Niche Axes Research Articles

Niche Relationships and Interspecific Interactions in Antiguan Lizard Communities

Anolis lizards are the focus of most Caribbean lizard community ecology studies with few studies including other common species that might influence community structure. To study niche relationships and interspecific interactions in Antiguan lizard communities, we used five offshore islands with varying combinations of three diurnal lizards: Ameiva griswoldi, Anolis leachii, and Anolis wattsi. We collected data on perch height, substrate, thermal microhabitat, body size, head length, daily activity, and abundance to characterize the ecological niche of each species. Ameiva griswoldi was more similar to A. leachii in size and daily activity, but more similar to A. wattsi in perch height, and A. leachii and A. wattsi were more similar in thermal microhabitat. This pattern of niche differentiation was consistent with niche complementarity, where species are similar on some niche axes but differ on others. Using the same niche characteristics as in species comparisons, we tested for a niche shift among islands for A. wattsi. In the absence of A. griswoldi, A. wattsi used lower perches, sunnier microhabitats, and was found more often on the ground. In contrast, with A. leachii absent, A. wattsi perched higher, more often in the shade, and on trunks. Furthermore, A. wattsi was most abundant when with A. leachii only, but least abundant when alone with A. griswoldi. These results suggest interspecific interactions, most likely due to competition and intraguild predation, are important for structuring Antiguan lizard communities.

Does niche overlap control relative abundance in French lacustrine fish communities? A new method incorporating functional traits

1. The mechanisms that structure biological communities hold the key to understanding ecosystem functioning and the maintenance of biodiversity. Patterns of species abundances have been proposed as a means of differentiation between niche-based and neutral processes, but abundance information alone cannot provide unequivocal discrimination. 2. We combined species niche information and species' relative abundances to test the effects of two opposing structuring mechanisms (environmental filtering and niche complementarity) on species' relative abundances in French lacustrine fish communities. The test involved a novel method comparing the abundance-weighted niche overlap within communities against that expected when relative abundances were randomized among species within the community. 3. Observed overlap was consistently significantly lower than expected at random for two (swimming ability and trophic status) of four primary niche axes across lakes of differing physical environments. Thus, for these niche axes, pairs of abundant species tended to have relatively low niche overlap, while rare species tended to have relatively high niche overlap with abundant species. 4. This suggests that niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities. The method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure.

CAUSES OF HABITAT DIVERGENCE IN TWO SPECIES OF AGAMID LIZARDS IN ARID CENTRAL AUSTRALIA

The deserts of central Australia contain richer communities of lizards than any other arid regions, with the highest diversity occurring in sand dune habitats dominated by hummock-forming spinifex grasses. To investigate the mechanisms that permit coexistence, we studied two species of coexisting agamid lizards that exhibit striking divergence in their use of habitat in the Simpson Desert of central Australia. Here, the military dragon Ctenophorus isolepis is restricted primarily to sites providing > 30% cover of hard spinifex Triodia basedowii, whereas the central netted dragon C. nuchalis occurs in areas with much sparser (< 10%) cover. We constructed four mechanistic models to explain this pattern and then derived hypotheses to test them. One hypothesis, that competition restricts each species to its preferred habitat, was rejected after dyad encounters in field enclosures failed to elicit any habitat shift or any overt interactions between the species. Our next hypotheses were that each species exhibits preferences for different thermal environments or different prey types and that each selects the habitats that maximize access to them. Both were supported. C. isolepis preferred lower temperatures when active and specialized in eating ants < 5 mm long and selected spinifex-dominated areas where these requirements were met. In contrast, C. nuchalis preferred higher temperatures and a diversity of prey, both of which were available mostly in open areas. Finally, we used plasticine models to test the hypothesis that each species faced lower risk of predation in its selected habitat. This was partly supported, as models of both species were attacked more often in the open than under spinifex cover. The results show that habitat divergence occurs along several, probably covarying, niche axes. We suggest that different levels of spinifex cover provide the template for a broad range of ecological interactions, allowing lizard species to partition biotic and abiotic resources and achieve the extraordinarily high levels of local diversity that are observed.

