Propagule Predation Research Articles

Limited Mangrove Propagule Retention at a Latitudinal Range Limit: Spatiotemporal Patterns at the Patch Scale

Dispersal and establishment dynamics are critical in understanding shifts in species’ ranges. We seek to illuminate patch-level dispersal dynamics by examining the shifting salt marsh-mangrove ecotone. Specifically, we ask the following: (1) How are mangrove propagules dispersed, retained, and exported within a discrete patch? (2) How do differences across a flooding gradient influence propagule dispersal dynamics? (3) How does the distribution of established seedlings compare to propagule movements? Avicennia germinans is the most temperate mangrove species in the northern Gulf of Mexico forming an ecotone with Spartina alterniflora marshes in coastal Louisiana. Sets of 500 distinctively marked mangrove propagules were placed at five different elevations. After their release, we observed dispersal dynamics for 1 month. Retention was limited in the study area (< 10%) with ~ 70–80% of propagules exporting out of the system and ~ 20% propagule predation. Retained propagules largely remained at their original elevations and were generally found at the highest elevation. Seedling establishment was also studied and unlike propagule dispersal distributions, peak seedling density occurred at elevations flooded 20–40% of the time. Our study highlights the mass export of mangrove propagules, the disparity between dispersal and establishment dynamics, and the need to explore dispersal at biologically relevant temporal and patch-level spatial scales. By understanding dispersal and establishment dynamics within the ecotonal boundary, we provide one of the first studies on dispersal at a temperature-controlled latitudinal limit for mangroves and highlight some of the drivers needed to better connect plot-, patch-, and landscape-level dynamics at this and other range limits.

Restoration versus natural regeneration in a neotropical mangrove: Effects on plant biomass and crab communities

Mangrove restoration is a powerful tool for reconstructing degraded tropical estuaries worldwide. The level of intervention necessary for rebuilding a mangrove area is still under study because this system is quite dynamic and some degraded areas are able to recover by themselves. We compared a Restored Area planted with Rhizophora mangle and a Self-recovered Area, to investigate how they would differ in respect to their plant and crab assemblages. In each area, 10 plots were randomly sampled for measuring tree richness, density, diameter at breast height and biomass, as well as crab richness and density after five years from recover. Plant height and biomass, as well as crab density, were significantly higher in the Restored in relation to the Self-recovered Area. However, tree richness was higher in the Self-recovered Area and crab assemblage reached in five years a similar composition in assisted, naturally recovered and natural areas, showing the high resilience of this important functional group. Active planting of R. mangle propagules can significantly improve mangrove recovery in sites with high propagule predation, increasing tree biomass and accelerating the return of animal functional groups such as crabs. However, if resources for active restoration are scarce, passive recovery can be a successful low cost alternative for mangrove restoration that preserves tree diversity and crab assemblages.

Propagule predation by crabs limits establishment of an endemic mangrove

Recruitment of seedlings is crucial to the establishment and maintenance of plant populations and species ranges. Seed predation by crabs is prevalent in mangroves globally, but the importance of predation on mangrove abundance and distribution relative to other factors remain unclear. The rare Neotropical mangrove Pelliciera rhizophorae occurs in freshwater-influenced sites, and may lose many of its propagules to crab predation in other locations. Propagules of P. rhizophorae were placed in either Rhizophora mangle or P. rhizophorae-dominated forests on the Caribbean and Pacific coasts of Panama. On the Caribbean coast, crabs consumed 86.7% of propagules in the R. mangle forest but only 3.3% of propagules in the P. rhizophorae zone. On the Pacific coast, crabs consumed 90% of propagules in the R. mangle zone and 66.7% in the P. rhizophorae zone. When crab exclosures were used, seedling survival did not vary between forest types, suggesting that predation, rather than seedling survival once established, limits P. rhizophorae populations. Rhizophora mangle produces an order of magnitude more propagules per capita than P. rhizophorae and may well satiate its crab predators. The low salinity forest stands may provide a refuge from crab predation for the rare mangrove P. rhizophorae.

What regulates crab predation on mangrove propagules?

