Description of a Tetraclita stalactifera panamensis community on a rocky intertidal Pacific shore of Panama

The midlittoral zone at Paitilla Beach, Panama, Is occupied by a group of organisms closely associated with Tetraclita stalactifera panamensis. Upon death, this cirripede provides housing for a number of species bound by common substrata and interacting with each other due to their close physical proximity; the faunistic group associated with each barnacle qualifies then, as a small, well-defined community which represents a subsample of the more comprehensive midlittoral community dominated by T. stalactifera panamensis. In patches where it is abundant, T. stalactifera panamensis occupies 28% of the surface area available. It has a relative abundance of only 1% of the specimens found in the zone, but all the other 95 species (with an average of 26, 810 individuals/m²) identified from the area are associated either with the parietal canals, or the external or internal surfaces of T. stalactifera panamensis tests. Each one of these structures represents a habitat which provides surface for attachment, shelter, or both, to 37 species of polychaetes, 32 of mollusks, 20 of crustaceans, 3 of actiniarians, 2 of pycogonids, 2 of sipunculans and an unknown number of tanaid, nemertean and turbellarian species. The single most diverse space-niche is represented by the parietal canals (H=0.783), occupied mostly by polychaetes. dead barnacles harbor an average of 131 individuals per test, which compose a fauna of higher diversity (H=0.678) than that associated with live barnacles (H=0.163). The latter contain 53 specimens per test on the average. Although there is considerable overlapping of species associated with the different T. stalactifera panamensis space-niches, mollusks are clearly more abundant on internal surfaces, polychaetes in parietal, canals; Balanus spp. on external surfaces, and anemones at the base of the parapet on the external surfaces of the tests. Over 220 species of macro-invertebrates were identified from the rocky intertidal at Paitilla. Most of the species are concentrated in the lower (157 species, 1824 specimens/m²) and midlittoral (96 species, 26, 810 specimens/m²) intertidal zones. Splash zone is depauperate (6 species, 1578 specimens/m²). Of the entire fauna identified from the intertidal, 73 species (33%) occur in more than one zone and 147 (67%) are restricted to a particular belt. This last category of zone-restricted species breaks down into 42% for the lower intertidal, 17% for the midlittoral, 6% for the intermediate zone, and 2% for the splash zone.     

Role of competition and predation in determining habitat occupancy of Cerithiidae (Gastropoda: Prosobranchia) on the rocky,intertidal, Red Sea coasts of Sinai

Six cerithiid snail species occur on rocky intertidal flats along the Sinai coasts of the Red Sea: Clypeomorus moniliferum, Cerithium caeruleum, C. scabridum, C. columna, Clypeomorus tuberculatum and Cerithium echinatum. The present study, conducted at 22 stations covering almost the whole length of Sinai, covered the 4 yr period from October 1972 to August 1976, and describes the habitat occupancy of 5 of these species (C. echinatum is excluded for lack of data). Several of these species (sometimes all) often occur together, and in such cases are distincly segregated by habitat. However all species considerably overlap in their distribution along the axes of four major interrelated abiotic gradients, thus excluding the possibility that habitat segregation is determined by larval settlement preferences. Other distributional patterns observed at some sites, such as lack of overlap or contact between belts of the various species and the relative abundance of food available to all species, make postlarval competitive interactions unlikely. The existence and the degree of proximity of a coral reef with its associated predatory fishes, influence the cerithiids' distributional patterns. Differences between the cerithiid species in their vulnerability to fish predation, associated with differences between sites in the abundance and the accessibility of predatory fishes, and in the availability of refuges for each cerithiid species, can satisfactorily explain the observed distributional patterns including co-occurrence with habitat segregation. It is proposed that habitat segregation is caused by predation on young stages by generalist fishes which may totally eliminate a certain species at a given site; the same site may provide refuges for recruits of another species, allowing these to survive to an advanced age. In general, the flat's structural complexity is associated with its diversity of refuges from predation, and hence with the number of co-existing species. This mechanism for co-existence and habitat segregation in tropical Cerithiidae may also be instrumental in maintaining the high species diversity of other tropical benthic communities.Paper No. 12 in the series Colonization of the Eastern Mediterrancan by Red Sea species immigrating through the Suez Canal     

