Are island plant communities more invaded than their mainland counterparts?

Questions: Are island vegetation communities more invaded than their mainland counterparts? Is this pattern consistent among community types? Location: The coastal provinces of Catalonia and the para‐oceanic Balearic Islands, both in NE Spain. These islands were connected to the continent more than 5.35 million years ago and are now located <200 km from the coast. Methods: We compiled a database of almost 3000 phytosociological relevés from the Balearic Islands and Catalonia and compared the level of invasion by alien plants in island versus mainland communities. Twenty distinct plant community types were compared between island and mainland counterparts. Results: The percentage of plots with alien species, number, percentage and cover percentage of alien species per plot was greater in Catalonia than in the Balearic Islands in most communities. Overall, across communities, more alien species were found in the mainland (53) compared to the islands (only nine). Despite these differences, patterns of the level of invasion in communities were highly consistent between the islands and mainland. The most invaded communities were ruderal and riparian. Main conclusion: Our results indicate that para‐oceanic island communities such as the Balearic Islands are less invaded than their mainland counterparts. This difference reflects a smaller regional alien species pool in the Balearic Islands than in the adjacent mainland, probably due to differences in landscape heterogeneity and propagule pressure.     

Ethnobotany of Atlantic Forest coastal communities: Diversity of plant uses in Gamboa (Itacuruçá island,Brazil)

Local plants are a very important resource for the community of Gamboa, located at Itacuruçá Island, Sepetiba Bay, State of Rio de Janeiro, Brazil Ninety species of plants, belonging to 40 families, are used for a variety of purposes, such as food, construction, handicraft, and medicine. In a survey medicinal uses for plants were the most quoted by the community. Uses of medicinal plants within Gamboa and with other coastal communities are analyzed using diversity indices. Use by different categories of people based on sex, age, and economic activity was compared and significant differences were found among the groups compared, except for economic categories (fishermen and non-fishermen). The theory of island biogeography is shown to be useful for analyzing different levels of resource uses on different islands.     

Structure of small tropical island freshwater fish and crustacean communities: A niche‐or dispersal‐based process?

Determining the relative importance of niche‐ and dispersal‐based processes in the structuring of animal communities is central in ecology. Freshwater fish and crustacean communities of small tropical islands can bring new insights for understanding these processes as all their species present a pelagic larval stage which gives them important dispersal capacities. Consequently, we hypothesized that dispersal‐based process may be preponderant for structuring these communities from the regional to the local, that is, survey site, scales. Gradient forest analyses allowed us to model the effect of 12 environmental variables on freshwater taxa abundances in two southwestern Indian Ocean islands: Mayotte (26 taxa) and Reunion (21). A total of 153 surveys in Mayotte and 266 in Reunion were used for building the models. Despite the strong heterogeneity of environmental conditions between the two islands, the main factors structuring freshwater fish and crustacean communities in both islands were the elevation and the slope of the sites. The observed structure appeared more pronounced for predatory species than for primary consumers and omnivores. As predators generally have limited locomotor capacities, it is concluded that dispersal‐based process structures these communities not only at large geographical scales but also at the intra‐watershed scale, by limiting the inland penetration (or dispersion) of species in relation to their locomotor capacities. However, more knowledge concerning ecological traits and taxonomic status of many species is needed to confirm this assumption. Abstract in French is available with online material     

Ecology and dynamics of two intertidal algal communities on the littoral of the island of São Miguel (Azores)

