Foraging behaviour of Nuthatches(Sitta europaea) in relation to the presence of mates and mixed flocks

Summary Nuthatches (Sitta europaea) usually forage in pairs outside the breeding season. I investigated whether foraging site selection in winter (tree species, height, substrate size) differed between sexes and whether this difference was related to the presence of mixed-species flocks. Foraging sites of pair members foraging together were highly correlated. In the rare cases when each used different tree species, males foraged significantly more often on the preferred oak trees than females. Males foraged significantly lower than females in some territories, but not in others. The reasons for this significant variation between pairs remains unexplained. Males also foraged lower than females in the company of mixed flocks. These differences are probably better explained as avoidance of the dominant male by the subordinate female, rather than specialization in resource use to avoid competition.

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Nahrungssuche beim Kleiber (Sitta europaea) im Paarverbund und in gemischten Schwärmen

Zusammenfassung Außerhalb der Brutzeit fouragieren Kleiber gewöhnlich in Paaren. Ich untersuchte, ob sich die Geschlechter in der Wahl des winterlichen Nahrungsortes (Baumart, Höhe, Astdicke) unterscheiden und ob solche Unterschiede durch die Anwesenheit von gemischten Schwärmen beeinflußt ist. Die Ortswahl der gemeinschaftlich auf Nahrungssuche gehenden Paarpartner war hoch korreliert. In den seltenen Fällen, in denen sich die Partner an verschiedenen Bäumen aufhielten, fanden sich die Männchen signifikant mehr an den der Art bevorzugten Eichen als die Weibchen. In einigen Revieren fouragierten die Männchen an signifikant niedrigeren Standorten als die Weibchen, doch war dies nicht in allen Revieren der Fall. Die Ursachen für diese Unterschiede sind nicht bekannt. Auch in gemischten Schwärmen erfolgte die Nahrungssuche der Männchen niedriger als die der Weibchen. Möglicherweise vermeiden die Weibchen so die Konkurrenz durch die dominanten Männchen.

     

Editorial

Postlarvae of two atherinid species, Menidia menidia (L.) and Menidia beryllina (Cope), were tested in two 14-day experiments at four densities of fish to determine effects of interspecific versus intraspecific competition for food. In one experiment, when M. menidia was slightly larger than M. beryllina (simulating a slightly earlier hatching time in nature) M. menidia exhibited a clear competitive advantage over M. beryllina, as demonstrated by covariance analysis of fish biomass change on fish density. In the second experiment, when M. beryllina was larger than M. menidia, neither species showed competitive advantage. The results suggest that M. menidia has an inherently superior ability to compete for food. Results of the experiment in which M. menidia was the larger species indicate the selective pressures which could have caused the two species to evolve spawning times ≈1 month apart.In another set of experiments, post-larval and juvenile M. menidia and M. beryllina were fed wet weight rations of Artemia nauplii ranging from 10–640% of initial body weight per day during a series of three 10-day feeding trials. As Menidia menidia grew, they required a smaller percentage of body weight per day in food for maximum growth (640, 320, and 160% for the three feeding trials). The same trend was observed with M. beryllina, although the percentages (160, 160, and 80%) were much less than those for M. menidia. Menidia menidia also required more food per day (20–135% of body weight) as a minimum ration for survival and growth than did M. beryllina (5–60%). The explanation for these differences may be that M. menidia in nature hatch earlier in the year, when Zooplankton is more abundant, than do M. beryllina. The two species have apparently evolved different feeding requirements as adaptations to the different food availabilities that they encounter.     

PHOTOSYNTHESIS-IRRADIANCE PATTERNS IN BENTHIC MICROALGAE: VARIATIONS AS A FUNCTION OF ASSEMBLAGE THICKNESS AND COMMUNITY STRUCTURE