Functional identity and functional structure change through succession in a rocky intertidal marine herbivore assemblage

Despite the great interest in characterizing the functional structure and resilience of functional groups in natural communities, few studies have examined in which way the roles and relationships of coexisting species change during community succession, a fundamental and natural process that follows the release of new resources in terrestrial and aquatic ecosystems. Variation in algal traits that characterize different phases and stages of community succession on rocky shores are likely to influence the magnitude, direction of effects, and the level of redundancy and complementarity in the diverse assemblage of herbivores. Two separate field experiments were conducted to quantify per capita and population effects and the functional relationship (i.e., redundancy or complementarity) of four herbivore species found in central Chile during early and late algal succession. The first experiment examined grazer effects on the colonization and establishment of early-succession algal species. The second experiment examined effects on the late-successional, dominant corticated alga Mazzaella laminarioides. Complementary laboratory experiments with all species and under natural environmental conditions allowed us to further characterize the collective effects of these species. We found that, during early community succession, all herbivore species had similar effects on the ephemeral algae, ulvoids, but only during the phase of colonization. Once these algae were established, only a subset of the species was able to control their abundance. During late succession, only the keyhole limpet Fissurella crassa could control corticated Mazzaella. The functional relationships among these species changed dramatically from redundant effects on ephemeral algae during early colonization, to a more complementary role on established early-successional algae, to a dominant (i.e., keystone) effect on late succession. This study highlights that functional relationship within consumer assemblages can vary at different phases and times of community succession. Differentiation in herbivore roles emphasizes the need to evaluate consumer's impacts through different times of community succession, and through experimental manipulations to make even broad predictions about the resilience or vulnerability of diverse intertidal assemblages to human disturbances.     

High intertidal community organization on a rocky headland in Maine,USA

A mosaic patchwork of the barnacle Balanus balanoides L., the mussel Mytilus edulis L., and the alga Fucus vesiculosus L. was found in the transitional region between the mid and high intertidal zones on a rocky headland on Mount Desert Island, Maine, USA. The development of the mosaic was observed by following recruitment and survival of B. balanoides in denuded patches located at the same tidal level along a 60 m stretch of shore. Barnacle recruitment was least under canopies of F. vesiculosus and greatest in open areas kept moist at low tide by surf. Barnacle survival after settlement was least under the F. vesiculosus canopy due to the whiplash effect of the algal fronds in the surf and greatest in open areas free from competition from mussels. In open areas, early mortality was correlated with settlement density. In areas of dense settlement (60 spat cm^-2) up to 90% mortality resulted within 5 months from crowding associated with growth. In older individuals crowding produced hummocks of elongated, weakly attached barnacles which were more prone to removal by surf than uncrowded barnacles. Mussels exerted competitive dominance over barnacles for space and the presence of mussel beds prevented further barnacle recruitment. Mussels suffered extensive mortality during winter storms when surf removed dense mats of weakly attached mussels. The patchy distribution of mussels and barnacles results from irregular rock substrata producing numerous environmental patches with respect to wave exposure and drainage at low tide, and from densitydependent mortality of both mussels and barnacles which creates patches of new colonizable space within each environmental patch.     

Amphipods and isopods in the rocky intertidal: dispersal and movements during high tide

Animals in the intertidal, both mobile and sessile, generally exhibit some zonation pattern, in which each species shows a preference for, or is confined to, some height levels. The study of zonation patterns is, however, almost exclusively based on surveys made during low tide, when many animals are relatively inactive. We studied zonation patterns of amphipods and isopods on rocky shores in southwestern Iceland, both by traditional sampling at low tide as well as by sampling during high tide. The distributional patterns seen at high tide differed significantly from that at low tide. One amphipod, Anonyx sarsi, was common around baits at all levels at high tide but absent from the intertidal at low tide. Several other species were either relatively more common or tended to be recorded higher, or in one instance, lower on the shore when the tide was in than at low tide. There was also evidence of some species changing habitats within the intertidal with the tidal cycle. Many species, however, moved little away from their respective zones occupied at low tide, and for some species, including some capable of rapid swimming, very limited mobility was indicated. We conclude that low-tide surveys of the intertidal give an incomplete picture of the community structure, and even key species may be missed in such surveys.Communicated by L. Hagermann, Helsingør     