Do lizard communities partition the trophic niche? A worldwide meta‐analysis using null models

Lizards have been widely used as model organisms in community ecology over the past 30 years. I have reviewed more than 50 studies from the literature on the community ecology of lizards worldwide, with a focus on studies from 1990–2007. I determined if there is support for the hypotheses that many lizard communities 1) are non‐randomly organized along the trophic niche dimension, and 2) partition the available food resource to minimize interspecific competition. I used data on number of prey items and percentage of prey volume, to calculate dietary overlap among species. I compared true datasets to randomly generate new datasets produced by 3×104 Monte Carlo permutations using two algorithms (RA2 and RA3). The great majority (more than 80%) of the communities were randomly organized along the trophic niche dimension using RA2 or RA3; the frequency of occurrence of detecting a non‐random structure in lizard community dietary studies did not differ significantly between datasets based on either number of prey items or prey volume. Thus, lizard communities usually do not partition the trophic niche axis. Concerning the number of prey items, logistic regression models showed that the presence of a structure in the dataset did not depend on number of species, method employed to obtain dietary data, matrix size, or location (continent), but instead significantly depended on whether a community was tropical. Concerning prey volume, presence of a structure in the dataset also did not depend on number of species, method employed to obtain dietary data, and tropical vs non‐tropical origin, but was marginally dependent on continent (South America was favoured for identifying a structure in the dataset) and matrix size. In general, a structure in the dataset was more often uncovered by using RA2 than RA3. Overall, I conclude that 1) lizard communities are unlikely to partition available food resources, and 2) as the method for identifying dietary items does not allow for accurate prey identification (at least at the genus level), there is a risk of obtaining false null results about community structure. This is because structure along the trophic niche axis is genuinely rare in lizards and also because inappropriate methods may erroneously lead to this conclusion. Therefore, conclusions from studies utilizing stomach dissection as a source of diet data (still common in the literature) should be cautiously considered for ethical reasons and because it is difficult to find any difference between stomach‐dissection and fecal‐pellet data when assessing the presence of a structure in datasets. My data also showed there are minor differences in the probability of detecting a structure in datasets using prey item number or prey volume data for lizards. The biases, both statistical and biological, associated with this meta‐analysis are discussed.

Ecological Relationships between Sympatric Vipera Aspis and Vipera Ursinii in High-Altitude Habitats of Central Italy

The ecological relationships between two sympatric vipers (Vipera aspis and Vipera ursinii) were studied at a mountainous area in central Italy (Gran Sasso, Abruzzo), between spring 2004 and autumn 2006. The two species differed significantly in their altitudinal distribution: V. ursinii being confined to the higher elevations (above 1650 m elevation), and V. aspis being present from the lowest to the highest altitudes (1200-1900 m). We recorded a wide sector of approximately 250 m elevation (between 1650 and 1900 m elevation) where the two species overlapped. In these areas, the two species exhibited significant difference in microhabitat use and diet composition. Male and female V. aspis were larger (SVL) than male and female V. ursinii. Furthermore, the two species exhibited a significant difference in sexual size dimorphism, with females being larger than males in V. ursinii, whereas no such difference in SVL was observed in V. aspis. However, we did not observe any difference in SVL of male and female vipers when comparing sympatric and allopatric populations. Overall, the potential for competition between these two species appears to be low because of their clearly different realized ecological niches. The ecology of European vipers has received considerable interest since the pioneering stud- ies conducted by Saint Girons (1952, 1978, 1980). Some European viperids (notably Vipera berus) are currently among the ecologically best known snake species in the world (e.g., Madsen, 1988; Madsen and Stille, 1988; Madsen and Shine, 1992a). However, most studies have focused on the population biology of a single species (e.g., Madsen and Shine, 1992b, c), whereas comparatively few studies have ex- plored the population biology of sympatric populations (for a review, see Luiselli, 2006a). The first study on the ecology of sympatric viper populations was a field study on Vipera aspis and V. berus in Western France (Saint Girons, 1975). After this pioneer study, only a few detailed studies have been conducted, the most significant being those on sympatric V. aspis and V. berus populations in the mountains of Switzerland (Monney, 1996) and on sympat- ric Vipera seoanei and Vipera latastei populations in the northwestern Iberian peninsula (Brito and Crespo, 2002; Martinez et al., 2006). In general, these studies suggest that sympatric vipers tend to partition the habitat niche axis to minimize competition, whereas their diets and activity patterns are very similar (Luiselli, 2006a). However, much still needs to be known concerning the ecological relationships of sym- patric European viper populations to further test competition/niche partitioning issues and also to explore conservation implications of threatened species/populations. In this paper, we describe the ecological relationships be- tween two viper species, the European Asp (V. aspis) and the Meadow Viper (Vipera ursinii), in a mountainous area of central Italy. The Meadow Viper is an endangered species which inhabits a few scattered mountain peaks in Western Europe (Bruno, 1985; Bombi et al., 2006).

