Competition For Food Resources Research Articles

Competition between perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus): the advantage of turning night into day

Summary1. The outcome of interspecific competition for food resources depends both on the competitors’ sensory abilities and on environmental conditions. In laboratory experiments we tested the influence of daylight and darkness on feeding behaviour and specific growth rate (SGR) of two species with different sensory abilities.2. We used perch (Perca fluviatilis) as a visually orientated, and ruffe (Gymnocephalus cernuus) as a mechano‐sensory oriented predator and tested their growth rates and behaviour under conditions of interspecific and intraspecific competition. Three different foraging conditions were used: food supplied (i) only during the day, (ii) only during the night or (iii) during both day and night.3. In perch neither SGR nor feeding behaviour were influenced substantially by interspecific competition during daylight. During darkness their foraging behaviour changed markedly and their access to the food source as well as their SGR were negatively affected by the presence of ruffe.4. Ruffe's foraging behaviour did not change during either day or night with interspecific competition. During the night ruffe's SGR was higher with interspecific competition, probably because of a release from intraspecific competition and the competitive inferiority of perch during the night.5. Because of its seonsory abilities ruffe feeds predominantly at night, thereby reducing competitive interference from perch.

Diet and daily ration of two nototheniid fish on the shelf of the sub-Antarctic Prince Edward Islands

The seasonal dietary composition and estimates of daily consumption rate of Lepidonotothen larseni and Gobionotothen marionensis juveniles were obtained for the first time using fish collected near sub-Antarctic Prince Edward Islands during April 1999–2003. The diet of L. larseni consisted mainly of pelagic prey, with copepods and arrow worms making up the most significant prey groups and accounting for 46% and 40% of prey mass, respectively. The diet of G. marionensis was more diverse than that of L. larseni and was composed mainly of benthic prey, including bottom-dwelling decapods (Nauticaris marionis) and sedentary polychaetes, which accounted for 54% and 30% of prey mass, respectively. During the present study, dietary overlap between juveniles of L. larseni and G. marionensis was very low (<5%) indicating that competition for food resources between them was negligible. They not only relied on different prey species, both also exhibited different diel feeding regimes. Daily consumption rate of L. larseni and G. marionensis juveniles was estimated to be 4.5% and 5.2% of body dry mass, respectively. Stomach contents and stable isotope analyses suggested, that both L. larseni and G. marionensis occupy the forth-trophic level of the sub-Antarctic food web but depend mainly on allochthonous and autochthonous (kelp derived) organic matter, respectively.

First record of Corbicula fluminalis (O. F. Müller, 1774) (Bivalvia: Corbiculidae) in Poland

A human-modified environment may provide a favourable habitat for acclimation of invasive species. The resulting new niche is often unavailable for the native fauna and, consequently, can be successfully inhabited by alien invaders. This is also due to relatively low competition for food resources and space. New species inhabiting heated waters are being reported increasingly often. Recently, the presence of an Asiatic clam, Corbicula fluminalis (O. F. Muller, 1774), was confirmed in the lower course of the Odra River. This is the second taxon of the genus Corbicula found in Poland, in Western Pomerania, in a heated channel.

Ontogenetic dietary shifts of fishes in an Australian floodplain river

The early life of fishes is marked by rapid development when individuals are thought to optimise their success through ontogeny by altering ecological niches and resources. Since most fish larvae are thought to require small prey items at first feeding, competition for potentially limiting food resources may occur between species creating a juvenile bottleneck, which may, in turn, influence future recruitment strength. The diets of the early life stages of most Australian freshwater fish are poorly known. The present study investigated the ontogenetic dietary patterns of six species of fish in an Australian floodplain river. A large proportion of first-feeding larvae of three species (Murray cod, Australian smelt and carp) were able to feed externally while still retaining their yolk sac. All species demonstrated major dietary shifts from newly hatched larvae through development into juvenile stages and adulthood. Only a few minor overlaps in diet were found, with greater overlaps commonly occurring between sequential stages of the same species, reflecting subtle ontogenetic changes. Despite two co-occurrences of an introduced and a native species using the same rearing habitat as larvae, dietary preferences did not significantly overlap, suggesting that a recruitment bottleneck caused by competition for food resources is unlikely for these species.

