Epiphyte Load Research Articles

Response of seagrass epiphyte loading to field manipulations of fertilization, gastropod grazing and leaf turnover rates

This study evaluates the bottom-up and top-down controls on epiphyte loads under low nutrient additions. Nutrients and gastropod grazers were manipulated in a field experiment conducted within a Thalassia testudinum meadow in Florida Bay, FL, USA. The effect of seagrass leaf turnover rate on epiphyte loading was also evaluated using novel seagrass short-shoot mimics that “grow,” allowing for the manipulation of leaf turnover rates. During the summer growing season and over the course of one seagrass leaf turnover period, low-level water column nutrient additions increased total epiphyte load, epiphyte chlorophyll a, and epiphyte autotrophic index. T. testudinum leaf nutrients (N and P) and leaf productivity also increased. Epiphyte loading and T. testudinum shoot biomass and productivity did not respond to a 60% mean increase in gastropod abundance. Manipulations of seagrass leaf turnover rates at minimum wintertime and maximum summertime rates resulted in a 20% difference in epiphyte loading. Despite elevated grazer abundances and increased leaf turnover rates, epiphyte loads increased with nutrient addition. These results emphasize the sensitivity of T. testudinum and associated epiphytes to low-level nutrient addition in a nutrient-limited environment such as Florida Bay.

A comparative biodiversity study of the associated fauna of perennial fucoids and filamentous algae

Anthropogenic activities worldwide have contributed to vegetation changes in many coastal areas, changes that may in turn affect faunal and algal assemblages in the involved ecosystems. In the northernmost part of the Baltic Sea the salinity is extremely low (3–4) and the only structurally complex alga present is Fucus radicans. Since in this area F. radicans is living at its salinity tolerance limit, it is potentially very sensitive to environmental changes. Any change in salinity could thus alter the overall algal community, changing it to one dominated solely by filamentous algae. To determine the importance of F. radicans to the associated faunal community, we examined differences between the 2 main vegetation types present, i.e., F. radicans and filamentous algae, in the Kronören marine reserve in the northernmost part of the Baltic Sea. A similar study was conducted in the Askö area in the northern Baltic Proper, where the more-investigated Fucus vesiculosus is the only large fucoid present. The biomass of associated fauna was significantly higher in both the F. radicans and F. vesiculosus than in the filamentous algal vegetation at some, but not all, sites. The F. radicans community also displayed a greater diversity of associated fauna in 3 of 5 investigated Kronören sites, whereas no difference in diversity was detected between F. vesiculosus and the filamentous algal vegetations in the Askö sites. Furthermore, the F. radicans community displayed a different faunal community, being the only investigated algal community with a faunal community dominated by K-strategy species, according to abundance–biomass comparison curves. This pattern may be due to the low epiphytic load on these Fucus plants. In contrast, the F. vesiculosus community, as well as the algal communities with no Fucus in both areas, had high biomasses of filamentous algae and an invertebrate fauna dominated by Chironomidae, occurring in great abundance but only with a low biomass. ANOSIM analyses of faunal composition demonstrated a significant difference between the 2 vegetation types in both areas, largely due to greater abundance of Gammarus spp. and Theodoxus fluviatilis in the fucoid vegetation. Differences observed between the F. radicans and filamentous algal vegetation types were generally more pronounced than those between F. vesiculosus and nearby filamentous algal vegetation. These observations may be due to abiotic factors that differ between the 2 investigated areas, factors such as depth distribution, wave action and eutrophication level. This study has demonstrated that the less-investigated F. radicans may be as important as the larger F. vesiculosus for the associated faunal assemblages. At the same time, the limited extent of F. radicans at shallower depths makes F. radicans vegetation potentially more vulnerable to anthropogenic changes, as declines in fucoid vegetation are usually first manifested in populations at their lower depth limits, whereas shallow populations are less affected.