Functional characters combined with null models reveal inconsistency in mechanisms of species turnover in lacustrine fish communities

Functional characters have the potential to act as indicators of species turnover between local communities. Null models provide a powerful statistical approach to test for patterns using functional character information. A combined null model/functional character approach provides the ability to distinguish between the effect of competition and environmental filtering on species turnover. We measured 13 functional characters relating directly to resource use for the fish species found in French lakes. We combined this functional character data with a null model approach to test whether co-occurring species overlapped more or less than expected at random for four primary niche axes. We used an environmentally constrained null model approach to determine if the same mechanisms were responsible for species turnover at different sections of the altitudinal gradient. Functional diversity indices were used to examine the variation in functional character diversity with altitude, as a test of the hypothesis that competitive intensity decreases with increasing environmental adversity. The unconstrained null model showed that environmental filtering was the dominant influence on species turnover between lakes. In the constrained null model, there was much less evidence for environmental filtering, emphasising the strong effect of altitude on turnover in functional character values between local communities. Different results were obtained for low-altitude and high-altitude lake subsets, with more evidence for the effect of environmental filtering being found in the high-altitude lakes. This demonstrates that different processes may influence species turnover throughout an environmental gradient. Functional diversity values showed a slight decrease with altitude, indicating that there was only weak evidence that competitive intensity decreased with increasing altitude. Variation resource availability and environmental stress probably cause the observed turnover in functional characters along the altitudinal gradient, though the effects of dispersal limitation and species introductions in high-altitude lakes cannot be ruled out.

Microhabitat separation and niche overlap among five assemblages of tropical skinks

The niche axes of microhabitat use, activity and size for 21 skink species from five assemblages (Alligator Creek, Cairns, Chillagoe, Cooktown and Mt Bartle Frere) in the biodiverse tropics of north-east Queensland were examined. Species within the same assemblage separated predominantly along two structural microhabitat gradients; one that ranged from microhabitats dominated by large rocks to leaf litter and total ground cover, and a second defined by increasing leaf litter, ground cover, undergrowth, proximity to vegetation and increased canopy cover. All species used available microhabitats non-randomly, with species from the same ecotype (arboreal, generalist, litter-dwelling, rock-using) clustering in multivariate ecological space. Despite evidence of within-assemblage niche overlap, null-model comparisons revealed that only one assemblage (Chillagoe) had greater niche overlap than would be expected by chance. Assemblages with more species occupied smaller niche space, indicating species packing. However, species with more diverse niches were less evenly packed. While most species overlapped in activity time and body size, differences among species from the same ecotype were observed. Despite subtle differences in temporal activity and body size, differences in structural microhabitat use appears to be the dominant niche axis allowing the coexistence of species within these assemblages of tropical skinks.

Feeding ecology of elasmobranch fishes in coastal waters of the Colombian Eastern Tropical Pacific

BackgroundStomach contents of 131 specimens of five elasmobranch species (Mustelus lunulatus, Dasyatis longa, Rhinobatos leucorhynchus, Raja velezi and Zapteryx xyster) caught in the central fishing zone in the Pacific Ocean of Colombia were counted and weighed to describe feeding habits and dietary overlaps.ResultsTwenty-one prey items belonging to four major groups (stomatopods, decapods, mollusks and fish) were identified. Decapod crustaceans were the most abundant prey found in stomachs. The mantis shrimp Squilla panamensis was the main prey item in the diet of M. lunulatus; tiger shrimp Trachypenaeus sp. was the main prey item in the diet of Rhinobatos leucorhynchus and Raja velezi, and Penaeidae shrimp were the main prey items in the diet of Z. xyster. Furthermore, fish were important in the diet of Raja velezi, Z. xyster and D. longa. The greatest diet breadth corresponded to Z. xyster whereas M. lunulatus was the most specialized predator. Finally, four significant diet overlaps between the five species were found, attributable mainly to Squillidae, Penaeidae and Fish.ConclusionShrimps (Penaeidae and stomatopods) and benthic fishes were the most important food types in the diet of the elasmobranch species studied. Diet breadth and overlap were relatively low. Determination of food resource partitioning among the batoid species studied was not possible. However, we identified partitions in other niche axes (time of feeding activity and habitat utilization). It is possible to assume that diffuse competition could be exceeding the biunivocal competition among the studied species. Therefore, this assemblage would have a strong tendency to trophic guild formation.

