Community Recovery After Storm Damage: A Case of Facilitation in Primary Succession

Urchin grazing of kelp gametophytes in warming oceans

Sea urchins can cause extensive damage to kelp forests, and their overgrazing can create extensive barren areas, leading to a loss of biodiversity. Barrens may persist when the recruitment of kelp, which occurs through the microscopic haploid gametophyte stage, is suppressed. However, the ecology of kelp gametophytes is poorly understood, and here we investigate if grazing by juvenile urchins on kelp gametophytes can suppress kelp recruitment and if this is exacerbated by climate change. We compared grazing of Ecklonia radiata gametophytes by two species of juvenile urchins, the tropical Tripneustes gratilla and the temperate Centrostephanus rodgersii, at winter (19°C), summer (23°C), and ocean warming (26°C) temperatures for the low-latitude range edge of E. radiata, which is vulnerable to ocean warming. We examined the rate of recovery of gametophytes following grazing and determined whether they survived and formed sporophytes after ingestion by sea urchins. Both T. gratilla and C. rodgersii grazed E. radiata gametophytes, reducing their abundance compared to no grazing controls. Surprisingly, temperature did not influence grazing rates, but gametophytes did not recover from grazing in the ocean warming (26°C) treatment. Gametophytes survived ingestion by both species of sea urchin and formed sporophytes after ingestion by T. gratilla, but not C. rodgersii. These results suggest complex grazer-gametophyte interactions, in which both negative (reduced abundance and poor recovery with warming) and positive (facilitated recruitment) effects are possible. Small grazers may play a more important role in kelp ecosystem function than previously thought and should be considered in our understanding of alternate stable states.

Environmental context dependency in species interactions

Ecological interactions are not uniform across time and can vary with environmental conditions. Yet, interactions among species are often measured with short-term controlled experiments whose outcomes can depend greatly on the particular environmental conditions under which they are performed. As an alternative, we use empirical dynamic modeling to estimate species interactions across a wide range of environmental conditions directly from existing long-term monitoring data. In our case study from a southern California kelp forest, we test whether interactions between multiple kelp and sea urchin species can be reliably reconstructed from time-series data and whether those interactions vary predictably in strength and direction across observed fluctuations in temperature, disturbance, and low-frequency oceanographic regimes. We show that environmental context greatly alters the strength and direction of species interactions. In particular, the state of the North Pacific Gyre Oscillation seems to drive the competitive balance between kelp species, asserting bottom-up control on kelp ecosystem dynamics. We show the importance of specifically studying variation in interaction strength, rather than mean interaction outcomes, when trying to understand the dynamics of complex ecosystems. The significant context dependency in species interactions found in this study argues for a greater utilization of long-term data and empirical dynamic modeling in studies of the dynamics of other ecosystems.

Small mammal assemblage in beaver-modified habitats

One of the less studied positive interactions among organisms is facilitation. Facilitation may have significant impact on diversity of species especially in low productive environments. We studied the effects of well-known facilitator and ecosystem engineer, the North American beaver (Castor canadensis), on small mammal assemblages in the boreal zone. The small mammals, namely voles, mice, and shrews, were trapped over 2 years in ten beaver-modified habitats and in ten control sites. Contrary to our expectations, we did not observe any differences between species or individual numbers between beaver-modified and control sites. However, there were differences in species composition between sites; grass-eating field voles (Microtus agrestis) and invertebrate-eating shrews (Sorex araneus, Neomys fodiens) tended to be more common in beaver sites while forest-dwelling wood lemmings (Myopus schisticolor) and yellow-necked mice (Apodemus flavicollis) were only captured in control sites. The most common species in both habitats was the bank vole (Myodes glareolus), which is a generalist in its habitat requirements. The bank vole’s population structures were similar between the two habitat types. The actions of beavers in water bodies within boreal forests seem to have no effect on the small mammal diversity and their numbers at the regional scale but may have positive effect on them at the larger landscape level as beavers are increasing the overall habitat diversity in the landscape.

Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls

The construction of artificial structures, such as seawalls, is increasing globally, resulting in loss of habitat complexity and native species biodiversity. There is increasing interest in mitigating this biodiversity loss by adding topographic habitat to these structures, and/or seeding them with habitat-forming species. Settlement tile experiments, comparing colonisation of species to more and less complex habitats, have been used to inform eco-engineering interventions prior to their large-scale implementation. Most studies have focused on applying one type of intervention (either adding habitat structure or seeding with native organisms), so it is unclear whether there are greater benefits to biodiversity when multiple interventions are combined. Using a fully orthogonal experiment, we assessed the independent and interactive effects of habitat structure (flat vs. crevice/ridges) and seeding with native oysters (unseeded vs. seeded) on the biodiversity of four different functional groups (sessile and mobile taxa, cryptobenthic and pelagic fishes). Concrete tiles (flat unseeded, flat seeded, complex unseeded and complex seeded) were deployed at two sites in Sydney Harbour and monitored over 12 months, for the survival and colonisation of oysters and the species density and abundances of the four functional groups. The survival of seeded oysters was greater on the complex than flat tiles, at one of the two sites, due to the protective role of crevices. Despite this, after 12 months, the species density of sessile invertebrates and the percentage cover of seeded and colonising oysters did not differ between complex and seeded tiles each of which supported more of these variables than the flat unseeded tiles. In contrast, the species density of mobile invertebrates and cryptobenthic fishes and the MaxN of pelagic fishes, at 1 month, were only positively influenced by seeding with oysters, which provided food as well as habitat. Within the complex seeded and unseeded tiles, there was a greater species density of sessile taxa, survival and percentage cover of oysters in the crevices, which were more humid and darker at month 12, had lower high temperature extremes at months 1 and 12, than on the ridges or flat tiles. Our results suggest that eco-engineering projects which seek to maximise the biodiversity of multiple functional groups on seawalls, should apply a variety of different microhabitats and habitat-forming species, to alter the environmental conditions available to organisms.

Tropicalization strengthens consumer pressure on habitat-forming seaweeds

Ocean warming is driving species poleward, causing a 'tropicalization' of temperate ecosystems around the world. Increasing abundances of tropical herbivores on temperate reefs could accelerate declines in habitat-forming seaweeds with devastating consequences for these important marine ecosystems. Here we document an expansion of rabbitfish (Siganus fuscescens), a tropical herbivore, on temperate reefs in Western Australia following a marine heatwave and demonstrate their impact on local kelp forests (Ecklonia radiata). Before the heatwave there were no rabbitfish and low rates of kelp herbivory but after the heatwave rabbitfish were common at most reefs and consumption of kelp was high. Herbivory increased 30-fold and kelp abundance decreased by 70% at reefs where rabbitfish had established. In contrast, where rabbitfish were absent, kelp abundance and herbivory did not change. Video-analysis confirmed that rabbitfish were the main consumers of kelp, followed by silver drummers (Kyphosus sydneyanus), a temperate herbivore. These results represent a likely indirect effect of the heatwave beyond its acute impacts, and they provide evidence that range-shifting tropical herbivores can contribute to declines in habitat-forming seaweeds within a few years of their establishment.

Phase shift facilitation following cyclone disturbance on coral reefs

While positive interactions have been observed to influence patterns of recruitment and succession in marine and terrestrial plant communities, the role of facilitation in macroalgal phase shifts is relatively unknown. In December 2012, typhoon Bopha caused catastrophic losses of corals on the eastern reefs of Palau. Within weeks of the typhoon, an ephemeral bloom of monospecific macroalgae (Liagora sp.) was observed, reaching a peak of 38.6% cover in February 2013. At this peak, we observed a proliferation of a second macroalgal species, Lobophora variegata. Lobophora was distributed non-randomly, with higher abundances occurring within the shelter of Liagora canopies than on exposed substrates. Bite rates of two common herbivorous fish (Chlorurus sordidus and Ctenochaetus striatus) were significantly higher outside canopies (2.5- and sixfold, respectively), and cage exclusion resulted in a significant increase in Lobophora cover. Experimental removal of Liagora canopies resulted in a 53.1% decline in the surface area of Lobophora after 12 days, compared to a 51.7% increase within canopies. Collectively, these results indicate that Liagora canopies act as ecological facilitators, providing a 'nursery' exclusion zone from the impact of herbivorous fish, allowing for the establishment of understory Lobophora. While the ephemeral Liagora bloom had disappeared entirely 9 months post-typhoon, the facilitated shift to Lobophora has persisted for over 18 months, dominating ~40% of the reef substrate. While acute disturbance events such as typhoons have been suggested as a mechanism to reverse algal phase shifts, our results suggest that typhoons may also trigger, rather than just reverse, phase shifts.