Impact of the brine from a desalination plant on a shallow seagrass ( Posidonia oceanica) meadow

Although seawater desalination has increased significantly over recent decades, little attention has been paid to the impact of the main by-product (hypersaline water: brine) on ecosystems. In the Mediterranean, potentially the most affected ecosystems are meadows of the endemic seagrass Posidonia oceanica. We studied the effect of brine on a shallow P. oceanica meadow exposed to reverse osmosis brine discharge for more than 6 years. P. oceanica proved to be very sensitive to both eutrophication and high salinities derived from the brine discharge. Affected plants showed high epiphyte load and nitrogen content in the leaves, high frequencies of necrosis marks, low total non-structural carbohydrates and low glutamine synthetase activity, compared to control plants. However, there was no indication of extensive decline of the affected meadow. This is probably due to its very shallow situation, which results in high incident radiation as well as fast dilution and dispersion of the brine plume.

Resource use and impact of three herbivorous damselfishes on coral reef communities

Territorial damselfish are considered to be functionally important herbivores on coral reefs, promoting the abundance of preferred algae and depleting corals through a variety of ‘farming’ activities. However, the composition of algal and coral communities inside territories and patterns of feeding selectivity are known for only a few species. Here we compare substrata inside and next to territories of Plectroglyphidodon lacrymatus, P. dickii and Stegastes nigricans, 3 common damselfish in Papua New Guinea, and interpret patterns in relation to diets and patterns of feeding selectivity. The coral assemblages inside territories differed among fish species and also between defended and non-defended areas. Live corals dominated P. dickii territories, while those of P. lacrymatus and S. nigricans were primarily associated with algal substrata. Territories of P. lacrymatus were similar in coral composition to adjacent areas, while those of P. dickii and S. nigricans were associated with particular coral taxa. Territories of the 3 damselfish were similar in algal composition, with mixed turf algae (primarily blue-green algae Polysiphonia spp. and other filamentous red algae) more prominent inside territories and crustose coralline algae more prominent outside. Diets for the 3 species were also similar, as each species was primarily herbivorous and diets consisted of diatoms, blue-green algae, Polysiphonia spp. and other filamentous red algae. An experimental removal of P. lacrymatus showed that they promoted epiphyte loads without influencing macro-algal or coral cover. Our results suggest that while these 3 species of damselfish vary in their interactions with coral reef habitat, they selectively cultivate a similar range of epiphytic food algae on different substrata.

Spatio‐temporal variation in the structure of a deep water Posidonia oceanica meadow assessed using non‐destructive techniques

AbstractThe Malta‐Comino Channel (Maltese islands, central Mediterranean), supports extensive meadows of the seagrass Posidonia oceanica that in some places extend to a depth of around 43 m, which is rare for this seagrass. To assess spatial and temporal variation in the state of the deeper parts of the P. oceanica meadow with time, data on the structural characteristics of the seagrass meadow at its lower bathymetric limit were collected during the summers of 2001, 2003 and 2004 from four stations (two stations within each of two sites) located at a similar depth, over a spatial extent of 500 m. Shoot density was estimated in situ, while data on plant architecture (number of leaves, mean leaf length, and epiphyte load) were successfully obtained using an underwater photographic technique that was specifically designed to avoid destructive sampling of the seagrass. Results indicated that P. oceanica shoot density was lower than that recorded from the same meadow during a study undertaken in 1995; the observed decrease was attributed to the activities of an offshore aquaculture farm that operated during the period 1995–2000 in the vicinity of the meadow. ANOVA indicated significant spatial and temporal variations in meadow structural attributes at both sites during the 3‐year study; for example, shoot density values increased overall with time at site A; a indication of potential recovery of the meadow following cessation of the aquaculture operations. Lower shoot density values recorded from site B (compared with site A) were attributed to higher epiphyte loads on the seagrass, relative to those at site A. The findings, which include new data on the structural characteristics of P. oceanica occurring at depths >40 m, are discussed with reference to the use of the non‐destructive photographic technique to monitor the state of health of deep water seagrass meadows.

