Wayne P. Sousa

DISTURBANCE IN MARINE INTERTIDAL BOULDER FIELDS: THE NONEQUILIBRIUM MAINTENANCE OF SPECIES DIVERSITY'

The effects of disturbance on local species diversity were investigated in an algal- dominated intertidal boulder field in southern California. In this habitat, the major form of disturbance occurs when waves, generated by winter storms, overturn boulders. These natural physical distur- bances open space, interrupt successional sequences, and determine local levels of species diversity. Because small boulders are more frequently overturned than larger ones, the plants and sessile animals of boulder fields are distributed in a patchwork of successional stages. Boulders which are subjected to intermediate disturbance frequencies are usually less dominated than those which are very frequently disturbed, and always less dominated than those which are seldom disturbed. In all seasons most small boulders have fewer species than those of intermediate size. Large boulders also usually have fewer species, except in the spring, when defoliation of the algal canopy during the previous winter has opened space for colonization. Species richness on these boulders declines during summer months, and is less than that on boulders of intermediate size in the fall. Small boulders, with a shorter disturbance interval, support only sparse early successional com- munities of the green alga, U/va, and barnacles. Large, infrequently disturbed boulders are dominated by the late successional red alga, Gigartina canaliculata. Intermediate-sized boulders support the most diverse communities composed of U/va, barnacles, several middle successional species of red algae, and Gigartina canaliclaata. Comparison of the pattern of succession on experimentally sta- bilized boulders with that on unstable ones confirms that differences in the frequency of disturbance are responsible for the above patterns of species composition. The frequency of disturbance also determines the degree of between-boulder variation in species composition and diversity. Small boulders which are frequently overturned sample the available pool of spores and larvae more often. As a result, a greater number of different species occur as single dominants on these boulders. Boulders with an intermediate probability of being disturbed are most variable in species diversity. Assemblages on these boulders range from being dominated by a single species to being very diverse while most communities on boulders which are frequently or seldom disturbed are strongly dominated. Observations on the local densities of three species of middle successional red algae over two year-long periods indicate that most of these are variable in time. More local populations went extinct or became newly established on boulders than remained constant in size. These species persist glob- ally in the boulder field mosaic by colonizing recent openings created by disturbances. These results lend support to a nonequilibrium view of community structure and, along with other studies suggest that disturbances which open space are necessary for the maintenance of diversity in most commu- nities of sessile organisms.

Experimental Investigations of Disturbance and Ecological Succession in a Rocky Intertidal Algal Community

Mechanisms of ecological succession were investigated by field experiments in a rocky intertidal algal community in southern California. The study site was an algal-dominated boulder field in the low intertidal zone. The major form of natural disturbance which clears space in this system is the overturning of boulders by wave action. Algal populations recolonize cleared surfaces either through vegetative regrowth of surviving individuals or by recruitment from spores. Boulders which are experimentally cleared and concrete blocks are colonized within the first month by a mat of the green alga, Ulva. In the fall and winter of the first year after clearing, several species of perennial red algae including Gelidium coulteri, Gigartina leptorhynchos, Rhodoglossum affine, and Gigartina canaliculata colonize the surface. If there is no intervening disturbance, Gi- gartina canaliculata gradually dominates the community holding 60-90W% of the cover after a period of 2 to 3 years. If undisturbed, this monoculture persists through vegetative reproduction, resisting invasion by all other species. During succession diversity increases initially as species colonize a bare surface but declines later as one species monopolizes the space. Several contemporary theories concerning the mechanisms of ecological succession were tested. The early successional alga, Ulva, was found to inhibit the recruitment of perennial red algae. This competition for settling space is an important feature of the successional process. Ulva is the best competitor for this space; it reproduces throughout the year and quickly becomes established on newly cleared substrates. As long as these early colonists remain healthy and undamaged, they preempt colonization by perennial red algae which have highly seasonal recruitment and slower growth. Selective grazing on Ulva by the crab, Pachygrapsus crassipes, breaks this inhibition and accel- erates succession to a community of long-lived red algae. Grazing by small molluscs, especially limpets, has no long-term effect on the successional sequence. Their grazing temporarily enhances the recruitment of the barnacle, Chthamalus fissus, by clearing space in the mat of algal sporelings and diatoms which develops on recently denuded rock surfaces. Where locally abundant, middle successional red algae also slow the invasion and growth of the late successional dominant, Gigartina canaliculata. This alga replaces middle successional species because it is less susceptible to damage by desiccation and overgrowth by epiphytes. The results of this study do not support either the classical facilitation model or the tolerance (competitive) model of ecological succession. Once early colonists secure the available space/light, they resist rather than facilitate the invasion of subsequent colonists. Early colonists are not killed by direct interference competition with late successional species which grow up through their canopy; rather, early colonists can successfully inhibit the recruitment and growth of these species. Succes- sional sequences occur because species which dominate early in a succession are more susceptible to the rigors of the physical environment and to attacks by natural enemies than late successional species. Late species colonize and grow to maturity when early species are killed and space is opened. Only late in a successional sequence, when large clearings become a mosaic of small openings, does direct competition with surrounding adult plants of late successional species contribute to the decline in cover of the remaining early species. Studies of succession in a number of terrestrial and marine communities lend support to this inhibition model.

