Experimental evidence for mediation of competition by habitat succession

Habitat succession is thought to influence the importance of competition in assemblages. Competitive interactions are considered of critical importance in structuring ant assemblages, but field experiments show varied effects. I tested how succession in managed boreal forests affects the outcome of competition from dominant red wood ants, Formica aquilonia, through a removal experiment in replicated stands of three different ages (0-4, 30-40, and 80-100 years old). F. aquilonia abundance was reduced by 87%, and procedural controls showed no nontarget effects. The succession gradient revealed the full range of possible responses from ant species: decreases in 1-4-year-old stands, increases in 30-40-year-old stands, and no effects in 80-100-year-old stands, where diversity was lowest. Habitat succession thus regulates competitive interactions in this system. I propose a model for this system, where competitive effects depend on time since disturbance. In this case, soon after disturbance the dominant species facilitates increases in the abundance of other species. At intermediate times, competition reduces the abundance of some species. Finally, in long-undisturbed habitats, competitors may fail to evolve, particularly in high-stress environments. Interactions between competition and habitat succession may explain why structuring effects of ecologically dominant species appear inconsistent.     

Functional identity and functional structure change through succession in a rocky intertidal marine herbivore assemblage

Despite the great interest in characterizing the functional structure and resilience of functional groups in natural communities, few studies have examined in which way the roles and relationships of coexisting species change during community succession, a fundamental and natural process that follows the release of new resources in terrestrial and aquatic ecosystems. Variation in algal traits that characterize different phases and stages of community succession on rocky shores are likely to influence the magnitude, direction of effects, and the level of redundancy and complementarity in the diverse assemblage of herbivores. Two separate field experiments were conducted to quantify per capita and population effects and the functional relationship (i.e., redundancy or complementarity) of four herbivore species found in central Chile during early and late algal succession. The first experiment examined grazer effects on the colonization and establishment of early-succession algal species. The second experiment examined effects on the late-successional, dominant corticated alga Mazzaella laminarioides. Complementary laboratory experiments with all species and under natural environmental conditions allowed us to further characterize the collective effects of these species. We found that, during early community succession, all herbivore species had similar effects on the ephemeral algae, ulvoids, but only during the phase of colonization. Once these algae were established, only a subset of the species was able to control their abundance. During late succession, only the keyhole limpet Fissurella crassa could control corticated Mazzaella. The functional relationships among these species changed dramatically from redundant effects on ephemeral algae during early colonization, to a more complementary role on established early-successional algae, to a dominant (i.e., keystone) effect on late succession. This study highlights that functional relationship within consumer assemblages can vary at different phases and times of community succession. Differentiation in herbivore roles emphasizes the need to evaluate consumer's impacts through different times of community succession, and through experimental manipulations to make even broad predictions about the resilience or vulnerability of diverse intertidal assemblages to human disturbances.     

Connell and Slatyer's models of succession in the biodiversity era

Understanding how species interactions drive succession is a key issue in ecology. In this study we show the utility of combining the concepts and methodologies developed within the biodiversity-ecosystem functioning research program with J. H. Connell and R. O. Slatyer's classic framework to understand succession in assemblages where multiple interactions between early and late colonists may include both inhibitory and facilitative effects. We assessed the net effect of multiple species interactions on successional changes by manipulating the richness, composition, and abundance of early colonists in a low-shore assemblage of algae and invertebrates of the northwestern Mediterranean. Results revealed how concomitant changes in species richness and abundance can strongly alter the net effect of inhibitory vs. facilitative interactions on succession. Increasing richness of early colonists inhibited succession, but only under high levels of initial abundance, probably reflecting the formation of a highly intricate matrix that prevented further colonization. In contrast, increasing initial abundance of early colonists tended to facilitate succession under low richness. Thus, changes in abundance of early colonists mediated the effects of richness on succession.     