Herbivory affects salt marsh succession dynamics by suppressing the recovery of dominant species

Disturbance can generate heterogeneous environments and profoundly influence plant diversity by creating patches at different successional stages. Herbivores, in turn, can govern plant succession dynamics by determining the rate of species replacement, ultimately affecting plant community structure. In a south-western Atlantic salt marsh, we experimentally evaluated the role of herbivory in the recovery following disturbance of the plant community and assessed whether herbivory affects the relative importance of sexual and clonal reproduction on these dynamics. Our results show that herbivory strongly affects salt marsh secondary succession by suppressing seedlings and limiting clonal colonization of the dominant marsh grass, allowing subordinate species to dominate disturbed patches. These results demonstrate that herbivores can have an important role in salt marsh community structure and function, and can be a key force during succession dynamics.

Experimental evaluation of the anti-attachment effect of microalgal mats on grazing activity of the sea urchin Strongylocentrotus nudus in oscillating flows

Algal mats can hinder the adhesion of the tube feet of sea urchins. This leads to the hypothesis that the restriction of sea urchin feeding activity by wave action can potentially be enhanced by the presence of algal mats, which will facilitate the survival of kelp recruits at sites with wave action in urchin barrens. To evaluate the potential anti-attachment effect of algal mats on sea urchins, a laboratory tank experiment was performed on the movement of Strongylocentrotus nudus sea urchins and their grazing on juvenile kelp plants at the center of 30×30 cm flat test substrates with or without a thin-layer microalgal mat at four levels of oscillatory flow (maximum orbital velocity: 10, 20, 30 and 40 cm s(-1)). The grazing loss of kelp slightly increased with increasing velocity up to 30 cm s(-1) in the absence of microalgal mats, while in contrast the loss substantially decreased at 30 cm s(-1) in their presence. Sea urchins were dislodged more frequently at 20 cm s(-1) or higher velocities in the presence of microalgal mats. Mats were frequently abraded by scraping by the adoral spines during urchin movement at high velocities (30 and 40 cm s(-1)) but were subject to no or only slight urchin grazing in most cases. The results indicate that the overall decrease in grazing loss of kelp within the microalgal mats was attributable to the anti-attachment effect on urchins during incursions rather than due to urchins grazing on the mats.

Roles of transport and mixing processes in kelp forest ecology

Fluid-dynamic transport and mixing processes affect birth, death, immigration and emigration rates in kelp forests, and can modulate broader community interactions. In the most highly studied canopy-forming kelp, Macrocystis pyrifera (the giant kelp), models of hydrodynamic and oceanographic phenomena influencing spore movement provide bounds on reproduction, quantify patterns of local and regional propagule supply, identify scales of population connectivity, and establish context for agents of early life mortality. Other analyses yield insight into flow-mediated species interactions within kelp forests. In each case, advances emerge from the use of ecomechanical approaches that propagate physical-biological connections at the scale of the individual to higher levels of ecological organization. In systems where physical factors strongly influence population, community or ecosystem properties, such mechanics-based methods promote crucial progress but are just beginning to realize their full potential.