Integration of object-based and pixel-based classification for mapping mangroves with IKONOS imagery

IKONOS 1-m panchromatic and 4-m multispectral images were used to map mangroves in a study site located at Punta Galeta on the Caribbean coast of Panama. We hypothesized that spectral separability among mangrove species would be enhanced by taking the object as the basic spatial unit as opposed to the pixel. Three different classification methods were investigated: maximum likelihood classification (MLC) at the pixel level, nearest neighbour (NN) classification at the object level, and a hybrid classification that integrates the pixel and object-based methods (MLCNN). Specifically for object segmentation, which is the key step in object-based classification, we developed a new method to choose the optimal scale parameter with the aid of Bhattacharya Distance (BD), a well-known index of class separability in traditional pixel-based classification. A comparison of BD values at the pixel level and a series of larger scales not only supported our initial hypothesis, but also helped us to determine an optimal scale at which the segmented objects have the potential to achieve the best classification accuracy. Among the three classification methods, MLCNN achieved the best average accuracy of 91.4%. The merits and restrictions of pixel-based and object-based classification methods are discussed.

Host life history and the effect of parasitic castration on growth: A field study of Cerithidea californica Haldeman (Gastropoda : Prosobranchia) and its trematode parasites

Abstract The prevalence of parasitic infection by larval digenetic trematodes in natural populations of the mud snail, Cerithidea californica Haldeman, was found to increase with snail length; all snails ≥ 33 mm were infected. Distributions of infections by the seven most common larval trematodes were heterogeneous due to two species being more common than expected in the smaller size classes of snails, two being more common than expected in the larger size-classes of snails and three species being most prevalent in snails of intermediate length. The relative abundances of trematodes in different size-classes reflected these distributional patterns. A mark-recapture field study of snail growth rates failed to demonstrate that parasitic infection causes gigantism in Cerithidea. Parasitism tended to stunt the growth of juvenile snails and to a lesser degree, that of adult snails. The effects of trematodes on snail growth was shown to be species specific. This finding contrasts with those of earlier studies in which gigantic growth was observed in infected snails. This discrepancy is attributed to differences in the life histories of the host snails. It is predicted that gigantism will occur commonly in short-lived or semelparous species of snails but rarely, if ever, in long-lived iteroparous species which are predominately marine.