Territorial defence and agonistic behaviour of breeding bearded vultures Gypaetus barbatus toward conspecifics and heterospecifics

Territoriality and agonistic behaviour against conspecifics and heterospecifics was recorded for nine breeding pairs of bearded vulture Gypaetus barbatus between 1992 and 1996 in the Pyrenees (NE Spain). For both sexes, the defence intensity increased from the pre-laying to the chick-rearing period. The intensity of nest defence was significantly higher in males during the pre-laying period but no sex differences were found during the incubation and chick-rearing periods. Competition for nest sites, food and the potential risk of predation may affect the bearded vulture's nest defence behaviour towards other species. Most agonistic interactions took place against Eurasian griffon vultures (the most abundant species and the most important competing for nest sites) and common ravens (the most likely potential predator and a kleptoparasitic species). Nest defence against conspecifics mainly took place during the pre-laying period and may be linked to sperm competition and food resource competition.

Trophic Relationships of Demersal Fishes in the Shrimping Zone Off Alvarado Lagoon, Veracruz, Mexico

The diet of demersal piscivorous fishes captured as bycatch of the commercial shrimping fleet off the Alvarado lagoonal system, Veracruz, Mexico, was studied. Nine collections distributed throughout the nortes (windy), wet, and dry seasons were made from November 1993 to January 1995. Sampling yielded a total of 646 fishes representing 10 families and 14 species, of which 44.9% had empty digestive tracts and were excluded from analysis. Trichiurus lepturus and Synodus foetens were the most abundant demersal predators in the collections. Differences in food consumption of the 7 most abundant predators were observed among the 3 seasons, with the greatest variety of prey (20 species) taken during the nortes season and the lowest variety (9 species) during the dry season. Five distinct trophic guilds were determined based on an index of relative importance of prey. Prey type and location of prey within the water column helped determine guild classification. The occurrence of different trophic guilds may allow for decreased competition for food resources on the continental shelf off Alvarado, Mexico.

Macroborings (Gastrochaenolites) in Lower Ordovician Hardgrounds of Utah: Sedimentologic, Paleoecologic, and Evolutionary Implications

Other| December 01, 2004 Macroborings (Gastrochaenolites) in Lower Ordovician Hardgrounds of Utah: Sedimentologic, Paleoecologic, and Evolutionary Implications JACOB S. BENNER; JACOB S. BENNER 1Department of Geology, Tufts University, Lane Hall, Medford, MA 02155, jacob.benner@tufts.edu Search for other works by this author on: GSW Google Scholar A.A. EKDALE; A.A. EKDALE 2Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Room WBB 719, Salt Lake City, UT 84112 Search for other works by this author on: GSW Google Scholar JORDI M. DE GIBERT JORDI M. DE GIBERT 3Departament d'Estratigraphia, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí Franquès s/n, 08028 Barcelona, Spain Search for other works by this author on: GSW Google Scholar Author and Article Information JACOB S. BENNER 1Department of Geology, Tufts University, Lane Hall, Medford, MA 02155, jacob.benner@tufts.edu A.A. EKDALE 2Department of Geology and Geophysics, University of Utah, 135 South 1460 East, Room WBB 719, Salt Lake City, UT 84112 JORDI M. DE GIBERT 3Departament d'Estratigraphia, Paleontologia i Geociències Marines, Universitat de Barcelona, Martí Franquès s/n, 08028 Barcelona, Spain Publisher: SEPM Society for Sedimentary Geology Accepted: 27 Apr 2004 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2004) 19 (6): 543–550. https://doi.org/10.1669/0883-1351(2004)019<0543:MGILOH>2.0.CO;2 Article history Accepted: 27 Apr 2004 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JACOB S. BENNER, A.A. EKDALE, JORDI M. DE GIBERT; Macroborings (Gastrochaenolites) in Lower Ordovician Hardgrounds of Utah: Sedimentologic, Paleoecologic, and Evolutionary Implications. PALAIOS 2004;; 19 (6): 543–550. doi: https://doi.org/10.1669/0883-1351(2004)019<0543:MGILOH>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract New evidence of fossil macroborings in the Lower Ordovician (Ibexian) of western Utah demonstrates that the macroboring behavioral strategy was firmly established in the earliest stages of the great Ordovician diversification of the marine biosphere. In Utah, borings were excavated in hardgrounds that had developed on sponge-algal mounds and flat-pebble conglomerates in the Fillmore Formation (Ibexian). The most complete specimens possess a neck up to 1 cm in length that opens into a teardrop-shaped chamber with a maximum diameter of 1 cm. The chamber terminates at a depth of 3–4 cm below the hardground surface. These borings belong to the ichnogenus Gastrochaenolites. The organisms responsible for creating the borings are unknown. Sedimentologically, the effect of boring on hardgrounds was to break them into pebble- and cobble-sized clasts. The endolithic lifestyle represented by the borings may have evolved in response to ecologic pressures such as predation or competition for food resources. The macroborings from the Fillmore Formation represent an innovative strategy that may have resulted in the later development of new body plans and the early establishment of endolithic macroinvertebrates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Feeding ecology of central Baltic Sea herring and sprat