Niche overlap and resource partitioning between two sympatric fox species in southern Brazil

AbstractMechanisms that favour the co‐occurrence of morphologically and ecologically similar species in South America are potentially relevant, because two or more species often occur in sympatry. In the present study, we investigated possible mechanisms of resource partitioning between two sympatric species of foxes, the crab‐eating foxCerdocyon thousand the pampas foxPseudalopex gymnocercus, in the National Park of Aparados da Serra in southern Brazil (29°10′S, 50°05′W). We considered three main niche dimensions – habitat, diet and time – and evaluated the overlap between the two species in these dimensions. We conducted the study from June 2000 to October 2001 in an area composed of open grasslands andAraucariaforests. We studied the foxes by using a combination of scat analysis and direct observation. A comparison of their diet indicated that it was very similar for both species in all seasons (Pianka's indexes ranging from 0.920 to 0.957). The most common food item for both species of foxes was rodents, followed by coleopterans, birds and plant remains (seeds and fruits). As expected by the niche‐complementarity hypothesis, we detected differences along other niche axes.Pseudalopex gymnocercusseemed to occur only in open areas, including grasslands and dirt roads, whereasC. thouswas more generalist in habitat use, occurring both inside and at the edge of forests. However, both fox species were common in grassland habitats, where they might compete for resources. The differences were more marked for activity time, with the crab‐eating fox being significantly more nocturnal (circular mean of time of sighting=23:39 h,se=56 min) than the pampas fox (circular mean=19:56 h,se=64 min; Watson'sF=6.06,P=0.02). We suggest that the largerC.thouscould be limiting the activity ofP.gymnocercusby interference competition.

On the role of phylogeny in determining activity patterns of rodents

Evolutionary plasticity is limited, to a certain extent, by phylogenetic constraints. We asked whether the diel activity patterns of animals reflect their phylogenies by analyzing daily activity patterns in the order Rodentia. We carried out a literature survey of activity patterns of 700 species, placing each in an activity time category: diurnal, nocturnal, or active at both periods (a-rhythmic). The proportion of rodents active at these categories in the entire order, was compared to the activity patterns of species of different families for which we had data for over ten species each: Dipodidae, Echimyidae, Geomyidae, Heteromyidae, Muridae, and Sciuridae. Activity times of rodents from different habitat types were also compared to the ordinal activity time pattern. We also calculated the probability that two random species (from a particular subgroup: family, habitat, etc.) will be active in the same period of the day and compared it to this probability with species drawn from the entire order. Activity patterns at the family level were significantly different from the ordinal pattern, emphasizing the strong relationship between intra-family taxonomic affiliation and daily activity patterns. Large families (Muridae and Sciuridae) analyzed by subfamilies and tribes showed a similar but stronger pattern than that of the family level. Thus it is clear that phylogeny constrains the evolution of activity patterns in rodents, and may limit their ability to use the time niche axis for ecological separation. Rodents living in cold habitats differed significantly from the ordinal pattern, showing more diurnal and a-rhythmic activity patterns, possibly due to physiological constraints. Ground-dwelling rodents differed significantly, showing a high tendency towards a-rhythmic activity, perhaps reflecting their specialized habitat.

Temporal resource partitioning by bats at water holes

AbstractThe temporal axis of niche (temporally based resource partitioning) is an understudied mechanism for resource partitioning in mammals even though it provides a potential means for species coexistence. Herein we evaluate species‐specific use curves based upon capture times during the overlapping activity of bat species at two water holes in an arid environment where bats likely suffer high levels of evaporative water loss during roosting. During the first 75 min after sunset, intense bat activity at each water hole ensued and, although visitation times overlapped, there was a significant difference among species use curves. In addition, pairwise comparisons showed high similarity in temporal visitation patterns across sites for species in which capture numbers were comparable, whereas other species shifted to significantly earlier arrival times when their capture numbers were higher. There were no significant differences in mean roost site emergence times among species, nor in distance of roost sites from water holes. Our data provide one of the few statistically verified examples of fine‐grain temporal partitioning by mammals simultaneously using the same resource. We conclude that temporal partitioning helps facilitate bat species coexistence in water‐stressed environments.

