Food-Chains and the Food Cycle in Hawaiian Fish Ponds.–Part II. Biotic Interaction

The distribution of Notonecta in relation to aquatic vegetation and water depth was studied in two ponds, one with predaceous fish, another without. Notonectid density was lower but the number of species was greater in the pond with fish. The dominant species in the fish pond (N. lunata) was strongly associated with vegetation throughout its life cycle, while the dominant species in the pond without fish (N. undulata) was associated with vegetation only during the early instars. N. undulata also occurred in the pond with fish, but at greatly reduced densities. The overall distribution of the two dominant notonectid species was random among the three water depth classes studied (< 15 cm, 15-30 cm, 30-65 cm) in both ponds. Each pond also contained a large notonectid species in low densities. The large species in the pond without fish (N. insulata) was characteristically found in open, deeper water where the substrate was bare or sparsely vegetated. The large species in the fish pond (N. irrorata) appeared to be randomly distributed with respect to vegetation density and water depth, and was mainly restricted to heavily shaded parts of the habitat.

The food and feeding habits of the three most important market fish in Hawaiian ponds (Mugil cephalus, Chanos chanos, and Elops machnata) were analyzed to ascertain the position of these fish in the food-chains and the food cycle occurring within the ponds. Mullet (M. cephalus) subsist primarily on littoral diatoms and blue-green algae, the presence of a unique pharyngeal filtering device prohibiting them from swallowing larger forms. No variation in food because of season or size of fish was found. Milkfish (C. chanos) are also herbivores; juveniles consume unicellular and other minute algae while larger fish take an increasing amount of larger filamentous algae. Competition for food between mullet and milkfish is greater than hitherto supposed and cannot be diminished under present operational procedures. An increased food supply for both species probably could be achieved by the application of inorganic fertilizers together with the construction of additional shallow areas in the ponds. Ten-pounders (E. machnata) subsist largely on mosquito fish and shrimps; no juvenile mullet or milkfish were found in 164 specimens of various sizes taken throughout the year. This species apparently damages pond fisheries less than is commonly supposed; in fact, ten-pounders may prove an asset to the industry.

Intra- and interspecific competition for food (Myzus persicae) in larvae ofSyrphus ribesii andS. corollae were investigated in the laboratory at 20°, 16 hrs light. A competing unit consisted originally of six newly hatched larvae, trhee of each species or six of one species.S. ribesii had a low larval survival (about 35–40%) and a prolonged larval period (about 16–17 days) under both intra- and interspecific conditions, although food consumption was greater when mixed withS. corollae. InS. corollae, larval survival was significantly smaller when mixed withS. ribesii than under intraspecific conditions (25.0% and 73.1%). Larval period was somewhat increased in the mixed series.

David Lack (1947, 1969) argued that interspecific competition for food has played a significant role in the adaptive radiation of Darwin's Finches. He regularly invoked competition to explain morphological and distributional patterns in the finches. An important example is the altitudinal distribution of Geospiza difficilis in relation to G. fuliginosa (Fig. 1). Low elevation, dry islands in the Galapagos are occupied by one of the two species, but never both. Islands possessing a highland vegetation zone may have both species but here they are altitudinally segregated, with G. fuliginosa occupying the lowlands and G. difficilis the highlands. Lack noted the morphological similarity of these finch species and suggested that G. difficilis had been excluded on low islands or altitudinally restricted on high islands through competition for food by G. fuliginosa wherever the species had come into contact. Bowman (1961), however, suggested instead that the unique food requirements of the two species and differences in food supply among islands, once known, would suffice to explain the pattern. The ecology of these two species was recently investigated over a calendar year on Isla Pinta where they coexist (Schluter, 1982a, 1982b). Surveys conducted on this island and on Islas San Salvador and Fernandina (unpubl. observ.) revealed that G. difficilis and G. fuliginosa are not allopatric as Lack believed. Their altitudinal ranges overlap extensively year-round although they are not coextensive (Fig. 2). Detailed observations of behavior and abundance strongly suggested that interspecific competition between them is presently weak. Food supply was probably limiting over part of the year, but interspecific aggression (e.g., territoriality) was not observed, and their diets and feeding positions were very different. The distributions and abundance of both species along the elevational gradient on Isla Pinta could easily be interpreted as being determined by the availability of their different foods. The question of whether past competition has influenced the distribution of G. difficilis in relation to G. fuliginosa remains. Significantly, G. difficilis varies in size and shape from island to island. For example on Isla Genovesa the species resembles the missing G. fuliginosa (Lack, 1947; see also Grant, in press). Possibly G. difficilis has adapted to exploit lowland environments on islands without a highland zone, and it is this lowland form which cannot coexist with G. fuliginosa. Observed differences in sympatry might also be the result of past competition. Lack's competition hypothesis is therefore still consistent with the observed insular distributions of these two species. Here we report the results of a field study which tested the argument that competition has influenced the distributions on low islands, against two alternative hypotheses based on Bowman's (1961) suggestions. We use information gained on diets, density and food supply of G. difficilis and G. fuliginosa on Pinta (Schluter, 1982b) to predict attributes of the two species in allopatry. The three hypotheses make different predictions, and these are tested with field data. The allopatric populations we used were G. difficilis on Genovesa and G. fuliginosa on Marchena (Fig. 2).