A unique dataset of stomach contents sampled between 1977 and 1999 in the central Baltic Sea was used to perform a comprehensive study of the feeding ecology of Central Baltic herring Clupea harengus and sprat Sprattus sprattus. Both fish species were mainly preying upon calanoid copepods with Pseudocalanus sp. dominating the diet of herring, whereas sprat generally preferred Temora longicornis. Sprat preyed upon older copepodite stages, indicating size‐selective particulate feeding, whereas herring additionally fed on smaller copepodite stages, indicating occasional low food supply inducing filter‐feeding. Additional food sources other than copepods were mysids in winter and autumn for medium to large herring, as well as cladocerans for sprat in spring and summer, determined by the seasonal occurrence of these plankton species. Seasonally the highest feeding activity of both fishes species occurred in spring and summer, the main reproductive periods of calanoid copepods. The most important food item for both predators in spring was Pseudocalanus sp. In summer sprat switched to T. longicornis and Acartia spp. Since the late 1970s, the total stomach fullness decreased and the fraction of empty stomachs increased. In parallel the amount of Pseudocalanus sp. in the diets of both fish species decreased. Further, a considerable dietary overlap between both species in spring indicated considerable competition for food resources, especially due to an enlarged sprat stock. The results of this study support the hypothesis that growth reductions observed in Baltic herring and sprat are due to combination of a change in food availability and an increase in density‐dependent competition.

Interpopulation differences in the mandible size of the coastal tiger beetle Lophyridia angulata associated with different sympatric species

AbstractThe coastal tiger beetle Lophyridia angulata (Fabricius) co‐occurs with one or two tiger beetle species at any one locality (in a sandy beach habitat) along the Japan Sea coast of Honshu, from eastern to southern Kyushu, and on Tanega‐shima, an island south of Kyushu, in Japan. Lophyridia angulata displays sexual dimorphism in mandible length (the mandible is longer in females), and shows large interpopulation differences in mandible size. We analyzed variation in mandible length between L. angulata populations associated with three different sympatric species, and examined morphological differences that may have resulted from interaction with these other species. Lophyridia angulata showed a significant size shift towards larger mandibles in a population that co‐occurred with Cicindela lewisii, which has a similar mandible length to that of L. angulata, as compared to a neighboring population of L. angulata that co‐occurred with another species. This pattern may be interpreted as a type of character displacement associated with food resource competition. Males in L. angulata populations co‐occurring with Chaetodera laetescripta showed little variation in mandible length, as compared to other populations. These males may experience some pressure from C. laetescripta and from conspecific females. We found no significant changes in mandible length or in size variation of L. angulata before and 20 years after the extinction of Abroscelis anchoralis at one site historically co‐inhabited by both species; that is, we did not detect a character release over the 20 years.

