Lack of recovery from disturbance in high-arctic boulder communities

High-arctic boulder communities that are impacted by anthropogenic and natural influences can result in the removal or scouring of sessile organisms leaving either open space or damaged organisms. This project asks how sessile communities recover after disturbances by determining (1) timing of recolonization, (2) grazer effects on recolonization, and (3) vegetative regrowth rates for encrusting sponges and corallines. Cleared boulders were monitored over 7 years to determine recolonization timing and grazer impacts. Vegetative regrowth rates were determined by monitoring partial clearings for 4 years. This study found that recolonization was slow with less than 10 % of the boulder surfaces being colonized after 7 years. Recolonization was so slow that it was difficult to ascertain grazer impacts, although it appeared there were no impacts. Lastly, this study showed that vegetative regrowth of sponges and encrusting coralline algae was fast. Sponges averaged 100 % regrowth after 2 years and corallines averaged 40 % after 4 years. Coralline regrowth was slower when exposed to higher sedimentation. This study showed that community recovery from disturbances is very slow in the Beaufort Sea, especially if entire organisms are removed. However, if removal is partial, recovery can be quicker, particularly in low sediment areas. Complete community recovery in this system after a disturbance may take a decade or more.     

Boulders increase resistance to clear-cut logging but not subsequent recolonization rates of boreal bryophytes

The extent to which a plant assemblage might recolonize a disturbed system is in general related to the availability of propagule sources and sites with appropriate conditions for establishment. Both these factors might be sensitive to aspects of spatial heterogeneity. Microtopographic variation may enhance initial resistance by reducing the impact of the disturbance and facilitating establishment of incoming propagules by providing shaded “safe-sites”. This study explores the influence of microtopographic heterogeneity (caused by variation in surface boulder cover) on the recolonization of closed-canopy forest floor bryophytes using a chronosequence of 75 spruce-dominated forests in south-central Sweden (2–163 years after clear-cutting). We found that high boulder cover did increase survival and subsequent persistence in young forests at both investigated scales (i.e. 1,000 and 100 m²), although this pattern became less evident on the smaller spatial scale. Species accumulation in boulder-poor subplots was not different when surrounded by boulder-rich compared with boulder-poor subplots suggesting short-distance recolonization from boulder-created refugia to be of little importance during recolonization. To conclude, it seems that boulders increase initial resistance to clear-cutting for this bryophyte guild, but that the subsequent recolonization process is more likely to depend on external propagule sources and factors affecting establishment such as the microclimate in the developing stand.     

Temporal vegetation dynamics and recolonization mechanisms on different-sized soil disturbances in tallgrass prairie

Assessing the various mechanisms by which plants revegetate disturbances is important for understanding the effects of disturbances on plant population dynamics, plant community structure, community assembly processes, and ecosystem function. We initiated a 2-yr experiment examining temporal vegetation dynamics and mechanisms of recolonization on different-sized soil disturbances created to simulate pocket gopher mounds in North American tallgrass prairie. Treatments were designed to assess potential contributions of the seed rain, soil seed bank, clonal propagation from the edges of a soil mound, and regrowth of buried plants. Small mounds were more rapidly recolonized than large mounds. Vegetative regrowth strategies were the dominant recolonization mechanisms, while the seed rain was considerably less important in maintaining the diversity of forbs and annuals than previously believed. All recolonization mechanisms influenced plant succession, but stem densities and plant mass on soil mounds remained significantly lower than undisturbed controls after two growing seasons. Because natural pocket gopher mounds are indistinguishable from undisturbed areas after two seasons, these results suggest that multiple modes of recruitment concurrently, albeit differentially, contribute to the recolonization of soil disturbances and influence tallgrass prairie plant community structure and successional dynamics.     

Natural physical disturbance and predation: their importance in structuring a marine sessile community

Settling panels were used to determine the distribution of sessile organisms. Experimental and control cages were located at two study sites that were of different physical exposures. Twenty-nine species of sessile organisms were recorded for the two sites. Data on the presence or absence of each species, for each sample, were classified. The resulting hierarchy showed natural physical disturbance to be the major structuring factor in the community. Predation was of less importance in the physically stressed study site and had no effect on species distribution in the protected site.     