The intertidal benthic algal communities of two sites located on opposite coasts of São Miguel Island (Azores), were studied over a 2-year period (September 1993–September 1995). At both sites (São Roque on the south coast and São Vicente on the north), the littoral region was surveyed from the upper intertidal down to the sublittoral fringe. The survey revealed five distinct zones, with a variable degree of overlapping. The two upper zones were characterized by animals (littorinids and barnacles, respectively). Lower down, algal communities formed three distinct zones: an upper Fucus spiralis/Gelidium microdon association, a more extensive turf zone, and a belt featuring erect or frondose algae. Upper in the eulittoral, the turf was mainly monospecific, and dominated by Caulacanthus ustulatus. The lower eulittoral turf was dominated by articulated coralline algae, the associated species differing between the two sites studied. Mainly erect algae (Pterocladiella, Asparagopsis, etc.) occurred furthest down the shore and extended into the nearby shallow sublittoral. Two intertidal communities were studied at each locality: the upper eulittoral (Caulacanthus turf in São Roque and the Fucus spiralis/Gelidium microdon association at São Vicente), and the lower eulittoral (the coralline turf). The lower littoral communities had a higher algal diversity. A general pattern was observed in the seasonal variation of biomass: the lower levels exhibited the higher values in late summer/early autumn, the period in which the upper levels had the lowest standing crops. Physical factors are proposed to account for this. No significant inter-annual variations could be detected, indicating relatively stable communities, at least on a short-term basis.     

When is an island not an island? Insular effects and their causes in fynbos shrublands

Summary According to the equilibrium theory of island biogeography, insularisation will lead to species loss from habitat remnants. Extinctions will continue untill species number equilibrates at a level appropriate for the size and isolation of the island remnants. We tested whether insularisation leads to species loss by comparing plant species numbers on islands of fynbos shrublands surrounded by Afrotemperate evergreen forest with extensive mainland tracts of fynbos. Species area curves for islands and subsamples of mainland had significantly different slopes (z _island=0.43, z _mainland=0.16). Small islands had the fewest species (less than one fifth) relative to mainland samples of similar size. The species area curves intersect at 590 ha so that reserve sizes of this order of magnitude are needed to avoid species losses relative to extensive areas of fynbos.We compared traits of species on islands and mainlands to determine processes most affected by insularisation. Island floras did not differ from the mainland in the mix of dispersal types, pollinator syndromes or proportion of dioecious species. Islands did have significantly fewer species of low stature and significantly more species that survive fire only as seed and not by resprouting. We infer that the main cause of species loss is change in disturbance frequency. Islands have fewer fires and lose species dependent on frequent fires. We predict that island effects could be reduced by judicious fire management of small reserves.     

A fascinating island: 2n = 4

Abstract

Among Angiosperms, only six species are known to possess the lowest identified chromosome number, i.e. 2n = 4. These plants are the monocotyledons Zingeria biebersteiniana, Colpodium versicolor, Ornithogalum tenuifolium and Rhynchospora tenuis, and the dicotyledons Haplopappus gracilis and Brachyscome dichromosomatica. The low chromosome number may be cytogenetically derived from a different ancestral basic number, characteristic of each genus, by different processes, including tandem fusion or unequal reciprocal translocations with loss of centric fragments. All these plants possess low nuclear 4C DNA contents, ranging from 1.56 pg (R. tenuis) to 8.20 pg (H. gracilis), and they generally display a similar chromosome size and a similar position of the nucleolus organising region (NOR), that is often located in the terminal or subterminal region of the small chromosome pair. All these characteristics could be a consequence of common adaptative mechanisms. Peculiar characteristics within these karyotypes are the holocentric chromosomes of R. tenuis and the presence of B chromosomes in B. dichromosomatica. Plants with a very low chromosome number may be considered to constitute a fascinating “island of interest”; moreover, they represent simple systems helpful for the examination of the structural organisation and evolution of Angiosperm chromosomes.     

New Caledonia: a very old Darwinian island?

New Caledonia has generally been considered a continental island, the biota of which largely dates back to Gondwanan times owing to its geological origin and the presence of phylogenetic relicts. This view is contradicted by geological evidence indicating long Palaeocene and Eocene submersions and by recent biogeographic and phylogenetic studies, with molecular or geophysical dating placing the biota no older than the Oligocene. Phylogenetic relicts do not provide conclusive information in this respect, as their presence cannot be explained by simple hypotheses but requires assumption of many ad hoc extinction events. The implication of this new scenario is that all the New Caledonian biota colonized the island since 37 Ma Local richness can be explained by local radiation and adaptation after colonization but also by many dispersal events, often repeated within the same groups of organisms. Local microendemism is another remarkable feature of the biota. It seems to be related to recent speciation mediated by climate, orography, soil type and perhaps unbalanced biotic interactions created by colonization disharmonies. New Caledonia must be considered as a very old Darwinian island, a concept that offers many more fascinating opportunities of study.     