Trophic ecology of the emerald notothen Trematomus bernacchii (Pisces, Nototheniidae) from Terra Nova Bay, Ross Sea, Antarctica

The trophic ecology of the emerald notothen Trematomus bernacchii was investigated using a sample of 284 specimens collected in the coastal waters of Terra Nova Bay (Ross Sea) during the summer season 1990–1991. The stomach-content analysis provided data on feeding habits and trophic niche breadth. Overall, 72 taxa of food items were identified; the most important as prey were infaunal and epifaunal polychaetes, amphipods and molluscs. Food of secondary importance comprised isopods, pycnogonids, fish and fish eggs. Several other taxa, such as euphausiids, mysids, decapods, echinoids, holothurians, priapulids and thaliaceans constituted food eaten in very small amounts and only occasionally. On the basis of their diet, T. bernacchii can be considered generalized feeders, with a wide niche breadth composed almost exclusively of benthic organisms. A cluster analysis was conducted to compare stomach-content data of subsamples selected on the basis of fish size and sampling depth, aiming to determine diet diversity and niche breadth, as well as factors involved in reducing intraspecific competition for food resources. Diet was different for fish sampled in shallow (≤90 m depth) and deeper waters (>90 m depth): while the first group fed mostly on bivalves (Adamussium colbecki) and amphipods, the latter preferred infaunal-epifaunal polychaetes. Conversely, fish size did not contribute significantly to diet diversification. According to our data and published data on feeding behaviour and sensory ecology, we conclude that in this species the intraspecific competition for food is reduced, either through habitat heterogeneity or different foraging strategies.

Population-Genetic Structure of Beaver (Castor fiber L., 1758) Communities and Estimation of Effective Reproductive Size Neof an Elementary Population

The absence of panmixia at all hierarchical levels of the European beaver communities down to individual families implies a complex organization of the population-genetic structures of the species, in particular, a large intergroup component of gene diversity in the populations. Testing this assumption by analysis of 39 allozyme loci in the communities of reintroduced beaver from the Vyatka river basin (Kirov oblast) has shown that only the beaver colonies exhibit high intergroup gene diversity (G st = 0.32) whereas this parameter is much lower when estimated among beaver groups from individual Vyatka River tributaries and among localities of one of the tributaries (0.07 and 0.11, respectively). The data suggesting genetic heterogeneity among individual settles within colonies have been obtained. The factors affecting the structure of the beaver communities of the lower hierarchical ranks are considered: the common origin, founder effect, selection, gene drift, assortative mating, and social and behavior features of the species. The conclusion is drawn that the founder effect could be the primary factor of population differentiation only at the time of their formation. The heterogeneity among colonies and among settles is maintained largely by isolation of colonies from one another. The strong interspecific competition for food resources, which is behaviorally implemented in the species at the level of minimal structural units (individual settles) creates a profound and unique population-genetic subdivision of the species. These results substantiate the suggestion that an elementary population (micropopulation) of European beaver is a colony, i.e., a set of related settles of different types. Based on ecological and genetic parameters, the effective reproductive size N e of the minimum beaver population was estimated to be equal to three animals. This extremely low value of effective reproductive population size largely explains the high tolerance of European beaver to inbreeding and striking viability of the species, which from the early 19th century has been for more than hundred years on the brink of survival in the condition which would made any other mammalian species vanish from the Earth.

Spatial patterns in diet and distribution of juveniles of four fish species in Delaware Bay marsh creeks: factors influencing fish abundance

Delaware Bay is one of the largest estuaries on the east coast of the United States and provides juvenile habitat for many fish species. We investigated spatial patterns in the diet and dis- tribution of the young-of-the-year of 3 marine transients — the weakfish Cynoscion regalis, the spot Leiostomus xanthurus and the Atlantic croaker Micropogonias undulatus — and the juveniles and adults of 1 estuarine resident, the white perch Morone americana, in 5 marsh creeks systems along a salinity gradient running from the poly/mesohaline (12 to 20) lower bay to the meso/oligohaline upper bay (0 to 12). Sampling was conducted monthly from April through November in 1997 and 1998, resulting in over 1700 otter trawl (4.9 m, 6 mm cod-end mesh) samples and 5703 fish stomachs examined, making this one of the largest studies of its kind. We employed a bootstrap technique to generate 95% confidence intervals for proportional consumption of the major prey categories. The diets of the target species differed significantly among the 5 marsh creeks. Mysid consumption was high for all species at the 3 most saline marshes and low at the 2 freshest sites, suggesting either low mysid abundance or availability at the latter sites. Consumption of insect larvae was important only at the freshest site. L. xanthurus consumed 80% annelids at the most saline sites, but less that 32% at the other sites. The other important consumer of annelids, Micropogonias undulatus, consumed similar quantities of annelids at all sites (32 to 41%). Total stomach fullness was almost always higher at the sites of highest fish abundance. We conclude that the marine transients appear to recruit pre- ferentially to sites of high resource abundance, while M. americana is restricted by physico-chemical tolerance during the spring through fall to limited juvenile habitats where it may experience food resource competition.

