Differences between understorey and canopy in herbivore community composition and leaf quality for two oak species in Missouri

1. From July 1994 to September 1995, at six censuses, the herbivore community associated with understorey (< 2.5 m height) and canopy (15–20 m) leaves of Quercus alba and Q. velutina was sampled in south-eastern Missouri, U.S.A. 2. Across all censuses, herbivore densities were not significantly different between canopy and understorey for Q. alba and Q. velutina, except in August 1994 when herbivore densities were 60% higher in the canopy on Q. alba. Little significant spatial variation in herbivore densities or community composition was found during the study years. 3. The herbivore community was diverse, consisting of 138 species of leaf-chewing insects. Species richness was significantly greater (by 5–20%) in the understorey than in the canopy for both tree species, and the relative abundance of the main families, different feeding guilds, and most common species differed significantly between understorey and canopy. 4. To determine the extent to which leaf quality explained the observed patterns, percentage nitrogen and protein binding capacity were measured in canopy and understorey leaves of Q. alba and Q. velutina. Per cent nitrogen was higher in canopy leaves for Q. velutina while protein binding capacity was higher in canopy leaves for Q. alba. 5. These results suggest that the herbivore community associated with these two species of Quercus comprises species that appear to respond individually to environmental and biological conditions encountered in the understorey and the canopy.     

Investigations of density interactions among breeding birds in ponderosa pine forests: correlative and experimental evidence

Summary We assessed the importance of interspecific competition among insectivorous birds breeding in northcentral Arizona's ponderosa pine forests. We examined density interactions among species using two analytic approaches; correlative and experimental. The correlative approach examined patterns of change in breeding densities over four years at the community level and within two foraging guilds; picker-gleaners and aerial feeders. The relationships between morphological and behavioral similarity with pairwise density interactions were also assessed. Our experimental approach involved placement of nest boxes on two treatment plots to increase breeding densities of secondary cavity nesting birds that were in foraging guilds with open nesting insectivores.We found little evidence of interspecific competition. Patterns of density fluctuations indicated large positive covariances among species at both the community level and within guilds. Pairwise density interactions were independent of morphological or behavioral similarity. Nest boxes significantly increased breeding densities of the secondary cavity nesters. However, these increases did not induce reprocal density changes in the open nesting species. Interspecific competition for food during the breeding season appears to be unimportant in ponderosa pine bird communities.     

Feeding habits and seasonal trophic guilds structuring fish community in the bay mouth region of a tropical estuarine habitat

Understanding trophic relationships of fish in estuarine ecosystem is an important element for sustainable resource management. This study examined the feeding habits of 29 dominant fish species, characterized the trophic guilds, assessed the impact of season and clarified the role of diets in structuring the fish community in the mouth region of Pattani Bay, Thailand. Samples of 5792 fishes collected monthly by gillnets from March 2019 to February 2020 were used for stomach content analyses. It was found that the number of food types and fullness index differed between fish taxa (P < 0.001). Most fishes were specialist feeders feeding on specific food components and were categorized into five trophic guilds: piscivore, shrimp-fish feeder, polychaete feeder, zooplanktivore and planktivore. Six species were piscivorous, considered as apex predators, that fed almost entirely on fishes. High diet overlaps among some species (>0.6) were recorded. Not much variation in seasonal guilds was observed: four guilds in the dry season, three in the moderate rainy season and four in the rainy season. Some species remained in the same guild the whole year round, but some fishes changed seasonally. Two fish communities from different regions of the bay were segregated based on feeding habits. The inner bay community comprised mainly copepod and plankton feeders, but there were more piscivores in the deeper bay mouth area. Results from this study help us to understand the feeding habits and trophic guilds of dominant fish species at the mouth of this tropical estuarine bay.     