Discontinuities in ecological data

Historically, ecology has focused on continuous distributions and smooth transitions. Only recently have discontinuities and thresholds become an explicit focus in some areas of ecology, especially in the realm of complex systems. The study of animal body mass distributions has been recognized for its potential to provide insight into the underlying processes shaping animal communities. Hutchinson (1) formalized the understanding of species niches and the potential for competition to shape body mass distributions. However, despite a long history of theoretical and empirical pursuit, the mechanisms driving patterns in body mass distributions remain poorly understood. The work of Scheffer and van Nes (2) in this issue of PNAS demonstrates that community interactions alone can create discontinuous, lumpy distributions of simulated species along a niche axis. Their contribution comes at a time of heightened interest in understanding the mechanisms that may lead to discontinuities in body mass or biomass distributions. Much of the renewed interest in body mass distributions has followed the publication of a provocative ecological monograph that suggested animal body mass distributions are entrained by landscape structure (3). Holling’s paper (3) initially spawned skepticism that body mass distributions are characterized by what Holling termed “lumps” and “gaps.” Currently, many ecologists accept that body size distributions are discontinuous, but there remains disagreement regarding the mechanisms responsible. One mechanism proposed focuses on interactions among species living in the same habitat. The strongest and clearest species interaction, other than predation, is competition. However, facilitative interactions are also increasingly recognized for their potential to shape community structure. Scheffer and van Nes (2) demonstrate that species interactions may result in both repulsion and attraction along a niche axis. Attraction occurs when species are similar enough to avoid competition and results … *E-mail: allencr{at}unl.edu

Self-organized similarity, the evolutionary emergence of groups of similar species

Ecologists have long been puzzled by the fact that there are so many similar species in nature. Here we show that self-organized clusters of look-a-likes may emerge spontaneously from coevolution of competitors. The explanation is that there are two alternative ways to survive together: being sufficiently different or being sufficiently similar. Using a model based on classical competition theory, we demonstrate a tendency for evolutionary emergence of regularly spaced lumps of similar species along a niche axis. Indeed, such lumpy patterns are commonly observed in size distributions of organisms ranging from algae, zooplankton, and beetles to birds and mammals, and could not be well explained by earlier theory. Our results suggest that these patterns may represent self-constructed niches emerging from competitive interactions. A corollary of our findings is that, whereas in species-poor communities sympatric speciation and invasion of open niches is possible, species-saturated communities may be characterized by convergent evolution and invasion by look-a-likes.

Diversidad y segregación de nichos en anfibios de montaña en la Reserva La Forzosa (Anorí, Antioquia, Colombia)

The northern Central Cordillera of the Andes in Colombia contains a diverse anuran fauna that is poorly known and highly threatened due to intensive processes that alter forests in this region. Human activity has created modified habitats that may differ in species composition and abundance. In a one-year study of the La Forzosa Reserve (Anori municipality, Antioquia), differences in species richness, composition, and abundance was studied in two anuran assemblages in two premontane forests (highly impacted primary forest and tall scrub forest) that differed in vegetative structure (complexity and heterogeneity), with the less perturbed forest exhibiting more variable arboreal vegetation but more homogeneous herbaceous vegetation. Although species richness did not differ (14 species each), the less perturbed forest was more diverse with greater species evenness, while the other forest was dominated by two species (Eleutherodactylus factiosus and E. viejas). In general, the anuran fauna of the area was dominated by Eleutherodactylus and centrolenid species, a typical pattern for Andean forests, presumably due to the relative absence of lentic water bodies and presence of streams. Three niche axes were quantified (activity period and habitat and microhabitat use) by means of a pseudocommunity analysis (null model) in order to inspect for ecological segregation among the anuran species in the assemblage (irrespective of forest type). Activity periods overlapped broadly among most species, with the majority nocturnal and a few strictly diurnal (Ranitomeya opisthomelas and E. mantipus) or occasionally diurnal. With respect to habitat and microhabitat, there also was no evident spatial segregation, and the null model analysis showed no significant differences among the species, although there was a suggestion that diurnal leaf litter species and principally nocturnal perching species differed and that species confined to the forest interior were more generalists

