Conspecifics as informers and competitors: an experimental study in foraging bumble-bees

Conspecifics are usually considered competitors negatively affecting food intake rates. However, their presence can also inform about resource quality by providing inadvertent social information. Few studies have investigated whether foragers perceive conspecifics as informers or competitors. Here, we experimentally tested whether variation in the density of demonstrators ('none', 'low' and 'high'), whose location indicated flower profitability, affected decision-making of bumble-bees Bombus terrestris. Bumble-bees foraged on either 'simple' (two colours) or 'complex' (four colours) artificial floral communities. We found that conspecifics at low density may be used as sources of information in first flower choices, whereas they appeared as competitors over the whole foraging sequence. Low conspecific densities improved foragers' first-visit success rate in the simple environment, and decreased time to first landing, especially in the complex environment. High conspecific densities did not affect these behavioural parameters, but reduced flower constancy in both floral communities, which may alter the efficiency of pollinating visits. These results suggest that the balance of the costs and benefits of conspecific presence varies with foraging experience, floral community and density. Spatio-temporal scales could thus be an important determinant of social information use. This behavioural flexibility should allow bumble-bees to better exploit their environment.     

Impacts of savanna trees on forage quality for a large African herbivore

Recently, cover of large trees in African savannas has rapidly declined due to elephant pressure, frequent fires and charcoal production. The reduction in large trees could have consequences for large herbivores through a change in forage quality. In Tarangire National Park, in Northern Tanzania, we studied the impact of large savanna trees on forage quality for wildebeest by collecting samples of dominant grass species in open grassland and under and around large Acacia tortilis trees. Grasses growing under trees had a much higher forage quality than grasses from the open field indicated by a more favourable leaf/stem ratio and higher protein and lower fibre concentrations. Analysing the grass leaf data with a linear programming model indicated that large savanna trees could be essential for the survival of wildebeest, the dominant herbivore in Tarangire. Due to the high fibre content and low nutrient and protein concentrations of grasses from the open field, maximum fibre intake is reached before nutrient requirements are satisfied. All requirements can only be satisfied by combining forage from open grassland with either forage from under or around tree canopies. Forage quality was also higher around dead trees than in the open field. So forage quality does not reduce immediately after trees die which explains why negative effects of reduced tree numbers probably go initially unnoticed. In conclusion our results suggest that continued destruction of large trees could affect future numbers of large herbivores in African savannas and better protection of large trees is probably necessary to sustain high animal densities in these ecosystems.     

Large herbivore responses to surface water and land use in an East African savanna: implications for conservation and human-wildlife conflicts

Water, forage and predation constrain ungulate distributions in savannas. To understand these constraints, we characterized distributions of 15 herbivore species from water, locations of peak density and degree of clustering around the peaks using zero-inflated count data models and mapping census data collected in the Mara reserve and the adjoining pastoral ranches in Kenya during a wet and dry year. Herbivores followed a humped pattern (n = 46), suggesting constrained foraging in which they balance the benefits of proximity to water with the costs of foraging where food is depleted near water and travelling to more abundant food distant from water; an exponentially decreasing pattern (n = 11), indicating strong attraction to water or vegetation near water; or a uniform (n = 3) pattern. The details rather than the types of these patterns varied between years. Herbivores concentrated farther from water and more tightly around locations of their peak densities in the ranches than the reserve. Herbivores were more abundant and widely distributed from water in the wet than the dry year, and segregated along the distance-to-water gradient, presumably to minimize interspecific competition for food. Pastoralism compressed herbivore distributions and partially excluded some species (warthog, hartebeest, topi, wildebeest, zebra, eland, buffalo and elephant) from, while attracting others (Grant’s and Thomson’s gazelles, impala, giraffe) to the ranches, relative to the reserve. Regulating cultivation, fencing, settlements and livestock stocking levels in the ranches would allow continued wildlife access to water, reduce competition with, displacement or harassment of wildlife by people, livestock and dogs near water.     