Contrasting multispecies patterns in larval fish production trace inter-annual variability in oceanographic conditions over the N.E. Aegean Sea continental shelf (Eastern Mediterranean)

The response of larval fish communities of the northeastern Aegean Sea (NEA) to interannual environmental changes is analyzed using data from four ichthyoplankton surveys covering the NEA continental shelf during June 1993, 1994, 1995, and 1996. Waters were significantly cooler, less saline and richer in zooplankton in 1993 and 1996 (‘cold’ years) than in 1994 and 1995 (‘warm’ years). A comparison of monthly SST series (1993–1997) between the NEA, the Marmara Sea, and the Western Black Sea revealed high correlations and similar trend components among these areas implying that oceanographic conditions over the NEA (and observed inter-annual differences) were most likely dominated by the properties and relative amount of Black Sea water inflow in the NEA. The relative composition of the larval fish community was significantly related to the ‘cold/warm’ regime and larval diversity was higher during the warm years. Larvae of the small-sized pelagic species, such as anchovy (Engraulis encrasicolus), and most mesopelagic fishes were relatively more abundant during the cold, zooplankton-rich years. Larvae of the middle-sized pelagics (Sardinella aurita, Scomber japonicus, Trachurus mediterraneaus, Auxis rochei) and certain benthopelagic species exhibited an opposite trend, i.e., they were more abundant during the warm years or absent during the cold years. Most of these species are known to be typical summer spawners (e.g., Serranus cabrilla, Lisa saliens, Trachinus draco, and Symphurus nigrescens). Co-variation in larval fish production might be indicative of similar responses among species to changing physical and/or trophic regimes.     

Size-abundance relationships in an Amazonian bird community: implications for the energetic equivalence rule

We studied size-abundance relationships in a species-rich Amazonian bird community and found that the slope of the logarithmic relationship between population density and bodymass (b = -0.22) is significantly shallower than expected under Damuth's energetic equivalence rule (EER), which states that population energy use (PEU) is independent of species body mass. We used estimates of avian field metabolic rates to examine the logarithmic relationship between PEU and body mass and its variation among ecological guilds. The relationship for all species had a significantly positive slope (b = 0.46), indicating that PEU of larger species was greater than that of smaller species. Analyses of guilds revealed significant variation. The slopes of the frugivore-omnivore, insectivore, and granivore guilds were all significantly positive, with that of the frugivore-omnivore guild being the steepest. In contrast, PEU did not vary significantly with species body mass among raptors. These results were confirmed, in analyses using both species values and phylogenetically independent contrasts, and the results do not support the EER in this community. The spatial distribution of resources and mechanisms of interference competition within guilds may explain why most patterns differed from the predictions of the EER. Other sources of variation, including the effects of scale, are also discussed.     

Variation in the guild composition of herbivorous insect assemblages among co‐occurring plant species

Variation in plant traits among plant species may promote the development of a characteristic functional assemblage of insect herbivores associated with each plant species. However, only a small number of studies have detailed the representation of several herbivore guilds among co‐occurring plant species to determine whether the functional structure of herbivorous insect assemblages varies widely and consistently among plant species. The present study provides one of the few published data sets reporting on the density of several guilds of insect herbivores among numerous plant species. Variation in guild associations with plant phenology and season are also described. Insect herbivores were divided into 10 guilds, and the representation of these guilds was examined for 18 co‐occurring plant species. Guild densities and assemblage composition varied significantly among plant species, even when variation over time was taken into account. Variation in guild densities and assemblage composition were not strongly related to the taxonomic relationships of the plants. The highest densities of several guilds occurred in spring and summer, although other guilds were not strongly seasonal. Certain guilds were strongly associated with the presence of new leaves, whereas other guilds appeared to prefer mature leaves. This resulted in assemblage differences between samples containing new and mature leaves and samples containing mature leaves only. Even though the timing and duration of leaf and flower production varied among plant species, this did not explain all variation in guild densities among plant species. It is suggested that additional factors, including plant traits, are contributing to the wide and consistent variation in herbivore assemblage composition among plant species.     