Crabs play a major role in some ecosystems. To increase our knowledge about the factors that influence crab predation on propagules in mangrove forests, we performed experiments in Gazi Bay, Kenya in July 2009. We tested whether: (1) crab density influences propagule predation rate; (2) crab size influences food competition and predation rate; (3) crabs depredate at different rates according to propagule and canopy cover species; (4) vegetation density is correlated with crab density; (5) food preferences of herbivorous crabs are determined by size, shape and nutritional value. We found that (1) propagule predation rate was positively correlated to crab density. (2) Crab competitive abilities were unrelated to their size. (3) Avicennia marina propagules were consumed more quickly than Ceriops tagal except under C. tagal canopies. (4) Crab density was negatively correlated with the density of A. marina trees and pneumatophores. (5) Crabs prefer small items with a lower C:N ratio.Vegetation density influences crab density, and crab density affects propagule availability and hence vegetation recruitment rate. Consequently, the mutual relationships between vegetation and crab populations could be important for forest restoration success and management.

Feeding ecology of mangrove crabs in Cameroon

The feeding ecology of several species of mangrove crabs was studied in Cameroon mangrove forest through tethering experiments and gut analyses from September 2010 to March 2011. Field observations were two-fold: to investigate mangrove propagules predation and feeding preferences of mangrove leaves. Feeding preference was determined on fresh and senescent leaves of Laguncularia racemosa, Avicennia germinans and Rhizophora species. Propagule predation was investigated on Rhizophora mangle, Rhizophora harrisonii and Rhizophora racemosa. In the laboratory, gut content analysis was done on Metagrapsus curvatus, Sesarma huzardi, Sesarma elegans, Sesarma alberti, Goniopsis pelii and Grapsus grapsus to assess their natural diets. The field observations suggest that 65.9% of the propagules studied was predated, 71.3% of the leaves was damaged. Mangrove material was the main component of crabs diet, it constituted 45.4% of Metagrapsus curvatus diet, 55% Sesarma elegans diet, 62.5% Sesarma alberti, 65.9% Goniopsis pelii diet, 47.8% Sesarma huzardi diet and 37.3% Grapsus grapsus diet. Damage on young mangrove trees was highly correlated to the number of crabs on trees (r2 = 0.75). The high damage rate of mangrove materials indicate that crabs play a significant ecological role in the functioning of the mangrove ecosystem, by affecting mangrove recruitment and biogeochemical functioning.

Propagule predation in a Neotropical mangrove: the role of the Grapsid crab Goniopsis cruentata

In Neotropical mangroves the crabs Ucides cordatus and Goniopsis cruentata have been considered the most significant propagule consumers, but their relative importance has not been investigated. The aim of this study was to compare the magnitude of predation by these crabs on three mangrove species propagules: Avicennia schaueriana, Laguncularia racemosa and Rhizophora mangle. We found that G. cruentata is a more important predator than U. cordatus in both natural and restored areas. We also tested the hypothesis that Ucides and Goniopsis have antagonistic effects on propagules predation using a cage experiment where the presence/absence of these species was manipulated in a 2 × 2 factorial design. The effects of Goniopsis were stronger in the absence of Ucides due to negative interactions between these predator species. Moreover, we found that Goniopsis preference for A. schaueriana and L. racemosa can favor the dominance of R. mangle in Neotropical mangroves. This study suggests that propagule predation by Goniopsis should be controlled in mangrove restoration programs at abandoned shrimp farms and destroyed areas, if dominance by R. mangle is undesirable relative to mixed species communities.

Human hydrographical changes interact with propagule predation behaviour in Sri Lankan mangrove forests

