Evidence For Niche Partitioning Research Articles

Toward a Beech-Dominated Alternative Stable State in Dinaric Mixed Montane Forests: A Long-Term Study of the Pecka Old-Growth Forest

In the last century, a synchronous beech expansion has been observed for many mixed mountain forests in southeastern Europe. This change is associated with the interaction of various disturbances. We analyzed structural changes in the Pecka old-growth forest in Slovenia during the last century, using several inventories of the tree layer, regeneration, and site factors. Throughout the observation period, the density of silver fir in the regeneration layer and in the overstory steadily decreased. In 1893, silver fir accounted for about 60% of the growing stock, whereas in 2013 it accounted for less than 13%. This is likely because of silver fir’s decline in the canopy layer due to air pollution, successive windthrows, and overbrowsing. However, climate change may also have played an important role, as silver fir also declined in southeastern European old-growth forests where air pollution was less pronounced and ungulate densities were low. A gradual decline of silver fir in the overstory resulted in a decrease of overall tree density to 231 trees ha–1, while growing stock remained relatively high at 712 m3 ha–1. Median diffuse light at 1.3 m was 3.7% and regeneration density was 19,954 ha–1. Beech was dominant (94%), followed by silver fir (4%), and sycamore maple (2%). No silver fir seedlings larger than 0.2 m were recorded. Silver fir, sycamore maple, and beech regeneration showed 87, 76, and 45% browsing damage, respectively. Regression models indicated some evidence of niche partitioning between silver fir and beech. However, many processes may be masked by the silver fir’s avoidance strategy. Given current red deer densities, climate change, and existing forest structure, the Pecka old-growth forest will likely reach an alternative stable state dominated by beech in a few decades. This calls for immediate reduction of ungulate populations. Despite the interaction of multiple disturbances, the Pecka old-growth forest has maintained a relatively high overall growing stock, a favorable microclimate, and succession pathway with shade-tolerant beech. This indicates the intrinsic resilience of natural forests. The mechanisms discussed here can be applied to the future governance of old-growth and managed montane mixed forests.

Niche partitioning among dead wood-dependent beetles

Niche partitioning among species with virtually the same requirements is a fundamental concept in ecology. Nevertheless, some authors suggest that niches have little involvement in structuring communities. This study was done in the Pardubice Region (Czech Republic) on saproxylic beetles with morphologically similar larvae and very specific requirements, which are related to their obligatory dependence on dead wood material: Cucujus cinnaberinus, Pyrochroa coccinea, and Schizotus pectinicornis. This work was performed on 232 dead wood pieces at the landscape scale over six years. Based on the factors studied, the relationships among these species indicated that their co-occurrence based on species presence and absence was low, which indicated niche partitioning. However, based on analyses of habitat requirements and species composition using observed species abundances, there was no strong evidence for niche partitioning at either studied habitat levels, the tree and the microhabitat. The most likely reasons for the lack of strong niche partitioning were that dead wood is a rich resource and co-occurrence of saproxylic community was not driven by resource competition. This might be consistent with the theory that biodiversity could be controlled by the neutral drift of species abundance. Nevertheless, niche partitioning could be ongoing, meaning that the expanding C. cinnaberinus may have an advantage over the pyrochroids and could dominate in the long term.

Evidence for niche partitioning among ground-height browsing sauropods from the Upper Jurassic Morrison Formation of North America