潜在洞巢资源差异对次级洞巢鸟及繁殖鸟类群落的影响

为了解次生林中潜在洞巢资源(包括各种啄木鸟的啄洞和人工巢箱)的多寡对次级洞巢鸟集团及繁殖鸟类群落结构的影响, 2007年11月至2008年7月, 我们在吉林省吉林市大岗林场选择洞巢密度不同的样地, 对其次级洞巢鸟及鸟类群落结构进行了比较研究。根据洞巢资源密度我们将9块样地分为3组, 即巢箱区(啄洞密度最低, 悬挂人工巢箱使其潜在洞巢资源总密度大幅提高)、低密度区(啄洞密度较低, 无巢箱)和高密度区(啄洞密度较高, 无巢箱), 调查了3组样地内鸟类的组成和密度、潜在洞巢资源的利用情况等。3组样地中均调查到4种初级洞巢鸟, 其种类组成略有不同; 4种次级洞巢鸟在3组样地广泛分布, 分别为白眉姬鹟(Ficedula zanthopygia)、大山雀(Parus major)、沼泽山雀(P. palustris)和普通鳾(Sitta europaea)。巢箱区和高密度区的次级洞巢鸟总密度显著高于低密度区。巢箱区同高密度区一样, 大山雀和白眉姬鹟的密度显著高于低密度区, 这是由于大山雀和白眉姬鹟是人工巢箱的主要利用鸟种, 而沼泽山雀和普通鳾的密度在三组样地间差异不显著。初级洞巢鸟总密度与啄洞密度、次级洞巢鸟总密度与潜在洞巢资源总密度都呈显著正相关关系。潜在洞巢资源丰富的样地中鸟类群落多样性指数显著高于潜在洞巢资源贫乏样地中的鸟类群落多样性指数, 人为增加洞巢资源可以改变鸟类群落组成并显著提高群落的多样性指数。三组样地中鸟类群落的均匀性、丰富度指数和种间相遇率没有显著差异, 群落相似性指数也相近。高密度区和低密度区鸟类群落集团结构相似。次级洞巢鸟密度的增加短时期内未对群落内其他主要鸟种的密度产生显著影响。研究结果显示, 初级洞巢鸟的密度决定了啄洞的丰富程度, 而洞巢资源的差异会对次级洞巢鸟集团的分布模式产生影响, 进而影响整个繁殖鸟类群落的结构。     

Interdecadal trends in composition,density, size,and mean trophic level of fish species and guilds before and after coastal development in the Mexican Caribbean

This study explores the possible influence of human coastal development (before and after) and protected area status (within and outside a marine protected area, MPA) on composition, density, and maximum size of fish species and guilds, including mean trophic level of the fish community, in four localities of the Yucatan Peninsula, Mexican Caribbean. Reef fish density, maximum length, species composition, and trophic guilds were recorded by SCUBA belt transects and stationary points in fore reef and lagoon reef areas at decadal intervals (1995–1998, 2006–2010, 2014–2015). Mean density of most species and guilds decreased significantly through the years, as also did mean trophic level of the fish community. Some fish species increased in length. Fish density for many species was larger outside than inside the MPA in 1995–1998; however, the difference tended to disappear in the more recent decades, which reflects either a positive effect of the MPA, or a detrimental effect of coastal development in the non-protected area. Nevertheless, the overall negative trends suggest a regional or global rather than a local cause.     

Spatial structure of fish assemblages in a tropical estuarine lagoon: combining multivariate and geostatistical techniques

The spatial structure and seasonal changes of estuarine fish assemblages in the Ciénaga Grande de Santa Marta (CGSM) were analysed based on four seasonal comprehensive surveys conducted in 1993-1994 and 1997. Geostatistical and multivariate techniques were used to: (a) determine seasonal changes in spatial distribution of the species richness, and (b) identify assemblages of estuarine fish and their relation to abiotic factors. Potential biotic interactions affecting the assemblage structure were also explored. A total of 11075 individuals representing 39 species were collected, with Eugerres plumieri, Diapterus rhombeus, Micropogonias furnieri, Mugil incilis, Cathorops spixii, Elops saurus and Anchovia clupeoides as dominant species between seasons. Spatial distribution of fish richness differed between rainy and dry seasons in each year, whereas species mapping showed spatial patchiness in 1993-1994 and gradients during 1997. Strong evidence of species saturation was found in all seasons, suggesting biotic interactions limiting species richness at a threshold density of ca. 50 ind./5000 m². Marine, marine-estuarine and freshwater species were classified in each season according to their capability to cope with salinity fluctuations. Associations defined by functional feeding guilds were also identified. Empirical and statistic evidence showed that fish assemblages differed between seasons within each year, and each assemblage was always dominated by a small number of species, notably E. plumieri in both years 1993-1994 and 1997. Between-season differences in fish assemblage structure in the CGSM seem to be driven by abiotic factors; however, evidence of species saturation could suggest the existence of density-dependent factors operating together.     

