摘要:

The mechanism that control the distribution and abundance of major benthic organisms in subtidal coral reef communities are poorly understood. Through field experimentation and manipulations, this study investigated the factors that account for the vertical zonation patterns found on a fringing reef in the Gulf of Panama (Pacific). In this community, pocilloporid corals form a near—monotypic stand (80—85% live coral cover) in shallow water (0—6 m depth) while massive corals, particularly Pavona gigantea, predominate in low density (°18% cover) in the deeper areas of the reef (6—10 m depth). The paucity of massive corals in shallow water results mainly from chronic and severe mortality caused by the omnivorous damselfish Eupomacentrus acapulcoensis (Pomacentridae). These highly territorial fish establish algal mats (used as a source of food and nesting site) on hard substrata, including coral. The effect of this activity was found to be differential with respect to (1) species (i.e., Pavona suffered higher mortality compared to Pocillopora) and (2) depth (i.e., the frequency and extent of damaged Pavona colonies were higher in shallow compared to deep areas). Pavona suffered higher mortality due to differences in coral morphology: damselfish are more effective at removing tissue from a massive (open) surface than from a tightly branched pocilloporid coral. Pavona was most severely damaged in shallow water because of depth—related differences in topographic complexity. By removing the topographic complexity surrounding colonies with damselfish and by adding complex substratum around colonies without damselfish, it was found that damselfish abandon territories when substratum complexity is reduced and immigrate to colonies when complexity is increased. Immigration eventually led to coral mortality. Hence, shelter is a necessary requisite for the establishment of a damselfish territory. Censuses revealed that shelter site availability surrounding Pavona colonies and damselfish abundance (both overall and per individual Pavona colony) were greater in shallow compared to deep water. Damselfish were conspicuously absent from small colonies (<1.5 m2surface area) that lacked holes or crevices within the colony or immediately adjacent to it; such colonies were only present in deep water. Controlled experiments using live coral fragments (simulating 1—1.5 yr old juveniles) attached to hard substratum demonstrated that pocilloporid corals suffer high mortality and substantially reduced growth rates in deep reef areas due to grazing by pufferfish and parrotfish. The effectiveness of this predation/grazing activity was related to the position of the coral fragments relative to defended damselfish territories. Outside the territory, growth and survivorship of fragments were low for Pocillopora but high for Pavona. Within areas defended by damselfish, Pocillopora and Pavona fragments grew equally well both caged and uncaged. On the algal mat itself, however, Pavona fragments suffered high mortality from damselfish: Pocillopora growth under these conditions was only temporarily reduced. The data in this study indicate that damselfish territories provide a refuge against juvenile mortality for Pocillopora within defended areas, thus facilitating the establishment of branching corals in shallow water and concomitantly reducing or eliminating its potential space competitor, Pavona. A proposed successional model suggests that Pocillopora abundance may ultimately control the abundance of damselfish. This study indicates that community structure on some eastern Pacific reefs may be controlled by the direct and indirect behavioral interactions between damselfish and transient fish corallivores, and physical factors (availability of shelters) limiting the distribution of damselfish.