Two tooth-bearing snout fragments from a diplodocid sauropod from the Brushy Basin Member of the Morrison Formation (Upper Jurassic) excavated from the Mygatt-Moore Quarry in Rabbit Valley, Colora­do are described. The Mygatt-Moore Quarry has produced thousands of vertebrate fossils from the Brushy Basin Member, with the diplodocid Apatosaurus cf. louisae and the tetanuran Allosaurus fragilis dominat­ing the assemblage. Additionally, remains of another diplodocid, Diplodocus sp., have been found near the quarry within Rabbit Valley. Both specimens in this study preserve eight teeth per alveolar position, as observed through broken surfaces at the gross anatomical level and also through computed tomography (CT) scans. This is inconsistent with the genus Diplodocus sp., which has been previously shown to have a maximum of six teeth per alveolus. The presence of eight replacement teeth per alveolus has previously only been reported in the Cretaceous rebbachisaurid Nigersaurus taqueti, which has been interpreted to have occupied a similar ground-height browsing feeding strategy to both Diplodocus and Apatosaurus. This is the first report of this type of high-count replacement teeth in a diplodocid sauropod from the Morrison Formation. The high number of replacement teeth in a close relative to the contemporaneous Diplodocus provides evidence for niche partitioning among the contemporary ground-height browsing diplodocid sauropods of the Late Jurassic Period in North America.

Evidence for niche partitioning among ground-height browsing sauropods from the Upper Jurassic Morrison Formation of North America

Two tooth-bearing snout fragments from a diplodocid sauropod from the Brushy Basin Member of the Morrison Formation (Upper Jurassic) excavated from the Mygatt-Moore Quarry in Rabbit Valley, Colora­do are described. The Mygatt-Moore Quarry has produced thousands of vertebrate fossils from the Brushy Basin Member, with the diplodocid Apatosaurus cf. louisae and the tetanuran Allosaurus fragilis dominat­ing the assemblage. Additionally, remains of another diplodocid, Diplodocus sp., have been found near the quarry within Rabbit Valley. Both specimens in this study preserve eight teeth per alveolar position, as observed through broken surfaces at the gross anatomical level and also through computed tomography (CT) scans. This is inconsistent with the genus Diplodocus sp., which has been previously shown to have a maximum of six teeth per alveolus. The presence of eight replacement teeth per alveolus has previously only been reported in the Cretaceous rebbachisaurid Nigersaurus taqueti, which has been interpreted to have occupied a similar ground-height browsing feeding strategy to both Diplodocus and Apatosaurus. This is the first report of this type of high-count replacement teeth in a diplodocid sauropod from the Morrison Formation. The high number of replacement teeth in a close relative to the contemporaneous Diplodocus provides evidence for niche partitioning among the contemporary ground-height browsing diplodocid sauropods of the Late Jurassic Period in North America.

Evidence of niche partitioning among bacteria living on plastics, organic particles and surrounding seawaters

Plastic pollution is widespread in ocean ecosystems worldwide, but it is unknown if plastic offers a unique habitat for bacteria compared to communities in the water column and attached to naturally-occurring organic particles. The large set of samples taken during the Tara-Mediterranean expedition revealed for the first time a clear niche partitioning between free-living (FL), organic particle-attached (PA) and the recently introduced plastic marine debris (PMD). Bacterial counts in PMD presented higher cell enrichment factors than generally observed for PA fraction, when compared to FL bacteria in the surrounding waters. Taxonomic diversity was also higher in the PMD communities, where higher evenness indicated a favorable environment for a very large number of species. Cyanobacteria were particularly overrepresented in PMD, together with essential functions for biofilm formation and maturation. The community distinction between the three habitats was consistent across the large-scale sampling in the Western Mediterranean basin. 'Plastic specific bacteria' recovered only on the PMD represented half of the OTUs, thus forming a distinct habitat that should be further considered for understanding microbial biodiversity in changing marine ecosystems.

Paleoecology of Miocene crocodylians in Florida: Insights from stable isotope analysis

Fossils from late Miocene localities in north Florida offer new information about the paleoecological relationships between sympatric alligatorine and tomistomine crocodylians. Large, robust tomistomines (traditionally referred to Gavialosuchus americanus) outnumber Alligator fossils at localities representing coastal/estuarine depositional environments. Stable isotope analysis of carbon (δ13C) and oxygen (δ18O) values in fossil crocodylian tooth enamel carbonate (CO3) provides evidence for niche partitioning between these two taxa, which may have co-inhabited estuarine paleoenvironments. Our isotopic results imply that alligators procured prey from primarily freshwater or terrestrial sources, whereas tomistomines fed mainly on marine-based prey items. As indicators of saltwater paleoenvironments, the fossil tomistomines may therefore be informative for tracking Miocene sea level change.

Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA.

Conclusive evidence for sexual dimorphism in non-avian dinosaurs has been elusive. Here it is shown that dimorphism in the shape of the dermal plates of Stegosaurus mjosi (Upper Jurassic, western USA) does not result from non-sex-related individual, interspecific, or ontogenetic variation and is most likely a sexually dimorphic feature. One morph possessed wide, oval plates 45% larger in surface area than the tall, narrow plates of the other morph. Intermediate morphologies are lacking as principal component analysis supports marked size- and shape-based dimorphism. In contrast, many non-sex-related individual variations are expected to show intermediate morphologies. Taphonomy of a new quarry in Montana (JRDI 5ES Quarry) shows that at least five individuals were buried in a single horizon and were not brought together by water or scavenger transportation. This new site demonstrates co-existence, and possibly suggests sociality, between two morphs that only show dimorphism in their plates. Without evidence for niche partitioning, it is unlikely that the two morphs represent different species. Histology of the new specimens in combination with studies on previous specimens indicates that both morphs occur in fully-grown individuals. Therefore, the dimorphism is not a result of ontogenetic change. Furthermore, the two morphs of plates do not simply come from different positions on the back of a single individual. Plates from all positions on the body can be classified as one of the two morphs, and previously discovered, isolated specimens possess only one morph of plates. Based on the seemingly display-oriented morphology of plates, female mate choice was likely the driving evolutionary mechanism rather than male-male competition. Dinosaur ornamentation possibly served similar functions to the ornamentation of modern species. Comparisons to ornamentation involved in sexual selection of extant species, such as the horns of bovids, may be appropriate in predicting the function of some dinosaur ornamentation.

De novo sequences of Haloquadratum walsbyi from Lake Tyrrell, Australia, reveal a variable genomic landscape.

Hypersaline systems near salt saturation levels represent an extreme environment, in which organisms grow and survive near the limits of life. One of the abundant members of the microbial communities in hypersaline systems is the square archaeon, Haloquadratum walsbyi. Utilizing a short-read metagenome from Lake Tyrrell, a hypersaline ecosystem in Victoria, Australia, we performed a comparative genomic analysis of H. walsbyi to better understand the extent of variation between strains/subspecies. Results revealed that previously isolated strains/subspecies do not fully describe the complete repertoire of the genomic landscape present in H. walsbyi. Rearrangements, insertions, and deletions were observed for the Lake Tyrrell derived Haloquadratum genomes and were supported by environmental de novo sequences, including shifts in the dominant genomic landscape of the two most abundant strains. Analysis pertaining to halomucins indicated that homologs for this large protein are not a feature common for all species of Haloquadratum. Further, we analyzed ATP-binding cassette transporters (ABC-type transporters) for evidence of niche partitioning between different strains/subspecies. We were able to identify unique and variable transporter subunits from all five genomes analyzed and the de novo environmental sequences, suggesting that differences in nutrient and carbon source acquisition may play a role in maintaining distinct strains/subspecies.

Evidence of niche partitioning under ontogenetic influences among three morphologically similar siluriformes in small subtropical streams.

