Intra-population niche differences in generalist foragers have captured the interest of ecologists, because such individuality can have important ecological and evolutionary implications. Few ...researchers have investigated how these differences affect the relationships among ecologically similar, sympatric species. Using stable isotopes, stomach contents, morphology and habitat preference, we examined niche partitioning within a group of five anurans and determined whether variation within species could facilitate resource partitioning. Species partitioned their niches by trophic level and by foraging habitat. However, there was considerable intraspecific variation in trophic level, with larger individuals generally feeding at higher trophic levels. For species at intermediate trophic levels, smaller individuals overlapped in trophic level with individuals of smaller species and larger individuals overlapped with the smallest individuals from larger species. Species varied in carbon isotopes; species with enriched carbon isotope ratios foraged farther from ponds, whereas species with depleted carbon isotope values foraged closer to ponds. Our study shows that these species partition their niches by feeding at different trophic levels and foraging at different distances from ponds. The intraspecific variation in trophic level decreased the number of individuals from each species that overlapped in trophic level with individuals from other species, which can facilitate species coexistence.
Terrestrial migrations of salamanders are influenced by many factors, particularly temperature and moisture availability. Although photoperiod is generally an important migratory cue, it has not been ...specifically considered for salamanders. We examine the importance of the above abiotic factors at the macro- and microclimatic scales in the subterranean migration of a population of Cave Salamanders (Eurycea lucifuga) in Kentucky. This species exhibited a cyclic, seasonal migratory pattern between the twilight and dark zones in a cave. Our regression analysis of macroclimate indicated this migration was influenced largely by photoperiod, but also highly correlated with temperature and relative humidity; principle components analysis confirmed the importance of similar microclimate variables (illumination, temperature, and substrate wetness), explaining 88% of the variability in the data. These environmental variables fluctuate little in the dark zone of caves and thus likely do not provide cues on seasonal conditions to salamanders. However, these cues do vary in the twilight zone and may inform salamanders of epigean seasonal conditions. This information is critical to these cave-dwelling salamanders, particularly for finding suitable food resources.
Public and private flower gardens could be valuable for slowing pollinator decline in urbanized areas, as they can potentially provide crucial foraging and reproductive resources in fragmented ...landscapes. We conducted surveys of adult butterflies at 26 gardens that contained a majority of native species; we then evaluated how the impervious surface percentage (IS%) surrounding each site and the gardens’ local characteristics (garden area, plant species richness, and planting density) influenced butterfly communities. Butterfly diversity and abundance were strongly influenced by interactions between IS% and local characteristics. IS% interacted significantly with plant species richness to affect butterfly species richness (p = 0.027) and also interacted significantly with both garden area (p < 0.001) and planting density (p = 0.001) to affect butterfly abundance. In each of these interactions, increasing IS% had a negative effect on butterfly abundance, but that effect was mitigated by increases in the interacting factor. In all cases, the strength of this mitigation was greater in more urban gardens, i.e. those with higher IS%. For example, while larger gardens always had higher butterfly abundances, this difference was greatest when comparing large urban gardens with small urban gardens. Garden area is also critical; in addition to the interaction with IS%, garden area significantly affected butterfly species richness (p = 0.037). As gardens increased in size, so did butterfly species richness regardless of IS%. Our results show that gardens can positively affect urban butterfly diversity and abundance. Urban conservation efforts should focus on establishing new gardens/habitat patches, as well as increasing the size of currently established gardens.
Niche partitioning is an important mechanism for allowing ecologically similar species to coexist, contributing to biodiversity and the functioning of ecological communities. Species partition niches ...by taking advantage of environmental heterogeneity. However, niche partitioning and species coexistence investigations often do not include intraspecific variation or individual differences like sex and body size even though these factors can have important ecological consequences. Such intrapopulation factors can reduce the number of individuals among species that overlap in resource use and potentially facilitate coexistence. Using stable isotopes (δ
13
C and δ
15
N), we quantified dietary differences among three ecologically similar, sympatric watersnake species:
Nerodia erythrogaster
,
N. rhombifer
and
N. sipedon
. Additionally for each species, we determined intraspecific dietary patterns and determined how those within-species patterns may contribute to dietary niche partitioning among species.