ISSN:0012-9615    

DOI:10.2307/2937329

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Population densities, reproduction, and survival of the lizard Uta stansburiana were measured at the Nevada Test Site in southern Nevada, USA, between 1964 and 1974. These data were used to develop a model of the population dynamics of this species. Results of irrigation experiments in 0.4—ha enclosures near Mercury, Nevada, were used to formulate multiple—regression equations predicting frequency and size of clutches laid by two age—classes of females in terms of winter rainfall, March air temperatures, and Uta population density. Densities of Uta in these enclosures were manipulated, and age—specific survival modeled in terms of spring densities of Uta. Experiments in which an important predator on Uta (the leopard lizard, Crotaphytus wislizeni) was removed from enclosures were used to estimate the influence of the predator on basic survival rates of hatchling and older Uta. The model was generally developed from data acquired in the small enclosures, but predictions were compared with actual observations of changes in Uta populations in Rock Valley (19 km west of Mercury, Nevada) between 1966 and 1972. Agreement between model predictions and actual numbers was fair. The model predicted a decrease in density from 1966 to 1967, but numbers of Uta actually increased conspicuously at this time. This was the only major discrepancy between predictions and observations. The observed mean spring density (d) between 1967 and 1972 was 41.4 Uta/ha (Sd = 20.8), while the model predicted a mean density of 37.8 Uta/ha (SD = 13.6). Observed and predicted mean proportions of yearlings in spring populations were identical (0.78). The basic version of the model estimated different survival rates for two age—groups of adult Uta. A simpler version of the model, using a common survival rate for both age—groups, gave predictions essentially identical with those of the basic model. Other tests of the basic model showed it to be most sensitive to changes in winter rainfall and predation pressure, much less so to air temperatures. Fifteen— and 30—yr synthetic sequences of predator densities were used to examine model stability over longer periods of time. When predator densities were drawn randomly from distributions with a mean of 2 individuals/ha, model populations exhibited lower mean numbers and amplitudes than actually observed during 9 yr in Rock Valley. The basic model included three density—dependent parameters: clutch frequency, clutch size, and adult survival. The model was modified so that (1) egg production was density independent, while adult survival was not; (2) adult survival was density independent, but egg production was not; and (3) there was no density dependence in the model. Thirty—year tests showed that cases 1 and 2 did not differ markedly from the basic model, although the removal of one density—dependent constraint resulted in slightly higher mean densities. In case 3, the model lacked stability and predicted numbers increased to unrealistic levels within 5 yr. We conclude that processes relating to egg production were modeled more effectively than those influencing survival, and that improvement of the model will depend on more detailed studies of the impact of predation on age—specific survival rates of Uta.

ISSN:0012-9615    

DOI:10.2307/2937330

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

Using mark—recapture experiments, I studied the foraging movements of two crucifer feeding flea beetles, Phyllotreta cruciferae and P. striolata. To determine the influence of distance between host patches, and their quality, on beetle movement, I released marked beetles in experimental arrays of collards (Brassica oleracea). I quantified and interpreted movement behavior by applying a passive diffusion model and a continuous—time Markov model to recapture distributions. These models provided both a standard protocol for interspecific comparisons and a formal tool for translating movements into steady—state foraging patterns. In homogeneous arrays (all collard patches of similar quality), P. cruciferae and P. striolata moved in accordance with a constant—coefficient diffusion model. The dispersal rates of both beetles increased as the distance between patches declined; thus, patches of collards acted as stepping—stones to accelerate beetle dispersal. Beetle movement was hampered by old—field vegetation; beetles moved along cultivated ground in lieu of entering dense stands of goldenrod. Under all experimental conditions, P. cruciferae was consistently more mobile than P. striolata. By differentially fertilizing and pruning collards, I created "lush" and "stunted" patches of collards, which were arranged alternately along linear arrays. To determine the influence of patch dispersion on foraging selectivity, I varied the distance between these patches of unequal quality. The degree of foraging discrimination exhibited by the beetles was greatest where patches were closest together, and declined as distance between patches increased. At comparable spacings, P. cruciferae consistently exhibited a higher degree of foraging discrimination than did P. striolata. I developed a Markov model of patch—to—patch dispersal in which each flea beetle's probability of movement depends on patch quality and distance to neighboring patches. Using 60—min observations of marked flea beetles to estimate model parameters, I found that the steady states of foraging discrimination predicted by the model matched the discrimination levels exhibited by undisturbed flea beetles in comparable control arrays of collard patches. Thus, simple movement rules were extrapolated into expected foraging behavior; the dependence of that foraging behavior on patch dispersion was shown to be mediated by varying levels of mobility. In particular, proximity between heterogeneous patches affords beetles greater opportunity for nonrandom foraging than when patches are widely scattered. Since movement is the mechanism by which beetles respond to heterogeneous environments, rates and patterns of movement determine each species' ability to concentrate its numbers on the best foodplants. P. cruciferae is more mobile than P. striolata and in turn can better adjust to variation in collard quality. The interplay between beetle movement and plant dispersion may have consequences well beyond the simple distribution of feeding activity among heterogeneous host patches.