Ontogenetic influences in patterns of niche breadth and feeding overlap were investigated in three species of Siluriformes (Heptapterus sp., Rhamdia quelen and Trichomycterus poikilos) aiming at understanding the species coexistence. Samplings were conducted bimonthly by electrofishing technique from June/2012 to June/2013 in ten streams of the northwestern state of Rio Grande do Sul, Brazil. The stomach contents of 1,948 individuals were analyzed by volumetric method, with 59 food items identified. In general Heptapterus sp. consumed a high proportion of Aegla sp., terrestrial plant remains and Megaloptera; R. quelen consumed fish, and Oligochaeta, followed by Aegla sp.; while the diet of T. poikilos was based on Simuliidae, Ephemeroptera and Trichoptera. Specie segregation was observed in the NMDS. Through PERMANOVA analysis feeding differences among species, and between a combination of species plus size classes were observed. IndVal showed which items were indicators of these differences. Niche breadth values were high for all species. The niche breadth values were low only for the larger size of R. quelen and Heptapterus sp. while T. poikilos values were more similar. Overall the species were a low feeding overlap values. The higher frequency of high feeding overlap was observed for interaction between Heptapterus sp. and T. poikilos. The null model confirmed the niche partitioning between the species. The higher frequency of high and intermediate feeding overlap values were reported to smaller size classes. The null model showed resource sharing between the species/size class. Therefore, overall species showed a resource partitioning because of the use of occasional items. However, these species share resources mainly in the early ontogenetic stages until the emphasized change of morphological characteristics leading to trophic niche expansion and the apparent segregation observed.

Do woody and herbaceous species compete for soil water across topographic gradients? Evidence for niche partitioning in a Neotropical savanna

Savannas are characterized by sparsely distributed woody species within a continuous herbaceous cover, composed mainly by grasses and small eudicot herbs. This vegetation structure is variable across the landscape, with shifts from open grassland to savanna woodland determined by factors that control tree density. These shifts often appear coupled with environmental variations, such as topographic gradients. Here we investigated whether herbaceous and woody savanna species differ in their use of soil water along a topographic gradient of about 110m, spanning several vegetation physiognomies generally associated with Neotropical savannas. We measured the δ2H and δ18O signatures of plants, soils, groundwater and rainfall, determining the depth of plant water uptake and examining variations in water uptake patterns along the gradient. We found that woody species use water from deeper soil layers compared to herbaceous species, regardless of their position in the topographic gradient. However, the presence of a shallow water table restricted plant water uptake to the superficial soil layers at lower portions of the gradient. We confirmed that woody and herbaceous species are plastic with respect to their water use strategy, which determines niche partitioning across topographic gradients. Abiotic factors such as groundwater level, affect water uptake patterns independently of plant growth form, reinforcing vegetation gradients by exerting divergent selective pressures across topographic gradients.

Little evidence for niche partitioning among ectomycorrhizal fungi on spruce seedlings planted in decayed wood versus mineral soil microsites

Ectomycorrhizal fungal (EMF) communities vary among microhabitats, supporting a dominant role for deterministic processes in EMF community assemblage. EMF communities also differ between forest and clearcut environments, responding to this disturbance in a directional manner over time by returning to the species composition of the original forest. Accordingly, we examined EMF community composition on roots of spruce seedlings planted in three different microhabitats in forest and clearcut plots: decayed wood, mineral soil adjacent to downed wood, or control mineral soil, to determine the effect of retained downed wood on EMF communities over the medium and long term. If downed and decayed wood provide refuge habitat distinct from that of mineral soil, we would expect EMF communities on seedlings in woody habitats in clearcuts to be similar to those on seedlings planted in the adjacent forest. As expected, we found EMF species richness to be higher in forests than clearcuts (P ≤ 0.01), even though soil nutrient status did not differ greatly between the two plot types (P ≥ 0.05). Communities on forest seedlings were dominated by Tylospora spp., whereas those in clearcuts were dominated by Amphinema byssoides and Thelephora terrestris. Surprisingly, while substrate conditions varied among microsites (P ≤ 0.03), especially between decayed wood and mineral soil, EMF communities were not distinctly different among microhabitats. Our data suggest that niche partitioning by substrate does not occur among EMF species on very young seedlings in high elevation spruce-fir forests. Further, dispersal limitations shape EMF community assembly in clearcuts in these forests.