For mangroves with almost no tidal influence and a subsequent mosaic vegetation structure we describe the relation between propagule predators and both vegetation structure and environmental factors on a vegetation assemblage or ‘forest patch’ level. We then report the interaction between natural propagule predation and human influence. In different forest patches dominated by Avicennia officinalis, Excoecaria agallocha, Lumnitzera racemosa, Rhizophora apiculata or R. mucronata or in open or mixed areas identified from aerial photographs, the predation on propagules of A. officinalis, B. gymnorrhiza, R. apiculata and R. mucronata was monitored in a total of 24 experimental plots (3 per forest patch). Relationships to environmental factors (topography, water level, rainfall and season) on the level of predation were investigated. The highest predation intensity by crabs was on A. officinalis propagules, and in E. agallocha forest patches. Predation by crabs, snails, insects and mammals was observed, and the significant differences in predation intensity across species and across forest patches were explained directly by propagule predator identity and abundance, and by the interaction between rainfall, water level and microtopography. We illustrated how hydrography changes alter the behaviour of propagule predators and play a role in the shaping of vegetation structure by reconstructing the lagoon water level of a site over a period of 50 years based on rainfall data. This time frame covered both a condition without and with human impacts, and past and present propagule predation settings were confronted with the simulated hydrology. The importance of spatial and temporal microhabitat variations in opening multiple successional pathways in vegetation dynamics is illustrated, and is highly relevant for ecosystems with unpredictable or short-lived (< 20 years) patchy vegetation structures and microhabitats.

The growth of Kandelia candel seedlings in mangrove habitats of the Zhangjiang estuary in Fujian, China

Abstract Propagules of Kandelia candel collected from the Zhangjiang estuary were planted in mangrove habitats along the intertidal gradient. The rooting rates of K. candel propagules varied spatially. The lowest rate occurred in Avicennia marina forest (69.7%). The rates were higher in K. candel forest (90.0%), at the fringe of the mangrove forest (89.3%) and on the bare tidal flat outside the mangrove forest (82.7%). After one year, the survival rates of seedlings planted under A. marina forest, K. candel forest, at the fringe of the mangrove forest, and on the bare tidal flat were 13.7%, 54.7%, 76.0%, and 34.7%, respectively. Among the surviving K. candel seedlings, those at the fringe of the mangrove forest and on the bare tidal flat had greater height, stem diameter, leaf number, leaf area, and biomass than those under A. marina and K. candel forests. These results demonstrated that establishment and growth of K. candel seedlings occurred successfully at the fringe of the mangrove forest, but were the worst under A. marina forest. The performance of K. candel seedlings was independent of physico-chemical characters of sediment. However, interspecies competition, propagule predation by insects and crabs, and the incident light had significant effects on seedling survival and growth.

Predation on propagules regulates regeneration in a high-density reforested mangrove plantation

Seedling recruitment underpins natural regeneration and contributes to the restocking of forest stands, which in the long term determines the structure and functioning of mangrove ecosys- tems. Propagule predation (among other factors) has been found to play an important role in deter- mining seedling distribution patterns in many mangrove stands. The role of propagule predation in influencing the observed regeneration in a Rhizophora mucronata reforested stand (9 yr old) with various spontaneously established mangrove species was investigated by looking at the effects of pruning, position (planted/vertical or prone/horizontal on the forest floor to simulate stranding) and species (Rhizophora mucronata, Ceriops tagal and Bruguiera gymnorrhiza) found to be sponta- neously growing in the plantation. Predation intensity was higher (p = 0.05) in the pruned part of the stand than in the unpruned part. Prone propagules were preyed on more (p = 0.05) than the planted category, while R. mucronata was the species least preyed on compared to C. tagal and B. gymnor- rhiza, suggesting that predation favors the recruitment of R. mucronata into this conspecific stand. Significant differences among prone and planted propagules suggest that predation is intense during the stranding phase, and this may play a critical role in limiting seedling establishment and subse- quent recruitment. Predation is suggested to be a regulator of competition in this high-density stand by eliminating or reducing potential competitors, alluding to a 'mutual relationship' between crab predators and mangrove trees.

Regeneration of Rhizophora mangle in a Caribbean mangrove forest: interacting effects of canopy disturbance and a stem-boring beetle.