Alternative usage of common feeding grounds by large predators: the case of two hakes ( Merluccius hubbsi and M. australis) in the southwest Atlantic

Monthly variations in spatial and depth distributions, sex ratios, and maturity status in two species of hakes, Merluccius hubbsi and M. australis, were analysed in an area where their ranges overlap spatially on the shelf and slope around the Falkland Islands, and in international waters at 45–47° S (High Seas), using data collected by scientific observers on commercial fishing vessels. A variety of exploratory analyses were carried out on the raw data before patterns were quantified using generalised additive models. Both species use the areas studied as their feeding grounds. M. australis occur mainly on the Falkland shelf south of 51° S, whereas M. hubbsi is widely distributed throughout both the Falkland and High Seas shelf areas. Preliminary schemes of the seasonal migrations of both hakes in Falkland waters and on the High Seas are suggested and discussed. M. hubbsi and M. australis are found to be strongly segregated, both spatially and temporally, on their common feeding grounds, thus avoiding potential inter-specific competition for food resources.

Forced dispersal of juvenile guanacos ( Lama guanicoe ): causes, variation, and fates of individuals dispersing at different times

We examined adult-juvenile conflict in the guanaco (Lama guanicoe). During spring, territorial males become increasingly aggressive toward all juveniles born the previous year and begin expelling them from family groups. In an apparent effort to reduce aggression, juveniles display submissive crouches when being observed, approached, or attacked by the territorial male. Therefore, we assessed the influence of juvenile submissive behavior on the timing of dispersal and also examined if dispersal time was related to survival and reproductive performance as adults. We also evaluated hypotheses regarding the evolution of juvenile mammalian dispersal in the context of if and how each may favor the forced dispersal of juvenile guanacos by territorial males. Juveniles generally dispersed in late spring and early summer, and a nearly equal proportion of females (n=46; 48%) and males (n=49; 52%) dispersed. More-submissive animals generally dispersed later than less-submissive animals. Juvenile sex and dispersal time were not related to survival. In contrast, juvenile sex and dispersal time were related to reproductive performance. The probability of reproducing was highest when juveniles dispersed early and decreased with increasing time in family groups prior to dispersal. The largest proportion of juveniles was forced to disperse during a 2-week interval following the peak of the breeding season. Competition for food resources is likely very intense at this juncture and territorial males may force older juveniles to disperse in order to divert food resources to younger neonates. Additionally, juveniles may be forced to disperse after territorial males mate their mothers to prevent lost mating opportunities, because females leave territories when their offspring disperse and possibly prior to mating with males. We conclude that the forced dispersal of juvenile guanacos by territorial males is ultimately driven by competition for food resources on territories. The timing of dispersal, however, may be tempered by the chronology of matings between territorial males and particular adult females, and/or genetic relatedness between territorial males and juveniles.