Widespread density-dependent seedling mortality promotes species coexistence in a highly diverse Amazonian rain forest

Negative density-dependent mortality can promote species coexistence through a spacing mechanism that prevents species from becoming too locally abundant. Negative density-dependent seedling mortality can be caused by interactions among seedlings or between seedlings and neighboring adults if the density of neighbors affects the strength of competition or facilitates the attack of natural enemies. We investigated the effects of seedling and adult neighborhoods on the survival of newly recruited seedlings for multiple cohorts of known age from 163 species in Yasuni National Park, Ecuador, an ever-wet, hyper-diverse lowland Amazonian rain forest. At local scales, we found a strong negative impact on first-year survival of conspecific seedling densities and adult abundance in multiple neighborhood sizes and a beneficial effect of a local tree neighborhood that is distantly related to the focal seedling. Once seedlings have survived their first year, they also benefit from a more phylogenetically dispersed seedling neighborhood. Across species, we did not find evidence that rare species have an advantage relative to more common species, or a community compensatory trend. These results suggest that the local biotic neighborhood is a strong influence on early seedling survival for species that range widely in their abundance and life history. These patterns in seedling survival demonstrate the role of density-dependent seedling dynamics in promoting and maintaining diversity in understory seedling assemblages. The assemblage-wide impacts of species abundance distributions may multiply with repeated cycles of recruitment and density-dependent seedling mortality and impact forest diversity or the abundance of individual species over longer time scales.

Latitudinal variation in intertidal algal community structure: the influence of grazing and vegetative propagation

SummaryThe hypothesis that sea urchin grazing and interactions with turf-forming red algae prevent large brown algae from forming an extensive canopy in the low intertidal zone of southern California was tested with field experiments at two study sites. Experimental removal of sea urchins resulted in rapid algal recruitment. Crustose coralline algae which typically dominate the substratum in areas with dense urchin populations were quickly overgrown by several species of short-lived green, brown and red algae. The removal of urchins also significantly increased the recruitment of two long-lived species of large brown algae (Egregia laevigata and Cystoseira osmundacea at one study site and E. laevigata and Halidrys dioica at the other). The experimental plots at both sites were eventually dominated by perennial red algae.A two-factorial experiment demonstrated that sea urchin grazing and preemption of space by red algae in areas where urchins are less abundant are responsible for the rarity of large brown algae in the low intertidal of southern California. The three dominant perennial red algae, Gigartina canaliculata, Laurencia pacifica and Gastroclonium coulteri, recruit seasonally from settled spores but can rapidly fill open space with vigorous vegetative growth throughout the year. These species encroach laterally into space created by the deaths of large brown algae or by other disturbances. Once extensive turfs of these red algae are established further invasion is inhibited. This interaction of algae which proliferate vegetatively with algae which recruit only from settled spores is analogous to those which occur between solitary and colonial marine invertebrates and between solitary and cloning terrestrial plants.It is suggested that a north-south gradient in the abundance of vegetatively propagating species, in grazing intensity and in the frequency of space-clearing disturbances, may account for latitudinal variation in intertidal algal community structure along the Pacific coast of North America.

Patterns and processes in communities of heirninth parasites

What processes account for the structure and dynamics of helminth parasite assemblages within vertebrate and invertebrate hosts? Attempts to answer this question form the basis for the emerging subdiscipline of parasite community ecology. Negative interspecific interactions strongly affect the distribution and abundance of parasites in some systems, but only intermittently, if ever, in others. Empirical results provide only mixed support for recent theories that attempt to explain this variation.

SUPPLY‐SIDE ECOLOGY IN MANGROVES: DO PROPAGULE DISPERSAL AND SEEDLING ESTABLISHMENT EXPLAIN FOREST STRUCTURE?