Daphnia–rotifer interactions in Patagonian communities

In fishless ponds of extra-Andean Patagonia, large Daphnia often may severely limit the abundance of small rotifers by mechanical interference (ingestion and damage after rejection). In ten laboratory experiments, 15–20 rotifers were exposed to one large daphniid for 1 day in 80 ml water with Cryptomonas erosa (5 × 10³ cells ml⁻¹). Both D. cf. pulex (3.6 mm) and D. cf. obtusa (3.4 mm) imposed significant mortality on Keratella tropica and newborn Brachionus angularis and Brachionus calyciflorus, but not on the larger neonates of Brachionus rubens or adults of B. angularis and B. calyciflorus. B. rubens neonates avoided the possibility of interference by rapidly attaching to Daphnia. The number of susceptible rotifers killed per Daphnia day⁻¹ was 2–8 for D. cf. obtusa and 8–15 for D. cf. pulex; rotifer-based clearance rates per Daphnia day⁻¹ were 10–35 and 35–65 ml, respectively. When large daphniids occurred in Laguna Los Juncos in late September and October 1997, the only common plankton rotifer was the large and probably well-protected Keratella morenoi (total length = 210 μm). Still, microscopic examination of gut contents revealed three K. morenoi in a 3-mm D. cf. obtusa and one in a 3.5-mm D. dadayana.     

Are arctic soft-sediment macrobenthic communities impoverished?

 Samples of the macrobenthic fauna of soft sediments were collected from around Svalbard during the 1991 Arctic EPOS cruise of RVPolarstern. Although faunal variability could be related to sediment granulometry and depth, all stations to the south and east of the Archipelago lay within the broadly defined central Barents Sea community. In this community, α-diversity was variable and sampled between 77 and 337 m showed no clear relationship to depth. Diversity in the area close to the polar front was notably high, rarefaction predicting that 43±5.5 species might occur in a sample of 201 individuals. k-Dominance plots suggested that muddy sand communities around Svalbard were no less diverse than similar assemblages in the North Sea. Received: 27 March 1995/Accepted: 30 July 1995     

The island rule: made to be broken?

The island rule is a hypothesis whereby small mammals evolve larger size on islands while large insular mammals dwarf. The rule is believed to emanate from small mammals growing larger to control more resources and enhance metabolic efficiency, while large mammals evolve smaller size to reduce resource requirements and increase reproductive output. We show that there is no evidence for the existence of the island rule when phylogenetic comparative methods are applied to a large, high-quality dataset. Rather, there are just a few clade-specific patterns: carnivores; heteromyid rodents; and artiodactyls typically evolve smaller size on islands whereas murid rodents usually grow larger. The island rule is probably an artefact of comparing distantly related groups showing clade-specific responses to insularity. Instead of a rule, size evolution on islands is likely to be governed by the biotic and abiotic characteristics of different islands, the biology of the species in question and contingency.     

Indirect interactions in aphid–parasitoid communities

Indirect interactions between populations of different species can be important in structuring natural communities. Indirect effects are either mediated by changes in population densities (trophic or density-mediated effects) or by changes in the behavior of species that are not trophically connected (behavioral or trait-mediated effects). We reviewed the literature on aphids and their parasitoids to explore the various possible indirect interactions that can occur in such communities. The review was motivated by our study of a particular aphid–parasitoid community in a natural (i.e., nonagricultural) habitat, and by the wealth of information that exists about aphid–parasitoid systems in agricultural settings. We focused our review on aphid–parasitoid interactions, but considered how these were influenced by the other aphid natural enemies and also by aphid mutualists and host plants. We conclude that indirect effects are likely to have a major effect in structuring aphid–parasitoid communities, and that the latter are a valuable model system for testing ideas about community interactions. Received: December 20, 1998 / Accepted: January 12, 1999     

A sponge diversity centre within a marine ‘island’

