Determining Community Structure Research Articles

Highly Structured Fish Communities in Australian Desert Springs

To assess the pattern and causes of community structure, we sampled 38 isolated springs in the Dalhousie Basin of South Australia to determine the distributions of five taxa of native fishes. We visited each spring on at least two consecutive days, used several collecting methods to determine the presence of absence of each taxon, and took standardized measurements of abiotic environmental variables. Community organization was highly predictable: (1) number of species increased with spring size, (2) each species occurred in nearly all springs larger than a certain size, and (3) species composition exhibited nearly perfect nestedness. These results suggest that much of the variation in the composition of other communities may not be stochastic. When the influence of historical and environmental factors can be assessed, the colonization—extinction processes and ecological relationships that determine community structure may be highly deterministic.

Scale dependence of vegetation‐environment correlations: A case study of a North Carolina piedmont woodland

Abstract. Vegetation and its correlation with environment has been traditionally studied at a single scale of observation. If different ecological processes are dominant at different spatial and temporal scales, the results obtained from such observations will be specific to the single scale of observation employed and will lack generality. Consequently, it is important to assess whether the processes that determine community structure and function are similar at different scales, or whether, how rapidly, and under what circumstances the dominant processes change with scale of observation. Indeed, early work by Greig‐Smith and associates (Greig‐Smith 1952; Austin & Greig‐Smith 1968; see Greig‐Smith 1979; Kershaw & Looney 1985; Austin & Nicholls 1988) suggested that plant‐plant interactions are typically important at small scales, but that the physical environment dominates at large scales.Using a gridded and mapped 6.6 ha portion of the Duke Forest on the North Carolina piedmont for a case study, we examined the importance of scale in vegetation studies by testing four hypotheses. First, we hypothesized that the correlation between vegetation composition and environment should increase with increasing grain (quadrat) size. Our results support this hypothesis. Second, we hypothesized that the environmental factors most highly correlated with species composition should be similar at all grain sizes within the 6.6‐ha study area, and should be among the environmental factors strongly correlated with species composition over the much larger extent of the ca. 3500 ha Duke Forest. Our data are not consistent with either portion of this hypothesis. Third, we hypothesized that at the smaller grain sizes employed in this study (< 256 m2), the composition of the tree canopy should contribute significantly to the vegetation pattern in the under‐story. Our results do not support this hypothesis. Finally, we predicted that with increased extent of sampling, the correlation between environment and vegetation should increase. Our data suggest the opposite may be true.This study confirms that results of vegetation analyses can depend greatly on the grain and extent of the samples employed. Whenever possible, sampling should include a variety of grain sizes and a carefully selected sample extent so as to ensure that the results obtained are robust. Application of the methods used here to a variety of vegetation types could lead to a better understanding of whether different ecological processes typically dominate at different spatial scales.

The Effect of Ontogeny on Interspecific Interactions in Larval Amphibians

We used 3 x 2 x 2 factorial experiments in a temporary pond to assess the relative importance of predator density (0, 2, or 4 salamander larvae), prey density (20 or 40 tadpoles), and time of prey introduction (early or late) on interactions between predaceous larval salamanders (Ambystoma tigrinum) and their anuran prey (Pseudacris triseriata). We examined how these factors and their interactions affected mass at metamorphosis, length of larval period, growth rate, and survival. These experiments demonstrated the effect of body size on predator—prey interactions by manipulating time of introduction of size—structured prey in a system with a gape—limited predator. Microhabitat differences in primary productivity, sources of predation other than salamanders, and abiotic factors were important determinants of community structure. High levels of salamander predation negatively affected tadpole survival, but affected no other response variable. We detected no density—dependent effects in either taxon. Tadpoles in the early treatment had longer larval periods and lower survival rates than those in the late treatment. Interactions of factors, including time of introduction, were common. Biotic and abiotic factors differentially affecting growth of predator or prey appear to be responsible for these differences. Priority effects in arrival to breeding sites, or climatic events affecting the growth rate of predator, prey, or both, play a role in determining community structure. Interactions of factors complicate the predator—prey relationship. In our experiment, the effects of time of introduction are highly correlated with body size in the two temporal treatments. Our results indicate that more realistic models of predator—prey systems, or population dynamics, should incorporate size structure of predator and prey.