Abundance of Fucus spiralis and ephemeral seaweeds in a high eulittoral zone: effects of grazers,canopy and substratum type

According to a widely accepted view of intertidal community organization in the north-west Atlantic Ocean, the abundance of perennial seaweeds is regulated by the activities of herbivores (which control ephemeral algal competitors) and carnivores (which control populations of competing filterfeeders). This concept was examined in 1987–1988 in a eulittoral belt on the Atlantic shores of Nova Scotia, Canada, dominated by a closed canopy of Fucus spiralis. A factorial design was used to test the effects of (a) grazers (there were no carnivores present, (b) adult F. spiralis canopy, and (c) substratum type on the abundance of perennial and ephemeral seaweeds. Grazers had no significant (p>0.05) effect on the density or cover of juvenile F. spiralis, but significantly reduced canopy cover of adults through the winter. The presence of grazers significantly enhanced the cover of ephemeral algae in early spring. Barnacle presence enhanced the cover of juvenile F. spiralis, but reduced the canopy of adult plants through abrasion. The most important regulator of recruitment density in F. spiralis was the presence of a canopy of conspecific adults. Canopy greatly reduced juvenile development. There was no significant relationship between the covers of perennial F. spiralis and ephemeral seaweeds. This study demonstrates that the recruitment of the perennial rockweed population examined is not dependent on the activities consumer animals. The results contrast with work in the midshore intertidal of New England, where the activities of herbivores are thought to regulate the abundances of perennial seaweeds.     

Effects of species diversity on community biomass production change over the course of succession

Over the past decade an increasing number of studies have experimentally manipulated the number of species in a community and examined how this alters the aggregate production of species biomass. Many of these studies have shown that the effects of richness on biomass change through time, but we have limited understanding of the mechanisms that produce these dynamic trends. Here we report the results of an experiment in which we manipulated the richness of freshwater algae in laboratory microcosms. We used two experimental designs (additive and substitutive) that make different assumptions about how patches are initially colonized, and then tracked the development of community biomass from the point of initial colonization through a period of 6-12 generations of the focal species. We found that the effect of initial species richness on biomass production qualitatively shifted twice over the course of the experiment. The first shift occurred as species transitioned from density-independent to dependent phases of population growth. At this time, intraspecific competition caused monocultures to approach their respective carrying capacities more slowly than polycultures. As a consequence, species tended to over-yield for a brief time, generating a positive, but transient effect of diversity on community biomass. The second shift occurred as communities approached carrying capacity. At this time, strong interspecific interactions caused biomass to be dominated by the competitively superior species in polycultures. As this species had the lowest carrying capacity, a negative effect of diversity on biomass resulted in late succession. Although these two shifts produced dynamics that appeared complex, we show that the patterns can be fit to a simple Lotka-Volterra model of competition. Our results suggest that the effects of algal diversity on primary production change in a predictable sequence through successional time.     

Horizontal,vertical and seasonal distributions of epiphytic hydrozoa on the algaHalimeda tuna in the Northwestern Mediterranean Sea

The thalli ofHalimeda tuna (Ellis & Solander) grow horizontally from rocky surfaces and present an upper and lower surface which can be colonized by epiphytes. Thalli ofH. tuna were collected twice a month from March 1985 to March 1986 at Tossa de Mar (northeast Spain). Thirty five species of hydroid were recorded living onH. tuna thalli; of these, the 15 most abundant species were classified into three groups on the basis of their horizontal zonation on the thalli: those most abundant on the proximal, medial or distal articles. Those hydroids common on the proximal (oldest) articles were relatively large and were present throughout the year, those common on the medial articles were smaller and often occurred in dense monospecific patches, while those common on the distal (youngest) articles were present for only short periods and were the most opportunistic hydroids found onH. tuna. We hypothesize that this zonation reflects interspecific niche-selection, enabling successful competition for space with other hydroids, algae and bryozonas. Three hydroid species were more abundant on the upper surfaces of the algae than on the lower, probably because of gravitationally enhanced settlement of planula larvae coupled with (unusually) limited hydrorhizal growth on the underside. Three opportunistic species were more common on the lower surfaces of the thalli, possibly reflecting the greater availability of substratum in the shade, where epiphytic algae were less abundant. Many hydroids declined in abundance in summer because of the death of old thalli, the growth of new thalli and of apical articles on existing thalli, and possibly because of interspecific competition with epiphytic algae. The death of old thalli and the growth of new thalli reduced the abundance of epiphytic algae; this may have facilitated the increase in abundance of hydroids recorded in the autumn.     