Interactions between environmental variability and immigration rate control patterns of species diversity

Theories focussing on local competition processes predict that species diversity is maximised with high-frequency environmental variability or alternatively with intermediate frequencies. The models coupling regional processes to local explanations of diversity patterns predict that the immigration rate from a regional species pool can strongly increase diversity. However, the interaction between local and regional processes in temporally fluctuating environment has received little attention. We explored in a simulation study how the patterns of species diversity are affected by the frequency spectrum of stochastic environmental variations and density independent immigration rate in a competition model. Environmental variations were assumed to affect the growth rate of competing species. The growth rate of a species had a unimodal response to a continuous environmental variable, such as salinity and temperature. The location of optimum and breadth of the growth response in the environmental niche axis is treated as a species-dependent trait. Consequently, environmental variations can affect competitive ranking of the species. Decreasing the frequency of environmental variations produced monotonically decreasing or unimodal “intermediate disturbance type” diversity patterns in the competition model. The level of diversity and type of the pattern was determined by immigration rate, and the number and traits of the species initially placed along the environmental niche axis. Immigration generally increased diversity, but the amount of increase in intermediate immigration rates was strongly modified by the frequency spectrum of the environmental variation. Decreasing the frequency of the environmental variation always produced a monotonically decreasing diversity pattern in a two-species system where the growth optima are positioned symmetrically around the mode of the environmental noise. However, increasing the species number and asymmetry of optima makes the diversity pattern sensitive to immigration rate: diversity can be very low and unaffected by the frequency of environmental variation in the absence of immigration, peak in low-frequency environment with small immigration rate (i.e. produce the IDH type pattern), or decrease monotonically with high immigration rate. Diversity is most sensitive to small changes in the initial species number and position of the species growth optima when environmental fluctuations have high frequency. That immigration can change the impact of the frequency of the environmental variation on diversity from monotonically decreasing to unimodal (IDH type) pattern emphasizes that the regional process can be important for explanations of the diversity patterns, especially when the immigration rates are uncorrelated to the local conditions. The strong interactions between community properties, immigration rate, and the spectrum of environmental variation suggest that small changes in community properties or in dispersal rates can have a large impact on species diversity.

Web-Construction Behavior of Linyphiid Spiders (Araneae, Linyphiidae): Competition and Co-Existence Within a Generalist Predator Guild

The web-construction behavior of three species of linyphiid spider (Erigone autumnalis, Meioneta unimaculata and Bathyphantes pallida) was studied in the laboratory to examine competition and co-existence within predator guilds. Competitive interactions between spiders potentially reduce their role in biological control. We tested the hypothesis that at high densities, intraguild competition for web-sites would occur but spatial separation of microhabitat would reduce interguild competition, thus allowing co-existence. High mortality and reduced web-size were observed at high B. pallida densities but Linyphiinae co-existed with Erigoninae which constructed webs at different strata. Competitive exclusion by larger individuals occurred between species whose microhabitat niche overlapped. The biocontrol potential of spider or arthropod predator guilds could ultimately be enhanced by maximizing the diversity of species whose niche axes vary.

Juvenile growth and palatability in co‐occurring, congeneric British herbs

Differential sensitivity (DS) storage dynamics describe a temporal niche axis that determines coexistence of competing taxa through a trade-off between environmental insensitivity and competitive ability at the recruitment stage. In DS storage dynamics, when the relevant environmental factor is low, the more sensitive, better competitor preferentially recruits; when the environmental factor is high, the environmentally sensitive species suffers high mortality and the environmentally insensitive taxon preferentially recruits. A herbivore defense/growth rate trade-off at the seedling/juvenile stage could support this dynamic. We therefore compared juvenile palatability, a measure of anti-herbivore defense, and early growth rate for five congeneric pairs of native British herbs. All five comparisons showed a positive association between average individual growth rate and average palatability to a native slug species. We observed no evidence of associations between early growth rate and adult palatability or between early growth rate and life history strategy (annual vs. perennial). Seed mass was not associated with either early growth rate or with life history strategy whether or not relatedness was taken into account. We offer two explanations as to why we found statistically significant support for a growth rate- defense trade-off when within-species studies so often produce only equivocal results.