Bird Community Composition in a Shaded Coffee Agro-ecological Matrix in Puebla,Mexico: The Effects of Landscape Heterogeneity at Multiple Spatial Scales

This study examined the importance of habitat heterogeneity on the avian community composition, and investigated the scale at which species abundances respond to habitat variables. The study was conducted within a diverse landscape matrix of a shaded coffee region in Mexico. To detect at which characteristic spatial scale different species and foraging guilds respond most strongly we analyzed the effect of plot-, patch- and landscape-level variables at different spatial extent (i.e., different kilometer radii) on species composition and foraging guilds. We used redundancy analysis to identify species–environment correlations, and to identify predictor variables that best explained the bird community structure, quantified the influence of plot-, patch- and landscape-level variables on the bird community composition. In addition, we used the 4th-corner method to detect significant relationships between the dietary guilds and plot-, patch- and landscape-level variables. We recorded 12,335 individuals of 181 bird species; 105 bird species were recorded foraging within the shaded coffee plantations. We found that plot- and landscape-level variables significantly explained the bird community composition best across all scales, and were significantly correlated with the abundance of the dietary guilds. In contrast, patch-level variables were less important. Habitat composition variables (i.e., coffee, forest and agricultural area) were among the most important predictors. Canopy structure was more important than other vegetation structure variables in explaining dietary guild structure. Hence, the maintenance of a heterogeneous landscape with a high-quality matrix within an agro-ecological region enhances bird conservation.     

Lake ecosystem effects associated with top-predator removal due to selenium toxicity

We examined the fish and zooplankton composition of Belews Lake, a 1,564-ha impoundment located in north-central North Carolina, which experienced a temporary surge of selenium that subsequently eliminated piscivorous fish stocks from the lake basin. Beginning in 1974 and ending in 2004, we focused on three 14 month sampling periods. In 1974/1975 the piscivorous fish community was comparable to that of near-by lakes. In the high selenium impact period of 1985/1986 piscivorous fish species were eliminated from the lake basin. In 2003/2004 piscivorous species again constituted a significant component of the fish community of the lake, although fish species diversity declined significantly. Lepomis cyanellus, which had been limited to <2% of all fish sampled prior to 1984, increased dramatically, constituting >63% from 1993 to 2004. Macrozooplankton density was >17 times higher in 1974/1975 than in 1985/1986. During 1985/1986, all cladocera except the smallest species were eliminated or were present at extremely low densities. As the piscivore population at Belews Lake returned to its pre-impact density, macrozooplankton recovered to baseline levels for density, raw species counts, and Shannon–Wiener diversity. Since zooplankton is resistant to selenium at the exposure levels experienced at Belews Lake, we attribute the changes in the restructuring of the zooplankton community and phytoplankton densities to changes in top-down predation.     

Re-establishment of epibiotic communities in reforested mangroves of Gazi Bay, Kenya

Recolonization of epibiotic flora and fauna in two fringing Sonneratia alba reforestation plots was investigated and compared to a natural mangrove stand and a denuded site in Gazi Bay, Kenya. The reforested sites differed with respect to land history and planting density. Habitat availability in the form of pneumatophore surface differed among forested sites (P<0.001), and between landward and seaward zones (P<0.05). Eighteen algal species were found in the natural area compared to 23 and 10 in replanted sites. Only one species was encountered in the denuded area. SIMPER analysis distinguished Enteromorpha ramulosa, Polysiphonia sp., Hypnea sp. and Caloglossa leprieuri as the main algal species responsible for differences between sites. Algal biomass was positively correlated to pneumatophores area (P<0.001). Total algal biomass differed markedly between forested sites: 1.4 (matrix replantation), 28.6 (natural stand) and 44.3 g m⁻² (integrated replantation) in the seaward zones. The matrix replantation showed strong differences in algal community assemblages compared to the other forested sites, and this site also had significantly lower biomass of sessile benthic fauna (P<0.001). Statistical differences in algal (P<0.01) and sponge (P<0.05) community composition between landward and seaward zones were observed in all sites and trunk fouling fauna was distinctly different between sites. Reasons for the above patterns are discussed and it is suggested that zonation patterns affecting pneumatophore surface and inundation time, in combination with proximity of sites to natural seeding areas, are the most likely explanations for observed patterns of epibiotic community distribution in this study.     