Biotic communities in highly variable, frequently perturbed habitats are usually ex- pected to be structured mainly by abiotic factors. In the highly variable deep-sea hydrothermal vent environment, physical and chemical factors are known to play an important role in limiting the dis- tribution and abundance of species, but the importance of biotic interactions remains largely unre- solved. The high density and biomass attained by the vent macrofauna suggest that resource parti- tioning and competition may be significant in these communities. This study of food resource utilisation at northeast Pacific deep-sea hydrothermal vents uses an approach based on stable carbon and nitrogen isotope analyses to characterise trophic interactions between the 3 dominant alvinellid polychaetes, Paralvinella palmiformis, P. sulfincola, and P. pandorae. We also examine size structure in sympatric and allopatric populations of P. palmiformis and P. pandorae. Results indicate that food resource partitioning occurs both intra- and interspecifically in P. palmiformis and P. sulfincola, and we advocate that this process contributes to explaining their co-existence at very high densities. In contrast, P. pandorae has a much more restricted trophic niche, overlaps P. palmiformis in diet and is much smaller in size when found in sympatry with P. palmiformis. P. pandorae is the most likely of the 3 species to be affected by intra- and/or interspecific competition for food, and this may explain the drastic change in the population size structure observed between successive years. Our work indi- cates that within the limits imposed by environmental conditions, biotic interactions such as food resource partitioning and competition can be significant factors structuring deep-sea hydrothermal vent communities.

The wood frog, Rana sylvatica, and the leopard frog, Rana pipiens, occur sympatrically over much of northern North America. Within this area the two appear to show habitat separation. Previous studies have given circumstantial evidence that competition leading to the habitat separation might be occurring between tadpoles of these species. In a pond on the University of Michigan's Edwin S. George Reserve, tadpoles were raised in enclosures where species composition and density of experimental populations were controlled. A preliminary experiment was designed to determine the influence of density and mixing of species on tadpole survival and production of biomass. In a second experiment, in addition to density, food and predation levels were manipulated to determine which were influential in the responses observed. The behavior of tadpoles in the 2 yr was markedly different. In the first experiment tadpoles behaved as if there were no limiting carrying capacity to the pond; mean survivorship was about 14%. There was no evidence of competition. In the second experiment the pond's carrying capacity appeared to be near the lowest density level tested. Both species reacted strongly to each other, behaving like ecological equals. Tadpole survivorship proved sensitive to both predation and food levels. At high predation levels, the effect of mixing species was reduced. Although mixed populations performed less well than those raised alone, I was unable to alter the interaction by adding food to experimental populations even though intraspecies competitive effects were reduced. The original concept of this ecological system was that it would be controlled either by food or by predators. The results of both experiments suggest that this concept was erroneous. Both factors act jointly. The role of food must not be studied without recognition of the extent to which predators can change the impact of tadpoles on their food supply, and vice versa. The results of the first experiment suggest that interspecific competition for food under natural conditions can eventually be demonstrated, because food levels can be sufficiently increased to negate the influence of one species on the other. As ecological systems are feedback systems, future field studies must become more experimental if they are to reveal the underlying mechanisms determining the structure and function of ecosystems.