Theory and empirical evidence suggest that spatial and temporal variation in propagule availability can have as great or greater an influence on community structure as post-recruitment biotic interactions, a phenomenon known as supply-side ecology. One of the first theories to invoke supply-side dynamics, D. Rabinowitzs Tidal Sorting Hypothesis (TSH), attributed the tidal zonation of mangrove tree species to the interacting effects of water depth and propagule size on dispersal and establishment. According to the TSH, smaller propagules are carried farther inland by flood tides than larger ones, stranding and establishing in greater numbers at upper tidal elevations. In contrast, larger propagules are better able to establish in deeper, more seaward areas because their size affords greater access to the soil surface and resistance to buffeting by moving water. Here we present results of the first comprehensive investigation of the TSH. In a series of experiments conducted at Punta Galeta on the Caribbean coast of Panama, we quantified patterns of propagule dispersal and establishment of the three dominant species: Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle. To measure dispersal patterns, we monitored movements (directions and distances) of marked propagules released at three elevations along each of three transects. On all transects, regardless of species or elevation, propagules moved seaward rather than being carried inland by tides, as predicted by the TSH. Rates of seedling establishment were monitored in experiments conducted at different tidal elevations. Contrary to the TSH, all three species established best in the lower intertidal, where they were in prolonged contact with the soil surface during low tides, and established more poorly in the upper intertidal basin, where standing water made rooting difficult. Seedling establishment was lowest in the wave-exposed, back reef habitat, but in accordance with the TSH, the large propagules of Rhizophora rooted and persisted better in this turbulent habitat than did the smaller propagules of the other species. We conclude that the TSH does not explain the vertical distributions of mangrove species on Punta Galeta. Rather, a different form of supply-side dynamics imposed by the strong seaward movement of dispersing propagules is important in structuring our study forests.

High-fidelity national carbon mapping for resource management and REDD+

BackgroundHigh fidelity carbon mapping has the potential to greatly advance national resource management and to encourage international action toward climate change mitigation. However, carbon inventories based on field plots alone cannot capture the heterogeneity of carbon stocks, and thus remote sensing-assisted approaches are critically important to carbon mapping at regional to global scales. We advanced a high-resolution, national-scale carbon mapping approach applied to the Republic of Panama – one of the first UN REDD + partner countries.ResultsIntegrating measurements of vegetation structure collected by airborne Light Detection and Ranging (LiDAR) with field inventory plots, we report LiDAR-estimated aboveground carbon stock errors of ~10% on any 1-ha land parcel across a wide range of ecological conditions. Critically, this shows that LiDAR provides a highly reliable replacement for inventory plots in areas lacking field data, both in humid tropical forests and among drier tropical vegetation types. We then scale up a systematically aligned LiDAR sampling of Panama using satellite data on topography, rainfall, and vegetation cover to model carbon stocks at 1-ha resolution with estimated average pixel-level uncertainty of 20.5 Mg C ha-1 nationwide.ConclusionsThe national carbon map revealed strong abiotic and human controls over Panamanian carbon stocks, and the new level of detail with estimated uncertainties for every individual hectare in the country sets Panama at the forefront in high-resolution ecosystem management. With this repeatable approach, carbon resource decision-making can be made on a geospatially explicit basis, enhancing human welfare and environmental protection.

Distinguishing mangrove species with laboratory measurements of hyperspectral leaf reflectance

As a first step in developing classification procedures for remotely acquired hyperspectral mapping of mangrove canopies, we conducted a laboratory study of mangrove leaf spectral reflectance at a study site on the Caribbean coast of Panama, where the mangrove forest canopy is dominated by Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle. Using a high‐resolution spectrometer, we measured the reflectance of leaves collected from replicate trees of three mangrove species growing in productive and physiologically stressful habitats. The reflectance data were analysed in the following ways. First, a one‐way ANOVA was performed to identify bands that exhibited significant differences (P value<0.01) in the mean reflectance across tree species. The selected bands then formed the basis for a linear discriminant analysis (LDA) that classified the three types of mangrove leaves. The contribution of each narrow band to the classification was assessed by the absolute value of standardised coefficients associated with each discriminant function. Finally, to investigate the capability of hyperspectral data to diagnose the stress condition across the three mangrove species, four narrow band ratios (R 695/R 420, R 605/R 760, R 695/R 760, and R 710/R 760 where R 695 represents reflectance at wavelength of 695nm, and so on) were calculated and compared between stressed and non‐stressed tree leaves using ANOVA. Results indicate a good discrimination was achieved with an average kappa value of 0.9. Wavebands at 780, 790, 800, 1480, 1530, and 1550 nm were identified as the most useful bands for mangrove species classification. At least one of the four reflectance ratio indices proved useful in detecting stress associated with any of the three mangrove species. Overall, hyperspectral data appear to have great potential for discriminating mangrove canopies of differing species composition and for detecting stress in mangrove vegetation.