on decaying bamboo culms

Fungal communities on decaying culms of a bamboo host (Phyllostachys bambusoides) from freshwater and adjacent terrestrial habitats were identified. Collections were made at Xiao Bai Long Mountain, Yiliang, Yunnan, China in the winter and summer. In each collection, 100 similar-sized bamboo culms were collected, comprising 50 submerged samples from a stream and 50 terrestrial samples from adjacent riparian vegetation. A total of 82 fungal taxa were recorded from the samples, including 30 ascomycetes and 52 anamorphic fungi. The frequency of occurrence of these fungi were recorded and the Shannon–Weiner indices (H′) were applied to evaluate fungal diversity. The results showed that variation of the fungal diversity between the summer and winter collections was insignificant (0.2<p<0.5). Fungal diversity on submerged bamboo however, was significantly higher than that on terrestrial bamboo (p<0.001). Further findings were that: (1) some commonly recorded freshwater and terrestrial taxa were found in both habitats, but overall there were only 15 overlapping species between the two habitats; (2) the dominant species in each habitat were considerably different, and (3) only a few fungi were dominant, while most species were rare, being recorded only once or twice. Factors responsible for the distribution patterns and variations in composition of the fungal communities are discussed.     

Biogeography and the structure of coral reef fish communities on isolated islands

Aim To determine the applicability of biogeographical and ecological theory to marine species at two remote island locations. This study examines how biogeography, isolation and species geographic range size influence patterns of species richness, endemism, species composition and the abundance of coral reef fishes. Location Christmas Island and the Cocos (Keeling) Islands in the tropical eastern Indian Ocean. Methods Published species lists and underwater visual surveys were used to determine species richness, endemism, species composition and abundance of reef fishes at the islands. These data were statistically compared with patterns of species composition and abundance from the neighbouring ‘mainland’ Indonesian region. Results The two isolated reef fish communities were species‐poor and contained a distinct taxonomic composition with an overrepresentation of species with high dispersal potential. Despite low species richness, we found no evidence of density compensation, with population densities on the islands similar to those of species‐rich mainland assemblages. The mix of Indian and Pacific Ocean species and the proportional representations of the various regional faunas in the assemblages were not influenced by the relative proximity of the islands to different biogeographical provinces. Moreover, species at the edge of their range did not have a lower abundance than species at the centre of their range, and endemic species had substantially higher abundances than widespread species. At both locations, endemism was low (less than 1.2% of the community); this may be because the locations are not sufficiently isolated or old enough to promote the evolution of endemic species. Main conclusions The patterns observed generally conform to terrestrial biogeographical theory, suggesting that similar processes may be influencing species richness and community composition in reef fish communities at these remote islands. However, species abundances differed from typical terrestrial patterns, and this may be because of the life history of reef fishes and the processes maintaining isolated populations.     

Peat respiration and decomposition in Antarctic terrestrial moss communities

Oxygen uptake by the peat of two Antarctic bryophyte communities (a moss turf and a moss carpet) is converted to organic matter loss and used to derive the rate of decomposition. The decay rates obtained in this way are evaluated in two mathematical models which simulate the accumulation of dead organic matter (DOM) in the communities from the litter production and decomposition rate. Litter production, the extent of DOM accumulations at present on the sites and mean decomposition rates (i.e. fraction of standing crop lost per year) were 409 g m^-2 year^-1, 33.5 kg m^-2 and 0.017 g g^-1 year^-1 in the moss turf and 392 g m^-2 year^-1, 29.6 kg m-² and 0.010 g g^-1 year^-1 in the moss carpet respectively (all weights expressed as dry weight). Aerobic decomposition rate declined with depth in both communities. From the model's predictions it is suggested that the observed decay rate was too high in the moss turf and too low in the carpet. Possible reasons for this are discussed and suggestions made for future work.     

Isolation‐driven functional assembly of plant communities on islands

The physical and biotic environment is often considered the primary driver of functional variation in plant communities. Here, we examine the hypothesis that spatial isolation may also be an important driver of functional variation in plant communities where disturbance and dispersal limitation may prevent species from occupying all suitable habitats. To test this hypothesis, we surveyed the vascular plant composition of 30 islands in the Gulf of Maine, USA, and used available functional trait and growth form data to quantify the functional composition of these islands. We categorized species based on dispersal mode and used a landscape metric of isolation to assess the potential role of dispersal limitation as a mechanism of isolation‐driven assembly. We tested for island and species level effects on functional composition using a hierarchical Bayesian framework to better assess the causal link between isolation and functional variation. Growth form composition and the community mean value of functional traits related to growth rate, stress tolerance, and nutrient use varied significantly with island isolation. Functional traits and growth forms were significantly associated with dispersal mode, and spatial isolation was the strongest driver of primary trait variation, while island properties associated with environmental drivers in our system were not strong predictors of trait variation. Despite the species‐level association of dispersal mode and functional traits, dispersal mode only accounted for a small proportion of the overall isolation effect on community‐level trait variation. Our study suggests that spatial isolation can be a key driver of functional assembly in plant communities on islands, though the role of particular dispersal processes remains unclear.