Competitive organization of the soft-bottom macrobenthic communities of southern California lagoons

The sandy-bottom macrobenthic community of Mugu Lagoon, a relatively pristine southern California (USA) marine lagoon, demonstrated (1) nearly constant community composition over 37 months of observation, (2) relatively little temporal variability in the population densities of the most abundant species over 37 months, and (3) a pattern of depth stratification in which very little vertical overlap existed among the six most abundant species. The only two species whose vertical distributions overlapped broadly showed horizontal spatial segregation, each abundant in different areas within the sand habitat. These community characteristics imply the importance of biological factors in structuring the sand benthos. The relatively large volume required for living space by these macrofauna suggests that competition for space may be the biological factor most important in determining the observed temporal and spatial abundance patterns. The muddy-sand community and the mud community of Mugu Lagoon also revealed similar patterns of stratification: new abundant species replaced species at the same sedimentary level while not greatly affecting species populations at other non-overlapping levels. In the sand community of Tijuana Slough, two of the abundant species of Mugu Lagoon's sand community were nearly absent as an apparent result of human over-exploitation. Probably in response, densities of species living at the sedimentary levels normally occupied by the missing species were much higher than would be predicted if competition for space were unimportant. In field experiments, removal of the deposit feeder Callianassa californiensis resulted in high recruitment of Sanguinolaria nuttallii, whereas control areas showed no S. nuttallii recruitment. Experiments also suggest that negative intraspecific interactions between Cryptomya californica individuals may explain the observed rapid emigration from areas of artificially high density. Perhaps the relatively great environmental predictability of southern California lagoons has permitted competitive interactions to play a singnificant role in determining the temporal and spatial abundance patterns of the soft-bottom macrobenthos.     

Secondary succession dynamics in estuarine marshes across landscape-scale salinity gradients

Secondary succession plays a critical role in driving community structure in natural communities, yet how succession dynamics vary with environmental context is generally unknown. We examined the importance of seedling and vegetative recruitment in the secondary succession of coastal marsh vegetation across a landscape-scale environmental stress gradient. Replicate bare patches were initiated in salt, brackish, and oligohaline marshes in Narragansett Bay, Rhode Island, USA, and allowed to recover unmanipulated or with colonizing seedlings or vegetative runners removed for three years. Seed dispersal and seed bank studies were conducted at the same sites. We found that rates of recovery were 3-10 times faster in brackish and oligohaline marshes than in salt marshes. The fast pace of recovery in oligohaline marshes was driven by seedling colonization, while recovery was dominated by vegetative runners in brackish marshes and by both seedlings and runners in salt marshes. Seed and seedling availability was much greater in oligohaline marshes with up to 24 times the seed bank density compared with salt marshes. In contrast to the facilitated succession generally found in salt marshes, oligohaline marshes follow the tolerance model of succession where numerous species colonize from seed and are slowly displaced by clonal grasses whose recovery is slowed by preemptive competition from seedlings, contributing to the higher species diversity of oligohaline marshes. These findings reveal fundamental differences in the dynamics and assembly of marsh plant communities along estuarine salinity gradients that are important for conceptually understanding wetlands and for guiding the management and restoration of various types of coastal marshes.     

