Interference competition and temporal niche shifts: elephants and herbivore communities at waterholes

Scarcity of resources may result in high levels of animal aggregation; interference competition can occur in such a scenario and play a role in resource acquisition. Here, we test the hypothesis that animals could minimize interference competition by shifting their temporal niches in relation to competitors. In Hwange National Park, Zimbabwe, we monitored waterholes in order to study agonistic interactions between elephants and other herbivore species. We also used a long-term data set from a yearly survey of waterhole attendance by herbivores to evaluate the influence of the presence of elephants on the use of waterholes by other herbivore species. Our results show that in drier years, waterholes are crowded with elephants early in the afternoon. In general, the species most affected by interference competition with elephants shift their temporal niches at the waterholes, thus maintaining a constant temporal overlaps with elephants. The species less affected by interference competition with elephants show no temporal niche shifts and increase their temporal overlap with elephants at waterholes, as predicted from a noncompetition hypothesis. This study provides evidence that interference competition with a behaviorally dominant large species influences the temporal niches of smaller species, and suggests that the potential costs associated with interference between elephants and other herbivores at waterholes are linked to shifts in diurnal activities rather than interactions and water acquisition itself.     

Scorpions,spiders and solpugids: predation and competition among distantly related taxa

Summary Scorpions, spiders and solpugids are generalist predators on the same types of arthropod prey. However, these potential competitors also frequently eat one another (=intraguild predation). In a 29 mo. experiment, >6,000 scorpions were removed from 300 (10x10m) quadrats. Significantly more spiders (but not solpugids) occurred in removal versus control quadrats. Two alternate hypotheses potentially explain this result: exploitation competition for jointly exploited prey or intraguild predation. There was no evidence of exploitation competition: no differences existed between removal and control quadrats in prey abundance or spider size and reproductive characteristics. It appears that the release from predation pressure in areas from which scorpions were removed produced the observed increase in spider abundance. Current ecological theory does not fully apply to situations whereby species at the same trophic level interact as both predators and potential competitors.     

Drought and the organization of tree-hole mosquito communities

Summary In southeastern North America (North Florida, USA), the duration, frequency, and timing of drought differentially affect the survivorship of pre-adult tree-hole mosquitoes. Drought affects survivorship both by the direct action of dehydration on developing larvae and pupae and by the indirect modulation of predation. The drought-susceptible species, Toxorhynchites rutilus, Orthopodomyia signifera, and Anopheles barberi co-occur in more permanent holes that are larger, with larger, more vertical openings, lower down in larger trees, and contain darker water with higher conductivity, pH, and tannin-lignin content than the holes occupied by Aedes triseriatus that has drought-resistant eggs and rapid larval development. Ovipositing mosquitoes cue on physical and chemical attributes of tree holes independently of host tree species. These same attributes differ among drought-prone and drought-resistant holes but mosquitoes track these attributes more faithfully than the attributes predict tree-hole stability.     

Competition and niche relationships among Eriosoma aphids occurring on the Japanese elm

Summary Galls of more than one species of Eriosoma (Aphidoidea) are found sympatrically even on single trees. Incipient galls are frequently invaded by conspecific and/or allospecific fundatrices. Eriosoma yangi, a component of Eriosoma communities, does not form its own galls but obligatorily usurps those of other species. There were interspecific differences in the timing of gall formation and the spatial distribution of galls. Nevertheless E. yangi fundatrices randomly invaded galls of any Eriosoma species and occupied 33%–41% of galls of each species. When more than one E. yangi fundatrix invaded one gall, mortal fights sometimes occurred. Fundatrices of gall-forming species also seemed to take part in such fights. Fundatrices of gall-forming species had a significant tendency to invade galls of a particular species. However, taking account of niche differences among species, invaders apparently entered available galls at random. Apparently E. yangi fundatrices search an extensive range within a branch for galls, while invaders of gall-forming species search a restricted speciesspecific range. The niche relation of gall-forming species in a northern community containing E. yangi were compared with those in a southern community lacking E. yangi. No obvious difference was found between them, suggesting that parasitism by E. yangi has not influenced niche divergence within the Eriosoma community.     