Patterns of distribution and abundance of chitons of the genus Ischnochiton in intertidal boulder fields

Abstract Chitons of the genus Ischnochiton are found predominantly on the undersurfaces of boulders, compared with other intertidal or subtidal habitats. They therefore appear to be habitat‐specialists at this scale. This, combined with the fact that boulder fields are relatively sparse compared with other intertidal habitats, makes these animals vulnerable to natural and anthropogenic disturbances. In addition, many species of Ischnochiton are relatively rare and appear to have very patchy abundances, making them likely to have very specific requirements for habitat. We need to understand the habitat requirements in order to manage, conserve and restore disturbed habitats. The present study was carried out at three intertidal boulder fields separated along approximately 200 km of the coast of New South Wales, Australia, centred around Sydney. The boulder fields were representative of those found in this region. The boulders were made of different materials: shale in the north, sandstone in Sydney and quartzitic sandstone in the south. Some boulders in each boulder field were covered by up to 0.4 m of water during low tide. The study showed that the seven species examined were overdispersed among boulders in each of these three intertidal boulder fields. Most boulders did not have associated chitons, but there were very large abundances on a very small number of boulders. Chitons were also overdispersed among boulders that they occupied. These patterns were consistent among shores and among species, even though patterns of abundance were extremely different among different species. These species appear therefore to show specific requirements for habitat at a small spatial scale, using only a small proportion of potential patches of habitat (i.e. boulders) in any place. Extremely patchy patterns of dispersion can be caused by variation in patterns of recruitment, mortality or behavioural responses to habitat or other species. Before performing experiments to investigate such processes, it is useful to test hypotheses of association with habitat using mensurative experiments to identify environmental correlates that might explain the observed patterns. In the present study, sizes of boulders and the associated sessile and mobile assemblages were proposed as mechanisms that could affect dispersion of chitons among boulders. None of these factors, however, showed strong associations with abundances of chitons. The lack of support of these models rules out some features of habitat to which species of Ischnochiton might respond, thereby precluding manipulative experiments involving these features, which are unlikely to be involved in the very patchy patterns of dispersion of species of Ischnochiton.     

The control of the development of a marine benthic community by predation on recruits

Recruitment is an important process in regulating many marine benthic communities and many studies have examined factors controlling the dispersal and distribution of larval immigrants. However, benthic species also have early post-settlement life-stages that are dramatically different from adult and larval stages. Predation on these stages potentially impacts measured recruitment and the benthic populations and communities that ultimately develop.We examined the consequences of post-settlement predation on 1-day-old to 1-month-old recruits of sessile invertebrates at two field sites in southern New England. One site (Breakwater) was in a protected area with few predators and the other (Pine Island) was <1 km away in an open coast area with three different predator guilds: small and large invertebrates and fish. The Breakwater site had been dominated for >10 years by colonial and solitary ascidians. These species were absent from the Pine Island site which was dominated by bryozoans. Our goal was to examine whether post-settlement predation influenced the development and subsequent structure of the epifaunal community.Here we examine long-term changes in community development resulting from post-settlement predation, and contrast these results to those of earlier experiments examining the reductions in observed recruitment by post-settlement predation. Our first long-term experiment examined natural community development at the two sites and whether transplanted communities changed when exposed to the different levels of predation at these sites. The communities that developed at both sites were consistently different from each other and similar to resident communities at their respective sites. On panels transplanted from the Breakwater to Pine Island, solitary ascidians and the colonial ascidian, Botryllus schlosseri, suffered high mortalities on both caged and uncaged treatments, indicative of predation by small predators that could enter cages. Some solitary ascidians did survive inside cages and the colonial ascidian, Botrylloides violaceus, became dominant on all transplanted treatments. On panels transplanted from Pine Island to the Breakwater, ascidians invaded and dominated all treatments except those that were originally caged at Pine Island.In the second long-term experiment, natural communities were allowed to develop on panels exposed at the Breakwater for 1, 2, 3, and 4 weeks. Each set was transplanted to three treatments at Pine Island: open uncaged pilings, caged pilings to exclude fish and large invertebrates, and racks suspended above the bottom to exclude all predators. When 1-week-old communities were transplanted, after 2-3 weeks only bryozoans were found on the open and caged pilings, while colonial ascidians dominated the suspended rack treatment. When older 2-week-old communities were transplanted, colonial ascidians also became dominant in the caged piling treatment and when 3- and 4-week-old communities were transplanted colonial ascidians dominated all three treatments. Solitary ascidians were never abundant on open pilings exposed to fish and large benthic invertebrate predators.Post-settlement predator-prey interactions involved newly settled and juvenile life-stages of a variety of prey species and many invertebrate and vertebrate predator species. The effects of these interactions on recruitment did result in differences in the development and eventual species composition of the communities, even though predators had little if any effect on the adults of the prey species.     