Modeling approach to regime shifts of primary production in shallow coastal ecosystems

Pristine coastal shallow systems are usually dominated by extensive meadows of seagrass species, which are assumed to take advantage of nutrient supply from sediment. An increasing nutrient input is thought to favour phytoplankton, epiphytic microalgae, as well as opportunistic ephemeral macroalgae that coexist with seagrasses. The primary cause of shifts and succession in the macrophyte community is the increase of nutrient load to water; however, temperature plays also an important role. A competition model between rooted seagrass (Zostera marina), macroalgae (Ulva sp.), and phytoplankton has been developed to analyse the succession of primary producer communities in these systems. Successions of dominance states, with different resilience characteristics, are found when modifying the input of nutrients and the seasonal temperature and light intensity forcing.     

Settlement of larval blacklip abalone, Haliotis rubra, in response to green and red macroalgae

Surfaces from the habitat of adult Haliotis rubra were tested as inducers of larval settlement to determine the cues that larvae may respond to in the field. Settlement was high on the green algal species Ulva australis and Ulva compressa (Chlorophyta), the articulated coralline algae Amphiroa anceps and Corallina officinalis, and encrusting coralline algae (Rhodophyta). Biofilmed abiotic surfaces such as rocks, sand and shells did not induce settlement. Ulvella lens was also included as a control. Treatment of U. australis, A. anceps and C. officinalis with antibiotics to reduce bacterial films on the surface did not reduce the settlement response of H. rubra larvae. Similarly, treatment of these species and encrusting coralline algae with germanium dioxide to reduce diatom growth did not significantly reduce larval settlement. These results suggest that macroalgae, particularly green algal species, may play an important role in the recruitment of H. rubra larvae in the field and can be used to induce larval settlement in hatchery culture.     

Algal succession in a Macrocystis pyrifera forest

Algal succession within a subtidal forest of the giant kelp Macrocystis pyrifera was studied by following colonization and community development on concrete blocks fastened to the bottom. Sets of blocks were placed in the bed at 3-month intervals. Subsequent algal development on each set was followed for over a year. All macroscopic species attached to the substrata were noted, and the number and lenght of basal branches determined every 1 to 3 months. Colonizing plants fell into 3 categories: rapid-growing ephemerals, and rapid and slow-growing perennials. Ephemerals such as Giffordia (Ectocarpus) mitchellae, Colpomenia peregrina, and diatom films generally produced and initial bloom on the blocks but were gradually replaced by perennials (articulated corallines, Rhodymenia spp., Gigartina spp.) characteristic of the mature kelp community. These stages, rather than representing “ecological” succession, seemed to reflect differences in growth rate and success in interspecific competition for space and light. Colonization on the blocks varied with season, indicating that most species have either a spring-summer or fall-winter period of maximum reproduction. M. pyrifera sporophyte colonization was greatest in spring. During community development, algal diversity (H), number of species (s) and evenness (J) all reached a peak within 100 to 200 days regardless of the time the blocks were started. Diversity and number of species then fell as ephemeral species disappeared. These species were apparently unable to compete with perennials and, once gone, did not recolonize. Evenness remained high.     

Adaptive significance of phytoplankton stickiness with emphasis on the diatom Skeletonema costatum

Diatom aggregate formation was analyzed using coagulation theory. Population dynamics models show that coagulation has an important impact on species succession during diatom blooms. When different species collide and form mixed aggregates this process causes interspecific interference competition within the diatom community. The outcome can be predicted by a set of simple differential equations. For a twospecies system the equations reduce to the Lotka-Volterra two-species competition model. The outcome of this interference competition depends on species-specific growth rates, cell sizes, stickiness and on the species composition of the seeding populations of a bloom. Due to mutual flocculation some species may disappear from the environment. Small and fast growing diatoms are favoured by high stickiness coefficients. The impact of stickiness on species succession was found to be most pronounced in eutrophic and hydrographically isolated environments. The sticking properties of the diatom Skeletonema costatum are discussed in an evolutionary context; we suggest that mutual coagulation increases the abundance of S. costatum relative to other diatom species in coastal areas. The model was tested on field data, and the predicted dynamics of a spring bloom was very similar to that observed.     