Trait evolution and the coexistence of a species swarm in the tropical forest understorey

Summary1. A small number of species‐rich plant genera make a substantial contribution to the α diversity of tropical forests. These ‘species swarms’ (Gentry 1982) challenge a view of community assembly that maintains that niche overlap, and hence, the likelihood of competitive exclusion should increase with phylogenetic affinity. Related species may, however, occupy different microhabitats within a forest stand or, alternatively, differ ecologically in ways that allow them to coexist syntopically.2. To explore the relationship between phylogenetic history and niche differentiation among sympatric, congeneric species, we performed phylogenetic analyses of microhabitat preferences, photosynthetic and hydraulic traits, and experimental responses to light and water availability for 20 species ofPsychotria(Rubiaceae) from Barro Colorado Island (BCI), Panama.3. ThePsychotriapresent in small (3‐m radius), circular plots were more closely related than expected by chance. Photosynthetic and hydraulic traits were both associated with species habitat distributions, but only hydraulic traits were conserved phylogenetically. Experimental responses to water availability were also conserved phylogenetically. Functional trait analyses revealed little or no evidence of niche partitioning within sites.4. We conclude thatPsychotriaspecies’ responses to both light and moisture availability determine their microhabitat distributions on BCI and that evolutionarily conserved hydraulic traits lead to phylogenetic clustering of co‐occurring species.5. Synthesis. The evolutionary conservation of hydraulic traits related to soil moisture tolerance largely explains phylogenetic clustering in the local assembly of the hyperdiverse genusPsychotria. We suggest that close relatives are unlikely to exclude one another from shared habitats because resource availability is determined largely by asymmetric competition with the overstorey, rather than by competition with neighbouring understorey plants. In light of the recent biogeographic admixture in central Panama, the phylogenetic niche conservatism exhibited byPsychotriaon BCI raises the possibility of an association between local microhabitats and the ancestral climatic regimes under which majorPsychotrialineages evolved before arriving in sympatry.

Habitat preferences and evidence for niche partitioning amongst cetaceans in the waters between Gabon and Angola, eastern tropical Atlantic

The habitat preferences and niches of eight cetacean species inhabiting eastern tropical Atlantic waters between Angola and Gabon (1°N–11°S latitude) were examined. A total of 2873 cetacean sightings, recorded between January 2004 and June 2009, was assigned to 10 × 10 km grid cells and linked to four ecogeographical variables (EGVs): water depth, seabed slope, sea surface temperature (SST) and relative frontal strength. Classification trees revealed that the habitat preferences (in terms of the habitats sampled) of most species were primarily determined by SST (for Bryde's whale, sperm whale, short-finned pilot whale and common dolphin) and water depth (for Risso's dolphin, bottlenose dolphin and Atlantic spotted dolphin). Seabed slope was the most important EGV describing the presence of the striped dolphin. A principal component analysis was used to compare the niches of the species with respect to the four EGVs. Nineteen species pairs (68%) differed significantly in median principal component (PC) score for the first PC axis, suggesting differences in their niche centres for that axis. Sixteen species pairs (57%) differed significantly in PC score variance suggesting differences in the niche widths along the first PC axis. Water depth and SST were the most important variables for the first PC axis. Together, these results suggest that cetacean species inhabiting the eastern tropical Atlantic exhibit interspecific variation in their habitat preferences, and so differ in the niches that they occupy. These differences are most likely related to variation in prey species and foraging strategy.

The critical scale of small-scale spatial variation in ecological patterns and processes in intertidal macrobenthic seagrass assemblages

Earlier work on benthic faunal assemblages in intertidal seagrass beds has shown maximal structural variation at spatial scales <150 m. To examine more closely spatial variation of species assemblages within this range, numbers of target taxa in the beds on North Stradbroke Island, Moreton Bay, Australia, were sampled at nested spatial scales of 0.5, 5, 30 and 90 m at two localities 900 m apart. Variance components were largest at the smallest scales, that at 0.5 m comprising 55% of the total and that at 5 m 13%. Individual species were patchily dispersed and their assemblages were randomly structured at all studied spatial scales. Although assemblage composition differed significantly between the two localities, overall faunal abundance and species richness were relatively uniform across the whole study area. These seagrass beds show little turn-over of dominant species and little evidence of niche partitioning through space over scales of <1 km. In this they contrast markedly with the equivalent intertidal dwarf-eelgrass beds of Knysna in South Africa that form a high turn-over, strongly structured, deterministic system.