ISSN:0012-9615    

DOI:10.2307/2937331

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The development of general theories concerning the origin and maintenance of community organization in marine sedimentary environments will benefit from studies of similar processes in the widest possible range of habitat types. The roles of predation and disturbance by large epibenthos are thought to be significant in many such habitats, but the bulk of recent experimental confirmation comes from shallow areas protected from oceanic swell. This field experimental study examines relationships among demersal predators, predator—caused local disturbance, infauna, and infaunal food resources in an exposed marine sand habitat at 17—m depth in southern California, USA. Manipulation of predator densities with exclusion cages, simulation of biological disturbance, and study of dispersal and habitat selection of infauna showed the importance of recurrent local disturbances by the rays Urolophus halleri and Myliobatis californica, which dig pits to expose prey but clear other infauna in the process. Benthic invertebrate populations show complex but reproducible patterns of reoccupation of disturbed sites. The most striking aspect of these patterns is active selection of recently formed pits by certain species. Ray pits are sites of accumulation for organic material on which most of the infauna feed. Experiments showed that populations which rapidly colonize new ray pits are responding to the concentration of food resources which are otherwise sparsely distributed. Responses of infauna to ray disturbance are correlated with postlarval swimming capability and method of feeding. Early colonists are active nocturnal swimmers that feed on detritus at the sand—water interface. Such features allow efficient exploitation of patchy, ephemeral concentrations of organic matter. Later arrivals are primarily subsurface feeders with limited swimming activity. The relative abundances of infauna are sensitive to seasonal changes in ray disturbance rates. Early pit colonists predominate when disturbance rates are high. The ray disturbance phenomenon produces a persistent mosaic of patches in various stages of infaunal recolonization. Other experiments showed the importance of predation by sea stars (Astropecten verrilli) and speckled sand dabs (Citharichthys stigmaeus). Sea stars consume crab larvae soon after they settle to the bottom and begin metamorphosis. During the study, recruitments of two crabs, Cancer gracilis and Portunus xantusii, were much reduced by sea star predation. A caging experiment indicated that high—density populations of P. xantusii have important negative effects on some infaunal populations. Thus, sea star predation on young crabs is important to the maintenance of infaunal community organization. Sand dabs consume infauna which are flushed into the water column or onto the sand surface by digging rays. This commensal behavior constitutes an important additional source of mortality for populationsthat are otherwise unavailable as food for sand dabs, which are visual predators.

ISSN:0012-9615    

DOI:10.2307/2937332

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY

摘要:

The developmental history of Lake Valencia, Venezuela, over the past 12 500 yr was studied by means of animal remains and some chemical and physical variables in three sediment cores. Radiocarbon dates of abandoned beach terraces also provided information for the interpretation of former lake levels. Transects of surficial sediments in the lake served as modern analogues for both animal communities in the sediments and characteristics of the sediment itself. Interpretation of cluster and multiple discriminant analyses led to the recognition of six stratigraphic zones of animal microfossils. The zones were expressions of climatically controlled fluctuations of water levels that have dominated successional processes within Lake Valencia. The animal remains were mostly carapaces of Ostracoda and Cladocera, with fewer head capsules and mouthparts of immature insects and flatworm (neorhabdocoel) egg cases. Study of the variation of animal assemblages with various environmental requirements led to the following interpretation of the zones. The basin was dry from at least 13 000 to 11 000 yr before present (BP). It filled rapidly °10 500 yr BP, and had an outflow for 2500 yr. The level then gradually declined during the time from 8000 yr BP to 2500 yr BP, but was periodically stable during which intervals several terraces were cut. The next 2000 yr included several rises, some to the outflow. As late as 1727 AD the lake was again at the outflow level, after which it began to desiccate. Cultural influences, both indigenous and European, are expressed in the sediments of the past 2000 yr. In 1979 the level was °25 m below the outlet and had been dropping at °0.1—0.2 m/yr.

ISSN:0012-9615    

DOI:10.2307/2937333

出版商:Ecological Society of America    

年代:1982

数据来源: WILEY