Photosynthesis-irradiance (P-I) characteristics of periphyton (microphytobenthos) have been considered primarily for entire assemblages. How P-I responses vary with mat thickness and with community composition has not been considered in detail. We used a combined approach of modeling, microscale determinations of photosynthetic rate and light attenuation, and whole-assemblage O₂ flux measurements to explore P-I relationships. The modeling approach suggested that the onset of photosynthetic saturation and photoinhibition will occur at higher irradiance and that whole-mat photoinhibition (decreased photosynthesis at very high irradiance), biomass-specific maximum photosynthetic rate, and initial slope of the P-I function (α) should decrease as assemblage thickness increases or light attenuation increases. Spherical light microsensor profiles for a variety of stream algae indicated a strongly compressed photic zone with attenuation coefficients of 70–1791 m^?1 for scalar photosynthetic photon fluence density. The O₂ microelectrode measurements showed little if any photoinhibition at 2 and 4 mm depths in one filamentous green algal (Ulothrix) assemblage, with a relatively low attenuation coefficient, and no photoinhibition in a second Ulothrix community. An assemblage dominated by a unicellular cyanobacterium exhibited little photoinhibition at 2 and 4 mm, and a dense cyanobacterial (Phormidium)/xanthophyte (Vaucheria) community exhibited no photoinhibition at all. The microelectrode data revealed increases in α over several millimeters of depth (photoacclimation). These data supported the model predictions with regard to the effects of mat optical thickness on whole-assemblage values for α and photoinhibition. Whole-community O₂ flux data from 15 intact assemblages revealed positive relationships between chlorophyll a density and maximum photosynthetic rate or α expressed per unit area; the relationships with chlorophyll a were negative when photosynthetic rates were expressed per unit chlorophyll a. None of the whole assemblages exhibited photoinhibition. Thus, the data from the whole communities were consistent with model predictions.     

Use of algal monocultures by larvae of Hydroptila waubesiana and Oxyethira pallida (Trichoptera: Hydroptilidae)

Larvae of the microcaddisflies Hydroptila waubesiana Betten and Oxyethira pallida (Banks) were given monocultures of eight genera of algae to test their suitability as food sources, and to investigate the degree of trophic specialization exhibited by these larvae. Only those larvae given green algae (Chlorophyta) survived and emerged as adults. Oxyethira pallida consumed Zygnema and Spirogyra, and exhibited the shortest larval-pupal period on Spirogyra (P < 0.05). Similarly, H. waubesiana consumed Zygnema and Cladophora, but exhibited the shortest larval-pupal period on Cladophora (P < 0.001). Diatoms and Hydrodictyon allowed H. waubesiana to attain partial growth only, while O. pallida did not consume either alga. Oscillatoria and Anabaena were not consumed by either species. The larvae of the two species tested appear to obtain adequate nourishment from only a narrow spectrum of algae, and one alga allowed significantly faster growth compared to the others tested in each case. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dietary and Temporal Niche Differentiation in Tropical Ants—Can They Explain Local Ant Coexistence?

How species with similar ecological requirements avoid competitive exclusion remains contentious, especially in the species‐rich tropics. Niche differentiation has been proposed as a major mechanism for species coexistence. However, different niche dimensions must be studied simultaneously to assess their combined effects on diversity and composition of a community. In most terrestrial ecosystems, ants are among the most abundant and ubiquitous animals. Since they display direct, aggressive competition and often competitively displace subordinate species from resources, niche differentiation may be especially relevant among ants. We studied temporal and trophic niche differentiation in a ground ant community in a forest fragment in French Guiana. Different baits were presented during day and night to assess the temporal and dietary niches of the local species. They represented natural food resources such as sugars, carrion, excrements, seeds, and live prey. In addition, pitfalls provided a background measure of ant diversity. The communities attracted to the different baits significantly differed from each other, and even less attractive baits yielded additional species. We detected species specialized on living grasshoppers, sucrose, seeds, or dead insects. Community‐level differences between day and night were larger than those between baits, and many species were temporally specialized. In contrast to commonness, foraging efficiency of species was correlated to food specialization. We conclude that many ant species occupy different temporal or dietary niches. However, for many generalized species, the dietary, and temporal niche differentiation brought forward through our sampling effort, cannot alone explain their coexistence.     

Resource use and co-existence: Experimental tests from a sequence of different-aged pastures

Summary Competitive relationships were estimated for plants from young and old pasture populations. Experimental treatments were conducted under both common garden field station conditions and actual grazed pasture conditions. The results suggest four conclusions. (1) Grazing and other pasture conditions did not prevent plant competition. (2) Interspecific competition appears to promote the use of different resources (niche divergence). (3) Intraspecific competition may lead to a broadening of resource use by a species and, thus, indirectly increase both niche overlap and interspecific competition. No evidence was found to support an alternative theory of co-existence in which interspecific competition is argued to promote a balancing of competitive abilities without changes to relative niche overlap. (4) Changes in resource use appears to occur within a few decades after pasture formation.     