Distribution, population structure and feeding of subtidal seastars in the northern Gulf of St. Lawrence

The distribution and size-structure of seastar populations in the Mingan Islands, northern Gulf of St. Lawrence, were examined using quantitative sampling by SCUBA Asterias vulgaris and Leptasterias polaris are the most abundant seastars. In both species, individuals measuring 1 to 5 cm in diameter are concentrated in the rocky subtidal zone, a t a slightly greater depth for A. vulgans (4 to 7 m) than for L. polaris (0 to 3 m). With increasing size and probably the onset of sexual maturity, both species aggregate near the low tide level and join concentrations of several earlier year classes. This shift in distribution is most marked for A vulgaris. Finally, adults measuring > 15 to 20 cm in diameter are associated with mixed and sediment substrata at greater depths. The aggregation in shallow water upon attaining 5 cm in diameter probably occurs because the seastars are attracted to the mussel beds in this zone, whereas the shift to habitats at greater depths at size 15 to 20 cm may be caused by a change in food preference towards larger prey which are concentrated in the sediment zone (e.g. large bivalves). Crossaster papposus and Solaster endeca are rarer species and also show different patterns of size partitioning related to depth and substratum. Seastars appear to be the dominant predators In the northern Gulf and probably play an important role in determining community structure. A variety of interactions (competition, predation, facilitation) exist between A. vulgaris and L. polaris, and between these seastars and the most abundant benthic carnivore, the whelk Buccinum undatum.

Interactions between geomorphological processes, benthic and hyporheic communities: First results on a by‐passed canal of the french upper rhône river

AbstractOn the basis of data collected from several previous studies on the Miribel Canal, a by‐passed section of the Upper Rhône river near Lyon (France), an ecological methodology, based on geomorphology and dealing with both benthic and hyporheic layers of the river bed, has been set up and tested at the scale of the whole reach. The methodology allowed the description, by single spatial survey of, a “functional profile” of the canal incorporating both hydrological and faunistic fluxes. The ecological importance of riffles is emphasized and the results indicate that, even in this kind of disturbed channel, geomorphological processes (degradation/aggradation…) act as ecological constraints, especially in the hyporheic layers where they determine community structure by influencing groundwater circulation. The methodology proved to be a good tool over different scales for ecological studies of fluvial systems.

Size Structure of a Lake Trout (Salvelinus namaycush) Population an Arctic Lake: Influence of Angling and Implications for Fish Community Structure

The lake trout (Salvelinus namaycush) population in Toolik Lake in arctic Alaska was assessed by gill net during 1977, prior to exploitation. In 1986 we replicated the gill net study to determine the effects of recent angling pressure on the fish community. Relative composition of the catch varied between samples. Round whitefish (Prosopium cylindraceum), which are not susceptible to angling, made up 42% of the catch in 1986 and 28% in 1977. Significant reductions in the median length and weight of lake trout between the sampling periods were observed. The median size of lake trout in 1986 was below the minimum 1977 reproductive size. Differences in median length and weight of grayling (Thymallus arcticus) and of round whitefish during this period were not significantly different. The condition factor of lake trout was significantly higher in 1986. We suggest that large lake trout in Toolik Lake control salmonid recruitment and determine community structure. With continued fishing pressure further changes in the fish community and lake trophic dynamics may result.