Harvesting of the Kelp Ecklonia Maxima in South Africa Affects Its Three Obligate, Red Algal Epiphytes

In South Africa, more than 7000 t (f wt) of kelp (Ecklonia maxima) fronds are harvested annually to feed cultured abalone. Carpoblepharis flaccida, Gelidium vittatum and Polysiphonia virgata are conspicuous red algal epiphytes on older kelps and provide habitat and food for numerous animals. Over 4.5 y, we examined the effects of one destructive harvest of E. maxima on these 3 epiphytes. Two 20 × 20m plots of kelp with similar epiphyte loads were demarcated. In one, all E. maxima sporophytes with stipes longer than 50 cm were harvested. The other plot served as a control. After 2.5 y the biomass of E. maxima in the harvested plot had recovered to control levels, but the epiphyte load (g epiphytes. kg kelp−1) was statistically lower in the harvested plot after 2.5 and 3.5 y, and only recovered after 4.5 y. While most commercial harvesters cut through the “heads” (primary blades) of the kelp, effectively killing them, a new, non-lethal method removes secondary blades 20–30 cm from their bases, leaving the meristems and primary blades intact. At 5 sites studied, G. vittatum and P. virgata were found almost entirely on stipes and primary blades, and harvesting only distal parts of secondary blades limited losses to about 50% of C. flaccida biomass. To protect epiphytes, non-lethal harvesting is recommended and permanent non-harvest zones have been established in addition to limiting kelp yields and disallowing harvesting in Marine Protected Areas.

Habitat characteristics and spatial arrangement affecting the diversity of fish and decapod assemblages of seagrass ( Zostera marina) beds around the coast of Jersey (English Channel)

Recent research and management plans for seagrass habitats have called for landscape level approaches. The present study examines the spatial utilisation of subtidal seagrass beds by fish and decapods around the coast of Jersey (49°N 02° W). A hierarchical scale of landscape configuration and the plant characteristics of eight seagrass beds were measured and the contributions of these variables as predictors of the properties of the fish and decapod assemblages were evaluated using multiple linear regression models. The results indicated that total diversity had a negative relationship with transect heterogeneity and total species number had a weak negative association with increasing fragmentation. Both total diversity and total species number showed a positive relationship with depth. In fact, in all models of species number and densities, values were higher in deeper seagrass beds. Total decapod density increased with aggregation of seagrass patches within a landscape. In addition to landscape configuration, smaller-scale structural changes in both canopy height and epiphyte load appeared to influence densities of decapod crustaceans. At night, fewer patterns could be explained by the independent variables in the model.

Seagrass complexity hierarchies: Influence on fish groups around the coast of Jersey (English Channel)

Recent research has identified a need for seagrass habitat management plans to be based on landscape-level approaches as they offer a more appropriate scale for large mobile fauna than smaller scales. Also, conservation decisions are more likely to be a choice between different seagrass beds rather than parts of individual beds. The present study examined the spatial utilisation of subtidal seagrass beds by fish around the coast of Jersey, English Channel (49°N 02°W) with the aim of identifying influential scales of complexity. A hierarchical-scale of landscape configuration and habitat characteristics was measured for eight seagrass beds using aerial photographic analysis (e.g. core area, contiguity and other landscape metrics), digital echo-sounder data (e.g. depth, canopy height) and diver surveys (e.g. epiphyte load). The contributions of these variables as predictors of functional fish groups were explored using multiple linear regression models. Results indicated that more fragmented seagrass beds supported lower numbers of fish species than more homogenous seagrass landscapes (squared semi-partial correlation coefficient, sr 2 = − 0.3). Densities of juveniles of larger fish species showed a negative relationship with increased fragmentation of the seagrass (sr 2 = − 0.34). At smaller scales of structural complexity the densities of cryptic fish were related positively to canopy height (sr 2 = 0.46). At night, fewer patterns could be explained by the independent variables in the model, which was attributed to the greater movement of fish between the seagrass and adjacent habitats to forage, and a breakdown in the association with seagrass habitat as a refuge from predation.