Effect of canopy removal on a rocky shore community metabolism and structure

The effects of the permanent removal of the canopy-forming alga Fucus serratus was studied in terms of both functioning and diversity on a mid–low rocky shore ecosystem in the south-west English Channel (48°N 43.686′, 3°W 59.282′). Ecosystem functioning was examined as net or gross primary productivity (NPP or GPP) and respiration (Resp) measured through CO₂ fluxes. Diversity was examined as number and composition of species. Measurements were performed in situ, during emersion times, without altering target assemblages. The experiment was designed with two treatments [control (C) or canopy removed (CR)] and five replicates, and was conducted over an 18-month period (from February 2006 to August 2007) to integrate the seasonal variability. The mean GPP and Resp were severely reduced in CR treatment compared to control throughout the survey. The mean NPP was not affected at first, due to the development of opportunistic green algae, but was drastically reduced after 9 months of experiment. The canopy removal affected neither the number of species nor their distribution among trophic groups, and the algal community was only slightly affected. The abundance and biomass of mobile invertebrates, however, were greatly reduced in the absence of canopy. This indicates an important effect of the dominant alga on the higher trophic levels of the community. At this tidal level, the canopy did not seem to affect the community by dampening the environmental stress but by providing food, habitat or both.     

Within-group spatial position in ring-tailed coatis: balancing predation, feeding competition, and social competition

A variety of factors can influence an individual’s choice of within-group spatial position. For terrestrial social animals, predation, feeding success, and social competition are thought to be three of the most important variables. The relative importance of these three factors was investigated in groups of ring-tailed coatis (Nasua nasua) in Iguazú, Argentina. Different age/sex classes responded differently to these three variables. Coatis were found in close proximity to their own age/sex class more often than random, and three out of four age/sex classes were found to exhibit within-group spatial position preferences which differed from random. Juveniles were located more often at the front edge and were rarely found at the back of the group. Juveniles appeared to choose spatial locations based on feeding success and not predation avoidance. Since juveniles are the most susceptible to predation and presumably have less prior knowledge of food source location, these results have important implications in relation to predator-sensitive foraging and models of democratic group leadership. Subadults were subordinate to adult females, and their relationships were characterized by high levels of aggression. This aggression was especially common during the first half of the coati year (Nov–April), and subadults were more peripheralized during this time period. Subadults likely chose spatial positions to avoid aggression and were actively excluded from the center of the group by adult females. In the Iguazú coati groups, it appeared that food acquisition and social agonism were the major determinants driving spatial choice, while predation played little or no role. This paper demonstrates that within-group spatial structure can be a complex process shaped by differences in body size and nutritional requirements, food patch size and depletion rate, and social dominance status. How and why these factors interact is important to understanding the costs and benefits of sociality and emergent properties of animal group formation.     

Impact of sewage on the distribution,abundance and community structure of rocky intertidal macro-organisms