Spatial and seasonal variation in niche partitioning between blue manakin (Chiroxiphia caudata) and greenish schiffornis (Schiffornis virescens) in southeastern Brazil

The diet of Chiroxiphia caudata and Schiffornis virescens were studied over three years in four forest sites in the southeastern Atlantic Forest of Brazil. Theory suggests that coexistence of similar species is possible by resource-use differences. To test this, patterns of niche overlap and niche breadth in diet were measured in different seasons and forest sites, based on fruit morphological traits and plant life forms. The results suggest seasonal and spatial variation in resource partitioning. During periods of high fruit availability, there was no evidence of fruit trait selection. In contrast, during periods of fruit scarcity in primary forests, there was evidence of niche partitioning, where C. caudata foraged for larger fruits in canopy and S. virescens searched for understory plants. The niche overlap was higher in secondary forest during periods of scarcity, when both species used small understory fruits. Foi estudada a dieta de Chiroxiphia caudata e Schiffornis virescens durante três anos, em quatro tipos de fisionomias florestais no sudeste da Mata Atlântica, Brasil. A teoria sugere que a coexistência de espécies semelhantes é possível pelas diferenças no uso dos recursos. Assim, nós mensuramos a amplitude e sobreposição de nicho considerando a morfologia dos frutos consumidos e a forma de vida das espécies de plantas utilizadas em quatro tipos florestais e em diferentes estações. Os resultados sugerem mudanças sazonais e espaciais na partilha de recursos. Durante períodos de alta disponibilidade de frutos, não há evidências de partilha de recursos. Já em períodos de escassez de frutos nas florestas primárias, C. caudata consumiu frutos grandes presentes no dossel enquanto S. virescens utilizou os frutos pequenos do sub-bosque. Na floresta secundária, o período de escassez foi marcado por sobreposição intensa de nicho onde ambas as espécies consumiram frutos pequenos de sub-bosque.

Leaf Gas Exchange, Water Status, Spatial Dispersion, and Gender in Gynodioecious Bidens sandvicensis (Asteraceae)

How the cost of reproduction is paid in dimorphic populations is a fundamental question in plant reproductive biology, and females of dioecious species in arid environments often show lowered drought tolerance and survivorship attributable to higher reproductive allocation. The cost of reproduction in gynodioecious populations is less understood but can provide information on the selective advantages of dimorphism. Leaf gas exchange rates, water status, and spatial dispersion were measured in the two sex morphs of gynodioecious Bidens sandvicensis from two neighboring populations during typical seasonal drought on Kauai, Hawaii, in which females produce on average 33% more seeds per flowering head. Both females and hermaphrodites showed instantaneous and integrated gas exchange behavior typical of mesophytic, herbaceous, heliophytic dicots, with values for Rubisco carboxylation capacity, electron transport capacity, stomatal limitation, maximum net carbon uptake, maximum water use efficiency, and carbon isotopic fractionation within the midrange of values reported for these species. Midday water potentials during the period of flowering and fruiting approached the limit of drought tolerance reported for mesophytic species. Overall, females had marginally significantly lower photosynthetic function and water use efficiency than hermaphrodites, consistent with a passive trade‐off between leaf function and reproductive allocation. The sex morphs were dispersed at random in relation to each other, with no evidence of niche partitioning on soil moisture or elevation between the sex morphs within the two populations, and thus no evidence of a fitness difference attributable to physiological costs. The absence of large physiological differences or niche partitioning in B. sandvicensis and other gynodioecious species may be due to less pronounced sex differences in reproductive allocation as well as the less derived status of gynodioecy relative to dioecy. The correlation between gynodioecy and drought in Hawaiian Bidens may result from higher self‐fertilization in larger floral displays, leading to an outcrossing advantage for unisexual females in arid habitats.