Variation in herbivore density among host plants and its consequences for community structure

Summary The densities of four species of gall-forming sawflies were found to vary significantly among willow host plant clones. Two of the speices varied among host plants at four sites in each of three years. The other two species varied in density among host plants at most of the sites in two of the three years. Total sawfly density also varied significantly among clones. Individual species densities on willow clones were significantly positively correlated between years when all sites were combined and frequently when sites were considered separately. Most pairwise species combinations were independent in density between years, but some negative correlations existed between the stem galler and the leaf galler. Gall-former densities also were largely independent among clones within years with all sites combined and with sites considered separately. The significant correlations were nearly all positive. At all four sites the combination of significant variation in sawfly densities among willow clones in the field and independence of species densities among clones resulted in significantly different communities (relative abundance of species) among willow clones in three years. Although sawfly abundances differed substantially among the four sites, this remained true. It is argued that the pattern of community structure among clones is the result of variation in host plant quality of clones. We propose an hypothesis to account for patterns of herbivore species associations based on intrapopulation host plant variation.     

Larger zooplankton in Danish lakes after cold winters: are winter fish kills of importance?

Winter fish kills can be intense under ice in shallow lakes, and have cascading effects on the food web and ultimately on lake water clarity. In maritime Western Europe, winters are usually mild, but occasional colder periods may also have strong effects on lake fish communities. Global warming may have disproportionate effects by delaying freezing and shortening the period of ice coverage. We studied differences in zooplankton (cladocerans, copepods, and rotifers): phytoplankton biomass, zooplankton community structure, and individual body size among 37 Danish lakes of various depths, chemical characteristics, and trophy, by comparing four winters of different severity (mean winter temperatures ranging from −1.19°C in 1996 to +2.9°C in 1995). We found that crustacean mean body sizes were significantly larger in the summer following a severely cold winter. The zooplankton communities in the summer after a cold winter had a significantly larger proportion of larger-bodied species and taxa. Phytoplankton biomass, expressed as chlorophyll-a (chl-a), was lower and zooplankton herbivory (chl-a:TP index), higher, in the summer after the severely cold winter of 1995/1996. All these effects were stronger in shallow lakes than in deep lakes. Changes in zooplankton during summer 1996, compared with other years, were likely caused by fish kills under ice during the preceding severe winter of 1995–1996. Fish kills due to under ice oxygen depletion would be expected to occur earlier and be more complete in the shorter water columns of shallow lakes. With climate change, severe winters are predicted to become less frequent and the winters to be milder and shorter. In general, this is likely to lead to higher winter survival of fish, lower zooplankton grazing of phytoplankton the following summer and more turbid waters, particularly in shallow eutrophic lakes.     

Variation in home-range size and movements of wintering dabbling ducks

Despite a long historical record of radio-tracking analyses, basic home-range information is still lacking for most common waterfowl species, especially during the winter. We investigated how dabbling duck home ranges and daily foraging movements are influenced by extrinsic (site, temperature, date) and intrinsic factors (species, sex, age). We radio-tagged and monitored 125 individuals of three duck species (mallard Anas platyrhynchos, Eurasian teal A. crecca crecca and northern pintail A. acuta) in three French wetlands over four winters. Home-range sizes for a given species varied greatly among our study sites. Moreover, species differed according to home-range structure and distance traveled to reach their foraging grounds (teal had a more patchy home range and traveled farther distances than mallards). Foraging distances increased with temperature and time (over the winter season), but this effect differed among species, suggesting that they behave differently in response to food depletion and/or cold weather. The commuting behavior (i.e., the decision to leave the roost at night for foraging) differed among species and season. Teals were more risk-prone because they were more likely to leave the roost at night. In our study, ducks foraged at distances of 1–2 km from roosts, whereas distances of 2–48 km have been recorded in North America. We suggest that food supply, hunting pressure or population density may account for these inter-continental differences.     