Disturbance and organisms on boulders

Summary The tops of intertidal boulders on the east coast of Australia may carry a half-dozen algae, while the undersides may support a dozen or more species of sessile animals; mobile forms such as molluscs, crustaceans, and echinoderms are also common. These organisms are disturbed when boulders are moved by waves or buried in sand. Experiments were done to test the hypothesis that these disturbances are responsible for the reduced diversity and abundance of the assemblages on the undersides of small boulders. Boulders on one shore were bolted to the substratum to prevent movement by waves. In two separate experiments on another shore, boulders with and without attached organisms were buried in a few centimetres of sand. All experiments included relevant controls and were done high and low on the shore using rocks of several sizes.The assemblages of algae developing on the tops of all stabilised boulders, regardless of size, were similar to those on boulders free to roll indicating that, in contrast to results elsewhere, disturbance by waves was too infrequent or slight to affect these species. The tops of rocks were also rarely buried in sand and the main influences on the community in this situation were apparently exposure at low-tide and grazing gastropods.More species, however, did settle or survive on the undersides of rocks which were free of sand or could not be moved by waves; thus disturbances were important in this situation. In the absence of disturbance low on the shore, much or all space was occupied and sessile species such as sponges and ascidians overgrew other forms and reduced diversity. This did not happen higher on the shore and here diversity was simply an increasing function of rock-size. Overall disturbance played a similar role in all places — it killed organisms and created free space — but the final effects on the community varied depending upon the species present and the actions of other factors.     

Effects of pulse versus steady recruitment on sessile marine communities

Variation in patterns of propagule establishment (recruitment) has important effects on population dynamics and the structure of some communities. Most experimental studies have varied recruitment by changing the nature of a single event early in community development, but recruitment can also vary from steady rates of arrival to highly episodic 'pulse' events, causing differences in the temporal spacing of individuals recruiting into patches. We examined whether two different temporal patterns of recruitment of sessile invertebrates affected temperate marine communities in southeastern Australia in two experiments that were run at different times at the same site and that manipulated several different species. Target species entered communities as either a single pulse of recruits within a 2-week period or steady input of the same total number of recruits over a longer time period (5-6 weeks). The pattern of recruitment had variable effects on community structure. The colonial ascidian Botryllus schlosseri did not have a strong influence on community structure whether it recruited in a single pulse or steadily. The cover of B. schlosseri was higher when recruitment occurred as a single pulse. In a second experiment, botryllid ascidians caused changes in the composition of communities when they recruited steadily compared to when they did not recruit or didemnids recruited, but caused no differences in communities when they recruited in a shorter pulse. In contrast, recruitment frequency of didemnid ascidians had little effect, though their presence/absence caused community differences. Though we found that different temporal recruitment patterns can alter community composition, the life history and ecology of particular taxa as well as differences in environmental background processes are likely to influence the strength of these effects.     