哀牢山木果石栎群落优势种的生态位宽度与重叠

通过多种方法比较研究了哀牢山木果石栎（Ｌｉｔｈｏｃａｒｐｕｓ ｘｙｌｏｃａｒｐｕｓ）群落８个优势种的生态位宽度和重叠,结果表明,该群落是以木果石栎、腾冲栲（Ｃａｓｔａｎｏｐｓｉｓ ｗａｔｔｉｉ）、滇木荷（Ｓｃｈｉｍａ ｎｏｒｏｎｈａｅ）和景东石栎（Ｌｉｔｈｏｃａｒｐｕｓ ｃｈｉｎｔｕｎｇｅｎｓｉｓ）为优势种的共优群落,它们的生态位宽度值分别为０．８７３２,０．７８５６,０．６７４２和０．５７９８。     

An examination of multiple factors affecting community structure in an aquatic amphibian community

Summary The potential effects of multiple factors structuring certain larval amphibian communities were studied using a pen experiment in a natural pond. Potential factors (predation and competition from other species) were allowed to act in a stepwise fashion such that their relative importance could be evaluated. Based on a previous study, it was hypothesized that predation by Ambystoma salamander larvae on other larval amphibian species would be the most important factor. Survival of Ambystoma jeffersonianum salamander larvae and Rana sylvatica tadpoles was significantly depressed only by Ambystoma opacum predation. Survival of Ambystoma maculatum salamander larvae was significantly greater in the absence of both A. opacum and A. jeffersonianum predators. The virtual elimination of Hyla chrysoscelis larvae in all treatments also can be largely attributed to Ambystoma predation. Thus, Ambystoma predation was the dominant factor determining larval survival of four amphibian prey species in the experimental communities.     

广东南澳岛台湾相思林主要种群生态位研究

以南澳岛台湾相思林不同生境梯度的种群作为一元资源位,共选取8个资源位进行调查,计测台湾相思林主要种群的生态位宽度、重叠和相似性比例。结果表明,台湾相思林种群各种的生态位宽度差别明显,重要值大的种生态位宽度基本较大,种群物种利用资源的相似程度普遍较小;而台湾相思与生态位较宽的树种马尾松、鸭脚木、中华楠、潺槁树等生态位相似程度较大,对生境有一定相似的要求,重叠值也偏高,易形成混交林。其结果对南澳的台湾相思林分及水源涵养林的生态学研究及生产实践有指导意义。     

Desiccation,predation, and mussel-barnacle interactions in the northern Gulf of California

Summary Field experiments were conducted in order to determine the potential for desiccation and predation to mediate the effect of mussels (Brachidontes semilaevis) on barnacles (Chthamalus anisopoma) in the highly seasonal northern Gulf of California. We did this by removing both mussels and a common mussel predator (Morula ferruginosa: Gastropoda) and by spraying selected sites with sea water during summertime spring low tides. We also determined the effect of crowding on resistance to desiccation in barnacles, and the effect of barnacles on colonization by mussels. The mussel-barnacle community was not affected by keeping experimental quadrats damp during daytime low tides throughout the summer. Exposure to summertime low tides, however, did affect the survivorship of isolated, but not crowded, barnacles; and barnacle clumps enhanced the recruitment of mussels. Hence crowding in barnacles had a positive effect on both barnacle survivorship and mussel recruitment. Morula had a negative effect on mussel density, and mussels had a negative effect on barnacle density. The effect of Morula on barnacle density was positive, presumably due to its selective removal of mussels. These results suggest an indirect mutualism between barnacles and the gastropod predator, because barnacles attract settlement or enhance the survival of mussels, and the predator reduces the competitive effect of mussels on barnacles.     

The dual importance of competition and predation as regulatory forces in terrestrial ecosystems: evidence from decomposer food-webs

The relative importance of predation and competition (resource limitation) in influencing the components of a below-ground food-web consisting of three trophic levels (bacteria and fungi; bacterial-feeding and fungal-feeding nematodes; and top predatory nematodes) was estimated using microbial biomass and nematode frequency data collected throughout a 1-year period in two agro-ecosystems. The study suggested that bacterial and fungal biomass were likely to be regulated by grazing and competition respectively, and that these differences were likely to be attributed to the biological (probably morphological) differences between bacteria and fungi, in contrast to the predictions of the hypothesis of Hairston et al. (1960). Top predatory nematodes were sometimes strongly related to the microbial but not microbial-feeding trophic levels, indicating that microbial biomass may directly influence top predator numbers, and that the intermediate level may simply serve as a conduit by which resources pass from the bottom to top trophic levels. This study also suggests that the detritus food-web acts as two distinct (bacterial-and fungal-based) compartments.     