Effects of territorial damselfish on an algal-dominated coastal coral reef

Territorial damselfish are important herbivores on coral reefs because they can occupy a large proportion of the substratum and modify the benthic community to promote the cover of food algae. However, on coastal coral reefs damselfish occupy habitats that are often dominated by unpalatable macroalgae. The aim of this study was to examine whether damselfish can maintain distinctive algal assemblages on a coastal reef that is seasonally dominated by Sargassum (Magnetic Island, Great Barrier Reef). Here, three abundant species (Pomacentrus tripunctatus, P. wardi and Stegastes apicalis) occupied up to 60% of the reef substrata. All three species promoted the abundance of food algae in their territories. The magnitudes of the effects varied among reef zones, but patterns were relatively stable over time. Damselfish appear to readily co-exist with large unpalatable macroalgae as they can use it as a substratum for promoting the growth of palatable epiphytes. Damselfish territories represent patches of increased epiphyte load on macroalgae, decreased sediment cover, and enhanced cover of palatable algal turf.

Nutrients increase epiphyte loads: broad-scale observations and an experimental assessment

There is a global trend towards elevated nutrients in coastal waters, especially on human-dominated coasts. We assessed local- to regional-scale relationships between the abundance of epiphytic algae on kelp (Ecklonia radiata) and nutrient concentrations across much of the temperate coast of Australia, thus assessing the spatial scales over which nutrients may affect benthic assemblages. We tested the hypotheses that (1) percentage cover of epiphytic algae would be greater in areas with higher water nutrient concentrations, and (2) that an experimental enhancement of nutrient concentrations on an oligotrophic coast, to match more eutrophic coasts, would cause an increase in percentage cover of epiphytic algae to match those in more nutrient rich waters. Percentage cover of epiphytes was most extensive around the coast of Sydney, the study location with the greatest concentration of coastal chlorophyll a (a proxy for water nutrient concentration). Elevation of nitrate concentrations at a South Australian location caused an increase in percentage cover of epiphytes that was comparable to percentage covers observed around Sydney’s coastline. This result was achieved despite our inability to match nutrient concentrations observed around Sydney (<5% of Sydney concentrations), suggesting that increases to nutrient concentrations may have disproportionately larger effects in oligotrophic waters.

Effects of herbivores on a Posidonia oceanica seagrass meadow: importance of epiphytes

There is increasing evidence that herbivory in seagrasses is more important than previ- ously thought. However, little experimental data evaluating the importance of seagrass grazing and the factors involved in these interactions is yet available. We carried out an experimental evaluation of the effects of macroherbivores (i.e. the sea urchin Paracentrotus lividus and the sparid fish Sarpa salpa) in a temperate seagrass (Posidonia oceanica) meadow by means of 2 macroherbivore density manipulation experiments in which several measurements of plant and epiphyte vitality, and abun- dance were carried out. The results show that P. oceanica can withstand high densities of sea urchins over long periods, as no significant effect on shoot density, shoot size, leaf growth or carbohydrate reserves was detected during the time course of both experiments. Conversely, epiphyte load was significantly higher in herbivore exclusion cages; even though this increase did not appear to reduce plant vitality. Furthermore, epiphyte biomass was greatly reduced when urchins were present even at relatively low densities (5 ind. m -2 ), suggesting that epiphytes are the most limiting food resource for sea urchins in P. oceanica meadows. Whereas epiphyte load was only reduced 30% by the pres- ence of fish alone, sea urchins at low densities (5 ind. m -2 ) decreased epiphyte load ca. 60%, reach- ing values as high as 80% in high density treatments, in which case the effect of fish was negligible. This study shows that in P. oceanica meadows, seagrass-herbivore interactions are further compli- cated by an underlying network of seagrass-epiphyte-herbivore, in which epiphytes appear to play a crucial role.

Nitrate reductase activity in macroalgae and its vertical distribution in macroalgal epiphytes of seagrasses