Disturbance,drought and dynamics of desert dune grassland,South Africa

A seven-year study of marked plants and plots in Stipagrostis ciliata (Desf.) de Winter dune grassland, in the arid (<100 mm yr^–1) Bushmanland area of the Northern Cape province of South Africa, was designed to test the hypothesis that establishment of ephemeral plants, and recruitment of perennial grasses was dependent upon disturbances that reduced the density of living perennial grass tussocks. In 1989, eight 4 m² plots were cleared of perennial vegetation by uprooting and removing all plants so as to resemble small-scale disturbances made by burrowing mammals or territorial antelope. The vegetation on the cleared plots and surroundings was monitored until 1996. Initial results supported our hypothesis. In wet years, when ephemeral plants were abundant, their average fresh mass was 2–3 times greater per unit area on the cleared plots than in control plots in adjacent, undisturbed grassland. Many Stipagrostis seedlings established in the cleared plots over the two years following clearing but were rare in adjacent areas among established conspecifics. However, a drought in 1992 (11 mm of rain over 12 months) lead to widespread mortality of the perennial grass, killing 56% (range 22–79%) of established tufts. High densities of Stipagrostis seedlings appeared following the drought-breaking rains in January 1993, both in the disturbed plots and in the surrounding `undisturbed' dune grassland. Ephemeral plants established in large numbers throughout the area during the high rainfall year of 1996 and were generally more numerous in the old disturbances than in control plots. Seven years after clearing the biomass of grass on the cleared plots was approximately 34% of the mass removed from the plots in 1989 whereas in the undisturbed grassland biomass was 66% of 1989 levels. Drought had little long-term effect on community composition, and Stipagrostis ciliata constituted 94–98% of plant community before and after drought. Cleared plots were recolonised by S. ciliata, but the contribution of other grass species increased by 6–9%. Synchronous recruitment following occasional drought-induced mortality can generate even-aged populations of the dominant desert dune grasses.     

Predators reduce abundance and species richness of coral reef fish recruits via non-selective predation

Predators have important effects on coral reef fish populations, but their effects on community structure have only recently been investigated and are not yet well understood. Here, the effect of predation on the diversity and abundance of young coral reef fishes was experimentally examined in Moorea, French Polynesia. Effects of predators were quantified by monitoring recruitment of fishes onto standardized patch reefs in predator-exclosure cages or uncaged reefs. At the end of the 54-day experiment, recruits were 74% less abundant on reefs exposed to predators than on caged ones, and species richness was 42% lower on reefs exposed to predators. Effects of predators varied somewhat among families, however, rarefaction analysis indicated that predators foraged non-selectively among species. These results indicate that predation can alter diversity of reef fish communities by indiscriminately reducing the abundance of fishes soon after settlement, thereby reducing the number of species present on reefs.     

Intertidal rocky shore biotopes of the Azores: a quantitative approach

Rocky littoral communities of the Azores are dominated by macroalgae where turf formations represent the main life form as on many subtropical and tropical shores. The present study aimed at describing and classifying benthic intertidal biotopes of the Azores using clear quantification and data analysis methodologies to be used in similar studies for spatial and/or temporal comparison. This numerical approach intends to have management and conservation applicability. Thirty study locations along the coast of São Miguel island were characterised by substratum type (cobbles, boulders or bedrock – different stability levels) and exposure to wave action (high, medium or low exposure), from June to August 2002. Algae, sessile and sedentary invertebrates were recorded along transects, and the more conspicuous taxa quantified. Community structure is mainly shaped by substratum stability – unstable cobble communities are separate from those of more stable boulders and bedrock. Boulders present an intermediate community composition between cobbles and bedrock. Exposure to wave action induces smaller variation in community composition. Sixteen biotopes have been recognised. Ultimately the present paper provides an objective set of biotope definition tools for a broad public whose interest focuses on spatial and temporal comparison of coastal communities and associated habitats.     