Current theory predicts that in low-density, seed-limited plant populations, seed predation will be more important than competition in determining the number of individuals that reach maturity. However, when plant density is high, competition for microsites suitable for establishment and growth is expected to have a relatively greater effect. This dichotomous perspective does not account for situations in which the risk of seed predation differs inside versus outside recruitment microsites. We report the results of a field experiment and sampling studies that demonstrate such an interaction between microsite quality and the risk of propagule predation in mangrove forests on the Caribbean coast of Panama, where it appears to play a key role in shaping the demography and dynamics of the mangrove, Rhizophora mangle. Rhizophora's water-borne propagules establish wherever they strand, but long-term sampling revealed that only those that do so in or near lightning-created canopy gaps survive and grow to maturity. These microsites afford better growth conditions than the surrounding understory and, as importantly, provide a refuge from predation by the scolytid beetle, Coccotrypes rhizophorae. This refuge effect was confirmed with a field experiment in which Rhizophora seedlings were planted at different positions relative to gap edges, from 5 m inside to 20 m outside the gap. Mortality due to beetle attack increased linearly from an average of 10% inside a gap to 72% at 20 m into the forest. The interaction between canopy disturbance and propagule predation may be having a large impact on the composition of our study forests. Being shade-tolerant, Rhizophora seedlings that escape or survive beetle attack can persist in the understory for years. However, the high rate of beetle-induced mortality effectively eliminates the contribution of advance regeneration by Rhizophora saplings to gap succession. This may explain why the shade-intolerant mangrove, Laguncularia racemosa, is able to co-dominate the canopy in low intertidal forests at our study sites.

Clonal diversity and structure within a population of the pondweed Potamogeton pectinatus foraged by Bewick's swans.

Clonal diversity within plant populations is affected by factors that influence genet (clone) survival and seed recruitment, such as resource availability, disturbance, seed dispersal mechanism, propagule predation and the age of the population. Here we studied a population of Potamogeton pectinatus, a pseudo-annual aquatic macrophyte. Within populations reproduction appears to be mainly asexually through subterranean propagules (tubers), while recruitment via seeds is believed to be relatively unimportant. RAPD markers were used to analyse clonal diversity and genetic variation within the population. Ninety-seven genets were identified among 128 samples taken from eight plots. The proportion of distinguishable genets (0.76) and Simpson's diversity index (0.99) exhibited high levels of clonal diversity compared to other clonal plants. According to an analysis of molecular variance (amova) most genetic variation occurred between individuals within plots (93-97%) rather than between plots (8-3%). These results imply that sexual reproduction plays an unexpectedly important role within the population. Nevertheless, autocorrelation statistics revealed a spatial genetic structure resulting from clonal growth. In contrast to genetic variation, clonal diversity was affected by several ecological factors. Water depth and silt content had direct negative effects on clonal diversity. Tuber predation by Bewick's swans had an unexpected indirect negative effect on clonal diversity through reducing the tuber-bank biomass in spring, which on its turn was positively correlated to clonal diversity. The disturbance by swans, therefore, did not enhance seed recruitment and thus clonal diversity; on the contrary, heavily foraged areas are probably more prone to stochastic loss of genets leading to reduced clonal diversity.

Factors influencing the regeneration of the mangrove Bruguiera gymnorrhiza (L.) Lamk. on a tropical Pacific island

Mangrove swamps occupy approximately two-thirds of the shoreline on Kosrae, Federated States of Micronesia (FSM), and also border the island’s most populated areas. Kosraeans depend on mangrove swamps for a supply of wood to support a growing handicraft industry, for a dependable source of fuelwood, and for habitat to support the harvest of fish and mangrove crabs. One of the more prominent mangrove species on Kosrae is Bruguiera gymnorrhiza, yet it is not the most preferred species for carving or cooking. To evaluate B. gymnorrhiza’s persistence in the intertidal and to develop a better understanding of factors influencing its regeneration, we investigated predispersal insect colonization of propagules, postdispersal propagule predation by crabs, and the relative effects of natural and artificial shade, salinity, and tidal flooding on early tree seedling survival and growth. Predispersal insect colonization of propagules by boring insects was very high (93%), but the damage did not seem to influence seedling survival. Postdispersal predation of B. gymnorrhiza propagules by crabs was low (17%) and did not change in gap versus understory plots. Predation did vary by intertidal location (lower intertidal>middle intertidal=upper intertidal), with lower predation occurring in an intertidal location with a B. gymnorrhiza-dominated overstory. Shade and tidal inundation reduced seedling growth more than salinity in greenhouse investigations, but sunlight had less positive influence on seedling growth in the field. In general, regeneration and growth occurred successfully under a variety of conditions, indicating that none of the factors investigated serve as strong regulators to B. gymnorrhiza regeneration and early growth on Kosrae.