Effects of large herbivore browsing on the functional groups of woody plants in a southern African savanna

This study examined the effect of different large herbivore species and stocking rates in savanna ecosystems of Zimbabwe on the richness and abundances of woody plant functional groups and woody plant functional attributes. Seven fence-lines with different herbivore species and stocking rates on either side of the fence were sampled. Plots were placed on both sides of each fence at each of 18 randomly selected positions. The size and species of each woody plant was recorded for each plot. It was found that the number of species with different functional attributes of spinescence, leaf longevity, fruit type and dispersal mechanism and in the functional groups of palatability were not different on the different sides of the fence. However, there were differences in plant abundances for 26 out of the 35 tests carried out on plant abundances with different functional attributes and functional groups. It was hypothesised that the time needed to change woody plant species richness is hundreds of years in these systems, whereas the time needed to change woody plant abundances is decades.     

Root foraging strategies and soil patchiness in a humid savanna

In Lamto (Côte d'Ivoire), the savanna is a patchy environment as far as soil is concerned: tree clumps and termite mounds lead to higher nutrient contents than in the surrounding savanna. Mature Borassus aethiopum (Mart.) specimens are tall palm trees dominating the community, with aerial parts located out of these nutrient-rich patches.Palm root densities were compared under tree clumps and in the surrounding savanna, and were also sampled along transects between palm trees and nutrient-rich patches (two clumps and one mound). Palm root densities were far higher (up to 10 times) in the nitrogen-rich soil of both clumps and termite mounds than in the surrounding savanna. Evidence is given that palm trees are able to extend their root system as far as 20 m towards these nutrient-rich patches where they proliferate. These results point out a particular root foraging strategy, which is one of the first known for a woody perennial. They also provide new insights for understanding nitrogen cycling and savannas high rate of primary production.     

Evolution of nocturnality in bats: Potential competitors and predators during their early history

Despite their taxonomic and ecological diversity, modern bats (Order Chiroptera) are almost exclusively nocturnal. This behaviour is too ubiquitous to be explained by common patterns of temporal variation in availability of their diverse food sources or by the risk of hyperthermia when flying during the day. Other explanations for bat nocturnality include competition and increased predation risk from birds during the day. In the early and mid Eocene, the known bat fauna consisted of several insectivorous species of sizes similar to those of the modern European assemblage. This fauna was contemporaneous with several species of predatory birds, including owls (Strigiformes), hawks (Accipitridae), falcons (Falconidae) and rollers (Coraciiformes), which were the same size as modern predators on bats. Predation risk could therefore have been a significant factor preventing the early bats from becoming diurnal. Competition from aerial insectivorous birds, however, was less likely to have been significant for bats during the early Eocene, since very few such groups, mainly small Aegialornithidae, were present, with most of the major groups of aerial insectivores evolving later.     

Reconstructing past biological invasions: niche shifts in response to invasive predators and competitors

Studying historic invasions can provide insight into the ongoing invasions that threaten global biodiversity. In this study, we reconsider the impacts of Littorina littorea and Carcinus maenas on the rocky intertidal community of the Gulf of Maine. Past research using invader-removal experiments demonstrated strong top-down effects of L. littorea on algal community structure; however, such removal experiments may overlook the long-term effects of niche shifts and local extinctions caused by invasive species. We considered how a niche-shift in the native littorine, Littorina saxatilis, may change the interpretation of L. littorea impacts. Using a factorial experiment crossing predator presence/absence with L. littorea presence/absence, we found that L. saxatilis is able to exert top-down control on ephemeral algae similar to that exerted by L.␣littorea and that both competition by L. littorea and predation by C. maenas have strong, negative impacts on L. saxatilis. We also found higher predation rates on protected shores and at lower tidal heights and preferential predation on L.␣saxatilis compared to L. littorea. While movement experiments demonstrate that behavioral response to tidal height is the proximate cause of L. saxatilis exclusion from the lower intertidal, our study suggests that the ultimate causes are the additive effects of competition from and predation by invasive species.     