Response of waterbird species to fluctuating water levels in tropical coastal wetlands

Recent upsurges in the incidence of dam construction over rivers for farming and hydro electric power in the West African sub‐region is a known promoter of fluctuating water levels on tropical coastal wetlands. Waterbirds, being one of the dominant fauna on wetlands, are key species that can be affected by fluctuating water levels. Waterbird census and water level monitoring at four coastal wetlands in Ghana revealed that different guilds (species assemblages) of waterbirds responded differently to fluctuating water levels. The populations density of birds in guilds 1 (ducks and cormorants), 3 (tactile surface foraging waders), 4 (pelagic foraging waders) and 5 (stalking herons and egrets) significantly (P < 0.05) increased linearly with decreasing water levels. The population density of birds in guilds 2 (visual surface foraging waders) and 7 (fishing terns) responded significantly (P < 0.05) in a second order polynomial function with optimum numbers occurring when water levels were neither too high nor too low. As far as farming and energy requirement are met from these dams, it is important that the ecological needs of waterbirds on wetlands are incorporated into the management of these dams so as to maintain appropriate water levels beneficial to waterbird populations.     

The Decapod Custacean Community of the Guadalquivir Estuary (SW Spain): Seasonal and Inter-Year Changes in Community Structure

Monthly samples were taken from May 1997 to March 2002 at three sampling sites within the last 32 km of the Guadalquivir Estuary. Twenty-four decapod crustacean species were recorded of which Crangon crangon (Linnaeus), Melicertus kerathurus (Forskal), and Palaemon spp. represented 99% of all collected individuals. These three dominant species showed a similar seasonal density pattern even though peaks in M. kerathurus were lower. Their densities were positively correlated (p < 0.01) with water temperature and salinity, but negatively with turbidity. The highest correlation corresponded to temperature in Palaemon spp. and to salinity in C. crangon and M. kerathurus. Therefore, the total estuarine decapod density also showed a regular seasonal pattern having the lowest figures in late autumn and winter and the highest in spring and summer. In addition, it was positively correlated with water temperature and salinity, but negatively with turbidity. Density decreased upstream, mainly due to the higher density of C. crangon and M. kerathurus in more saline waters. Non-metric multi-dimensional scaling ordination of samples also indicated a regular seasonal change in the community, even though inter-year differences between dry and rainy winters were especially great. The first ordination axis was significantly correlated with environmental variables, while the second axis seemed to split samples up following seasonal community changes in species’ composition and dominance.     

The influence of stream flow reduction on the energetics of endemic Hawaiian torrenticolous aquatic insects, <Emphasis Type="Italic">Telmatogeton</Emphasis> Schiner and <Emphasis Type="Italic">Procanace</Emphasis> Hendel

Tropical island stream ecosystems continue to be threatened by increasing anthropogenic demands for freshwater, with many streams dammed or diverted. Stream flow amendments can have substantial effects on aquatic insect populations of tropical archipelagoes. In Hawaiian streams, an endemic Diptera community of the following genera dominates cascades and other torrential habitats: Telmatogeton Schiner (Chironomidae), Procanace Hendel (Canacidae), Scatella Robineau-Desvoidy (Ephydridae). Larval densities, standing stock biomass (SSB, as ash-free dry mass [AFDM]), and monthly secondary production of Telmatogeton and Procanace were measured during two summers of significantly different stream discharge in Iao Valley, Maui, Hawaii. Very few Scatella larvae were collected (<20 larvae for both years combined), so they were omitted from subsequent analyses. Stream discharge decreased approximately 40% from 1994 to 1995, providing a ‘natural test’ of the effects of reduced stream flow on these torrenticolous populations between two years. Combined Telmatogeton and Procanace SSB (total torrential community SSB) was 3176 and 1683 mg AFDM m⁻² for 1994 and 1995, respectively, with Telmatogeton accounting for >95% in both years due to significantly larger body size and high density. The SSB of Telmatogeton significantly decreased from 3138 to 1622 mg AFDM m⁻² from 1994 to 1995 but increased for Procanace (37.6–60.9 mg AFDM m⁻², respectively). Total torrential community secondary production was 31% lower in 1994 (12,833 mg AFDM m⁻² mo⁻¹) compared to 1995 (8855 mg AFDM m⁻² mo⁻¹), reflecting the Telmatogeton proportion of total community production (99%); however, Procanace production increased by 40%. Monthly P/B ratios indicated that biomass turnover was generally high and increased for Telmatogeton from 1994 (3.8) to 1995 (5.1), whereas it remained lower and did not change between years for Procanace (1.7). A natural drought of the Iao Stream valley was associated with structural and functional changes in two endemic aquatic insects; these results are a conservative indication of permanent stream flow reductions from anthropogenic withdrawal (e.g., dams and diversions).     

