Gilberto M. de M. Santos

Flower-visiting guild associated with the Caatinga flora: trophic interaction networks formed by social bees and social wasps with plants.

We conducted a comparative analysis of bee-plant and wasp-plant interaction networks, aiming at the identification of similarities and differences between networks of flower-visiting groups with direct or indirect mutualism with plants. We measured for each network: number of social bees and social wasps, number of plants visited (P), degree of nestedness, number of observed (I) and possible interactions, connectance (C), and interaction density (D). The network formed by pooling together social bees and social wasps exhibited 25 species (12 bees and 13 wasps) and 49 visited plants, with a connectance of 15.34%. The wasp-plant network had higher connectance (C = 21.24) than the bee-plant network (C = 15.79). Both the social wasp-plant and the social bee-plant network were significantly nested, they presented structure more nested than all randomly generated matrices (n = 1 000). Both interaction networks have similar topologies and are nested, asymmetrical and modular structures.ZusammenfassungWir verglichen ein soziales BienenPflanzen Netzwerk (BPN) mit einem sozialen Wespen-Pflanzen Netzwerk (WPN) unter der Fragestellung, ob verschiedene Arten von Mutualismus mit demselben Partner zu unterschiedlichen Netzwerkstrukturen führen. Hierzu wurden soziale Bienen und Wespen in einem Sammelgebiet mit Caatinga-Vegetation in Bahia bei ihren Blütenbesuchen gefangen. Jedes zweiteilige Netzwerk wurde dann in Form einer R-Nachbarschaftsmatrize beschrieben. Wir beschrieben jedes Netzwerk in Hinblick auf Schachtelung (nestedness NODF und N), Modularität (M) und Bedeutung einzelner Spezies (Ij). Beide Netzwerke erwiesen sich als geschachtelt (Abb. 1). Wir untersuchten weiterhin, inwieweit die beiden Netzwerkmaße (NODF und N) die Schachtelung in entsprechenden Nullmodellmatrizen (mit n = 1000 Zufallswiederholungen) quantifizierten, wobei Präsenzen zufallsverteilt den jeweils einzelnen Zellen der Matrix zugeordnet wurden. Die aus unseren Daten gebildeten Bienen-Pflanzen- und Wespen-Pflanzen-Matrizen wiesen allesamt eine tiefere Schachtelung auf als die Zufallsmatrizen. Das WPN bildete 116 der 546 möglichen Interaktionen ab und zeigte eine Bindungsstärke von C = 21,24, während das BPN 72 der 456 möglichen Interaktionen aufwies, bei einer Konnektivität von C = 15,79. Sieben der blütenbesuchenden Spezies bildeten den Kernbereich des WPN und sechs Arten erwiesen sich als von besonderer Bedeutung für die Struktur des BPN. Die Topologie des BPN war stark von der Gegenwart von Apis mellifera beeinflusst, einer eingeführten Art, die als Generalist Interaktionen mit 33 der 38 Pflanzenarten einging. Das Gesamtnetzwerk (OFPN) wurde aus 25 blütenbesuchenden Arten und 49 besuchten Pflanzen gebildet und hatte eine Konnektivität von 15,34. Insgesamt konnten wir 188 von 1225 möglichen Interaktionen finden, wobei neun Arten für 78,19 % aller Verbindungen verantwortlich waren: Apis mellifera (33 Interaktionen), Polybia ignobilis (18), P. sericea (17), Polistes canadensis (17), P. paulista (15), Brachygastra lecheguana (13), Trigona spinipes (12), P. occidentalis (11) und Protonectarina sylveirae (11). Ein relativ hoher Anteil der Pflanzenarten (63,3 %, N = 31) wurde sowohl von sozialen Bienen als auch sozialen Wespen besucht. Unsere Analysen der Beziehungsnetzwerke zeigten einen klaren Unterschied hinsichtlich der Bedeutung einzelner Pflanzenarten als Nahrungsquelle für die jeweiligen blütenbesuchenden Gruppen (Abb. 2). Die Modularität des Gesamtnetzwerks aus beiden Gruppen war relativ gering (M = 0,07, P = 0,004), was auf eine Überlappung zwischen Bienen und Wespen in den Blütenbesuchen zurückzuführen ist. Wir fanden zudem eine höhere Nischengruppierung innerhalb der Bienen als innerhalb der Wespen. Trotz der Unterschiede im fakultativen Mutualismus zwischen Bienen-Pflanzen (direkter Mutualismus — Bestäubung) und WespenPflanzen (indirekter Mutualismus — Verteidigung) wiesen die beiden Beziehungsnetzwerke ähnliche Topologien auf, mit geschachtelten Mustern und asymmetrischen, modularen Strukturen.