Timing an ontogenetic niche shift: responses of emerging salmon alevins to chemical cues from predators and competitors

Ontogenetic niche shifts are ubiquitous in nature. However, in most species little is known about phenotypic plasticity in the timing of shifts and in the associated characters. Following a period of endogenous feeding on yolk, salmonid alevins emerge from their gravel nests into the open water and start feeding exogenously. We studied, by using replicated artificial nests, whether the responses of emerging Atlantic salmon ( Salmo salar ) alevins to chemical cues from two piscivorous fish predators and conspecific competitors are plastic. We hypothesised that the alevins should delay emergence in response to predator cues, whereas cues from competitors should induce earlier emergence. We found that chemical cues from predatory burbot ( Lota lota ) delayed emergence, whereas cues from brown trout ( S. trutta ) tended to induce earlier emergence. The earlier emergence in response to trout cues was associated with a smaller body size and more yolk resources remaining at emergence, but burbot cues did not alter these traits. Predator cues also influenced the daily pattern of emergence: more alevins emerged during the morning in the burbot treatment, and fewer alevins emerged during the afternoon in the trout treatment. Chemical cues from conspecific competitors had no effect on emergence patterns. The results indicate that salmon alevins exhibit flexible ontogenetic niche shifts in the timing of emergence in response to predator cues, and that the responses are predator-specific. As timing of emergence is a major determinant of territory acquisition, these responses are likely to have an impact on later fitness of the fry.     

Interactions of multiple predators with different foraging modes in an aquatic food web

Top predators can have different foraging modes that may alter their interactions and effects on food webs. Interactions between predators may be non-additive resulting from facilitation or interference, whereas their combined effects on a shared prey may result in emergent effects that are risk enhanced or risk reduced. To test the importance of multiple predators with different foraging modes, we examined the interaction between a cruising predator (largemouth bass, Micropterus salmoides) and an ambush predator (muskellunge, Esox masquinongy) foraging on a shared prey (bluegill sunfish, Lepomis macrochirus) with strong anti-predator defense behaviors. Additive and substitution designs were used to compare individual to combined predator treatments in experimental ponds. The multiple predator interaction facilitated growth of the cruising predator in the combined predator treatments, whereas predator species had substitutable effects on the growth of the ambush predator. The combined predator treatments created an emergent effect on the prey; however, the direction was dependent on the experimental design. The additive design found a risk-reducing effect, whereas the substitution design found a risk-enhancing effect for prey fish. Indirect effects from the predators weakly extended to lower trophic levels (i.e., zooplankton community). Our results highlight the need to consider differences in foraging mode of top predators, interactions between predators, and emergent effects on prey to understand food webs.     

Evolutionary rescue and the coexistence of generalist and specialist competitors: an experimental test

Competition for resources is thought to play a critical role in both the origins and maintenance of biodiversity. Although numerous laboratory evolution experiments have confirmed that competition can be a key driver of adaptive diversification, few have demonstrated its role in the maintenance of the resulting diversity. We investigate the conditions that favour the origin and maintenance of alternative generalist and specialist resource-use phenotypes within the same population. Previously, we confirmed that competition for hosts among φ6 bacteriophage in a mixed novel (non-permissive) and ancestral (permissive) host microcosm triggered the evolution of a generalist phenotype capable of infecting both hosts. However, because the newly evolved generalists tended to competitively exclude the ancestral specialists, coexistence between the two phenotypes was rare. Here, we show that reducing the relative abundance of the novel host slowed the increase in frequency of the generalist phenotype, allowing sufficient time for the specialist to further adapt to the ancestral host. This adaptation resulted in ‘evolutionary rescue’ of the specialists, preventing their competitive exclusion by the generalists. Thus, our results suggest that competition promotes both the origin and maintenance of biodiversity when it is strong enough to favour a novel resource-use phenotype, but weak enough to allow adaptation of both the novel and ancestral phenotypes to their respective niches.     

The effects of foraging ants on arboreal insect herbivores in an undisturbed woodland savanna

ABSTRACT.

• 1 A comparison was made of the insect fauna on paired Terminalia sericea, Burkea africana and Ochna pulchra trees; one tree in each pair was treated with Formex® to exclude ants from the canopy, and the other was designated the control.
• 2 Treatment with Formex banding to exclude ants did not influence plant phenology.
• 3 Pyrethrum knockdown samples from control trees had generally more insect individuals and insect species than samples from trees where ants were excluded.
• 4 Formex-treated trees had significantly less homopterous individuals and species than the control trees.
• 5 With the exception of ants and Homoptera, there was no difference in the insect guild composition and dominance ranking of various insect taxa, sampled by pyrethrum knockdown, between the control and Formex-treated trees.
• 6 T.sericea had significantly greater populations of both sessile and mobile Homoptera on the control trees than on the Formex-treated trees. A similar trend could be seen on B.africana. There were significantly more sessile homopterans on the control trees of O.pulchra than on the Formex-treated trees. Low numbers of mobile Homoptera were recorded on both control and Formex-treated O.pulchra trees, and numbers on control trees were fewer in comparison to numbers of mobile Homoptera on control T.sericea and B.africana trees.
• 7 It is concluded that the ants have similar effects on the insect communities of trees in a natural, undisturbed savanna as has been demonstrated on trees in agro-ecosystems, and on plants that are structurally adapted for mutualistic associations with ants.
• 8 Although slight, leaf damage by some leaf-feeding insects was greater on trees where ants had been excluded than on control trees that supported foraging ant populations.