Macroalgal epiphytes within seagrass meadows make a significant contribution to total primary production by assimilating water column N and transferring organic N to sediments. Assim- ilation of NO3 - requires nitrate reductase (NR, EC 1.6.6.1); NR activity represents the capacity for NO3 - assimilation. An optimised in vitro assay for determining NR activity in algal extracts was applied to a wide range of macroalgae and detected NR activity in all 22 species tested with activity 2 to 290 nmol NO3 - min -1 g -1 frozen thallus. With liquid-N2 freezing immediately after sample collec- tion, this method was practical for estimating NR activity in field samples. Vertical distribution of NR activity in macroalgal epiphytes was compared in contrasting Posidonia sinuosa and Amphibolis antarctica seagrass meadows. Epiphytes on P. sinuosa had higher mass-specific NR activity than those on A. antarctica. In P. sinuosa canopies, NR activity increased with distance from the sediment surface and was negatively correlated with (NH4 + ) in the water but uncorrelated with (NO3 - ). This supported the hypothesis that NH4 + released from the sediment suppresses NR in epiphytic algae. In contrast, the vertical variation in NR activity in macroalgae on A. antarctica was not statistically sig- nificant although there was a weak correlation with (NO3 - ), which increased with distance from the sediment. Estimated capacities for NO3 - assimilation in macroalgae epiphytic on seagrasses during summer (24 and 46 mmol N m -2 d -1 for P. sinuosa and A. antarctica, respectively) were more than twice the estimated N assimilation rates in similar seagrasses. When the estimates were based on annual average epiphyte loads for seagrass meadows in other locations, they were comparable to those of seagrasses. We conclude that epiphytic algae represent a potentially important sink for water-column nitrate within seagrass meadows.

The Use of Size as an Estimator of Age in the Subantarctic Cushion Plant, Azorella selago (Apiaceae)

Plant species with morphological features that enable the age of individuals to be estimated are potentially useful for understanding past environmental conditions. Here, the size and growth rate of the cushion plant, Azorella selago Hook. (Apiaceae), are examined to determine if an accurate and reliable age can be assigned to individual plants using the phytometric model detailed by Frenot et al. (1993). Plant size, growth rate, epiphyte load, nearest-neighbor characteristics, and spatial position (used as a surrogate, to encompass a range of abiotic conditions to which plants were exposed) were measured at three sites. Additionally, variation in some of these variables was quantified across three altitudinal transects. Relationships were examined using regression, trend surface and partial regression approaches. Growth rate was independent of plant size, differed between sites, and was related to abiotic as well as other biotic factors. As a result, the phytometric model’s age estimates may be biased by environmental variables. The results of the phytometric model, albeit in the absence of support for one assumption, estimated mean plant age at 30 yr, with some plants estimated as older than 80 yr. Using a simulation model, the accuracy of age estimates was shown to vary with temporal variation in plant growth rate and plant size. Nonetheless, even a conservative approach suggested these estimates to be accurate to within 2 to 15 yr. While further development of the phytometric model would improve its reliability, the model remains a valuable tool for estimating plant ages in an environment where many related techniques can not be applied.

Fine scale variation in microarthropod communities inhabiting the keystone species Azorella selago on Marion Island

Invertebrates contribute significantly to nutrient cycling on sub-Antarctic islands and thus their distribution patterns are of considerable interest. Few studies have, however, investigated the deterministic nature of fine-scale patterns in arthropod communities. This study investigated the relationship between the fine-scale distribution and abundance of mites (Acari: Arachnida) and springtails (Collembola: Hexapoda) in Azorella selago Hook. f. (Apiaceae) on Marion Island, and plant size, isolation, within-plant variability and epiphyte load. Microarthropod abundances were significantly higher on the southern, cold, dry, less frequently wind-blown sides of plants. Abundances were also significantly higher in association with the dominant epiphyte, a likely consequence of increased resource availability. No effects of cushion size or isolation on abundance or species richness were found. This study thus demonstrates that fine-scale variation in the microarthropod community is deterministic, a likely consequence of biotic and abiotic factors, and therefore of importance in the context of rapid climate change.