Compartilhamento de recursos florais por vespas sociais (Hymenoptera: Vespidae) e abelhas (Hymenoptera: Apoidea) em uma área de caatinga

Wasps and bees are important components of flower visitor guilds in the Neotropical region. The abundance of social wasps and bees, collected during flower visits, was used to calculate niche breadth and niche overlap indices, in order to characterize the utilization of the floral resources by those insects. Samples were taken monthly, for 13 months (156h), by two collectors which captured the specimens on flowers using entomological nets, while walking along a way in an area of caatinga at Itatim, State of Bahia, Brazil. Wasps and bees heavily visited a few sources of floral resources, but the most visited plants by each group were different, and few plant species were important for both groups, resulting in low niche overlap between bees and wasps. The niche overlap among wasp species was generally higher than among bee species. The general overlap, which takes into consideration all species together was low (< 30%).

The seasonal dynamic of ant-flower networks in a semi-arid tropical environment

1. Several studies have recently focused on the structure of ecological networks involving ants and plants with extrafloral nectaries; however, little is known about the effects of temporal variation in resource abundance on the structure of ant–plant networks mediated by floral nectar.

Trophic niche breadth and niche overlap in a guild of flower-visiting bees in a Brazilian dry forest

Trophic niche breadth and niche overlap of bees were studied in a region of Caatinga (a deciduous dry thorn scrub forest) in Brazil with the lowest mean annual rainfall of the country, highly seasonal environmental variation, and an unpredictable rainy season. A null model approach was used to determine if the observed niche overlap in the community differed from that expected by chance. In general, even bee species with wider trophic niches concentrated foraging efforts on flowers of only a few of the available plant species, with low trophic niche overlap between pairs of species and among all species in the community. A randomization test showed niche overlap among the 10 common species in the guild to be significantly less than expected by chance, suggesting the existence of functional complementarity. The structure of communities defined by high functional complementarity is likely to be more sensitive to variation in available resources (e.g., richness and abundance of floral resources for bees). Ecosystem services provided by such communities may be more greatly affected by environmental changes (anthropogenic activities and climate change) than are services provided by communities with greater functional redundancy.

The use of cactus fruit food resources by social wasp (Hymenoptera, Vespidae, Polistinae) in an area of Caatinga (lpirá, Bahia, Brazil)

This study aims to identify food resource plants used by social wasp species in a Caatinga area, as well as describe the local patterns of cactus fruit resources used by wasps. Nine hundreds and eight foraging wasps, belonging to nine species, were captured while visiting six cactus species. Cereus jamacaru DC. and Pilosocereus catingicola (Gurke) Byles & G.D. were the most frequently visited plants. Several wasp species and a great number of individuals visited them. Polybia paulista von Ihering, 1896, Polybia ignobilis (Haliday, 1836), Polistes versicolor (Olivier, 1791), Polistes simillimus Zikan, 1951, Polistes billardieri Fabricius, 1804, Polistes canadensis (Linnaeus, 1758), Polybia occidentalis (Olivier, 1791) and Polybia sericea (Olivier, 1705) had the larger trophic niche breadth. The highest trophic niche overlap was between Mischocyttarus lanei Zikan, 1949 and Polistes simillimus Zikan, 1951 (0.83). The high niche overlap among social wasps seems to be resulting of the generalist behavior of the social wasp, as well as of the intensive exploration of few cactus species.

Niche Overlap and Network Specialization of Flower-Visiting Bees in an Agricultural System.

Different resource use strategies manifest as differences in the realized niches of species. Niche segregation may involve several dimensions of the niche, such as diet, space, and time. We measured the level of redundancy and complementarity of a bee–plant interaction network in an agricultural system. Because flower resource diversity is high and resource abundance associated with flowering phenology varies throughout the year, we hypothesized that trophic overlap in the community would be low (i.e., high niche complementarity). In contrast, we expected a combination of physiological constraints and exploitation competition to create high temporal overlap, leading to high redundancy in the time of use of floral resources. Dietary overlap was low (NOih = 0.18): niches of 88% of species pairs had less than 30% overlap. In contrast, temporal overlap was intermediate (NOih = 0.49): niches of 65% of species pairs had 30% to 60% overlap. Network analysis showed that bees separated their dietary niches and had intermediate complementary specialization (H2′ = 0.46). In terms of their temporal niches (H2′ = 0.12), bees were generalists, with high temporal redundancy. Temperature was not a key factor in the determination of niche overlap, suggesting that environmental factors do not likely have a primary role in determining high redundancy in the temporal use of floral resources. Rather, temporal overlap is likely associated with the timing of nectar production by flowers. Our results suggest that bees partition a wide variety of available floral resources, resulting in low dietary overlap and intermediate temporal overlap.