The exposed and gulf-stream warmed south-west coast of Ireland has a Lusitanean fauna composed of elements of the colder waters to the north and east, and others from the warmer Mediterranean Sea. Lough Hyne, a small marine body, is unusual on this coast in being very sheltered, but also in being characterised by many different niches within a small space (1 km²). Sponges are particularly abundant, morphologically varied and more than 100 species have been described. Species diversity was measured at 6 m intervals on vertical and inclined profiles (to a maximum of 30 m) at six sites, spanning a range of flow rate and sedimentation regimes. Diversity, richness, evenness and density varied significantly with both flow regime and depth, but was much lower on the surrounding Atlantic coast. Four different sponge communities were differentiated on the basis of sponge species assemblages which correlated with different environmental conditions. At sites of turbulent and fast flow conditions, sponge diversity and richness were lowest, with the highest values being found at the sites of moderate and high sedimentation. Significant differences were observed in all four ecological variables with respect to substratum angle with the exception of the site experiencing the most turbulent flow conditions. Lough Hyne was found to possess the second highest sponge species diversity (H=3.626) and richness (77 species) of all available figures from temperate, polar and tropical areas (of similar sized sampling area). The uniqueness, diversity and species composition of the sponge community at this location suggests Lough Hyne is, biologically, a marine island within the island of Ireland.     

Does phosphorus limitation promote species-rich plant communities?

It is known that the number of limiting nutrients may affect the species richness of plant communities, but it is unclear whether the type of nutrient limitation is also important. I place the results from a study in Patagonia (elsewhere in this issue) in the context of the number and types of nutrients that are limiting. I present four mechanisms through which N or P limitation may potentially influence species richness. These mechanisms are related to: (i) the number of forms in which P or N are present in soil and the plant traits needed to acquire them, (ii) the mechanisms and traits that control species competition and coexistence under N or P limitation, (iii) the regional species pools of plants capable of growing under N- and P-limited conditions, and (iv) the interaction between the type of nutrient limitation and community productivity. It appears likely that P limitation can favour a higher species richness than N limitation, in at least in a variety of low productive plant communities, but evidence to support this conclusion is so far lacking. The four mechanisms proposed here offer a framework for exploring whether the type of nutrient limitation per se, or an interaction with productivity, is a potential driver for variation in species diversity.     

Do fitness-equalizing tradeoffs lead to neutral communities?

Neutral theory in ecology is aimed at describing communities where species coexist due to similarities rather than the classically posited niche differences. It assumes that all individuals, regardless of species identity, are demographically equivalent. However, Hubbell suggested that neutral theory may describe even niche communities because tradeoffs equalize fitness across species which differ in their traits. In fact, tradeoffs can involve stabilization as well as fitness equalization, and stabilization involves different dynamics and can lead to different community patterns than neutral theory. Yet the important question remains if neutral theory provides a robust picture of all fitness-equalized communities, of which communities with demographic equivalence are one special case. Here, I examine Hubbell’s suggestion for a purely fitness-equalizing interspecific birth–death tradeoff, expanding neutral theory to a theory describing this broader class of fitness-equalized communities. In particular, I use a flexible framework allowing examination of the influence of speciation dynamics. I find that the scaling of speciation rates with birth and death rates, which is poorly known, has large impacts on community structure. In most cases, the departure from the predictions of current neutral models is substantial. This work suggests that demographic and speciation complexities present a challenge to the future development and use of neutral theory in ecology as null model. The framework presented here will provide a starting point for meeting that challenge, and may also be useful in the development of stochastic niche models with speciation dynamics.     

Can transgenic maize affect soil microbial communities?