Simple SummaryOrganisms that are similar in size, morphological characteristics, and adaptations, including vertebrates, often coexist by partitioning the available resources (food, space, and time). So, studies of the dynamics of these cases of coexistence are scientifically interesting. Here, we study a coexistence case between two species of freshwater turtles inhabiting the forest waterbodies of West Africa, focusing on the dietary habits of the two species. We found that both turtle species are omnivorous generalists, eating both vegetal and animal matter abundantly. However, there were clear interspecific differences, with the larger of the two species (P. cupulatta) eating more vertebrates (mainly fish but occasionally other vertebrates), whereas P. castaneus consumed more invertebrates. We also showed that interspecific competition for food does not occur between these two species; instead, previous studies demonstrated that a clear partitioning of the habitat niche occurs.Organisms that are similar in size, morphological characteristics, and adaptations, including vertebrates, often coexist by partitioning the available resources (food, space, and time). So, studies of the dynamics of these cases of coexistence are scientifically interesting. Here, we study a coexistence case of two species of freshwater turtles inhabiting the forest waterbodies of West Africa, focusing on the dietary habits of the two species. We found that both turtle species are omnivorous generalists, eating both vegetal and animal matter abundantly. However, there were clear interspecific differences, with the larger of the two species (P. cupulatta) eating more vertebrates (mainly fish but occasionally other vertebrates), whereas P. castaneus consumed more invertebrates. These patterns appeared consistently within the species and across sites, highlighting that the same patterns were likely in other conspecific populations from the Upper Guinean forest streams (Côte d’Ivoire and Liberia). Our study also showed that interspecific competition for food does not occur between these two species; instead, previous studies uncovered that a clear partitioning of the habitat niche occurs. We conclude that the food resource is likely unlimited in the study areas, as it is not the case in more arid environments (since food shortages may occur during the dry season). We anticipate that, within the Pelomedusidae communities throughout Africa, intense competition for food probably occurs in the Sahel and Sudanian vegetation zones, particularly during the dry months, but is unlikely within the Guinea and wet savannah region and even less likely in the Guineo-Congolian rainforest region.

Warming increases competition among plant species in alpine communities by ameliorating harsh environmental conditions, such as low temperatures. Grazing, as the main human activity, may mitigate the effect of warming, as previously reported. However, it is critical to refine the effects of warming on biotic interactions among species, for example, by taking the competitive ability of species into consideration. Based on a 10-year warming and grazing experiment in a Tibetan alpine meadow, we evaluated interspecific biotic interactions of dominant and subordinate species, using the approach of interspecific spatial associations. Warming significantly increased competition between subordinate and dominant species as well as among subordinate species, but not among dominant species. Moreover, facilitation of dominant-subordinate species also increased under warming. Simulated rotational grazing had similar effects to warming, with increasing interspecific competition. Our results show that, when studying the effects of warming on biotic interactions among species, it is necessary to characterize different species pairs relative to their competitive ability, and that simulated rotational grazing does not mitigate the effects of warming in the long term. Our results also provide evidence that the spatial pattern of species is a critical mechanism in species coexistence.

The spatial distribution pattern and population structure of trees are shaped by multiple processes, such as species characteristics, environmental factors, and intraspecific and interspecific interactions. Studying the spatial distribution patterns of species, species associations, and their relationships with environmental factors is conducive to uncovering the mechanisms of biodiversity maintenance and exploring the underlying ecological processes of community stability and succession. This study was conducted in a 25-ha Qinling Huangguan forest (warm-temperate, deciduous, broad-leaved) dynamic monitoring plot. We used univariate and bivariate g(r) functions of the point pattern analysis method to evaluate the spatial distribution patterns of dominant tree species within the community, and the intra- and interspecific associations among different life-history stages. Complete spatial randomness and heterogeneous Poisson were used to reveal the potential process of community construction. We also used Berman’s test to determine the effect of three topographic variables on the distribution of dominant species. The results indicated that all dominant species in this community showed small-scale aggregation distribution. When we excluded the influence of environmental heterogeneity, the degree of aggregation distribution of each dominant species tended to decrease, and the trees mainly showed random or uniform distribution. This showed that environmental heterogeneity significantly affects the spatial distribution of tree species. Dominant species mainly showed positive associations with one another among different life-history stages, while negative associations prevailed among different tree species. Furthermore, we found that the associations between species were characterized by interspecific competition. Berman’s test results under the assumption of complete spatial randomness showed that the distribution of each dominant species was mainly affected by slope and convexity.

Here we provide an alternative interpretation to that of Pistorius et al. (2001), concerning density-dependent increases in fecundity resulting in population regulation of the southern elephant seal population at Marion Island. We do not contradict the findings of Pistorius et al. (2001), because it does appear: (1) that a change in fecundity has been observed, and (2) that some factor related to food supply is the most likely cause for an observed population decline and increase in reproductive performance. The main observation leading to the interpretation of density-dependent feedback in the population of southern elephant seals at Marion Island (one of the Prince Edward Islands) is that there has been a reduction in the population's rate of decline in recent years (reported by Pistorius et al. (1999b)), and that this could have resulted from a per capita increase in food availability. However, because rates of population change are rarely linearly constant, changes in population size should be expressed on a logarithmic, rather than a linear scale, as used by Pistorius et al. (1999b). Re-plotting the linear values of Pistorius et al. (1999b) on the natural logarithmic scale gave no clear change in the rate of population decline; therefore, we conclude that the rate of population change (decline) has remained constant from 1986 to 1997 (r=–0.0439). The Marion Island population is part of the larger Kerguelen population, and there might be considerable overlap in the foraging areas, and possibly prey, exploited by elephant seals from all sub-populations within this larger population. Changes in the number of intra-specific resource competitors at Marion Island are therefore unlikely to alter per capita food availability since the Marion population constitutes approximately 1% of the total Kerguelen population. We propose an alternative hypothesis that the present data support a mechanism driving the proposed increase in per capita food supply through changes in either: (1) inter-specific food competition, (2) rates of predation, (3) changes in weather pattern or (4) disease.

