33 research outputs found
Legacy Effects of Canopy Disturbance on Ecosystem Functioning in Macroalgal Assemblages
Macroalgal assemblages are some of the most productive systems on earth and they contribute significantly to nearshore ecosystems. Globally, macroalgal assemblages are increasingly threatened by anthropogenic activities such as sedimentation, eutrophication and climate change. Despite this, very little research has considered the potential effects of canopy loss on primary productivity, although the literature is rich with evidence showing the ecological effects of canopy disturbance. In this study we used experimental removal plots of habitat-dominating algae (Order Fucales) that had been initiated several years previously to construct a chronosequence of disturbed macroalgal communities and to test if there were legacy effects of canopy loss on primary productivity. We used in situ photo-respirometry to test the primary productivity of algal assemblages in control and removal plots at two intertidal elevations. In the mid tidal zone assemblage, the removal plots at two sites had average primary productivity values of only 40% and 60% that of control areas after 90 months. Differences in productivity were associated with lower biomass and density of the fucoid algal canopy and lower taxa richness in the removal plots after 90 months. Low-shore plots, established three years earlier, showed that the loss of the large, dominant fucoid resulted in at least 50% less primary productivity of the algal assemblage than controls, which lasted for 90 months; other smaller fucoid species had recruited but they were far less productive. The long term reduction in primary productivity following a single episode of canopy loss of a dominant species in two tidal zones suggests that these assemblages are not very resilient to large perturbations. Decreased production output may have severe and long-lasting consequences on the surrounding communities and has the potential to alter nutrient cycling in the wider nearshore environment
Polychaete invader enhances resource utilization in a species-poor system
Ecosystem consequences of biodiversity change are often studied from a species loss perspective, while the effects of invasive species on ecosystem functions are rarely quantified. In this experimental study, we used isotope tracers to measure the incorporation and burial of carbon and nitrogen from a simulated spring phytoplankton bloom by communities of one to four species of deposit-feeding macrofauna found in the species-poor Baltic Sea. The recently invading polychaete Marenzelleriaarctia, which has spread throughout the Baltic Sea, grows more rapidly than the native species Monoporeia affinis, Pontoporeia femorata (both amphipods) and Macoma balthica (a bivalve), resulting in higher biomass increase (biomass production) in treatments including the polychaete. Marenzelleria incorporated and buried bloom material at rates similar to the native species. Multi-species treatments generally had higher isotope incorporation, indicative of utilization of bloom material, than expected from monoculture yields of the respective species. The mechanism behind this observed over-yielding was mainly niche complementarity in utilization of the bloom input, and was more evident in communities including the invader. In contrast, multi-species treatments had generally lower biomass increase than expected. This contrasting pattern suggests that there is little overlap in resource use of freshly deposited bloom material between Marenzelleria and the native species but it is likely that interference competition acts to dampen resulting community biomass. In conclusion, an invasive species can enhance incorporation and burial of organic matter from settled phytoplankton blooms, two processes fundamental for marine productivity
Rare Species Support Vulnerable Functions in High-Diversity Ecosystems
Around the world, the human-induced collapses of populations and species have triggered a sixth mass extinction crisis, with rare species often being the first to disappear. Although the role of species diversity in the maintenance of ecosystem processes has been widely investigated, the role of rare species remains controversial. A critical issue is whether common species insure against the loss of functions supported by rare species. This issue is even more critical in species-rich ecosystems where high functional redundancy among species is likely and where it is thus often assumed that ecosystem functioning is buffered against species loss. Here, using extensive datasets of species occurrences and functional traits from three highly diverse ecosystems (846 coral reef fishes, 2,979 alpine plants, and 662 tropical trees), we demonstrate that the most distinct combinations of traits are supported predominantly by rare species both in terms of local abundance and regional occupancy. Moreover, species that have low functional redundancy and are likely to support the most vulnerable functions, with no other species carrying similar combinations of traits, are rarer than expected by chance in all three ecosystems. For instance, 63% and 98% of fish species that are likely to support highly vulnerable functions in coral reef ecosystems are locally and regionally rare, respectively. For alpine plants, 32% and 89% of such species are locally and regionally rare, respectively. Remarkably, 47% of fish species and 55% of tropical tree species that are likely to support highly vulnerable functions have only one individual per sample on average. Our results emphasize the importance of rare species conservation, even in highly diverse ecosystems, which are thought to exhibit high functional redundancy. Rare species offer more than aesthetic, cultural, or taxonomic diversity value; they disproportionately increase the potential breadth of functions provided by ecosystems across spatial scales. As such, they are likely to insure against future uncertainty arising from climate change and the ever-increasing anthropogenic pressures on ecosystems. Our results call for a more detailed understanding of the role of rarity and functional vulnerability in ecosystem functioning