Zooplankton communities and acidification processes (a review)

A review of the literature which deals the with zooplankton of acid-stressed ecosystems revealed several strong patterns, weaker patterns, areas of controversy or inconsistency, and poorly understood but potentially important factors. Those patterns which are strong, but not necessarily universal, include a general response to short term pH shocks suggesting that daphnids are least tolerant followed by other cladocerans, copepods, and chaoborids. In addition, the numbers of both crustacean and rotifer species are decreased, and the importance of daphnids is reduced in acidic lakes, while limnetic hemipterans are greatly increased in acidic fishless lakes. Weaker patterns include reduced zooplankton biomass, reduced importance of cyclopoids, and increased representation of generalist species in acidic lakes, and intraspecies differences in pH tolerance indicating that populations of acid-stressed ecosystems are more tolerant. But, zooplankton biomass is apparently a result of system productivity as opposed to pH per se. Inconsistencies within the literature exist as to whether chaoborids are more abundant in acidic fishless lakes, the influence of physiological stress contra predation in determining community structure, the immediate response of the zooplankton to lake neutralization, and the toxicity of Al. The importance of metals, humic substances, and the fauna in determining community dynamics is poorly understood. Although both phyto- and zooplankton communities can be severely simplified in acidic lakes, the influence of the phytoplankton on the zooplankton, beyond the relation edible biomass vs zooplankton standing stock, in these ecosystems is unclear. From the literature it is apparent that direct physiological influences of acid-stress are important, but that indirect (biotic) influences and variables which correlate with pH are often as important if not more so to the zooplankton.

The effect of stream regulation on the physico‐chemical properties of the palmiet river, South Africa

AbstractThe effect of regulation, by multiple impoundment, on ‘discontinuity’ (recovery) ‘distances’ (sensu Ward and Stanford, 1983) for various physico‐chemical attributes of the Palmiet River (a short, steep gradient, cool temperate, acid, low‐nutrient system) was investigated. Physico‐chemical features were measured monthly, from February 1986 to August 1987, at 11 sites along the river, but concentrating on two zones: one below an upper‐reach impoundment, Nuweberg Dam, and one below a middle‐reach impoundment, Arieskraal Dam. Median total suspended solids, nitrates, nitrites and ammonium all increased below both dams with varying levels of ‘recovery’. Median spot temperatures, pH and alkalinity increased below Nuweberg, but decreased below Arieskraal Dam, whereas the converse was true for soluble reactive phosphate. Nuweberg had no effect on conductivity and total dissolved solids, while Arieskraal caused a slight decrease. The river remained well oxygenated down its length.Distinct seasonal variations were recorded for: flow rate, with flow reversal below Nuweberg and flow constancy below Arieskraal; temperature, with seasonal shifts and dampening of annual temperature range below both dams; alkalinity, with seasonal shifts below Arieskraal, and nitrites and nitrates, but not for any other attributes. At least partial recovery was noted in all cases, but in some instances, recovery was incomplete to the estuary. The implications of such alterations, for invertebrates, in terms of reduction in habitat diversity as well as for food supply must be considerable and studies are currently under way to determine community structure and diversity.

The influence of predation and substratal complexity on recruitment to settlement plates: a test of the theory of alternative states

There are radical differences between the benthic communities occupying subtidal hard substrata at Marcus and Malgas Islands on the South African west coast, even although the islands are closely situated and experience similar physical conditions. Experimental settlement plates were installed at both islands to examine the process of succession by the benthos and as a partial test of the hypothesis that the two communities are alternative states of the same ecosystem. The plates were of different types of substrata of varying complexity, and half were caged to exclude predators while the remainder were left unprotected. The results indicate that predation by the abundant rock lobster Jasus lalandii is the main force maintaining the sublittoral community at Malgas Island. Because this major predator is virtually absent from Marcus Island, wave action and inter/intraspecific competition are considered more important in determining community structure there. At Malgas Island, unprotected plates became covered by algae within 4 months, thereby resembling the natural biota. In contrast, the protected plates at Malgas developed a community closely similar to that on the plates at Marcus Island, showing that, when protection is provided from rock lobsters, the community at Malgas has the capacity to converge on that of Marcus. In the absence of rock-lobster predation, plates which had pre-established mussels on them experienced the highest rate of recruitment and initially supported the largest variety of species. In the case of plates that experienced either extremely high or extremely low levels of predation, the complexity of the substratum ultimately had no influence on species richness: after 6 months richness was uniformly high on plates that were exposed to virtually no predation, and uniformly low on plates subjected to intense predation. Plates experiencing intermediate levels of predation consistently supported a higher number of species if they had a complex biotic substratum. At Malgas, the early colonists always comprised sessile space-occupying species which were established by larval settlement. At Marcus, postlarval juveniles and adults of mobile species rapidly moved onto the plates and influenced subsequent recruitment. These contrasts reflect the structure of the two communities which are determined by differing levels of rock-lobster predation.