Invasion of an indigenous Perna perna mussel bed on the south coast of South Africa by an alien mussel Mytilus galloprovincialis and its effect on the associated fauna

The introduced mussel Mytilus galloprovincialis is progressively increasing in abundance along the south coast of South Africa. Quantitative 0.1 m² samples were collected in the mid-zone of an indigenous Perna perna mussel bed in the 1980s prior to the arrival M. galloprovincialis (12) and in the 2000s during the M. galloprovincialis invasion (16). In addition, in situ counts of M. galloprovincialis were done on eight occasions between 1993 and 2005, and in the low- and high-zones on four occasions. In the mid-zone M. galloprovincialis was absent until 1987, its mean densities were low (< 15 individuals/0.1 m²) between 1993 and 1996, but thereafter increased steadily, peaking in 2004 (at 721 individuals/0.1 m²), before declining in 2005 (331 individuals/0.1 m²). The greatest densities of M. galloprovincialis were recorded at the high-zone (1121 individuals/0.1 m²) and the smallest in the low-zone. As M. galloprovincialis numbers increased, there was an associated, but smaller decline in P. perna numbers and the overall density of mussels increased significantly (P < 0.05). No major change was recorded in the size composition of P. perna. The density of associated fauna differed significantly (P < 0.01) between sampling dates with the lowest and highest values being recorded near the ‘beginning’ (2001) and ‘end’ (2005) of the invasion period respectively. These differences were largely due to variations in the density of barnacles, and the toothed barnacle Chthamalus dentatus appeared to be the only associated faunal species that was directly affected by the M. galloprovincialis invasion, experiencing a significantly (P ≤ 0.05), but temporary decline in density and biomass values.     

Effects of sediment type on macrobenthic infaunal colonization of laboratory microcosms

We tested the effects of four different sediment types collected from northern Gulf of Mexico estuarine systems on macroinfaunal colonization and community development in our laboratory flow-through microcosm system. Four sediments, types included , a beach sand, two fine-grained muds, but from different locations, and a 50:50 mixture of one of the mud sediments and beach sand. Our hypothesis was that the pattern of colonization would differ among sediment types based on empirical field data and theory but the differences would be expressed most strongly at sediment type extremes (e.g., mud versus sand). A total of 49 taxa colonized the four sediments. Unidentified Actiniaria (sea anemones) numerically dominated densities among all four sediments with densities ranging between 46.5 to 60.5 per microcosm (20 cm side^–1). Average taxa richness per microcosm (N: 10 replicates per sediment treatment) ranged from 10.4 in one of the mud treatments to 14.9 taxa in the sand. These were the only significant differences among sediment types (P0.05) in taxa richness and we detected no significant effects of sediment type on animal densities. Differences in community metrics, although statistically significant, were generally of a relatively small magnitude. Five of 10 microcosms per treatment were randomly selected to test for effects of sediment depth (e.g., top, mid, and bottom). In vertically sectioned microcosms, average taxa richness in sand treatments was significantly greater than those of the other three sediments. A non-parametric multivariate analysis (Primer) revealed that community structure in the vertically sectioned microcosms differed significantly between sand and one of the mud treatments. Mean taxa richness of top sections differed significantly from mid and bottom sections. We detected significantly higher animal densities and taxa richness (p0.05) in vertically sectioned versus non-sectioned microcosms. However, these differences were unexplained based on experimental protocols.