The importance of propagule predation in a forest of nonindigenous mangrove trees

Predation on propagules ofRhizophora mangle was compared in forests where this species is non-indigenous (Hawai’i, USA) and where it is native (American Samoa). Tree density and basal area of the non-indigenous stand were intermediate when compared to natural stands in other places whereRhizophora is common. Propagules were tethered on the forest floor at both sites for 14 days, predation was recorded, and survival rates of both attacked and control individuals were determined by placing them in a mist room. Mortality was significantly greater in American Samoa (x=25%) than in Hawai’i x=8%). The lower effectiveness of the non-indigenous predators in Hawai’i may help explain the unusually high rate at whichR. mangle propagules become established there.

The Effect of Seed Predators on Plant Distributions: Is There a General Pattern in Mangroves?

Selective crab predation has been hypothesized to maintain mangrove forest zonation by preventing the establishment of certain species' seedlings at particular tidal elevations. Support for this hypothesis consists of observations that the propagules of some mangrove species are consumed at a rate that is inversely related to the species' relative abundance in the adult canopy. In addition, one previous study found that rates of crab predation were lower in large light gaps than in small ones. We conducted a mensurative field experiment to assay rates of crab predation on dispersing propagules of the dominant mangrove species (Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle) in our study forests at Punta Galeta on the Caribbean coast of Panama. The experiment specifically examined whether rates of crab predation varied 1) with species of propagule, 2) with changes in stand composition along the tidal gradient, and 3) inside versus outside of light gaps. Lightning-created canopy gaps are the primary sites of tree regeneration in these and other tall-stature mangrove forests. Propagules of the three mangrove species were tethered inside and in the understory surrounding nine light gaps, with representatives in each of the major stand types that occur along the intertidal gradient. Fates of propagules were monitored over a four-wk period. Rates of propagule predation varied spatially, with highest rates of removal occurring in lower intertidal R. mangle/L. racemosa stands, where the large herbivorous crabs, Ucides cordatus and Goniopsis cruentata are common. In mid- and upper intertidal stands, dominated by A. germinans or L. racemosa, where detritivorous Uca spp. are abundant but larger herbivorous crabs are rare, few propagules were consumed. The propagules of A. germinans were preyed upon more heavily than those of the other two species; rates of predation on R. mangle and L. racemosa propagules did not differ. Light environment (i.e. inside versus outside a gap) had no significant effect on the rate of propagule predation. The enhanced survival of juvenile mangroves in light gaps that we have observed is probably due to other factors such as higher light and nutrient availability, and in some situations, lower insect herbivory. Only the propagules of A. germinans experienced a pattern of predation consistent with the dominance-predation model: they were consumed at higher rates in the low intertidal where adults of the species are rare, and at lower rates in high intertidal areas where the species dominates the canopy. This suggests that predation on mangrove propagules by herbivorous crabs could influence the species composition of low intertidal forests by differentially reducing the recruitment of A. germinan On the other hand, rates of crab predation on A. germinans propagules seem insufficient to prevent A. germinans from establishing in low intertidal areas. In opposition to the dominance-predation hypothesis, crab predation cannot account for the distribution patterns of R. mangle or L. racemosa whose propagules were preyed on most heavily in lower intertidal areas where these species are very abundant. Our results, together with those of two earlier studies, indicate that predation by crabs on dispersing propagules is not a general explanation for the zonation of tree species along tidal gradients in mangrove forests.