Fitness cost of parasitoid avoidance behavior in the arrowhead scale, Unaspis yanonensis Kuwana

AbstractWe examined trade‐offs between fitness components and the parasitoid avoidance behavior of the arrowhead scale, Unaspis yanonensis Kuwana (Homoptera: Diaspididae). We examined the performance of two settling modes: singleton and double settling with the burrower under the burrowee. Each of these modes differs in vulnerability to parasitism by Aphytis yanonensis DeBach et Rosen and Coccobius fulvus Compere et Annecke (Hymenoptera: Aphelinidae), and performance. Field and laboratory observations revealed smaller body size and lower parasitism of burrower scales, while both singletons and burrowees showed the opposite tendency (larger body size and a greater parasitism rate). Laboratory observations under parasitoid‐free conditions revealed greater mortality and reduced growth rate for simulated burrowers than for singletons during the 40‐day nymphal period. This suggests that competition for food resources reduces performance. The results also suggest trade‐offs between parasitoid avoidance and resource acquisition ability in U. yanonensis. The trade‐offs also explain our previous findings, that introduction of the parasitoids increased the proportion of burrowers, but that within seven years equilibrium was reached at fairly low levels (ca. 5%) in the field, even under intense parasitism.

Effects of Eutrophication on Young-of-the-year Freshwater Fish Communities in Coastal Areas of the Baltic

Abundance, growth, habitat and food choice of young-of-the-year (Y-O-Y) freshwater fishes were investigated in a gradient of eutrophication in a Baltic inlet. Secchi-disc depth decreased from 3 to 0.5 m along the gradient with an accompanying reduction in submerged vegetation coverage. There were positive trends in Y-O-Y abundance with increasing turbidity for cyprinids, pikeperch, ruffe and smelt, and a negative trend for perch, leading to a shift from a perch dominated community in the reference area to a cyprinid dominated in the eutrophic area. The species composition of adult fishes resembled the Y-O-Y species community, indicating that community structure was determined already during early juvenile interval. Perch and cyprinid Y-O-Y preferred areas with submerged macrophytes while pikeperch and smelt were found more frequently in open waters. Perch Y-O-Y growth was reduced in eutrophic areas compared to references, especially in the size range where they change from foraging on zooplankton to benthic organisms. The deterioration of visual conditions caused by high turbidity is suggested to be negative for feeding of perch especially in deeper water. There was no evident growth reduction for pikeperch, which may be explained by a visual physiology well adapted to turbid conditions. The combined effects of turbidity and competition for food resources on growth and survival in eutrophic waters are discussed.

Effect of intra- and interspecific interactions on the feeding behavior of two subtidal sea stars

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 232:149-162 (2002) - doi:10.3354/meps232149 Effect of intra- and interspecific interactions on the feeding behavior of two subtidal sea stars Carlos F. Gaymer*, John H. Himmelman*, Ladd E. Johnson Département de Biologie and GIROQ (Groupe Interuniversitaire de Recherches Océanographiques du Québec), Université Laval, Québec G1K 7P4, Canada *Corresponding author. E-mail: john.himmelman@bio.ulaval.ca ABSTRACT: Competition and its effects are difficult to demonstrate, as they vary depending on the species present and the environmental conditions in each community. We conducted laboratory and field experiments to evaluate the effects of intra- and interspecific interactions on the use of food resources and behavior of the sea stars Leptasterias polaris and Asterias vulgaris, 2 major predators in subtidal communities of the northern Gulf of St. Lawrence. We demonstrate interference (encounter) competition between the two. For both species, the presence of the other (as detected by both contact and chemical signals) decreased feeding activity and provoked behavioral changes (avoidance, decrease in percentage feeding). These effects were reversible and short-lived, and were more pronounced when there was a shortage of food and when sea stars were exposed to simulated attacks by the other species (involving pedicellarial pinching). L. polaris seemed to be the inferior competitor as (1) it switched to less profitable prey size (smaller mussels, Mytilus edulis) when A. vulgaris was present, and (2) it avoided A. vulgaris and reduced its feeding activity when food was scarce. The dominance of A. vulgaris is possibly attenuated at our study site (near its northern distributional limit) by the reduction of its fitness when water temperatures are low (most of the year) and during the winter when it migrates to deeper water. Competition for food resources seems to occur periodically (mainly summer), in localized areas (patches and beds of mussels) or when sea stars form dense aggregations in which they are likely to interact frequently. Interactions, and thus the chance of competition, are less likely at greater depths. Coexistence seems to be permitted by both the attenuation of the dominance of A. vulgaris and by the great variations in how L. polaris and A. vulgaris interact (strongly affected by variations in prey availability, sea star abundance, and in the patterns of prey use by both sea stars). KEY WORDS: Sea stars · Feeding ecology · Prey preference · Feeding experiments · Behavior · Interference · Competition · Avoidance Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 232. Online publication date: May 03, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Delayed maturity, group fission and the limits of group size in female Cape ground squirrels (Sciuridae: Xerus inauris)