Helminth Parasites of Sympatric Salamanders: Ecological Concepts at Infracommunity, Component and Compound Community Levels

Four species assemblages of desmognathine salamanders are sympatric in and along many mountain streams of the southern Appalachians. The species, Leurognathus marmorata, Desmognathus quadramaculatus, D. monticola and D. ochrophaeus, span an aquatic to terrestrial habitat continuum ranging from totally aquatic (L. marmorata) to primarily terrestrial (D. ochrophaeus). Such a system afforded the opportunity of examining concepts of helminth parasite ecology within the context of several recent theoretical predictions. Parasite infracommuinities (populations of all helminth species within individual hosts) in salamanders are isolationist in character. That is, parasite prevalence (percentage of hosts infected with a given parasite species) and mean intensity (mean number of parasites of a given species within a species of host) are low, and as a consequence there is little potential for competitive interactions. Isolationist infracommunities in salamanders arise from factors including a simple enteric system, restricted vagility and ectothermic generalist insectivory. Salamanders have a broad prey base and they do not focus on any particular prey species. Transmission of helminths by intermediate hosts is unimportant and contributes to depauperate helminth faunas dominated by nematodes with direct life cycles. The least diverse infracommunities were associated with Leurognathus marmorata while Desmognathus quadramaculatus had the most diverse fauna. Patterns of prevalence and intensity of infection revealed that salamander helminths were, in general, not host specific. Rather, they are generalists and their distributions can be correlated with host size (age), diet and habitat preferences. INTRODUCTION A principal focus of community ecology is to interpret the effects of competition, predation, mutualism and parasitism, as well as abiotic factors, in determining community structure. Ecological theory has been largely derived from studies on free-living animals with little attention focused on specialist organisms such as parasites. Price (1980, 1984) emphasized that small, highly specialized organisms are very different in many aspects of their biology from larger, more generalized animals so that communities of specialists may be organized in fundamentally different ways from generalist communities. The current literature reveals a predominance of nonequilibrial communities in nature among specialists, in which biotic interactions play a minor role in maintaining community structure (Price, 1984). These are in marked contrast to the equilibrium communities, many of which conform to the predictions of classic competition-based theory (see Strong et al., 1984; Price et al., 1984, and references therein). As stated by Price (1984), It is valuable to examine the extent to which specialist communities support or contradict the paradigms' Despite their biological uniqueness and complexity, helminth parasites possess certain attributes which should permit them to contribute significantly to several basic concepts in community theory. First, helminth communities have unambiguous boundaries

Is interspecific competition a major structuring force in animal communities?

Until recently the view that competition is the dominant ecological interaction was the prevailing one. Interspecific competition was widely regarded as a principal mechanism in determining community structure and organization and thus the distribution and abundance of species. The volume of literature that provides indirect evidence in favour of competition (observational approach) greatly exceeds the number of studies that provide direct evidence (experimental approach). In part for this reason the importance of competition in community ecology is questioned by some ecologists. The strongest evidence for competition is derived from controlled field experiments which manipulate the abundancies of putative competitor species. It is stressed that to be able to study competition in the field and to test its theories, interaction coefficients have to be measured. In community studies the question should be asked how important competition, relative to other processes, is. A mechanistic perspective could be a powerful heuristic tool for community ecologists.