The biological effects of a low-volume domestic sewage discharge were studied near Wilson Cove, San Clemente Island, California (USA), from February to June, 1972. There were fewer species and less cover near the outfall (7 macro-invertebrates, 17.6%; 13 macrophytes, 91.7%) than in nearby “unpolluted” control areas (9 macroinvertebrates, 9.2%; 30 macrophytes, 103.4%). The outfall biota was less diverse than that of the controls, as shown by 5 different diversity indices. A great reduction in community stratification (spatial heterogeneity) and, hence, community complexity occurred near the outfall; this reduction in stratification was primarilly due to the absence of Egregia laevigata, Halidrys dioica, Sargassum agardhianum and Phyllospadix torreyi. These were replaced in the mid-intertidal near the outfall by a low turf of blue-green algae, Ulva californica, Gelidium pusillum and small Pterocladia capillacea, and in the lower intertidal by Serpulorbis squamigerus covered with Corallina officinalis Var. chilensis. A statistically-based determination of assemblages or groups of organisms (i.e., cluster analysis) revealed 3 discrete outfall and 3 discrete control area groups; 3 assemblages contained samples from both areas. The distributional patterns of these groups indicate that near the outfall the degree of dilution of discharged sewage is more important in regulating zonation than is tidal height. The enhancement of the suspension feeder Serpulorbis squamigerus and the omnivores Ligia occidentalis, Pachygrapsus crassipes and Anthopleura elegantissima in the outer fringe of the outfall plume hypothetically is due to their ability to utilize sewage as a food source. A critical effect of the outfall may be to decrease environmental stability thereby favoring rapid-colonizers and more sewage-tolerant organisms. The outfall macrophytes were characterized by relatively higher net primary productivities, smaller growth forms, simpler and shorter life histories, and most were components of early successional stages.     

Interspecific competition and predation: relative effects on foragers and their densities

Predation and competition are both strong structuring forces in community dynamics, but their relative importance is disputed. In a laboratory experiment, we evaluated the relative importance of competition and predation from juvenile and adult brown trout, respectively, on foraging performance of groups of three stone loaches. We observed loach consumption rate, time spent inactive, and aggressive interactions between juvenile trout and loach in artificial stream sections. The controlled experiments were complemented by examining stone loach population densities in natural systems as functions of juvenile and adult trout. In the laboratory experiments, increasing numbers of competitors decreased prey availability, which ultimately led to lower consumption rates for loach. Loach responded to predation risk by increasing time being inactive, thereby decreasing consumption rates. However, there were no effects of juvenile trout competitors on loach consumption rates in treatments with adult trout presence, suggesting no additive effect of predation and competition on loach foraging success. Partial regressions of loach and trout densities in natural streams revealed a positive relationship between juvenile trout and loach, and a negative relationship between adult trout and loach. Our laboratory and field data thus suggest that predation is a limiting factor for loach success, and predator presence could mediate species coexistence at high interspecific densities.     

Dynamic zonation of Nerita plicata,N. undata and N. textilis (Prosobranchia: Neritacea) populations on a rocky shore in Kenya

Vertical zonation of Nerita plicata Linnaeus, N. undata Linnaeus, and N. textilis Dillwyn, inhabiting the rocky cliffs of Nyali, Kenya, was studied through several spring-neap tidal cycles from November to December (northeastern monsoon) 1981 and May to July (southeastern monsoon) 1982. For the first time, a monsoon-related cycle of migration in Nerita plicata and N. undata, superimposed on the shorter term spring-neap cycle is demonstrated. Both species rest predominantly in the upper eulittoral zone and the littoral fringe. N. plicata and N. undata rest at significantly higher levels during the southeastern monsoon (SEM) than during the northeastern monsoon (NEM) (P<0.001). On and around neap-tide days N. plicata, N. undata, and N. textilis occupy significantly distinct mean resting positions (P<0.001) in both seasons; N. undata being the highest in the SEM, followed by N. plicata, with N. textilis always in the lowest level. In the NEM, N. plicata and N. undata sometimes change order. On and around springtide days in both seasons, the upper two species, N. plicata and N. undata, rest close together with no significant difference in their mean resting positions, but with both significantly higher than N. textilis (P<0.001). Both the spring-neap and the monsoon migrations of N. plicata and N. undata are interpreted as either direct or indirect responses to wave action.     