AbstractFemale Cape ground squirrels Xerus inauris live in matrilineal kin groups in which the number of reproductive positions is limited to three females. Field observations on a population in Namibia revealed that the mechanisms by which group size was limited were reproductive suppression and group fission. Sub‐adult females attained sexual maturity at 8 months of age when only a single adult female was present in their social group, whereas sub‐adult females in groups with more than one adult female matured at 12 months of age. Social groups with more than three breeding females subsequently split into smaller groups. Several hypotheses for limiting group size were considered. There was no evidence of higher ectoparasite loads on females in larger groups, nor was survival of adult females affected by group size. Although adult females have lower body masses and higher mortality during the winter when food is more scarce, feeding competition alone does not seem to limit group size. Groups that split continued to overlap in their feeding ranges and there was no seasonal difference in juvenile survival. However, juvenile survival was affected by group size. The greatest cost of being in a larger group was lower juvenile survival. Larger groups use larger feeding ranges, and because squirrels return to their sleeping burrow when threatened, not just to the nearest refuge, slower juveniles have greater exposure to predators. Thus, although competition for food resources may be important, predation seems to be a major constraint on female group size as well as the primary selective force leading to female group formation.

Comparative Morphometrics and Associated Growth Trends of Two Benthophagous Cichlid Species from Lake Malawi (Pisces, Perciformes)

Body shape differences between two benthophagous, sympatric cichlid species, Ctenopharynx pictus and Otopharynx sp. “heterodon nankhumba” were examined for 14 morphometric characters using measurements obtained from 11 morphological landmarks and additional 6 characters from the head region using principal component and regression analyses. The two species, though not closely related, superficially resemble each other and, being benthic feeders, directly compete for food resources unless morphological differences lead to trophic specialization. Hence, the present study explores the presence of such morphological variations. Principal component analysis (PCA), revealed that only four characters loaded highly on PC II and, therefore, account for most of the variations in the external morphology between the two species. C. pictus has longer lower jaws reflecting longer gape, and a shorter abdominal cavity than O. sp. “heterodon nakhumba”. The regression analyses showed significant differences in the allometric coefficients of the lower jaw and abdominal cavity, which might have contributed to differences in overall shape detected between the two species. The difference in gape size between the two species may be related to different feeding habits employed by each species, and such specializations are suggested to reduce interspecific competition for food resources. We suggest that the ensuing ecological separation promotes sympatry of these two species.

INDIRECT COMPETITION BETWEEN A RESIDENT UNISEXUAL AND AN INVADING BISEXUAL GECKO

Abstract Established populations of the unisexual gecko, Lepidodactylus lugubris, decline around man-made lights when the bisexual gecko, Hemidactylus frenatus, invades the environment. Some of the decline in L. lugubris numbers could occur through the process of exploitative competition for food resources. Our experiments were designed to see if other variables were important in the decline. We found that L. lugubris were more likely to use a hiding platform in enclosures with 2 rather than 1 platform when conspecific or heterospecific pairs of geckos were housed in an enclosure. Additionally, when two H. frenatus were housed in the same enclosure, they maintained closer proximity to each other than when their cagemates were L. lugubris. L. lugubris developed and laid more eggs when housed with another L. lugubris than when housed with either a female or male H. frenatus. Most interestingly, L. lugubris housed in enclosures previously occupied by H. frenatus males required more time for egg development and laying than geckos housed in enclosures previously occupied by another L. lugubris. In conclusion, variables in addition to food competition may influence the declines in L. lugubris numbers when an area in which they are established is invaded by the bisexual gecko, H. frenatus. L. lugubris numbers may decline in response to their reluctance to share a hiding place with another gecko, leaving them more vulnerable to predators. Additionally, L. lugubris fecundity may be negatively affected by the exudates from H. frenatus femoral pores or the odors of their feces.