Interspecific competitive interactions among sessile epibenthos were studied by suspending PVC panels at Tomioka Bay, south Japan, for a maximum period of 16 mo. Interactions were monitored from photographs of a fixed area of the panels. Four panels were suspended during two different months in autumn 1991, and the development of the community was followed until December 1992. Altogether, 6511 interspecific overgrowth interactions were recorded, of which 37 resulted in standoffs and the rest in overgrowths. The competitive relationship observed in this sessile assemblage followed the pattern of a hierarchy with numerous backloops. Among the 36 species, belonging to the seven taxonomic groups encountered during the study, the colonial ascidian Didemnum moseleyi was recorded as the dominant species (with respect to competitive ability) while the barnacle Balanus trigonus was the weakest species. The month of panel exposure and whether or not the panel surface was shaded had a significant influence on the competitive ability of the sessile organisms. The order of hierarchy of the most dominant species changed with the month of panel submersion and its light conditions. Among the several abundant species tested, longer residence times were recorded for serpulid worms than for the colonial species. A significant, positive relationship was obtained between the areal cover of competitively dominant sessile organisms and the number of their interspecific interactions. From the short residence time of sessile organisms and the significant relationship between their areal cover and number of interspecific interactions, it is concluded that the interspecific interactions played important role in the species succession.

The quantitative food consumption and somatic growth of Atlantic salmonSalmo salarparr were compared between three sub‐Arctic rivers in northern Norway and Finland, addressing the potential occurrence of resource limitation and interspecific competition. In one of the rivers, previous resource partitioning studies have suggested severe dietary competition between juvenileS. salarand a dense population of alpine bullheadsCottus poecilopus. It was hypothesized thatS. salarparr in this river would have restricted food consumption and growth rates compared to theS. salarpopulations in the other two rivers where interspecific competition was less likely to occur. The feeding and growth performance differed significantly between theS. salarpopulations. The lowest food acquisition and growth rates were in theS. salarparr population living in sympatry withC. poecilopus, confirming a restricted food supply for theS. salarparr and providing empirical support for the presence of resource limitation and interspecific food competition in this river system. The study reveals thatS. salarparr in sub‐Arctic rivers may experience food limitations resulting in diminished growth rates.

– To elucidate the performances of perch and ruffe in oligotrophic lakes, we carried out a field study in reoligotrophic Upper Lake Constance. Both these percids used the same habitat, albeit with different activity patterns. Interspecific competition for food was relevant only in summer when both species fed on zoobenthos. Even then, niche overlap was low, while intraspecific diet overlap was moderate to high throughout the season. Perch did not perform fixed, ontogenetic diet shifts, but used a wide range of prey. During spring and early summer, all size classes were planktivorous, then switched to benthivory and cannibalism in summer, and part of the population reverted to planktivory in autumn. Ruffe, by contrast, fed mainly on chironomid larvae and pupae throughout the year. It is suggested that in lakes of low productivity the euryphagous characteristics of perch, including cannibalism, provide a clear advantage over the benthivorous specialist ruffe in two ways: (i) it allows perch to switch to alternative prey types if one prey type becomes scarce; and (ii) reduces both intra‐ and interspecific competition for food.

Impact of fouling organisms on mussel rope culture: interspecific competition for food among suspension-feeding invertebrates

Microalgae were collected from 48 locations in the Hawaiian Islands in 1985. The sites were an aquaculture tank; a coral reef; bays; a geothermal steam vent; Hawaiian fish ponds; a Hawaiian salt punawai (well); the ocean; river mouths; saline lakes; saline pools; saline ponds; a saline swamp; and the ponds, drainage ditches and sumps of commercial shrimp farms. From 4,800 isolations, 100 of the most productive clones were selected to be maintained by periodic transfer to sterile medium. Five clones were tested for growth rate and production in a full-spectrum-transmitting solarium.