Direct and indirect effects of predators on the dominant invertebrates of two freshwater littoral communities

Summary Two congeneric damselfly species, Enallagma traviatum and E. aspersum, dominate the littoral macroinvertebrates of Bays Mountain Lake and of the adjacent fish-free Ecology Pond, respectively (northeastern Tennessee, USA). Extending previous experimental studies, we test seven hypotheses concerning the role of fish (bluegill sunfish, Lepomis macrochirus) and larvaldragonfly (Anax junius) predation, competitive effects on damselflies, and the interaction between competition and predation, in determining invertebrate dominance in these communities. Three types of experiments were conducted: an enclosure experiment within Ecology Pond, an outdoor replicated tub experiment, and a laboratory behavior experiment. The in-situ enclosure experiment showed that E. traviatum larvae were more susceptible to Anax predation than were E. aspersum larvae; a tendency toward greater vulnerability to fish of E. aspersum compared with E. traviatum was not statistically significant. The outdoor tub experiment confirmed both of these trends with statistically significant results. In the tubs, both predators inhibited feeding of both zygopterans (as indicated by reduced fecal mass), particularly for E. aspersum in the presence of fish. This effect appears to have been primarily indirect, mediated through exploitation of the zooplankton. We also detected competitive effects of E. traviatum on E. aspersum: E. traviatum reduced the emergence and increased the exposure above the substrate of E. aspersum. In the absence of predators, E. traviatum inhibited feeding of E. aspersum via interference. In the laboratory behavior experiment, predators inhibited crawling by E. aspersum. E. aspersum was more exposed than was E. traviatum; it swam and crawled more than did E. traviatum, considerably increasing these movements at night. Over all, E. traviatum consistently appeared to be the more cryptic of the two species, and E. aspersum appeared to be much more active. Our results suggest an explanation for the clear difference in structure between communities like Bays Mountain Lake and Ecology Pond: predaceous fish eliminate large invertebrate predators and shift the community toward cryptic forms at relatively low densities, reflecting the effects of both predation and exploitation competition. In the absence of fish, large invertebrate predators are less able to deplete littoral invertebrates but may favor the more active forms, perhaps because these are better able to avoid invertebrate predators.     

Microhabitat selection and coexistence in feather mites (Acari: Analgoidea) on Alaskan seabirds

Summary Two species of feather mites, Alloptes (A.) sp. and Laronyssus martini, coexisted on the flight feathers of kittiwakes, while Alloptes (C.) sp. occurred alone on murres. All three species were found mostly on the ventral surface of mid-wing regions. On individual feathers, the mites were attached to the distal sides of barbs on the trailing vanes. The two species coexisted on kittiwakes but occupied different portions of feathers. The results suggest that the two species compete for the attachment site on the feather and Alloptes (A.) sp. is excluded by L. martini from the preferred microhabitat.     

Niche shifts in Greater Antillean Anolis communities: effects of niche metric and biological resolution on null model tests

Summary Using published data, a Monte Carlo approach was used to determine if niche shifts in communities of Anolis lizards have occurred within two islands in the Greater Antilles. Communities at 4 sites on Puerto Rico and 4 sites on Jamaica were randomly assembled using resource-use data from any of the 4 sites with the constraint that the species list of the random community was identical to that observed for each site. The observed niche structure and that of the randomly assembled community were compared at each site. The null hypothesis tested was that one of several functions of niche overlap did not produce lower values in the observed communities compared to those randomly assembled. The sensitivity of this method to details of the analysis was investigated by using 5 variations on the procedure for quantifying the niche structure of the communities. I investigated the effects of (1) using Pianka's versus Levins' metric of niche overlap, (2) using complete resource-use data versus the marginals of the distributions, (3) asymmetric responses to overlap due to size differences between overlapping species, (4) using estimated electivities versus the observed proportional use of resource states, and (5) methods of niche analysis based on different degrees of biological resolution (e.g., community-wide statistics versus individual species responses). The results of the simulations and statistical comparisons did not provide a clear, unambiguous answer to the question of the occurrence of niche shifts. Statistical evidence for niche shifts was more frequently detected using Levins' metric compared to Pianka's, and shifts were easier to detect using high biological resolution (examination of individual species) compared to community-wide measures of niche structure response.     