Soil nutrients and beta diversity in the Bornean Dipterocarpaceae: evidence for niche partitioning by tropical rain forest trees

Summary The relative importance of niche‐ and dispersal‐mediated processes in structuring diverse tropical plant communities remains poorly understood. Here, we link mesoscale beta diversity to soil variation throughout a lowland Bornean watershed underlain by alluvium, sedimentary and granite parent materials (c. 340 ha, 8–200 m a.s.l.). We test the hypothesis that species turnover across the habitat gradient reflects interspecific partitioning of soil resources. Floristic inventories (≥ 1 cm d.b.h.) of the Dipterocarpaceae, the dominant Bornean canopy tree family, were combined with extensive soil analyses in 30 (0.16 ha) plots. Six samples per plot were analysed for total C, N, P, K, Ca and Mg, exchangeable K, Ca and Mg, extractable P, texture, and pH. Extractable P, exchangeable K, and total C, N and P varied significantly among substrates and were highest on alluvium. Thirty‐one dipterocarp species (n = 2634 individuals, five genera) were recorded. Dipterocarp density was similar across substrates, but richness and diversity were highest on nutrient‐poor granite and lowest on nutrient‐rich alluvium. Eighteen of 22 species were positively or negatively associated with parent material. In 8 of 16 abundant species, tree distribution (≥ 10 cm d.b.h.) was more strongly non‐random than juveniles (1–10 cm d.b.h.), suggesting higher juvenile mortality in unsuitable habitats. The dominant species Dipterocarpus sublamellatus (&gt; 50% of stems) was indifferent to substrate, but nine of 11 ‘subdominant’ species (&gt; 8 individuals ha−1) were substrate specialists. Eighteen of 22 species were significantly associated with soil nutrients, especially P, Mg and Ca. Floristic variation was significantly correlated with edaphic and geographical distance for all stems ≥ 1 cm d.b.h. in Mantel analyses. However, juvenile variation (1–10 cm d.b.h.) was more strongly related to geographical distance than edaphic factors, while the converse held for established trees (≥ 10 cm d.b.h.), suggesting increased importance of niche processes with size class. Pervasive dipterocarp associations with soil factors suggest that niche partitioning structures dipterocarp tree communities. Yet, much floristic variation unrelated to soil was correlated with geographical distance between plots, suggesting that dispersal and niche processes jointly determine mesoscale beta diversity in the Bornean Dipterocarpaceae.

Niche Partitioning by Two Sympatric Goshawks in the Australian Wet Tropics: Ranging Behaviour

SummaryTwo medium-sized woodland hawks, the Grey Goshawk Accipiter novaehollandiae and the Brown Goshawk Accipiter fasciatus, are sympatric in the Australian wet tropics. Because females are considerably larger than males, the two species and sexes form a four-member guild. Typically, when similar raptor species occur together they partition resources. Radio-tracking was used to study habitat use and composition, and interactions between the Australian goshawks. Sample sizes were small and there was considerable individual variation. Nevertheless, there was evidence of niche partitioning according to home range size and overlap, habitat composition and habitat use. In general, both species frequented forests or woodlands and adjacent open country. Consistent with morphological, dietary and hunting differences, Grey Goshawks, especially the males, frequented closed forest types and Brown Goshawks more open habitats of woodlands, crops and edges. In the non-breeding season, females ranged widely and their ranges overlapped both inter- and intra-specifically, whereas those of males of each species tended not to overlap with males and females of either species. In the breeding season, home range overlap declined markedly between neighbouring pairs of the two species, and there was no overlap between the females. The two species have relatively small home ranges, attributable to their rich habitat. In turn, this may facilitate co-existence of two such similar-sized accipiters. Nevertheless, there was evidence of competition between the two most similar-sized members of the guild, the male Grey Goshawk and the female Brown Goshawk.