Nerodia erythrogaster
fed more on terrestrial prey, while
N. rhombifer
fed at higher trophic levels. Females across species fed at higher trophic levels than did males, and isotopic variance differed between the sexes in
N. sipedon
. Larger watersnakes foraged at higher trophic levels and fed more on terrestrial prey. Each watersnake species had a distinct diet that overlapped to some degree with the other species’ diets, but these diets varied both between sexes and among size groups within species. This inter- and intraspecific dietary variation can facilitate species coexistence by reducing the number of individuals from all species that use the same resources. Intraspecific variation can add important and nuanced layers to the evolution of species coexistence, and research on interspecific niche relationships needs to increasingly consider the effects of these intraspecific variations.
Individual generalist predators often have more specialized diets than their populations do. Individual specialization (IS) is influenced by ecological opportunity, intraspecific competition, and ...interspecific competition, although the effects of these parameters are inconsistent across studies. We investigated IS in five species of frogs and toads, Anaxyrus americanus, A. fowleri, Lithobates catesbeianus, L. clamitans, and L. sphenocephalus. We used the natural history and ecology of each species to predict which parameters would influence IS. Our predictions were supported for some species but not others. We predicted IS would be positively influenced by resource diversity in all species, but this prediction held for only three species, with the relationship significant in A. fowleri and L. catesbeianus and marginally significant in A. americanus. We also predicted that interspecific competition would have a negative relationship with IS in L. clamitans because L. catesbeianus is competitively superior to L. clamitans and likely to suppress its foraging options. This prediction was upheld. Finally, we predicted that IS in A. americanus, A. fowleri, and L. clamitans would be influenced by intraspecific competition. However, IS was not influenced by intraspecific competition in any species, a surprising result given that intraspecific competition has traditionally been assumed to be the ecological parameter with the strongest effects on IS. Many previous studies did not simultaneously consider all three ecological parameters, which may have increased the apparent importance of intraspecific competition for IS. Our results revealed that the ecological parameters affected IS differently even across closely related and ecologically similar species, and demonstrated that these differences are sometimes predictable based on natural history. This study also suggests that sympatric ecological speciation based on IS may be rare because the ecological parameters driving IS are inconsistent across species, and the strength of their effects on intraspecific diet variation varies in space.
Abstract
Habitat selection is driven by many factors, but no one location is likely to be best for all factors; thus, individuals are subject to trade-offs when selecting habitat. Caves provide a ...clear example of such trade-offs because these habitats are energy deprived. Cave salamanders (Eurycea lucifuga) commonly inhabit caves at least in part because this habitat is cool and wet. We tested the hypothesis that caves also provide cave salamanders with a reduction in predation risk. We used clay models to test for differences in predation risk in caves vs. forests and at low (e.g., ground) vs. elevated (e.g., cave wall) positions, and recorded locations of cave salamanders to assess vertical (i.e., wall) vs. non-vertical (e.g., ground) substrate selection in a cave. Overall, a mean of 3.2 models in caves were damaged and a mean of 8.2 were damaged in forests. Cave salamanders selected vertical substrate more often than non-vertical substrate (
χ
2
=
794.53,
p
<
0.001), and in caves, low-positioned models were more likely to be damaged than models on walls, although there was no effect of model height in forests. This study suggests that caves provide salamanders a refuge from increased predation pressure, a benefit that likely compensates for the costs of moving between caves and the richer foraging grounds outside them.
Habitat alteration via urbanization has very different effects on even closely related taxa. Most research investigating the ecological effects of urbanization has focused on birds or mammals, ...resulting in a relatively poor understanding of how the species richness and community composition of invertebrates may change. We quantified differences in species richness of adult odonates (dragonflies and damselflies) at lentic and lotic sites in urban and rural landscapes, and we examined environmental factors that might drive the differences in community composition that we observed. For lotic sites, species richness did not differ between urban versus rural sites for either dragonflies or damselflies. For lentic sites, urban and rural sites contained similar dragonfly species richness, but damselfly species richness was significantly lower at urban sites than at rural sites. Differences in lentic odonate community composition were associated with the amount of urban development within 150 m of each site, mean algal coverage, and distance to the urban center. At lotic sites, water temperature and distance to the urban center were correlated with differences in odonate community composition. The differing responses to urbanization observed in this study were probably a consequence of differences between lentic versus lotic ecosystems and between dragonflies versus damselflies in dispersal capability and habitat specificity. Given that different environmental factors affected these taxa differently in lentic and lotic sites, maintaining the highest level of odonate diversity possible across a landscape will require the use of different management practices for each ecosystem type.