Microfungal community structure from forest soils in southern Quebec, using discriminant function and factor analysis

Fungal isolation data, from four forest soils, were analyzed using both discriminant analysis and factor analysis, to investigate the structure of fungal communities. The analyses indicated that the forest of origin was a major factor determining fungal community structure in these soils. Discriminant analysis separated the fungal communities of coniferous-forest soil samples from those of deciduous-forest soil samples. The pine-forest soil, which was in an early stage of invasion by hardwoods, had a fungal community intermediate in structure between the extremes. Of the microfungi isolated, Botryotrichum piluliferum, Farrowia seminuda, Geomyces pannorus, Mucor hiemalis, Penicillium janthinellum, Trichoderma polysporum, and Zygorrhynchus moelleri were associated with the coniferous sites, whereas an Acremonium species, Gliomastix murorum, Paecilomyces carneus, Pae. fumosoroseus, Penicillium spinulosum, and Pen. thomii characterized the deciduous-forest soils. Many pine-forest soil fungi were species abundant in the other soils, a possible reflection of the successional stage of the forest. Both factor analysis and discriminant analysis yielded similar interpretations of the data and indicated the importance of fungal interactions in determining community structure. The discriminant analyses also showed that fungal isolation data gave a better separation between the soils of the four forests than did abiotic data.

INTERACTIONS AMONG TEMPORARY AND PERMANENT MEIOFAUNA: OBSERVATIONS ON THE FEEDING AND BEHAVIOR OF SELECTED TAXA.

Meiofauna diets and behavioral patterns are relatively unknown despite the fact that in any system, predatory relationships and behavioral responses may play an important role in determining community structure. Therefore, observations on food preferences, feeding behavior, and encounter interactions of members of a meiofauna assemblage were made in the laboratory in dishes of natural sediment. The diets and behavior of two turbellarian species, Neochildia fusca and Archiloa wilsoni were examined in detail. Both are predators and both feed on a variety of other taxa, including the temporary meiofauna. Based upon the results of these experiments, a potential food web was constructed among the temporary and permanent meiofauna. The behavioral responses of these turbellarians and other members of the meiofaunal assemblage at times of encounter were also observed, categorized, and quantified. Implications of these behaviors are discussed.

Notes on the Population Ecology of Perognathus fallax (Rodentia: Heteromyidae) in Southern California

-A population of the San Diego pocket mouse, Perognathus fallax, from San Diego County, California, was monitored by monthly live trapping from September, 1978, through August, 1980. Peaks in population size were observed each spring and the precise timing of these peaks varied between years. A dramatic decline in density occurred as the study was concluded in the summer of 1980. Reproductive activity displayed significant seasonal heterogeneity, with the highest proportions of breeding adults being observed in the spring. Similarly, survival was also found to vary seasonally, with significantly lower survival in the spring. There were no significant differences in survival rates between sexes. The average P. fallax remained on the study site for 5.2 ? 1.4 months after first capture. The deserts of North America support diverse rodent communities and the primary thrust of recent ecological research on these communities has been to identify factors which regulate species diversity and determine community structure (Brown, 1973; Hafner, 1977; Rosenzweig and Sterner, 1970). Although life history characteristics of component populations may be important to patterns of species diversity (M'Closkey, 1972), there is a general paucity of detailed demographic studies on rodents inhabiting arid environments (Conley et al., 1977). The San Diego pocket mouse (Perognathus fallax Merriam) is one of seven perognathine species occurring in San Diego County, California (Bond, 1977). It inhabits open, sandy areas of both the Upper and Lower Sonoran life-zones of southwestern California and northern Baja California (Bond, 1977; Ingles, 1965). Adult individuals weigh about 20 g and rely primarily on seeds for food (Meserve, 1976). Torpor is employed during periods of cold weather (Bartholomew and Cade, 1957; MacMillen, 1964) and it has been shown that P. fallax survival is independent of drinking water (MacMillen, 1964). Several workers have reported the presence of P. fallax on their study plots (M'Closkey, 1972; MacMillen, 1964; Meserve, 1976), but only general information about the population ecology of this species was gathered in these studies because of low densities and sporadic

Effect of epibenthic macropredators on community structure in subtidal organically enriched sediments in the inner Oslofjord

Predator exclusion experiments were performed on an unvegetated low-energy subtidal mudflat. Exclosures were maintained for periods from 2 to 5 mo to protect infauna from epibenthic macropredators. Recruitment was studied by plac~ng trays with azoic sediment on the bottom. Predation effects were determined by comparing faunal composition and abundance on natural sediment and in recruitment trays inside and outside the cages. Community structure was not affected by the cages. There was a tendency for several specles to increase In abundance inside the cages on the natural sediment, but only a few increases were significant. Habitat modifications under the cages were small. It is concluded that in the subtidal of the Oslofjord macropredators are not important in determining community structure in sediments, and that there is only a small effect of macropredators controlling densities of infauna.