Drift algae-epiphyte-seagrass interactions in a subtropical Thalassia testudinum meadow

Determination of the percentage cover and residence time of drift algae over perma- nently marked 0.25 m 2 plots at 8 study sites indicated that drift algal cover was relatively low (<20% at most sites) and did not remain in the same location longer than 2 to 3 mo. Experimental mainte- nance of high drift algal cover for 2 to 3 mo produced a ca. 25% reduction in above-ground biomass compared to plots free of drift algae, but the magnitude of the decline in biomass did not increase with an additional 3 mo of algal cover. Below-ground biomass and shoot density, however, were not affected by the presence of drift algae, suggesting that a change in plant morphometrics contributed to reduced above-ground biomass and limited long-term negative impacts as root reserves were not depleted. We investigated the effects of drift algal cover on short-term growth of Thalassia tes- tudinum at 2 different sites in 2 different seasons. In neither instance did we observe any decrease in growth rates of T. testudinum in the presence of drift algae. The lack of a negative effect of drift algae on seagrass growth is likely due to the clonal nature of the seagrass plants, their ability to share resources among shaded and unshaded short shoots, and the availability of stored reserves in robust roots and rhizomes. We also conducted growth experiments assessing the effects of epiphytes on growth of T. testudinum. Unlike drift algal cover, the presence of encrusting calcareous epiphytes was shown to have a negative impact on growth of T. testudinum. This is likely due to a combination of decreased light levels as well as a reduction in exchange of materials across the boundary layer at the leaf surface. Collections of seagrass leaves from under drift algae and from areas free of drift algae indicated that filamentous epiphyte loads are decreased in the presence of macroalgae. Our results suggest that temporary, moderate cover of macroalgae may benefit seagrass by reducing epiphyte loads if the epiphyte cover negatively impacts the seagrass.

Importance of within-shoot epiphyte distribution for the carbon budget of seagrasses: the example of Posidonia oceanica

To assess the effects of shading by epiphytes on the carbon balance of the seagrass Posidonia oceanica, we examined the patterns of within-shoot epiphyte abundance on leaves, and their variability with season and depth. Epiphyte biomass was found to depend on leaf age (larger epiphyte load on older tissues), leaf side (more epiphytes on the inner face than on the outer) and depth (more epiphytes in deep meadows). Depth differences were maximum in spring and disappeared in late summer. Percent light absorbed (absorptance) by epiphytes was measured; light absorptance followed an exponential- saturated model with epiphyte biomass. Combining these data of percent light absorbed, within-shoot epiphyte biomass distribution and an existing carbon balance model, we conclude that reduction in carbon gains caused by epiphyte shading is relatively small, and greater in deep meadows (8.8% on average, with values up to 14.2% in May) than that in shallow water (4.7% on average with maximum values of 7% in August). This is mainly due to the accumulation of epiphytes on old tissues, which contribute only marginally to the carbon gain of the plant. Using the same procedure, we modelled the effects of a doubling in epiphyte biomass, conserving the observed within-shoot distribution. The result was a very small additional carbon loss. However, using the same biomass but modifying the distribution (shifting the same abundance towards younger leaf age classes), the reduction in carbon gains was dramatic, particularly in deep meadows (between 21% and 41%, depending on the shift used). Therefore, it is the epiphyte growth timing rate rather than the final biomass reached which seems to be a key control for Posidonia oceanica survival, especially near the deep limit of its distribution.

Effects of epiphyte load on optical properties and photosynthetic potential of the seagrasses Thalassia testudinum Banks ex König and Zostera marina L.

The biomass and optical properties of seagrass leaf epiphytes were measured to evaluate their potential impact on the photosynthetic performance of the seagrasses Thalassia testudinum Banks ex König (turtlegrass) and Zostera marina L. (eelgrass). Turtlegrass was obtained from oligotrophic waters near Lee Stocking Island, Bahamas; eelgrass was collected from a eutrophic environment in Monterey Bay, California. Leaf‐epiphyte loads were characterized visually and quantified using measurements of their phospholipid biomass. Light absorption and reflectance of the intact epiphyte layer were determined spectrophotometrically. Turtlegrass epiphytes from the oligotrophic site absorbed a maximum of 36% of incident light in peak chlorophyll absorption bands, whereas higher epiphyte loads on eelgrass from the more eutrophic Monterey Bay absorbed 60% of incident light in peak chlorophyll absorption bands. The combination of intact epiphyte‐leaf complexes and spectral measurements enabled us to construct a quantitative relationship between epiphyte biomass and light attenuation, and, by extension, between epiphyte biomass and seagrass photosynthesis. The model yielded a robust, positive relationship between epiphyte biomass and the absorption of photons in photosynthetically important wavelengths, and it generated a strong negative relationship between epiphyte biomass and spectral photosynthesis of their seagrass hosts. Furthermore, the calculations of photosynthesis highlighted the significant differences between PAR and spectral models of photosynthesis, illustrating that the spectral quality of the incident flux must be considered when evaluating the effects of epiphyte load on seagrass leaf photosynthesis. Verification of the model—using direct measurements of photosynthesis and a variety of epiphyte and macrophyte combinations from different locations—is warranted.