Polychaete assemblages along a rocky shore on the South Adriatic coast (Mediterranean Sea): patterns of spatial distribution

This study reports on the distribution of polychaete assemblages along a depth gradient from 5 to 25 m, along a stretch of rocky coast near Otranto (South Adriatic sea, Italy). Three depths were sampled in May and November 2000. At each depth three sites (about 100 m apart) were sampled by scraping off three replicate quadrats of 20×20 cm. The experimental design enabled identification of across- and along-shore spatial patterns of variation of polychaete assemblages. A total of 4,168 specimens, belonging to 152 taxa were collected. Multivariate analysis showed that the polychaete assemblages differed significantly among depths with the clearest differences between the shallowest sites (5 m) and the deeper ones (15–25 m). A considerable source of variation among sites at each level of the shore was also exhibited by the analyses, with the greatest differences among sites within depths recorded at 5 m. The species most contributing to the differentiation of assemblages among depths and sites within each depth were identified. Some potential causes of the observed differences are hypothesized and discussed. The importance of quantitative observation to provide the context for studying the underlying ecological processes is also stressed.Communicated by R. Cattaneo-Vietti, Genova     

A comparison of macrofaunal and meiofaunal distribution and standing stock across a rocky shore,with an estimate of their productivities

In 1984 and 1985 algal, macrofaunal and meiofaunal standing stocks were estimated on a exposed rocky shore along the west coast of False Bay, South Africa, using comparable, area-based sampling techniques. The shore supported a rich growth of algae, particularly in summer, when a maximum standing crop of 403 g m^-2 was recorded in the low shore. In winter, the largest component of macrofaunal biomass comprised the filter-feeding barnacle Tetraclita serrata, which attained 75 g m^-2 in the middle balanoid; but as a result of late recruitment and high mortality of this species, the summer shore was dominated by herbivorous gastropods, particularly Patella cochlear, which attained a maximum biomass of 66 g m^-2 on the low shore. Meiofaunal numbers and biomass were closely correlated to the distribution of algal turfs and associated trapped sediments. Numerically, the most important components of the meiofauna were nematodes and copepods, while the biomass was more evenly shared among foraminifera, minute gastropods, copepods and insect larvae. Numbers and biomass peaked in the lower balanoid during winter (1.9×10⁶ individuals, or 8.5 g m^-2). Macrofauna:meiofauna numbers and biomass ratios are presented for each zone and the distribution patterns discussed in relation to the conditons in each. Numerically, meiofauna exceed macrofauna by an overall ratio of 1:391, with values ranging from 1:556 in the lower balanoid to 1:18 in the Littorina zone. Macrofaunal biomass exceeds that of meiofauna in all zones by an overall ratio of 10:1, but values range from 2.1:1 in the upper balanoid to 48:1 in the middle balanoid. By incorporating turnover ratios extrapolated from the literature, mean annual productivity ratios have been calculated. These indicate that macrofauna account for 75% of total secondary production and meiofauna for 25%. Failure to incorporate meiofauna in analyses of energy flow on rocky shore ecosystems would thus lead to considerable errors. The possible trophic role of meiofauna in such systems is discussed.     

Correlations between weather and recruitment of Nassarius pauperatus,an intertidal gastropod with a planktotrophic larval stage

Recruitment of the intertidal gastropod Nassarius pauperatus (Lamarck) was estimated annually for more than a decade at three sites in South Australia. Recruitment varied temporally, but was relatively synchronous amongst sites, even though these were at least 10 km apart. The density of recruits at each site was negatively correlated with evaporation and also with several indices of temperature for February. We postulate that either high temperatures or desiccation cause the death of recently settled individuals, which may be unable to avoid these extremes by burrowing deeply into the substratum.     

Demography and phenology of the intertidal green algaCodium setchellii: The enigma of local scarcity on sand-influenced rocky shores

Demographic and phenological attributes that characterize rare and scarce species have not been extensively investigated for marine benthic organisms. From 1984 to 1987, 1 investigated the population ecology of the low intertidal, psammophytic green algaCodium setchellii Gardner that forms low-density populations on sand-influenced rocky benches on N.E. Pacific shores. To address why the alga is not more common in the habitat in which it is most abundant, I quantified sand fluctuations and population dynamics ofC. setchellii at several sites along the central coast of Oregon, USA. Sand movement was highly unpredictable both spatially and temporally on a local scale. AlthoughC. setchellii became fertile in late autumn to winter, the alga and rocky surfaces were often deeply buried by sand in these seasons. Algal recruitment was also spatially and temporally variable, and mortality of small thalli was high. Thallus growth was generally slow, and the alga had the capacity to persist at a given size for long periods. On sand-influenced rocky shores, mortality of largethalli was low, with some thalli persisting for several years. In laboratory experiments,C. setchellii lost substantial biomass during 4 to 5 wk sand burials, whereas two sympatric species of psammophytic algae lost little to no biomass. The within-site scarcity ofC. setchellii may be a consequence of (1) largely unpredictable environmental conditions that reduce algal recruitment and survival, and (2) apparent lack of specialized anatomical or reproductive adaptations to persist on sand-scoured rocky shores. The alga's persistence may be due to its slow growth and long lifespan.     