The Pattern of Colonization of Epiphytes on Calabash Trees (Crescentia alata HBK.) in Guanacaste Province, Costa Rica

We examined the structure of the community of epiphytes growing on Crescentia alata trees in Santa Rosa National Park, Costa Rica. Four orchid species were dominant in this community, showing a distinct order of colonization of the calabash trees, with Oncidium cebolleta colonizing first, Encyclia cordigera second, Brassavola nodosa third, and Laelia rubescens fourth. We were unable to find any differentiation of the locations of these species based on the physical characteristics of the sites. It is hypothesized that the colonization pattern observed is due to the quantity of propagules produced by each species. THE NUMBER OF SPECIES found in a community is determined by five major factors-the time for colonization, the distance over which colonizers must travel, the number of potential colonizers, the size of the area to be colonized, and the biotic interactions between colonizers (MacArthur and Wilson 1967). Two approaches have been used to determine the effects of these factors upon the community organization of oceanic islands (e.g., Diamond 1969, Simberloff and Wilson 1969) and a variety of island-like situations (e.g., Vuilleumier 1970, Brown 1971, Culver, Holsinger, and Baroody 1973, Opler 1974). One approach applied to both plant (Johnson, Mason, and Raven 1965) and animal (Hamilton, Barth, and Rubinoff 1964, Wilson and Taylor 1967) communities has been to use correlative techniques to describe the patterns of species number and distribution from to on a large geographical scale. The second approach, applied exclusively to animal communities, has been to manipulate the five factors experimentally (e.g., Simberloff and Wilson 1969, Addicott 1974, Schoener 1974). In this paper we describe the community structure of plants occurring on a local scale using correlative techniques and suggest that this system is one in which experimental work on the relative roles of these five factors in determining community structures may be performed. STUDY AREA AND METHODS Individual Crescentia alata HBK, the calabash tree (Bignoniaceae), constitute an island-like situation for epiphytic plants. The calabash tree is an ideal epiphyte host because its soft, deep bark provides easy anchorage for epiphytes and recess for humus accumulations (Pittendrigh 1948). The study site, a 10 ha calabash orchard in Santa Rosa National Park, Guanacaste Province, Costa Rica, has been described by Janzen et al. (1980). It is surrounded by dry, deciduous thorn-forest, the dominant vegetation type in this region (Holdridge 1967). No individual calabash tree at this site is located more than 100 m from the edge of the thorn-forest. The spherical canopies of the calabash trees range from 0.5 to 7.5 m radius. Since trees are spaced at least 5 m apart, canopies of individuals rarely intertwine. Each calabash tree was considered an island which may be colonized by epiphytic plants: one cactus species, three bromeliads, and five species of orchids. Nomenclature is that of Janzen and Liesner ( 1980). We measured the canopy radius of a randomly chosen calabash tree and recorded the species of epiphytes present (sample sizes are indicated in figure 1). Canopy radius was used because it was both easily measured and accurate with repetition. The existence of a species-area relationship was investigated by means of regression analysis using a logarithmic curve fit after making a scatter plot of the data. To explore the temporal pattern of colonization, we calculated the percent occurrence of each epiphytic species in one-species, two-species, three-species, etc. communities. To study the role of biotic interactions, we contrasted the relative abundance of each epiphytic species on small trees (less than 2 m canopy radius) with that on large trees (greater than 4 m canopy radius). To obtain these numbers, at least two branches were randomly chosen on a minimum of five inBIOTROPICA 14(2): 137-14