Field biology of Halimeda tuna (Bryopsidales, Chlorophyta) across a depth gradient: comparative growth, survivorship, recruitment, and reproduction

Growth, survivorship, recruitment, and reproduction of Halimeda tuna, a dominant green alga in many reef systems of the Florida Keys, were monitored at a shallow back reef (4–7m) and deep reef slope (15–22 m) on Conch Reef. Despite lower light intensities and similar grazing pressures, amphipod infestations, and epiphyte loads at both sites, the deeper site exhibited significantly higher growth rates in summer months over a 4-year period than found for the shallow population, possibly because of higher nutrient levels at depth and photoinhibition of shallow plants. Sexual reproductive events occurred simultaneously across the entire reef, with up to 5% of the population at both sites developing gametangia. New upright axes formed from zygotes, asexual fragmentation, or vegetative runners. Plants appear to have persistent basal stumps that survive harsh environmental conditions, even if upright, photosynthetic axes are removed. Sexual reproduction and ‘smothering’ by epiphyte overgrowth are hypothesized to be two causes of death for individuals.

Use of Native Forest and Eucalyptus Plantations by Eleutherodactylus Frogs

We examined population densities, movements, and habitat use of common coqui (Eleutherodactylus coqui), the most common and widespread species of frog in Puerto Rico, in native forest and eucalyptus plantations in 2 forest reserves of the Cordillera Central between July 1997 and June 1998. We established 4 grids of bamboo retreat sites on stands of native forest and eucalyptus plantations in each reserve. We established sampling points within trapping grids to quantify vegetation and determine habitat use. Sampling was conducted during the wet and dry seasons. We used mark-recapture for population estimation. Population estimates and maximum distances moved were determined using program CAPTURE. During the dry season, common coqui population densities were similar in 3 of the 4 study grids, but 1 eucalyptus site had significantly lower densities. We found no differences in adult frog density among study sites during the wet season. Horizontal movements differed between forests, albeit not significantly. Our results indicate that plant species richness did not affect common coqui density. Coqui density was influenced more by vegetation structure than by plant species composition. Five habitat variables (midstory epiphyte load, midstory stem count, ground-cover stem count, total epiphyte load, total basal area) were correlated with adult common coqui density. Eucalyptus plantations in montane regions of Puerto Rico provide suitable habitat for the common coqui. However, suitability may be linked to threshold basal area levels of 18-28 m 2 /ha.

Epiphyte host preferences and host traits: mechanisms for species-specific interactions.

We investigated species-specific relationships among two species of vascular epiphytes and ten host tree species in a coastal plain forest in the southeastern United States. The epiphytes Tillandsia usneoides and Polypodium polypodioides were highly associated with particular host species in the field, but host traits that favored colonization were inadequate to fully explain the epiphyte-host associations for either epiphyte. Field transplant experiments that bypassed epiphyte colonization demonstrated that the growth of epiphytes was significantly higher on host tree species that naturally bore high epiphyte loads than on host species with few or no epiphytes. These species-specific relationships were highly correlated with the water-holding capacity of the host tree's bark. Positive and negative effects of throughfall, light attenuation by the canopy, and bark stability did not explain the overall patterns of host specificity, but did correlate with some epiphyte-host species relationships. The relative importance of particular host traits differed between the "atmospheric epiphyte" Tillandsia, and the fern Polypodium, which roots in the bark of its hosts. Species-specific interactions among plants, such as those described here, suggest that communities are more than individualistic assemblages of co-occurring species.