Niche differentiation depends on body size in a cichlid fish: a model system of a community structured according to size regularities

1. Communities of different species are often structured according to niche differentiation associated with competitive interactions. We show that similar principles may apply on an ecological time-scale when individuals of a species having a wide size variation compete for resources, using the Lake Tanganyika cichlid Lobochilotes labiatus (5-30 cm). This species has a mouth especially adapted to suck up invertebrates from rock crevices. 2. Individuals defended feeding territories against similar-sized conspecifics, but not against different-sized ones. Thus, territories of similar-sized fish rarely overlapped, but up to a total of seven individuals (of seven size-ranks) had broadly overlapping territories with dissimilar-sized individuals. Comparison with expectation from the null model demonstrated clearly that observed size ratios between adjacent size rank were determined non-randomly regardless of sexual combinations. 3. Larger individuals took larger prey types of larger average size, but more importantly used wider rock crevices from which to suck food than smaller individuals. We calculated pairwise values of Schoener's index of diet overlap C(d) and the values of Levin's index of diet breadth B(d) (prey type and prey size) and the same for the width of the rock crevices used for foraging (C(r) and B(r)). C(d) remained high among all combinations of the seven ranks. In contrast, C(r) declined strongly in combinations of adjacent ranks (to 0.27), and was low or zero among further different size ranks. This shows that fish with overlapping territories divided the food resources largely through foraging site partitioning. Accordingly, B(d) did not depend on the size difference to the nearest two coinhabiting fish, whereas B(r) did. 4. We conclude that this L. labiatus community is structured non-randomly: body size-dependent effects on foraging site usage result in competition with, and territorial exclusion of, similar-sized individuals, but not of dissimilar-sized individuals that were accepted as coinhabitants. Accordingly, mean body size ratios (large/small) between two adjacent ranks were consistently approximately 1.28 [standard deviation (SD) = 0.07, n = 104], while approximately 1.34 from the null model (SD = 0.34, n = 10 400 simulations). We discuss our results as an example of Hutchinson's rule, applied originally to size ratios of different species.     

Resistance and resilience of benthic biofilm communities from a temperate saltmarsh to desiccation and rewetting

Periods of desiccation and rewetting are regular, yet stressful events encountered by saltmarsh microbial communities. To examine the resistance and resilience of microbial biofilms to such stresses, sediments from saltmarsh creeks were allowed to desiccate for 23 days, followed by rewetting for 4 days, whereas control sediments were maintained under a natural tidal cycle. In the top 2 mm of the dry sediments, salinity increased steadily from 36 to 231 over 23 days, and returned to seawater salinity on rewetting. After 3 days, desiccated sediments had a lower chlorophyll a (Chl a) fluorescence signal as benthic diatoms ceased to migrate to the surface, with a recovery in cell migration and Chl a fluorescence on rewetting. Extracellular β-glucosidase and aminopeptidase activities decreased within the first week of drying, but increased sharply on rewetting. The bacterial community in the desiccating sediment changed significantly from the controls after 14 days of desiccation (salinity 144). Rewetting did not cause a return to the original community composition, but led to a further change. Pyrosequencing analysis of 16S rRNA genes amplified from the sediment revealed diverse microbial responses, for example desiccation enabled haloversatile Marinobacter species to increase their relative abundance, and thus take advantage of rewetting to grow rapidly and dominate the community. A temporal sequence of effects of desiccation and rewetting were thus observed, but the most notable feature was the overall resistance and resilience of the microbial community.     

Aggregation among resource patches can promote coexistence in stream-living shredders

1. We conducted a new single‐site study and a meta‐analysis of pre‐existing studies from multiple streams to assess whether intraspecific aggregation among leaf packs promotes coexistence among leaf‐eating invertebrates (shredders) in Swedish streams. 2. In the single‐site study, 48 standardised leaf bags were exposed for 1 month for shredder colonisation in a homogeneous glide of a forested stream. Current velocity, water depth and substratum composition were additionally assessed to investigate how these factors affected shredder distributions. 3. The meta‐analysis included information on shredder colonisation of leaf packs from seven other studies of detritus decomposition to assess patterns of aggregation. Intra‐ and interspecific aggregation and their relative strength were assessed using indices (J, C and A respectively) originally developed for terrestrial insects also dependent on patchy and ephemeral resources. 4. In both parts of the study, intraspecific aggregation was much stronger than interspecific aggregation, which was weak overall, indicating that the conditions under which aggregation is expected to facilitate coexistence were fulfilled in our shredder assemblages. 5. In the single‐site study, shredder abundances were weakly associated with environmental variables suggesting that habitat heterogeneity only partly explains aggregation patterns. 6. Our results strongly suggest that shredder diversity in streams, particularly during periods of leaf limitation (such as might occur in spring), is promoted by the aggregation of individual species among patches of resource.