Effects of the non-native amphibian species Discoglossus pictus on the recipient amphibian community: niche overlap, competition and community organization

The painted frog, Discoglossus pictus, was introduced into the Iberian Peninsula 100 years ago and its distribution has steadily increased since then. We studied the effects of this non-native amphibian species on the native ones in the northeastern area of the peninsula. We compared amphibian assemblages in regions with and without D. pictus to estimate niche overlap between species. Additionally, we carried out a laboratory evaluation of the effects of competition between the non-native and the two native species with which it overlaps most commonly: Bufo calamita and Pelodytes punctatus. The presence of D. pictus larvae reduced the survival, body mass and activity of B. calamita, and increased time to metamorphosis. Furthermore, D. pictus showed the highest consumption rate while P. punctatus showed the lowest. One possible consequence of these competitive interactions is an alteration of species co-occurrence patterns in the recipient community on a regional scale. In the non-invaded area, the checkerboard score (C-score) of co-occurrence indicated that the community was structured, whereas the standardized C-score in the invaded area indicated that the community did not differ significantly from having a random structure. These results suggest that competition between native and non-native species can cause recipient communities to become less structured.     

Predation risk and the structure of freshwater zooplankton communities

Summary Many predators inflict substantial mortality on their prey. The prey respond to these selective pressures with changes in their spatial and temporal overlap with the predator (density risk responses), or with changes in their vulnerability to the predator (prey vulnerability responses). Here we develop a simple predation model that permits quantification of the basic response types of the prey in nature. We then test the hypothesis that prey response will be proportional to the intensity of the predation mortality relative to all other sources of mortality and decreased natality acting on the prey. A significant regression relationship is obtained for the prey vulnerability response but not for any of the density risk responses. The individual response values and regression statistics are used to interpret the relative importance of the different response types and to assess the predator's influence on prey community structure.Supported by Lehigh University Environmental Studies Center     

Effects of competition and shading in planktonic communities

A model for several algal species which compete both for light and for nutrients, and which are also subject to settling and diffusion, is considered. The settling speeds and diffusion coefficients are assumed to be small, in a sense to be made precise later, and a singular perturbation argument is used. In certain cases vertical segregation of the algal species is observed, and the mechanism for this is interpreted biologically.Supported by the Danish Natural Science Research Council (Grant No. 11-8321)     

Foraging time and spatial patterns of predation in experimental populations

Summary Responses of the predaceous mites Phytoseiulus persimilis, Typhlodromus (=Metaseiulus) occidentalis, and Amblyseius andersoni to spatial variation in egg density of the phytophagous mite, Tetranychus urticae, were studied in the laboratory.The oligophagous predator P. persimilis showed initially a direct density dependent foraging time allocation and variation in foraging time increased with prey density. With changes in prey density due to predation, predator foraging rates (per hour) decreased with time and density dependent foraging gradually became density independence, because P. persimilis continued to respond to initial prey density, instead of the changing prey density and distribution. The consequent spatial pattern of predation by P. persimilis was density independent, although slopes of predation rate-prey density regressions increased with time.Compared with P. persimilis, the narrowly polyphagous predator T. occidentalis responded relatively slowly to the the presence or absence of prey eggs but not to prey density: the mean and variation of foraging time spent in patches with prey did not differ with prey density, but was significantly greater in patches with prey eggs than in patches without eggs. Prey density and distribution changed only slightly due to predation and overall foraging rates remained more or less constant. The consequent spatial pattern of predation by T. occidentalis was inversely density dependent. As with P. persimilis, slopes of predation rate-prey density regressions increased with time (i.e. the inverse density dependence in T. occidentalis became weaker through time).The broadly polyphagous predator A. andersoni showed density independent foraging time allocation with variation independent of prey density. With changes in prey density over time due to prey depletion, overall foraging rates decreased. The consequent spatial pattern of predation by A. andersoni also changed through time; it initially was inversely density dependent, but soon became density independent.Overall, P. persimilis and T. occidentalis spent more time in prey patches than A. andersoni, suggesting that A. andersoni tended to spend more time moving outside patches. The overall predation rates and searching efficiency were higher in P. persimilis than in A. andersoni and T. occidentalis. Predator reproduction was highest in P. persimilis, lower in T. occidentalis and the lowest A. andersoni.The differences in response to prey distribution among the three predaceous species probably reflect the evolution of these species in environments with different patterns of prey distribution. The degree of polyphagy is a major determinant of the aggregative response, but other attributes such as handling time are also important in other aspects of phytoseiid foraging behavior (e.g. searching efficiency or predation rate).     