Spatial processes that maintain biodiversity in plant communities

The composition of communities of sessile organisms, and the change in species diversity with time, is a spatially explicit phenomenon. Three spatial factors clearly affect diversity: (1) the structure and heterogeneity of the landscape that limits species immigration and ultimate community size; (2) neighborhood interactions that determine colonization and extinction rates and influence residence times of local populations; and (3) disturbances that open spatially contiguous areas for recolonization by less abundant species. The importance of these three factors was first reviewed and then examined with a spatially explicit, multi-species model of plant dispersal, competition and establishment, with an assumption of neutrality (all species had equivalent life histories) that reduced the initial dimensionality of the problem. The simulations assumed that the probability of immigration was a linear function of mainland abundance and distance to islands, similar to the equilibrium theory of island biogeography and the unified neutral theory of biodiversity. The rate of increase in species richness was not constant across island sizes, declining as island area became very large. This pattern was explained by the spatial dynamics of colonization and establishment, a non-random process that cannot be explained by passive sampling alone. Simulations showed that population establishment depended critically on rare long-distance dispersal events while population persistence was achieved by the formation of aggregated species distributions that developed through restricted dispersal and local competitive interactions. Nevertheless, species richness always declined to a single species in the absence of disturbances, while up to 40 species could persist to 10,000 years when spatially dependent mortality was added. Further explorations with spatially explicit models will be required to fully appreciate the consequence of land use change and altered disturbance regimes on patterns of species distribution and the maintenance of diversity.     

Inhibition of kelp recruitment by turfing algae and consequences for an Australian kelp community

In the sublittoral kelp forests of New South Wales, dictyotalean turfing algae dominate the substratum in areas where kelp canopies have been removed by storms. The effects of the presence of these algae on the recruitment of kelp plants and the subsequent structure of the benthic community were determined. Areas of turfing algae in clearings were experimentally cleared of turf at a time of dense kelp settlement, and areas of the kelp canopy were also removed. These treatments were compared with unmanipulated areas of turf in clearings and areas under the kelp canopy. The number of kelp recruits in each replicate plot was recorded throughout time as were the abundances of most of the macroscopic and microscopic species living on the substratum. Kelp was found to recruit very quickly to areas of substratum which had the overlying kelp canopy removed, but few recruits appeared under the natural canopy, or in areas with turfing algae, or in clearings where turfing algae had been removed. The results showed that whilst the presence of a natural kelp canopy and/or turfing algae inhibited the recruitment of kelps, removal of turfing species from clearings did not facilitate kelp recruitment. This indicated that the presence of turf did not solely inhibit kelp recruitment, but some lingering influence of turf seems to affect the substratum in clearings such that kelp cannot recruit to such areas. Turfing species returned and dominated the substratum in clearings where turf had been removed. After an initial decline, encrusting species increased in abundances in clearings where kelp recruited as these recruits developed a canopy. Turf and encrusting species remained great in cover in the respective control treatments. The results are discussed in terms of alternate stable states within this community. Such concepts are concluded to be dependent upon the spatial and temporal scales of one's investigation.     

Does disturbance enhance genotypic diversity in clonal organisms? A field test in the marine angiosperm Zostera marina

Physical disturbance has often been invoked to control genotypic diversity in sessile clonal organisms, yet experimental evidence is lacking. I studied the effects of physical disturbance on genet dynamics and genotypic diversity in a clonal marine angiosperm, Zostera marina (eelgrass). In replicated plots of 1 m2, the vegetation canopy was removed in gaps of zero (control), 25%, 50% and 75% of the area (n = 6 replicates). Before removal and during two consecutive years, the genotypic composition was determined using genetic markers (DNA microsatellites) in a 5 x 5 pixel grid per plot. An aggregate index of genet dynamics summarizing recruitment, increase, loss and decrease of clones was maximal at intermediate disturbance levels (quadratic polynomial P = 0.02). Physical disturbance also increased the occurrence of new genotypes, possibly reflecting recruitment (linear model, P < 0.05). Contrary to expectations, there was no competitive advantage of more heterozygous genotypes over less heterozygous ones. In the absence of disturbance, in particular, clones with lower individual heterozygosity were more likely to increase in area over a 1-year time period than more heterozygous ones, while there was no such correlation in plots with disturbance (logistic model, P(disturbance x heterozygosity) = 0.036). Undisturbed plots revealed background recruitment independent of canopy gaps, suggesting that Z. marina exhibits a strategy of continual recruitment. Effects of experimental disturbance (linear or quadratic) on clonal diversity were not detectable. Instead, initial (pretreatment) clonal diversity accounted for between 68% and 91% of the variance in diversity, indicating remarkable resilience of genotypic diversity in the face of physical disturbance.     