Early life histories in the bryozoan<Emphasis Type="Italic"> Schizobrachiella sanguinea</Emphasis>: a case study

Mediterranean populations of Schizobrachiella sanguinea seem to span a wide range of contrasting benthic habitats. We have studied adult, larval, and recruit populations of S. sanguinea from sublittoral (approximately 10 m depth) communities at Blanes (Spain, NW Mediterranean). At the study site, the species colonises semi-obscure caves and, to a lesser extent, communities of sciaphilic algae. Our first goal was to show the periods of brooding, larval release, and recruitment. Plankton was collected about twice a week between July 1998 and June 2000 over an artificial reef. A total of 303 larvae were collected in 23 of the 102 days of sampling. At the study site the species shows a single, annual larval release period, which occurs from late March to June–July every year. Recruits were found from late May to late August 1999 and in late May 2000. A recruitment peak was observed in July 1999 and again in late May 2000. The presence of brooding adults corresponded to the periods of larval supply and recruitment. Our second goal was to investigate the causes of the variable distribution of the species along several communities in the north-west Mediterranean Sea. We describe adult distribution and the dynamics of larval supply, recruitment of early (15 days from settlement) and late (4 months from settlement) recruits. Patterns of larval presence and early recruit distribution along the communities on the reef did not reflect the adult distributions, whereas this was closely matched by the distribution of late recruits. Post-recruitment mortality strongly affected recruits of the PA (photophilic algae) and SA (sciaphilic algae) communities, whereas it was very low in the communities of SOC (semi-obscure caves). Mortality of recruits was density independent and colony survivorship in the SA communities, where recruitment was the highest, appeared strongly affected by competition with brown algae. Like the majority of cheilostomate bryozoans, S. sanguinea releases a coronate larva capable of active swimming and positive reaction to light. Such swimming abilities may allow larvae to actively select the substrate on which to settle and to avoid unsuitable substrates. It seems that larval behaviour does not explain the preference of S. sanguinea for sciaphilic habitats. High post-recruitment mortality due to various factors, especially competition with fast-growing algae, seems particularly relevant in determining the adult distributions observed.Communicated by S.A. Poulet, Roscoff     

Effects of resource availability,predators, conspecifics and heterospecifics on decorating behaviour by the majid crab <Emphasis Type="Italic">Tiarinia cornigera</Emphasis>

Utilization of algae for decorating by the intertidal majid crab Tiarinia cornigera was examined by laboratory experiments in relation to availability of algae, presence of a predator, and the presence of conspecific and heterospecific crabs. Different availabilities of decorating materials had a positive correlation on decorating amount by juveniles, while the correlation was not so clear in subadults. The amount of decorating decreased with increasing density of conspecific crabs in the presence of a predator, but there was so clear relationship in the absence of a predator. The decrease in decorating under high density could be due to intraspecific aggression, because the superior crab, in fighting, was found to be decorated with more algae. Tiarinia cornigera was found to be superior to the co-occurring different majid species Micippa platipes in fighting. When T. cornigera and M. platipes were housed together, the former was decorated with more algae than the latter in the presence of a predator, but in the absence of a predator, the amount of algae was not different. Thus, presence of a predator may motivate intraspecific competition in T. cornigera as well as interspecific competition between T. cornigera and M. platipes for decorating materials.     

Effects of herbivory on zonation of Sargassum spp. within fringing reefs of the central Great Barrier Reef