Growing native plants in urban gardens is often promoted as a possible means of increasing lepidopteran populations. However, the efficacy of such efforts has not been well studied. Lepidopterans ...vary widely in their ability to survive in cities, and the few previous studies of caterpillar abundance or biomass across an urban-rural gradient have yielded mixed results. We placed clay caterpillar models in native plant gardens to assess whether the attack rate on these models varied with degree of urbanization (percent impervious surface within 1 km radius of each garden), and whether responses differed across predator taxa. We also examined how garden characteristics (plant biovolume density, plant species richness) affected probability of attack. Overall, attack rates on these models decreased with increasing impervious surface, although predator taxa varied in their sometimes complex responses. For parasitoid wasps, which accounted for 47% of all attacks, increasing biovolume density increased attack probability at impervious surface levels below 35%, but decreased the probability of attack at higher levels of impervious surface. In contrast, probabilities of attack by both predatory wasps and vertebrates decreased with increasing percent impervious surface, but did not vary with impervious surface for ants and spiders. Predation on caterpillars in urban gardens may be lower than in rural ones; however, this potential increase in survival may be a result of declines in some predator taxa, such as predatory wasps and insectivorous birds. More studies across an urban gradient are needed to measure factors other than predation that influence caterpillar survival in gardens.
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•Stable isotopes are increasingly used tool of biologists.•Researchers often assume that isotopic properties are thermally static.•Temperature increased incorporation rates via higher ...growth and catabolism rates.•Temperature decreases trophic fractionation.•Natural variation in temperature must be accounted for when using stable isotopes.
Stable isotopes are valuable tools in physiological and ecological research, as they can be used to estimate diet, habitat use, and resource allocation. However, in most cases a priori knowledge of two key properties of stable isotopes is required, namely their rate of incorporation into the body (incorporation rate) and the change of isotope values between consumers and resources that arises during incorporation of the isotopes into the consumer’s tissues (trophic discrimination). Previous studies have quantified these properties across species and tissue types, but little is known about how they vary with temperature, a key driver of many biological rates and times. Here, we explored for the first time how temperature affects both carbon incorporation rate and trophic discrimination via growth rates, using the domestic cricket, Acheta domesticus. We raised crickets at 16 °C, 21 °C, and 26 °C and showed that temperature increased carbon isotope incorporation rate, which was driven by both an increased growth rate and catabolism at higher temperatures. Trophic discrimination of carbon isotopes decreased at higher temperatures, which we attributed to either lower activation energies needed to synthesize non-essential amino acids at higher temperatures or the increased utilization of available resources of consumers at higher temperatures. Our results demonstrate that temperature is a key driver of both carbon isotope incorporation rate and trophic discrimination, via mechanisms that likely persist across all ectotherms. Experiments to determine incorporation rates and trophic discrimination factors in ectotherms must include temperature as a major factor, and natural variation in temperature might have significant effects on these isotopic properties that then can affect inferences made from isotope values.
Stable isotope analysis is an increasingly useful ecological tool, but its accuracy depends on quantifying the tissue-specific trophic discrimination factors (TDFs) and isotopic incorporation rates ...for focal taxa. Despite the technique’s ubiquity, most laboratory experiments determining TDFs and incorporation rates have focused on birds, mammals, and fish; we know little about terrestrial ectotherms, and amphibians in particular are understudied. In this study we used two controlled feeding experiments to determine carbon (δ13C) and nitrogen (δ15N) isotope TDFs for skin, whole blood, and bone collagen and incorporation rates for skin and whole blood in adult green frogs,Lithobates clamitans. The mean (±SD) TDFs for δ13C were 0.1‰ (±0.4‰) for skin, 0.5‰ (±0.5‰) for whole blood, and 1.6‰ (0.6‰) for bone collagen. The mean (±SD) TDFs for δ15N were 2.3‰ (±0.5‰) for skin, 2.3‰ (±0.4‰) for whole blood, and 3.1‰ (±0.6‰) for bone collagen. A combination of different isotopic incorporation models was best supported by our data. Carbon in skin was the only tissue in which incorporation was best explained by two compartments, which had half-lives of 89 and 8 d. The half-life of carbon in whole blood was 69 d. Half-lives for nitrogen were 75 d for skin and 71 d for whole blood. Our results help fill a taxonomic gap in our knowledge of stable isotope dynamics and provide ecologists with a method to measure anuran diets.
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