The development of sessile epifaunal communities in Kylesalia,Kilkieran bay (west coast of Ireland)

Test panels, which were suspended horizontally at three sites in a shallow inlet on the west coast of Ireland, were sampled monthly over a period of one year to investigate the development of epifaunal communities. In addition, the current regime and sedimentation at these sites were measured. The deposited materials, of high organic value, may enhance the food supply to the sessile organisms through resuspension. Heavy deposition at one site interrupted the study. At the other two sites, however, the experiment was not affected. At one site, initial settlement by colonial bryozoans, followed by solitary ascidians, was noted on the panels over the period studied. In the competition for space, the dense aggregations during settlement, the relative large size, and the rapid growth of these solitary animals may occlude the colonization by colonial bryozoan mats underneath. At the other site, a more diversified community with co-existence of many sedentary polychaete and colonial bryozoan species was recorded on the panels. The continuing availability of free space on the panels suggests that the development of sessile communities at this site may be hindered by the relatively slow current flow and heavy deposition and resuspension. Whilst differences in community development were observed on the panels at these two sites, it would be difficult, through such a short study period, to ascertain whether stable climax states had been reached.     

Impact of supply-side ecology on consumer-mediated coexistence: evidence from a meta-analysis

Studies of marine nearshore hard substrates have demonstrated that consumers and abiotic disturbances can remove biomass, clearing space for species that are competitively subordinate and subsequently increasing diversity. However, studies often examine the impact of these space-opening forces on diversity in isolation from other potentially interacting factors. In marine systems, space can be closed by recruitment decoupled from local populations. Therefore, we investigated how recruitment influences the impacts of consumers on diversity with a meta-analysis of 27 experiments of community development involving sessile species on marine hard substrates. These studies allowed quantification of recruitment rates, consumer pressure, and species richness of primary space occupants. This meta-analysis demonstrated that consumers generally increase diversity at high levels of recruitment but decrease diversity at low levels of recruitment. Therefore, species diversity of sessile species is controlled by the interaction between forces that open (predation and herbivory) and close (recruitment) space.     

Response of benthic invertebrates to natural versus experimental disturbance in a Swiss prealpine river

1. The crucial point of disturbance experiments in streams is the extent to which they can simulate a natural spate. Ideally, disturbance experiments should proceed side by side with a phenomenological study to allow a direct comparison. In the present study conducted in a prealpine Swiss river, the River Necker, fortuitous events made such a comparison possible. 2. In summer 1994, we took Surber samples one day before and on several sampling dates after a major flood (recurrence interval ≈ 5 years), which was followed by a long period of uniform discharge in a river characterized by frequent spates. Beginning 19 days after this flood, patches of the stream bed (≈ 9 m²) were physically disturbed by kicking and raking. 3. The degrees of reduction in the total number of individuals and the dominant taxa were similar after both types of disturbance, as were the recolonization patterns of Rhithrogena spp., Leuctra spp. and Hydracarina. Chironomidae, Baetis spp., Simuliidae, Pentaneurini and Corynoneura/Thienemanniella spp. showed a distinct lag phase after the flood before recolonization began, whereas there was no such lag phase after the experiment. Therefore, the time needed to recover to pre-flood densities was longer for these taxa. Nevertheless, recolonization rates and patterns after the lag phase were similar to those after the experimental disturbance. 4. Size-class measurements indicated that recruitment from egg hatching may have been more important after the flood than after the experimental disturbance for Rhithrogena spp., but not for Chironomidae, Baetis spp., Simuliidae, Pentaneurini and Leuctra spp. Invertebrate drift was probably the most important pathway of recolonization after both types of disturbance. 5. Our experiment allowed a realistic simulation of several important effects of the large flood on the invertebrate community. Smaller spates that induce substratum movement at a spatial scale similar to our experimental plots are much more common than large floods in the River Necker. For these spates, our experiment should provide an even more realistic simulation of natural disturbance.     