Stand Structure and Productivity of the Introduced Rhizophora mangle in Hawaii

Since its introduction in the early part of this century,Rhizophora mangle L. has spread extensively through most of the main islands of the Hawaiian Archipelago. We investigated the structural properties and estimated productivity of aR. mangle population at Nuupia Ponds Wildlife Management Area (NPWMA), on windward Oahu, where the mangroves were being controlled due to their propensity to overgrow archaeological sites and the habitat of endangered Hawaiian waterbirds. Mangroves within NPWMA were very dense (>24,000 trees ha−1) and most were relatively small (only 3.3% of the trees were ≥10 cm DBH). Mean basal area, aboveground biomass, and number of seedlings were all high, at 37.2 m2 ha−1, 279 t (dry wt) ha−1, and 121 m−2, respectively. The seedling, density may be particularly unusual and appears to be due to extremely high rates of propagule, production coupled with low rates of propagule predation. Stand productivity was estimated by stem growth (allometry), litterfall, and a light attenuation approach to determining net canopy photosynthetic production. All three methods yielded estimates that are higher than previously, reported forR. mangle and comparable with estimates of highly productiveRhizophora spp.-dominated stands in Australia and Asia. The high density, biomass, and productivity of this stand relative to stands within the species' native range may be due to a combination of favorable site conditions, lack of competition from other woody plants, and very low rates of herbivory and propagule predation.

Global Patterns of Pre‐Dispersal Propagule Predation in Mangrove Forests1

ABSTRACTMangroves in disparate families produce viviparous seedlings (propagules) that are attacked by many crab and insect predators both before and after dispersal. While post‐dispersal predation is viewed as an important factor in structuring many mangrove communities, pre‐dispersal predation rates and agents have been characterized for few species. Ten species of mangrove and 3299 propagules were surveyed for pre‐dispersal propagule predation at 42 sites around the world. Pre‐dispersal predation rates were variable among sites and species, ranging from 0 to 93 percent within sands, with a global total predation rate of 23.3 percent (across all propagules examined) and a mean level of 28.3 percent across sites. Grapsid crabs, Coleoptera and Lepidoptera were the primary predators identified. Forests near human population centers and stands occurring at high intertidal sites exhibited higher levels of propagule predation than those in unpopulated or low‐intertidal sites. Predation rates on a species were weakly, negatively correlated with conspecific seedling density at a site. To explore temporal variation in, and ramifications of pre‐dispersal predation for propagule growth and abscission dynamics, Rhizophora mangle propagules were monitored over two years at three sites in Belize, Central America. Predation did not significantly reduce hypocotylar growth of germinated propagules on the parent tree, but nearly doubled the abscission rate of premature propagules. Pre‐dispersal propagule predation is a ubiquitous feature of mangrove forests world‐wide, and must be accounted for in estimates of reproductive output, stand health, and propagule availability for forestry and restoration efforts.

Mangrove Species Distribution and Propagule Predation in Belize: An Exception to the Dominance-Predation Hypothesis

Predation on mangrove propagules was studied to assess its impact on the intertidal distribution of Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa. Field experiments conducted on a mangrove island range (Twin Cays) in Belize showed that rates of predation (percent of propagules rendered nonviable in 9 days) just after peak dispersal (December) were highest for A. germinans (60 ? 8%) and significantly lower for R. mangle (18 ? 5%) and L. racemosa (28 ? 9%), which were not significantly different. Rates varied seasonally, however, with highest losses of propagules at the beginning of dispersal (July). Major predators were the mangrove crabs, Goniopsis cruentata and Ucides cordatus. The high predation rate on A. germinans propagules was attributed to a relatively small size (facilitating burial in crab burrows), a higher nutritive content, and lower amounts of defensive chemicals such as tannins. Interspecific differences in predation rates, however, could not explain the relative dominance and spatial distribution of two of the three species that comprise this intertidal community. Comparison of predation rates in forest locations dominated by either R. mangle or A. germinans demonstrated that consumption of R. mangle propagules was highest in areas where it dominated the canopy and lowest where A. germinans was dominant. Predation also could not account for the infrequent occurrence of L. racemosa trees at Twin Cays, since rates of propagule consumption were not different from that of R. mangle and significantly less than that of A. germinans. High densities of L. racemosa seedlings further suggested that predators were not preventing this species' establishment here. Only the predation pattern for A. germinans was consistent with intertidal dominance of trees. The results indicate that differential predation plays a less important role in the zonation of these island mangrove forests than has been reported for other geographic regions.