Algal populations controlled by fish herbivory across a wave exposure gradient on southern temperate shores

Consumers that forage across habitats can affect communities by altering the abundance and distribution of key species. In marine communities, studies of trophic interactions have generally focused on the effects of herbivorous and predatory invertebrates on benthic algae and mussel populations. However, large mobile consumers that move across habitats, such as fishes, can strongly affect community dynamics through consumption of habitat-dominating species, but their effects often vary over environmental gradients. On temperate rocky shores, herbivorous fishes are generally a small part of the fish fauna compared to the tropics, and there is sparse evidence that they play a major direct role in algal community dynamics, particularly of large brown algae that dominate many reefs. In New Zealand, however, a wide-ranging herbivorous fish, Odax pullus, feeds exclusively on macroalgae, including Durvillaea antarctica, a large low-intertidal fucoid reaching 10 m in length and 70 kg in mass. In four experiments we tested the extent of fish herbivory and how it was affected by algal canopy structure across a gradient of wave exposure at multiple sites. Exclusion experiments showed that fish impacts greatly reduced the cover and biomass of Durvillaea and that these effects decreased with increasing wave stress and algal canopy cover, effectively restricting the alga to exposed conditions. Almost all plants were entirely removed by fish where there was a sparse algal canopy in sheltered and semi-exposed sites, but there was significantly less grazing in exposed sites. Recruit Durvillaea beneath canopies were less affected by fish grazing, but they grew slowly. Successful natural recruitment, therefore, occurred almost exclusively on exposed shores outside canopies where many plants escaped severe grazing, and growth to maturity was far greater than elsewhere. Such large and direct impacts on the local and regional distribution of large brown algal populations by mobile vertebrate consumers are rare and were mediated by an environmental gradient and plant density, both of which interact with algal demographics. The study highlights that, even though herbivorous fish diversity may be low, the impacts of particular species may still be high, even in cool temperate waters where fish herbivory is usually considered to be minimal.

Multiple controls of community structure and dynamics in a sublittoral marine environment

The structure and dynamics of ecological communities can be determined by both top-down (e.g., predation) and bottom-up (e.g., energy inputs) processes, which can act synergistically and across spatial and temporal scales. Here we aimed at understanding the role of multiple controls in a Mediterranean rocky sublittoral marine community that harbors a diverse algal community and strongly interacting herbivores, and which is subject to marked seasonality in energy inputs. We conducted an experiment by manipulating densities of the major consumers of benthic algae (fishes and sea urchins) in approximately 100-m2 enclosures in a marine reserve, and monitored algal assemblages over two and a half years. Most algae showed a marked annual cycle, with a biomass peak in late spring/summer and low biomass in winter, following seasonal fluctuations in resource availability, indicating the existence of bottom-up processes. Sea urchins reduced the abundance of most algal species, indicating the existence of top-down processes. The effect of fish grazing on algal abundances was significantly weaker. Sea urchin grazing was inhibited when predatory fish were present. Multivariate analysis showed that the interaction between seasonal resource inputs and herbivory induced the formation of algal assemblages characterized by different species abundances. The organization of algal assemblages was determined by the synergistic interaction between top-down and bottom-up processes: top-down control regulated total algal abundance, while bottom-up control determined seasonal fluctuations.

Detached kelps from distant sources are a food subsidy for sea urchins

Trophic subsidies link habitats and can determine community structure in the subsidised habitats. Knowledge of the spatial extents of trophic interactions is important for understanding food webs, and for making spatial management practices more efficient. We demonstrate trophic linkages between detached (drift) fragments of the kelp Ecklonia radiata and the purple sea urchin Heliocidaris erythrogramma among discrete rocky reefs separated by kilometres. Sea urchins were abundant at one inshore reef, where the biomass of drift was usually high. There, sea urchins trapped detached kelp at high rates, although local kelp abundance was low. Most detached kelp present on the reef was retained by sea urchins. Detached seagrass, which was abundant on the reef, was not retained by sea urchins in large quantities. Experiments with tethered pieces of kelp showed that sea urchins only consumed detached fragments, and did not consume attached kelps. Comparisons of the morphology of detached fragments of kelp collected from the inshore reef to attached kelps from reefs further offshore showed that a large proportion (30-95%, varying among dates) of the fragments originated at distant reefs (>or=2 km away). At the inshore reef, the sea urchin H. erythrogramma is subsidised by detached kelps, and detached kelp fragments have been transported across landscapes. Cross-habitat resource subsidies therefore link discrete reef habitats separated by kilometres of non-reef habitat.