Ants as Indicators of Restoration Success at a Uranium Mine in Tropical Australia

The composition and structure of ant communities were used to assess the success of the preliminary restoration program at Ranger uranium mine in the seasonal tropics of northern Australia. Ants were surveyed at eight sites, including two relatively undisturbed control sits, within the Ranger lease. The revegetated sites represented a range of variables likely to influence restoration success: revegetation age (two, four, and eight years), proximity to undisturbed sites (which act as potential sources of recolonization), and burning treatment. Revegetation at most sites was dominated by fast-growing species of Acacia. There was a clear succession of ant species across revegetated sites. Initial colonization was by species of Iridomyrmex, but as plant cover and litter development increased these were replaced by broadly adapted, opportunist species, especially the introduced Paratrechina longicornis. Ant recolonization was very slow at isolated sites, with only 12 species present after eight years (the oldest site available). This compares with 21 species after only four years at a site located close to potential sources of recolonization. The ant community at this site, however, was very similar to that at another site located close to colonization sources, but eight years old. Ant succession therefore appeared to have stalled at this point, with species richness and composition bearing little resemblance to that at control sites. The heavy shade and litter produced by acacias were considered to be the major impediment to further change. Results from a site that had undergone a prescribed burn after two years, thereby breaking dominance by acacias and allowing for the establishment of a wide variety of plant taxa, suggest that such management practices may promote further colonization by ant species.     

Effects of early recruits on temperate sessile marine community composition depend on other species recruiting at the time

In many environments recruitment of dispersive propagules (e.g. seeds, spores and larvae) can vary from situations when particular taxa recruit in relative isolation to times when they recruit simultaneously with other, functionally quite different taxa. Differences in the identity and density of recruiting taxa can have important consequences on community structure, but it is still not clear how the effects of individual taxa on communities are modified when they recruit together with other species. Using an experimental approach we compared early development of a temperate marine sessile community after the recruitment of mixtures of botryllid ascidians and barnacles to that when barnacles or botryllid ascidians recruited alone. Communities exposed to recruitment of botryllid ascidians in isolation differed from those that received barnacles, a mixture of botryllids and barnacles or no recruitment in 2-week-old communities. These early differences were driven by higher abundances of the species that were present as initial recruits in experimental treatments. After 2 months communities also differed between barnacle and mixed recruitment treatments but not mixed and botryllid or botryllid and barnacle treatments. These differences were not directly due to differences in the abundances of our manipulated taxa but occurred because of two abundant arborescent bryozoans, Bugula dentata, which occupied more space in communities that initially received mixed recruitment than in those that received barnacle or no recruitment, and Zoobotryon verticillatum, which occupied more space in communities that initially received only barnacle recruitment than those that initially received botryllid or mixed recruitment. These effects did not persist, and communities did not differ after 6 months. These results suggest that, more generally, species may influence community dynamics differently when they recruit alongside other species than when they recruit in relative isolation.     

Recruitment Variability of Coral Reef Sessile Communities of the Far North Great Barrier Reef

One of the key components in assessing marine sessile organism demography is determining recruitment patterns to benthic habitats. An analysis of serially deployed recruitment tiles across depth (6 and 12 m), seasons (summer and winter) and space (meters to kilometres) was used to quantify recruitment assemblage structure (abundance and percent cover) of corals, sponges, ascidians, algae and other sessile organisms from the northern sector of the Great Barrier Reef (GBR). Polychaetes were most abundant on recruitment titles, reaching almost 50% of total recruitment, yet covered <5% of each tile. In contrast, mean abundances of sponges, ascidians, algae, and bryozoans combined was generally less than 20% of total recruitment, with percentage cover ranging between 15–30% per tile. Coral recruitment was very low, with <1 recruit per tile identified. A hierarchal analysis of variation over a range of spatial and temporal scales showed significant spatio-temporal variation in recruitment patterns, but the highest variability occurred at the lowest spatial scale examined (1 m—among tiles). Temporal variability in recruitment of both numbers of taxa and percentage cover was also evident across both summer and winter. Recruitment across depth varied for some taxonomic groups like algae, sponges and ascidians, with greatest differences in summer. This study presents some of the first data on benthic recruitment within the northern GBR and provides a greater understanding of population ecology for coral reefs.