     

Belowground competitive suppression of seedling growth by grass in an African savanna

Coexistence of N₂-fixing legumes and non-legume trees with grasses in African savannas results in intense competition between these life-forms. We hypothesised that belowground competition might induce different nutritional constraints in N₂- versus non-N₂-fixing species. A field (Hluhluwe-imFolozi nature reserve, South Africa) competition experiment with two N₂-fixing legume species (Acacia burkei and Acacia karroo) and two non-N₂-fixing species (Schotia brachypetala and Spirostachys africana) both with clipped grass and without grass was established. Plants were supplied with no fertilizer, or generous amounts of fertilizer (200?kg?N?ha^?1, 100?kg P₂O₅ ha^?1, 7.1?kg K₂O ha^?1) supplied as either 28?C10 (N?CK), P or a combination of these fertilizers (NPK). Regularly clipped grass suppressed growth (by more than 90?%) of both N₂- and non-N₂-fixing seedlings equally. Biomass accumulation of seedlings grown with grass and the grasses themselves responded positively to NK and/or NPK, but not P, although P-fertilization did have effects on foliar [N] and ??¹⁵N values of trees and grasses showing that plants accessed the fertilizer. Tree ??¹⁵N values and foliar [N] were also modified by NPK, demonstrating access to fertilizer. However, the ameliorative effects of NPK on grass competition-induced biomass suppression were only partial. This may be due to ??non-resource competition?? (i.e. root gaps) imposed by dense grass roots. The fact that nutrients were able to partially ameliorate the effects of grass competition, however, indicates that such ??non-resource competition?? may be partially overcome by even more generous supply of fertilizers.     

Dynamic spatial partitioning and coexistence among tall grass grazers in an African savanna

Competitive relationships among mobile animals may be expressed through dynamically changing spatial relationships over different time frames. Less common species that are apparently inferior competitors may be able to coexist with more abundant species by concentrating in regions of the landscape little utilized by the former at spatio‐temporal scales from annual or seasonal ranges to the specific foraging localities exploited at different stages of the annual cycle. Spatial relationships may be influenced further by dependencies on other resources, predation risks and facilitatory interactions under certain conditions. Our study aimed to determine whether competition with more abundant zebra and buffalo restricted the abundance of sable antelope in a region where these three tall‐grass grazers overlapped in their herd distributions. We tracked the simultaneous movements of animals representing herds of these species over two dry seasons and one wet season using GPS‐GSM collars, and estimated seasonal or monthly range extents and their overlap. We also compared daily separation distances between these animals against the null pattern expected if their movements had been independent, and assessed how prior grazing by buffalo influenced the subsequent use of these localities by sable. The range of the sable herd was mostly separated from the seasonal range of the buffalo herd during the late dry season of 2006 and throughout the dry season of 2007. Seasonal home ranges of zebra herds overlapped partially with the range of the sable herd during most of the year. Even during times when their ranges overlapped, sable were rarely recorded within <1 km of the buffalo herd. Prior grazing by buffalo beyond a threshold level inhibited later use of these localities by sable, but the sable were nevertheless able to exploit places that were little utilized by buffalo at that time. Sable were less able to evade overlap with the small, mobile zebra herds, and hence more vulnerable to competitive exclusion by zebra than by buffalo. Our findings demonstrate how less abundant species can restrict competition from more abundant competitors through dynamic spatial partitioning in regions where their home ranges overlap.     