A NEW APPLICATION OF STORAGE DYNAMICS: DIFFERENTIAL SENSITIVITY, DIFFUSE COMPETITION, AND TEMPORAL NICHES

We have developed our recent demonstration that a temporal niche axis may support the stable coexistence of tree species. We previously applied a two-species lottery model assuming differences between species in environmental sensitivity to generate the predictions that the better competitor will show greater fluctuation in recruitment and be the less common species, predictions supported unanimously by data from a Mexican deciduous forest. Here, we expand on that model and explore the implication from our earlier study that competition among tree species may be focused, rather than the more commonly assumed diffuse, by applying a version of the model dealing with diffuse competition. We found focused competition to be the better explanation of the available data. Our results constitute a serious challenge to the entrenched opinion that competition among tree species is diffuse and to the proposition that competition may be unimportant in structuring tree communities. Another component of the earlier work was the comparison of closely related species. Although species may be ecologically similar for reasons other than phylogenetic relatedness, the additional underlying genetic similarity and the shared evolutionary history of close relatives makes them good candidates for the processes driving storage dynamics. We draw on the Gentry data set to show that congeneric species co-occur in plant communities in all habitat types worldwide, indicating that the dynamics we describe may take place in almost any community containing woody species, in any part of the world.

Niche partitioning in the cestode communities of two elasmobranchs

Several randomization methods have been used to investigate the influence of competitive interactions in shaping parasite community structure. Marine fish parasite communities have often been regarded as unstructured assemblages with little or no resource limitation and, therefore, not prone to competitive influences. In this study, null models were used to assess the niche overlap of cestode communities of two distinct but closely related elasmobranch hosts: the round stingray, Urobatis halleri, Cooper (1863) and the skate, Leucoraja naevus, Muller and Henle (1841). Cestode species distributions were analyzed for randomness with respect to two niche axes: host size class and location within host spiral intestine. Niche overlap of cestode species was calculated for each niche type using MacArthur and Levins’ and Pianka's indices, and compared to overlap values obtained from randomly generated communities. Cestodes of both host species were distributed heterogeneously among valves and host size‐classes. The majority of parasite species (including all common ones) within U. halleri and all species within L. naevus had significantly nonrandom distributions with respect to at least one niche category. Cestode species pairs tended to overlap significantly less than expected in spiral intestine valves and significantly more than expected among host size classes. We conclude that cestode communities are structured deterministically in a way consistent with expectations based on competition.

SEXUAL SEGREGATION IN RUMINANTS: DEFINITIONS, HYPOTHESES, AND IMPLICATIONS FOR CONSERVATION AND MANAGEMENT

The concept of sexual segregation was 1st formally proposed by Charles Darwin. Among mammals, ruminants have been the focus of most research on this phenomenon. Sexual segregation has been defined traditionally as the differential use of space (and often habitat and forage) by sexes outside the mating season, but other hypotheses related to activity patterns of sexes recently have been forwarded. These new hypotheses, however, cannot explain the spatial separation of sexes or their differential use of habitats and forages. Sexual segregation should be considered in a niche framework wherein overlap on 1 niche axis is accompanied by avoidance on another, including space, diet, and habitat. Moreover, sexual segregation is scale sensitive, which limits the usefulness of a comparative approach in investigating differences among species or populations. Failure to discriminate between the potential evolutionary causes of sexual dimorphism in ruminants has led to confusion over whether polygyny or intersexual competition has led to sexual segregation. Neither exploitive nor interference competition, wherein males are competitively excluded by females, is a likely cause of sexual segregation. I suggest that the gastrocentric model, which invokes allometric and life-history differences between sexes, or risk of predation are the only hypotheses necessary to explain sexual segregation. Additional research, however, is necessary to verify some aspects of those hypotheses. The management and conservation of ruminants requires consideration that sexes behave as if they were different species, which holds consequences for estimating populations, manipulating harvest and habitats, and the potential spread of diseases and parasites and might have genetic consequences for some populations. This review attempts to resolve long-standing problems related to studies of sexual segregation, but much research remains to be accomplished, including more manipulative experiments.