Dynamics of the predatory arthropod community in Spanish olive groves

1 A survey of the predatory fauna of olive pests, particularly the olive moth, Prays oleae, was undertaken in Granada, Spain, from May to September 1994–95. 2 At both experimental sites similar patterns of emergence were observed for all predatory species with peaks occurring in June, both in 1994 and 1995. More individuals and species were captured in the morning than in the evening. 3 A weak inverse relationship existed between the mean monthly temperatures and the total number of predators caught. 4 The number of predatory species and of individuals caught at both sites was, respectively, two and three times larger in 1994 than in 1995; This difference is assumed to have been due to the accumulated affect of 6 years of drought. 5 Synchrony between P. oleae and predator phenology was suggested by the coincidental timing of peak predator numbers with the oviposition and subsequent emergence of the most vulnerable stage of the olive moth’s lifecycle, i.e. the flower generation. 6 There were more individuals of ants caught, whilst more spider species were captured than all other groups; predatory Coleoptera and Heteroptera were poorly represented. 7 Ants appeared to have a detrimental affect on the number of all other predators.     

Niche differentiation and preferences of Elaphrus cupreus Duftschmid, 1812 and Elaphrus uliginosus (Fabricius, 1792) (Coleoptera: Carabidae) as reason for their different endangerment in Central Europe

Ecological demands were studied in Elaphrus cupreus, a frequent species, and E. uliginosus, one of the most endangered carabids in Europe. Ecological experiments were performed in the laboratory and the field in northern Germany. Abundance was measured using the mark-and-recatch method. Both species prefer sites with water content changes lower than 25%. E. uliginosus prefers higher temperatures than E. cupreus and niche breadth was on average wider in E. cupreus than in E. uliginosus. E. uliginosus is restricted to grassland sites with a low fraction (>0 and 25%) of bare soil. E. cupreus was mainly found on sites having >75–100% bare soil. This preference of E. uliginosus can be referred to its high temperature demands using open regions for warming up and its predatory behaviour using hideouts for a rapid predatory push. The wider niche structure and lower temperature demand of E. cupreus allows the inhabitation of cooler alder woods. It prefers large fractions of bare soil because its predatory behaviour is to run randomly on the soil. Thus, E. uliginosus demands wet extensively grazed grassland where cattle produces low fractions of bare soil, while E. cupreus prefers intensively grazed sites in grassland or alder woods.     

Direct and indirect effects of predation,herbivory and surface rugosity on mussel recruitment

Summary The predatory gastropod Nucella lapillus, commonly preys upon the mussel, Mytilus edulis, and is thought to control the distribution and abundance of mussels on the rocky shores of New England, USA. In this study, done in Maine, USA, not only the presence of Nucella lapillus but also the roughness of the experimental surface and the presence of the herbivorous gastropod, Littorina littorea, were manipulated. Four types of surfaces were used as recruitment substrata for mussels: smooth bare granite, aggregations of the barnacle, Semibalanus balanoides, fiberglass resin castings of smooth bare granite and resin castings of aggregations of S. balanoides. To ensure that caged N. lapillus were not starving, barnacles were provided as alternative prey. Experiments showed no detectable effect of N. lapillus on the recruitment of M. edulis. Mussel recruitment was enhanced by surface rugosity and depressed by the activities of L. littorea. Analysis of covariance, using the number of algal species as the covariate, suggested that L. littorea reduced the number of newlyrecruited mussels by removing algae that provided recruitment sites, but no manipulations were done to test this conjecture. It is likely that previous reports of N. lapillus controlling mussel abundance are attributable to N. lapillus preying upon barnacles, which increase surface rugosity and enhance mussel recruitment. Review of literature on feeding preferences of N. lapillus supports this view. When handling times and prey availability are taken into account, Nucella shows a clear preference for barnacles over mussels.     

Increasing community size and connectance can increase stability in competitive communities

The relationship between community complexity and stability has been the subject of an enduring debate in ecology over the last 50 years. Results from early model communities showed that increased complexity is associated with decreased local stability. I demonstrate that increasing both the number of species in a community and the connectance between these species results in an increased probability of local stability in discrete-time competitive communities, when some species would show unstable dynamics in the absence of competition. This is shown analytically for a simple case and across a wider range of community sizes using simulations, where individual species have dynamics that can range from stable point equilibria to periodic or more complex. Increasing the number of competitive links in the community reduces per-capita growth rates through an increase in competitive feedback, stabilising oscillating dynamics. This result was robust to the introduction of a trade-off between competitive ability and intrinsic growth rate and changes in species interaction strengths. This throws new light on the discrepancy between the theoretical view that increased complexity reduces stability and the empirical view that more complex systems are more likely to be stable, giving one explanation for the relative lack of complex dynamics found in natural systems. I examine how these results relate to diversity-biomass stability relationships and show that an analytical solution derived in the region of stable equilibrium dynamics captures many features of the change in biomass fluctuations with community size in communities including species with oscillating dynamics.