Temporal patterns of settlement, recruitment and post-settlement losses in a rocky reef fish assemblage in the South-Western Mediterranean Sea

Post-settlement events can significantly alter the density distribution of settlers and subsequently the adult population structure. The temporal and inter-annual variability of settlement and the effects of mortality on recruitment were investigated across 2 years in the vicinities of Cabo de Palos–Islas Hormigas Marine Reserve by visual census and light trap sampling. Settlement was seasonal with greater species richness and abundance in summer. Although temporal synchronization was observed between larval supply and settlement, densities of settlers could not be predicted from post-larval abundances. Timing of settlement was consistent between years but with high inter-annual variation in abundance. High mortality (~80 %) and general decoupling between post-larval and settlement stages suggest that early mortality is driving such patterns. Nevertheless, indications of habitat-mediated mortality were found for benthic species such as Symphodus and Diplodus, highlighting the importance of habitat in shaping population demography.     

Variations in cardiac performance and heat shock protein expression to thermal stress in two differently zoned limpets on a tropical rocky shore

Understanding variation in physiological adaptations to thermal stress is vital when investigating intertidal species?? distribution patterns. The thermal sensitivities of two limpets, Cellana grata and C. toreuma, differed in accordance with their vertical distributions. Cardiac performance was maintained at higher temperatures (~47°C) for the high-zone C. grata than the mid-zone C. toreuma (~42°C). At 40°C, C. grata maintained regular heart function for ~4?h, while heart function of C. toreuma decreased rapidly. Heat shock protein expression revealed that C. toreuma had two constitutive isoforms, Hsp77 and Hsp72, and C. grata one inducible form, Hsp75, which was upregulated at 40°C, suggesting C. grata has a more effective heat shock response than C. toreuma. The temperature-adaptive differences in cardiac thermal tolerance and Hsp expression match observed differences in thermally induced mortalities with the onset of summer and may help predict differential effects of climate change on the two congeners.     

Uniform predation risk in nature: common,inconspicuous, and a source of error to predation risk experiments

Previous studies indicate that when predation risk is uniform across habitats, foragers concentrate their exploitation in fewer patches. Although uniform predation risk may seem rare in nature, some scenarios might cause it. Testing all scenarios in a single experiment is unfeasible; therefore, we developed a model that points whether concentration of exploitation in specific habitats due to uniform risk requires parameter values similar to what is found in literature. This model was based on Brown’s (Behav Ecol Sociobiol 22:37–47, 1988) fitness function but rescaled to multiple habitats and predators, including uniform risk predators. Deriving function’s maximum allowed comparisons with giving-up density studies. Results showed that uniform predation risk had a u-shaped effect in habitat exploitation, causing a concentration of habitat exploitation at probabilities of survival from 0.2 to 0.8. However, the length of this interval and degree of concentration depended on the value of safety to forager fitness. Heterogeneous, nonuniform, predation risk decreases habitat exploitation where it was higher, therefore suppressing the effect of uniform risk on prey behavior. Time spent in the focal habitat and metabolic costs reduced the detectability of habitat concentration, while total time did not. We also found that uniform risk reduced accuracy of heterogeneous risk measurements. Future studies should aim to control all possible predators, as even the mild ones can induce complex behavior.     