A combination of small-scale transplants and herbivore exclusion was used to test the importance of herbivory, physiological tolerance limits, and recruitment and dispersal in regulating the distribution and abundance of the genus Sargassum on two nearshore fringing reefs of the central Great Barrier Reef, during 1992/1993. Sargassum (predominantly S. oligocystum and S. tenerrimum) were transplanted from reef-flat zones where they normally grow, to a seaward coral zone where they are not normally found. At Great Palm Island, coral-zone transplants only survived if protected from herbivores. At Brook Island, survival of uncaged coral-zone transplants was more variable but not significantly lower than plants returned to the Sargassum zone. Thus herbivory may be a major cause of the zonation patterns of adult Sargassum on these fringing reefs, but the importance of this factor varies between and within reefs. Since protected Sargassum survived and grew for up to 6 mo in the coral zone, the adult algae are not physiologically limited by any physical or chemical differences between zones. However, Sargassum recruitment to the coral zone was very low (mean 2.7 recruits m⁻² over 13 mo), and was not significantly affected by herbivores. Since rates of herbivory were relatively slow, effective exclusion of Sargassum from the coral zone by herbivores may depend on low recruitment of the algae. In a broader context, the distribution of Sargassum may depend on the combined spatial patterns of herbivory and recruitment. Received: 24 January 1997 / Accepted: 12 May 1997     

Does local competition increase the coexistence of species in intransitive networks?

Competitive intransitivity, a situation in which species' competitive ranks cannot be listed in a strict hierarchy, promotes species coexistence through "enemy's enemy indirect facilitation." Theory suggests that intransitivity-mediated coexistence is enhanced when competitive interactions occur at local spatial scales, although this hypothesis has not been thoroughly tested. Here, we use a lattice model to investigate the effect of local vs. global competition on intransitivity-mediated coexistence across a range of species richness values and levels of intransitivity. Our simulations show that local competition can enhance intransitivity-mediated coexistence in the short-term, yet hinder it in the long-term, when compared to global competition. This occurs because local competition slows species disaggregation, allowing weaker competitors to persist longer in the shifting spatial refuges of intransitive networks, enhancing short-term coexistence. Conversely, our simulations show that, in the long-term, local competition traps disaggregated species in unfavorable areas of the competitive arena, where they are excluded by superior competitors. As a result, in the long-term, global intransitive competition allows a greater number of species to coexist than local intransitive competition.     

Female rock shrimp<Emphasis Type="Italic"> Rhynchocinetes typus</Emphasis> mate in rapid succession up a male dominance hierarchy

The dynamics of male-male competition for mates and patterns of female choice depend critically on the social environment. We released newly molted sexually receptive females of the rock shrimp Rhynchocinetes typus in the field and recorded their interactions with males. In the dense aggregations in which these shrimp live, most females were encountered and seized by males within 2 min. Usually, females were first seized by subordinate males, and subsequently taken over by the dominant males. Many females (17 out of 23) had multiple mates during the 10-min observation period, and most of them received spermatophores from multiple males. Males used different mating tactics in accordance with their dominance status: subordinate males often used the sneaking tactic, seizing the female and immediately transferring spermatophores. In contrast, all dominant males used the primary mating tactic; they seized and stimulated the female before transferring spermatophores. Results from previous studies had indicated that females may reduce the fertilization chances of subordinate males by delaying spawning and removing spermatophores. We suggest that this capability in combination with the observed rapid mate succession may enable females to exploit male contest behaviors.Communicated by P. Backwell     

Temporal variation of recruits as a basis of ephemeral genetic heterogeneity in the western rock lobster Panulirus cygnus

Allozymes were examined in quantitative lunar monthly collections of larval recruits of the western rock lobster Panulirus cygnus George over three recruitment seasons at two sites nearly 350 km apart in Western Australia. At Alkimos, the southern site, recruitment occurs in a relatively narrow peak early in the spring, whereas at the northern Houtman Abrolhos Islands, recruitment extends into the summer months. In the 1995/1996 recruitment season, the frequency of the GPI ^* 100 allele increased from early to late in the season, but the frequencies were indistinguishable at the two sites in each monthly collection. The combination of this temporal variation in allelic frequencies with the contrasting patterns of recruitment at the Abrolhos Islands and Alkimos resulted in genetically different cohorts at the two sites. This pattern was ephemeral, as it was not repeated in the subsequent two years. Thus, ephemeral genetic patchiness in P. cygnus can be generated by the locally-specific genetic mix of recruits obtained from a common larval pool. This mechanism is the probable explanation of previously observed temporal and possible spatial genetic variation in adult P. cygnus, and highlights the importance of studying recruitment in order to understand the genetic structure of marine species. Received: 22 February 1999 / Accepted: 8 June 1999     