Optimal foraging when predation risk increases with patch resources: an analysis of pollinators and ambush predators

Pollinators and their predators share innate and learned preferences for high quality flowers. Consequently, pollinators are more likely to encounter predators when visiting the most rewarding flowers. I present a model of how different pollinator species can maximize lifetime resource gains depending on the density and distribution of predators, as well as their vulnerability to capture by predators. For pollinator species that are difficult for predators to capture, the optimal strategy is to visit the most rewarding flowers as long as predator density is low. At higher predator densities and for pollinators that are more vulnerable to predator capture, the lifetime resource gain from the most rewarding flowers declines and the optimal strategy depends on the predator distribution. In some cases, a wide range of floral rewards provides near‐maximum lifetime resource gains, which may favor generalization if searching for flowers is costly. In other cases, a low flower reward level provides the maximum lifetime resource gain and so pollinators should specialize on less rewarding flowers. Thus, the model suggests that predators can have qualitatively different top‐down effects on plant reproductive success depending on the pollinator species, the density of predators, and the distribution of predators across flower reward levels.     

Photosynthetic and gas exchange characteristics of dominant woody plants on a moisture gradient in an African savanna

We determined key photosynthetic gas exchange parameters, and their temperature dependence, in dominant woody plants at four savanna sites on a moisture gradient in Botswana, southern Africa. Leaf stable carbon and nitrogen (N) isotope and morphological measures were made concurrently. Sampling of these predominantly non‐N‐fixing species took place during an exceptional rainfall season, representing near‐optimum conditions for primary production at these sites. The mean specific leaf area and leaf size were positively related to mean annual rainfall (MAR); species with larger leaves of lower density were more abundant in wetter sites. Almost all species at all sites showed high net light‐saturated photosynthetic rates (A_max?10 μmol CO₂ m^?2 s^?1) due both to high CO₂ carboxylation (V_c,max) and RubP‐regeneration capacity (J_max). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of A_max showed no clear optimum, and a gradual drop from 25°C to 35°C, without notable temperature limitation at leaf temperatures in excess of 35°C. Dark respiration rate (R_day) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf δ¹³C (stable carbon isotope ratio) and MAR, suggesting higher leaf‐level water‐use efficiency at drier sites when integrated over the life of the leaf. At wetter sites, specific leaf [N] was lower and photosynthetic nitrogen‐use efficiency increased, a pattern reflected at the ecosystem level by less ¹⁵N enrichment of leaves at these sites. Taken together, the results suggest a switch from water‐use to nitrogen‐use efficiency constraints with increasing moisture availability. These constraints impact leaf form and function significantly, and may emerge at the ecosystem level in aspects of water and N cycling.     

Causes of increased nutrient concentrations in post-fire regrowth in an East African savanna

The aim of the present study was to investigate the causes of increased macronutrient concentrations in above-ground post-fire regrowth in an East African savanna (Northern Tanzania). Experiments were set up to discriminate between the following possible causes: (1) increased soil nutrient supply after fire, (2) relocation of nutrients from the roots to the new shoots, (3) rejuvenation and related changes in plant tissue composition and (4) changes in nutrient uptake in relation to above-ground carbon gains. N, P, K, Ca and Mg concentrations in post-burn graminoid vegetation were compared with clipped and with unburned, control vegetation during the post-burn growth season. One month after burning and clipping, nutrient concentrations in live grass shoots in the burned and clipped treatments were significantly higher than in the control. This effect, however, declined in the course of the season and, except for Ca, disappeared three months after onset of the treatments. There were no significant differences in live grass shoot nutrient concentrations between burned and clipped treatments which suggests that the increased nutrient concentration in post-fire regrowth is not due to increased soil nutrient supply via ash deposition. The relatively low input of nutrients through ash deposition, compared to the amount of nutrients released through mineralisation during the first month after burning and to the total nutrient pools, supports this suggestion. There was no difference between burned and unburned vegetation in total root biomass and root nutrient concentrations. Relocation of nutrients from the roots to the new shoots did not, therefore, appear to be a cause of higher post-fire shoot nutrient concentrations. The present study shows that in this relatively nutrient-rich savanna, the increased nutrient concentration in above-ground post-fire regrowth is primarily due to increased leaf:stem ratios, rejuvenation of plant material and the distribution of a similar amount of nutrients over less above-ground biomass. This revised version was published online in June 2006 with corrections to the Cover Date.