Relative intensity of predation on the French grunt, Haemulon flavolineatum, during diurnal, dusk, and nocturnal periods on a coral reef

Established hypotheses state that the rate of predation on coral reef fish should be highest during crepuscular periods (dawn and dusk) intermediate diurnally, and lowest nocturnally. We examined the relative risk of predation on juvenile French grunts (Haemulon flavolineatum Desmarest) during diurnal, dusk, and nocturnal periods on the fore- and back-reef at Teague Bay, U.S. Virgin Islands in July and August 1996. Tethering-devices recorded the exact amount of time between attaching a prey fish to its tether and subsequent predation on the prey fish. As tethering of prey usually inflates the actual rate of predation, times from our tethering devices were used to establish only the relative predation risk among treatments. During 3-h diurnal and nocturnal tethering experiments, relative predation was significantly higher during the nocturnal period, and differences between side of reef were not significant. In 30-min tethering experiments, which included all three time periods, the relative predation risk was significantly higher during dusk and nocturnal periods than during the diurnal period. Relative predation was not significantly different between the dusk and nocturnal periods, or between side of reef during any time period. The unexpected finding that the diurnal period had the lowest relative risk of predation indicates that the timing of predation events on reefs, as well as the adaptive reasons for nocturnal larval settlement, may need to be re-examined. Received: 11 February 1997 / Accepted: 21 October 1998     

Community development on subtidal temperate reefs: the influences of wave energy and the stochastic recruitment of a dominant kelp

Patterns of community development on subtidal rocky reefs in Marmion Lagoon, southwest Australia, were investigated with a settlement panel experiment. We tested the hypothesis that community development would differ between outer and inner reefs lines, because exposure to swell and wave energy was significantly greater on outer reefs. Following a 14-month deployment, we recorded pronounced variability between panels and sites, but did not detect any effect of wave exposure on the structure of panel assemblages. Subsequent data exploration suggested the importance of the presence of kelp recruits (Ecklonia radiata) in structuring the overall assemblage. Panel assemblages with kelp recruits were significantly different in structure to those without, principally because of greater space coverage of encrusting coralline algae and less coverage of red turfing algae, spirorbids, and bryozoans. Mature E. radiata act as ecosystem engineers in subtidal rocky reefs in southwest Australia. Our results suggested the importance of young, recruiting kelps in determining patterns of early community development on newly available hard substrata.     

Recovery of plant diversity following N cessation: effects of recruitment, litter, and elevated N cycling

Plant species richness has declined and composition shifted in response to elevated atmospheric deposition of biologically active nitrogen over much of the industrialized world. Litter thickness, litter nitrogen (N) content, and soil N mineralization rates often remain elevated long after inputs cease, clouding the prospects that plant community diversity and composition would recover should N inputs be reduced. Here we determined how N cycling, litter accumulation, and recruitment limitation influenced community recovery following cessation of long-term N inputs to prairie-like grasslands. We alleviated each of these potential inhibitors through a two-year full-factorial experiment involving organic carbon addition, litter removal, and seed addition. Seed addition had the largest effect on increasing seedling and species numbers and may be necessary to overcome long-term burial of seeds of target perennial grassland species. Litter removal increased light availability and bare sites for colonization, though it had little effect on reducing the biomass of competing neighbors or altering extractable soil N. Nonetheless, these positive influences were enough to lead to small increases in species richness within one year. We found that, although C addition quickly altered many factors assumed favorable for the target community (decreased N availability and biomass of nearby competitors, increased light and site availability), these changes were insufficient to positively impact species richness or seedling numbers over the experimental duration. However, only carbon addition had species-specific effects on the existing plant community, suggesting that its apparent limited utility may be more a result of slow recovery under ambient recruitment rather than from a lack of a restorative effect. There were dramatic interactions among treatments, with the positive effects of litter removal largely negated by carbon addition, and the positive effects of seed addition generally amplified by litter removal. It remains unclear whether each mechanism explored here will induce community recovery, but over different temporal scales. Long-term monitoring will help resolve these remaining questions. Regardless, our results suggest that reversal of species loss and compositional shifts from N deposition in prairies may be more inhibited by habitat fragmentation, recruitment limitation, and long-term suppression of fire than from continued effects of elevated N.