Genetic structure and reproduction dynamics of Salix reinii during primary succession on Mount Fuji, as revealed by nuclear and chloroplast microsatellite analysis

The early stage of volcanic desert succession is underway on the southeastern slope of Mount Fuji. We used markers of nuclear microsatellites (simple sequence repeats; SSR) and chloroplast microsatellites (cpSSR) to investigate the population genetic structure and reproduction dynamics of Salix reinii, one of the dominant pioneer shrubs in this area. The number of S. reinii genets in a patch and the area of the largest genet within the patch increased with patch area, suggesting that both clonal growth and seedling recruitment are involved in the reproduction dynamics of S. reinii. Five polymorphic cpSSR markers were developed for S. reinii by sequencing the noncoding regions between universal sequences in the chloroplast genome. Nineteen different cpSSR haplotypes were identified, indicating that S. reinii pioneer genets were created by the long-distance dispersal of seeds originating from different mother genets around the study site, where all vegetation was destroyed during the last eruption. Furthermore, the clustered distributions of different haplotypes within each patch or plot suggested that newly colonized genets tended to be generated from seeds dispersed near the initially established mother genets. These results revealed that the establishment of the S. reinii population on the southeastern slope of Mount Fuji involved two sequential modes of seed dispersal: long-distance dispersal followed by short-distance dispersal.

Correlation between CA19-9 production in vitro and histological grades of differentiation in vivo in clones isolated from a human pancreatic cancer cell line (SUIT-2)

The production of carcino-embryonic antigen (CEA) and carbohydrate antigen 19-9 (CA19-9) were investigated in 28 clones isolated from a human pancreatic cancer cell line (SUIT-2) and related to in vitro morphology of the clones and in vivo tumorigenicity. Clones of fusiform and polygonal cells could be morphologically distinguished in confluent cultures. There was no significant difference in CEA production between fusiform-cell clones (2.10 +/- 2.70 ng/L x 10(6) per 24 h) and polygonal-cell clones (6.01 +/- 7.30 ng/L x 10(6) per 24 h), but polygonal-cell clones had higher production of CA19-9 (1176.1 +/- 1628.4 U/L x 10(6) per 24 h) than fusiform (6.0 +/- 7.3 U/L x 10(6) per 24 h; P < 0.01). Production of CA19-9 in vitro correlated with the histological grade of differentiation in vivo in nude mice (r = 0.73, P < 0.001), but CEA production did not. The polygonal-cell clones developed well-differentiated carcinomas in vivo and produced significantly more CA19-9 (P < 0.001) than fusiform-cell clones, which generally developed into poorly differentiated tubular adenocarcinomas in vivo. This cell line may provide an appropriate system for further studies of the biology and therapy of pancreatic cancer.

High-Resolution Climatic Analysis and Southwest Biogeography

Meteorologists and climatologists have produced significant new data on the fluid dynamics of the atmosphere, thus allowing biologists to examine more closely the cause-effect relation between the large-scale structure of the atmosphere and the dominant patterns of global biogeography. The inability to characterize the high-frequency variability of the weather has constrained such efforts. A method that allows year-to-year patterns of weather variability to be characterized in the contexts of global warming and cooling trends is applied in a combined analysis of long-term monthly weather records and data from an ecological monitoring project in southern New Mexico. The analysis suggests a cause-effect hypothesis of recent desertification in the North American Southwest. The links between the atmosphere and the biosphere are based on the fundamentally different responses to specific weather regimes of semidesert grasses with a C4photosynthetic pathway and desert shrubs with a C3photosynthetic pathway. The hypothesis appears to be of sufficient generality to explain the complex, but well-documented, floristic changes that have occurred in the same region since the last glacial maximum.

Disturbance and Ecologic Succession in an Upper Ordovician Cobble-Dwelling Hardground Fauna

An Ordovician hardground fauna in northern Kentucky provides an example from the fossil record of the maintenance of species diversity by periodic disturbance of an autogenic ecologic succession. The marine invirtebrates found encrusting limestone cobbles show an order of community development from a low-diversity pioneer assemblage through a high-diversity association to a monospecific stable fauna. All species, including the late successional dominants, were present in the early stages of colonization. Periodic overturning of the cobbles killed the encrusters and opened up new spaces on which succession was reinitiated. These disturbances maintained high diversity within the encrusting community by limiting the spatial distribution of the most efficient space competitors.