Local and meso-scale patterns of recruitment and abundance of two intertidal crab species that compete for refuges

Interference competition for limited habitat or refuges is known to produce density-dependent mortality and generate patterns of micro-habitat distribution. While in mobile species the outcome of interference at a local scale can usually be determined from differences in body size and behavior, the population-level consequences of such interactions vary depending on rates of settlement and recruitment at a site, which are not directly correlated to local reproductive success. Previous experimental studies in central Chile demonstrated that interference competition for refuges is the primary factor driving microhabitat segregation between the predatory crabs Acanthocyclus gayi and Acanthocyclus hassleri, with the latter species monopolizing galleries inside mussel beds and excluding A. gayi to rock crevices. Between April 2001 and March 2006 we quantified monthly recruitment rates in artificial collectors at 17 sites over 900 km of the central coast of Chile. Results show that recruitment rates of A. hassleri are almost two orders of magnitude lower than those of A. gayi, and that they are tightly and positively correlated among sites across the region, suggesting that at scales of kilometers larval stages of these species are affected by similar oceanographic processes. Total crab densities per site were also positively correlated between species and strongly associated to mussel cover, with overall low crab densities at all sites where mussel cover was lower than about 60%. At all sites with mussel cover >60%, the ratio of A. gayi to A. hassleri density progressively decreased from recruits (2.6) to juveniles (0.5) to adults (0.04), overcoming initial differences in recruitment rates. The relative success of the inferior competitor at sites with low mussel cover does not appear to provide a potential mechanism favoring regional coexistence through dispersal to other sites (“mass effects”), because their densities were lower than at sites of high mussel cover. Yet, at many sites of low mussel cover the dominant competitor is virtually absent, allowing A. gayi to attain larger population sizes at the scale of the region. Thus, the factors limiting the dominant competitor from successfully utilizing other microhabitats seem to be the most critical factor in promoting both local and regional coexistence between these species.     

湛江特呈岛红树林植物群落的结构和动态特征

对特呈岛红树林植被进行多次和全面的调查,通过采用格子样的方法,选取面积为100～800 m^2的样地,每片样地再分成若干10 m×10 m的样方,对红树林样地内的植物进行每木测量,记录各植物的种名、胸径、高度、冠幅等,并进行重要性以及多样性等常规的群落分析,判定该地区植被的生长特征以及研究其演替动态变化。结果表明：①面积较大的群落主要有3个类型,即白骨壤群落（Avicennia marina）、白骨壤＋红海榄群落（Avicennia marina＋Rhizophora stylosa）、白骨壤-桐花树群落（Avicennia marina-Aegiceras corniculatum）;其它面积较小的群落有：白骨壤＋红海榄-木榄＋桐花树群落（vicennia marina＋Rhizophora stylosa-Bruguiera gymnorrhiza＋Aegiceras corniculatum）、红海榄＋木榄群落（Rhizophora stylosa＋Bru-guiera gymnorrhiza）;②在3个主要群落中,优势种的苗木和小树相对较少,优势种群的年龄结构均为衰退型;③各群落的Simpson指数关系为：白骨壤＋红海榄群落﹥红海榄＋木榄群落﹥白骨壤＋红海榄—木榄＋桐花树群落﹥白骨壤—桐花树群落﹥白骨壤群落;白骨壤群落的重要值为300,为典型的单优群落;④特呈岛的白骨壤群落是中国该类群落中面积最大的,目前长势良好,无病虫害,并且红树林外缘的白骨壤枝干在波浪冲刷或人为原因下,受到较严重的侵蚀,其幼树在林外缘基本无法存活,需要加强保护,对其生理生态学特征也值得进一步深入研究。