Habitat selection, facilitation, and biotic settlement cues affect distribution and performance of coral recruits in French Polynesia
Habitat selection can determine the distribution and performance of individuals if the precision with which sites are chosen corresponds with exposure to risks or resources. Contrastingly, facilitation can allow persistence of individuals arriving by chance and potentially maladapted to local abiotic conditions. For marine organisms, selection of a permanent attachment site at the end of their larval stage or the presence of a facilitator can be a critical determinant of recruitment success. In coral reef ecosystems, it is well known that settling planula larvae of reef-building corals use coarse environmental cues (i.e., light) for habitat selection. Although laboratory studies suggest that larvae can also use precise biotic cues produced by crustose coralline algae (CCA) to select attachment sites, the ecological consequences of biotic cues for corals are poorly understood in situ. In a field experiment exploring the relative importance of biotic cues and variability in habitat quality to recruitment of hard corals, pocilloporid and acroporid corals recruited more frequently to one species of CCA, Titanoderma prototypum, and significantly less so to other species of CCA; these results are consistent with laboratory assays from other studies. The provision of the biotic cue accurately predicted coral recruitment rates across habitats of varying quality. At the scale of CCA, corals attached to the “preferred” CCA experienced increased survivorship while recruits attached elsewhere had lower colony growth and survivorship. For reef-building corals, the behavioral selection of habitat using chemical cues both reduces the risk of incidental mortality and indicates the presence of a facilitator
Top-down modification of bottom-up processes: selective grazing reduces macroalgal nitrogen uptake
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Top-down modification of bottom-up processes: selective grazing reduces macroalgal nitrogen uptake
Although nutrient availability and oceanographic context can influence top-down processes in intertidal communities, the reciprocal effects of consumers on bottom-up processes, such as nitrogen uptake by macroalgae, are less well-documented. We used a combination of field observations, mesocosm experiments, and laboratory nitrate-uptake measurements to evaluate the interaction between the kelp crab Pugettia producta and its preferred food source, the kelp Egregia menziesii, in northern California, USA. We found that R producta fed selectively on E. menziesii, removing tissues with a high surface area to volume ratio (SA:V), which resulted in a 66% decrease in overall SA:V. Because these high SA:V tissues are disproportionately more responsible for nitrate uptake, selective herbivory by P. producta decreased E. menziesii's biomass-specific uptake of nitrogen, the primary limiting nutrient in this ecosystem, by 65% per gram of remaining algal tissue. In field surveys, where P. producta abundances were high, E. menziesii thalli more frequently exhibited signs of intense grazing and thus had lower SA:V ratios, suggesting that grazing is sufficiently intense in the field to generate differences in algal morphology and nitrate uptake. P. producta therefore influences E. menziesii biomass and productivity from the top down, by removing substantial amounts of biomass, and from the bottom up, by reducing the kelp's subsequent ability to acquire nutrients. Because of selective grazing on specialized tissues, the effects of mesoherbivores, like P. producta, on the structure and dynamics of marine communities may often be greater than predicted simply by the biomass they consume
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Quantifying the top-down effects of grazers on a rocky shore: Selective grazing and the potential for competition
The effect of grazers on the diversity, distribution, and composition of their principal food source has rarely been described for the high intertidal zone of rocky shores, a model system for studying the potential effects of climate change. Along rocky, wave-swept shores in central California, the microphytobenthos (MPB) supports diverse assemblages of limpets and littorine snails, which, at current benign temperatures, could potentially partition food resources in a complementary fashion, thereby enhancing secondary productivity. Two limpet species in particular, Lottia scabra and L. austrodigitalis, may partition components of the MPB, and are likely to affect the composition of the MPB on which they graze. In this study, we describe the composition, nutritional value (C:N ratio), and fluorescence (an index of chlorophyll density) of ungrazed, L. scabragrazed and L. austrodigitalis-grazed MPB, each as a function of temperature. Fluorescence de-creased with increased average daily maximum temperature for ungrazed MPB, but temperature had no discernible effects on either fluorescence or the composition of the MPB of grazed assemblages. L. austrodigitalis and L. scabra did not partition the MPB, and did not exhibit complementarity. Both species exhibited an ordered grazing scheme, in which limpets grazed down certain components of the MPB before others, and grazing increased the C:N ratio of the MPB, decreasing its nutritional value. Taken together, these results suggest that L. austrodigitalis and L. scabra may experience increased competition as warming temperatures reduce the available MPB