Structure and Function of Two Antarctic Terrestrial Moss Communities

Two bryophyte—dominated communities in the maritime Antarctic are analyzed in terms of the transfer and standing crops of organic matter within them. A moss turf dominated by Polytrichum alpestre and Chorisodontium aciphyllum and a moss carpet composed of Calliergon sarmentosum, Calliergidium austro—stramineum, and Drepanocladus uncinatus with the liverwort Cephaloziella varians were investigated at Signy Island, South Orkney Islands. Biomass, respiration, and production data for the primary producers (mosses, lichens, liverworts, and algae), for the fauna (Protozoa, Rotifera, Tardigrada, Nematoda, Acari, and Collembola), and for the microflora were synthesized, and annual rates of consumption, egestion, assimilation, and production were derived. The two communities showed similar levels of productivity, trophic structure, and efficiencies of organic matter transfer, but different Collembola and Acari standing crops, turnover of mosses, and accumulation of dead organic matter. These features are discussed in relation to the role of biotic and abiotic variables in determining community structure and function.

Benthic infaunal variability on a transect in the Gulf of Mexico

Macroinfaunal benthos off the South Texas Coast of the Gulf of Mexicoformed different assemblages distributed according to depth: shallow (22 m), mid-depth (36 to 49 m) and deep water (78 to 131 m). Species composition of shallower stations were less diverse, composed of eurytopic and opportunistic species adapted to a fluctuating environment. The deep water benthos, in a more stable environment, had a higher diversity. Sediment composition (high proportions of silt) at the mid-depth stations resulted in dominance of deposit feeders. The environmental gradient was related to a species continuum which changed from polychaete dominated groups in shallow water, to deposit feeding molluscs and crustaceans, to a deep water diverse fauna not dominated by any particular group. Environmental heterogeneity, including climatic variability, may be most important in controlling shallow water benthos. In deeper more stable shelf habitats where diversities are higher and species equilibrium is the case, species interactions may determine community structure.

The evolution of insect/vertebrate associations

The evolution of close vertebrate associations has occurred in seven orders of insects, resulting in a great diversity of interactions which range from commensalism to true parasitism. The evolution of each taxon of vertebrate associates is discussed in turn, some new ideas on the development of certain groups are presented and, on a broader scale, a general model for the evolution of ectoparasitic insects is proposed. It argues that all vertebrate associates have evolved along one of two macroevolutionary pathways which differ only in the sequencing of adaptations facilitating host association and host feeding. These pathways lead to parasite types which differ greatly in their life history and intimacy of host association. Some microevolutionary processes influencing the diversification of ectoparasites are discussed, in particular the process of insect/vertebrate coevolution and the forms this may take. Host specificity, one consequence of coevolution, is recognised as an important factor influencing the structure of ectoparasite communities, and a hypothesis is presented that competition between ectoparasite species, mediated by host defensive responses, is also important in determining community structure.

Limiting nutrient patchiness and its rôle in phytoplankton ecology

Limiting nutrient patchiness is examined as a factor affecting the community structure and species succession of natural phytoplankton communities held in ammonia limited continuous culture at a dilution rate of 0.3 day −1. It was found that under a homogeneous distribution of the limiting nutrient members of genus Chaetoceros dominated and when ammonia was added daily (patchy distribution), Skeletonema dominated. Intermediate patchiness gave rise to an assemblage dominated by both Chaetoceros and Skeletonema. The nutrient uptake ability of each assemblage was determined three weeks after experiment initiation. Each assemblage was best able to optimize uptake of ammonia under its particular patchy nutrient regime. Optimization of a patchy environment took place by an increased maximal uptake rate ( V max ) while optimization of a homogeneous environment appeared to take place by increased substrate affinity (i.e., low K s ). This study demonstrates that limiting nutrient patchiness can alter the relative abundance of populations within a community based on each population's ability to exploit the limiting resource under a particular degree of patchiness. We also show that coexistence of two populations might be expected due to the patchiness of a single limiting nutrient. The importance of patchiness in relation to other factors which determine community structure is discussed.