The aim of the experiment was to determine if temporal variations of belowground activity reflect the influence of the Cry1Ab protein from transgenic maize on soil bacteria and, hence, on a regulatory change of the microbial community (ability to metabolize sources belonging to different chemical guilds) and/or a change in numerical abundance of their cells. Litter placement is known for its strong influence on the soil decomposer communities. The effects of the addition of crop residues on respiration and catabolic activities of the bacterial community were examined in microcosm experiments. Four cultivars of Zea mays L. of two different isolines (each one including the conventional crop and its Bacillus thuringiensis cultivar) and one control of bulk soil were included in the experimental design. The growth models suggest a dichotomy between soils amended with either conventional or transgenic maize residues. The Cry1Ab protein appeared to influence the composition of the microbial community. The highly enhanced soil respiration observed during the first 72 h after the addition of Bt-maize residues can be interpreted as being related to the presence of the transgenic crop residues. This result was confirmed by agar plate counting, as the averages of the colony-forming units of soils in conventional treatments were about one-third of those treated with transgenic straw. Furthermore, the addition of Bt-maize appeared to induce increased microbial consumption of carbohydrates in BIOLOG EcoPlates. Three weeks after the addition of maize residues to the soils, no differences between the consumption rate of specific chemical guilds by bacteria in soils amended with transgenic maize and bacteria in soils amended with conventional maize were detectable. Reaped crop residues, comparable to post-harvest maize straw (a common practice in current agriculture), rapidly influence the soil bacterial cells at a functional level. Overall, these data support the existence of short Bt-induced ecological shifts in the microbial communities of croplands' soils.     

Are algal communities driven toward maximum biomass?

In this continental-scale study, we show that in major benthic and planktonic stream habitats, algal biovolume--a proxy measure of biomass--is a unimodal function of species richness (SR). The biovolume peak is observed at intermediate to high SR in the benthos but at low richness in the phytoplankton. The unimodal nature of the biomass-diversity relationship implies that a decline in algal biomass with potential harmful effects on all higher trophic levels, from invertebrates to fish, can result from either excessive species gain or species loss, both being common consequences of human-induced habitat alterations. SR frequency distributions indicate that the most frequent richness is habitat-specific and significantly higher in the benthos than in the plankton. In all studied stream environments, the most frequent SR is lower than the SR that yields the highest biovolume, probably as a result of anthropogenic influences, but always within one standard deviation from it, i.e. they are statistically indistinguishable. This suggests that algal communities may be driven toward maximum biomass.     

What in earth? Analysing soil bacterial communities

Walk into any field and you will at once see dozens of plant and animal species. However, below your feet and largely unnoticed, lies a population of unseen millions. Can the introduction of powerful molecular biological techniques increase our understanding of how these hidden bacterial communities function in their natural environment?     

The small‐island effect: fact or artefact?

Positive relationships between species richness and sampling area are perhaps the most pervasive patterns in nature. However, the shape of species–area relationships is often highly variable, for reasons that are poorly understood. One such source of variability is the "small-island effect", which refers to a decrease in the capacity of sampling area to predict species richness on small islands. Small-island effects have been attributed to a variety of processes, including spatial subsidies, habitat characteristics and ocean-born disturbances. Here, we show that small-island effects can be generated by logarithmic data transformations, which are commonly applied to both axes of species–area relationships. To overcome this problem, we derive several null models to test for non-random variability in the capacity of island area to predict species richness and apply them to data sets on island plant communities in Canada and New Zealand. Both archipelagos showed evidence for small-island effects using traditional breakpoint regression techniques on log-log axes. However, null model analyses revealed different results. The capacity of sampling area to predict species richness in the Canadian archipelago was actually lowest at intermediate island size classes. In the New Zealand archipelago, island area was similarly capable of predicting species richness across the full range of island sizes, indicating the small-island effect detected by breakpoint regression is an artifact of logarithm data transformation. Overall results show that commonly used regression techniques can generate spurious small-island effects and that alternative analytic procedures are needed to detect non-random patterns in species richness on small islands.     

The contribution of island populations to in situ genetic conservation

Genetic variation is often lower within island populations, however islands may also harbor divergent genetic variation. The likelihood that insular populations are genetically diverse or divergent should be influenced by island size and isolation. We tested this assumption by comparing patterns of genetic variation across all major island song sparrow populations along the Pacific North American coast. Allelic richness was moderately lowered even on islands which are close to large, potential sources. The most significant differences in allelic richness occurred on very small or highly remote islands. Gene diversity was significantly lower only on remote or very small islands. We found that island populations contribute to regional genetic variation through both the amount of genetic variation and the uniqueness of that variation. The partitioning of this contribution was associated with the size and isolation of the island populations.