Nonadaptive radiation: Pervasive diet specialization by drift in scale insects?

At least half of metazoan species are herbivorous insects. Why are they so diverse? Most herbivorous insects feed on few plant species, and adaptive host specialization is often invoked to explain their diversification. Nevertheless, it is possible that the narrow host ranges of many herbivorous insects are nonadaptive. Here, we test predictions of this hypothesis with comparative phylogenetic analyses of scale insects, a group for which there appear to be few host‐use trade‐offs that would select against polyphagy, and for which passive wind‐dispersal should make host specificity costly. We infer a strong positive relationship between host range and diversification rate, and a marked asymmetry in cladogenetic changes in diet breadth. These results are consonant with a system of pervasive nonadaptive host specialization in which small, drift‐ and extinction‐prone populations are frequently isolated from persistent and polyphagous source populations. They also contrast with the negative relationship between diet breadth and taxonomic diversification that has been estimated in butterflies, a disparity that likely stems from differences in the average costs and benefits of host specificity and generalism in scale insects versus butterflies. Our results indicate the potential for nonadaptive processes to be important to diet‐breadth evolution and taxonomic diversification across herbivorous insects.     

The ‘generalism’ debate: misinterpreting the term in the empirical literature focusing on dietary breadth in insects

We here attempt to show, using three broad insect groups – Lepidoptera (mainly leaf‐chewing larval herbivores and nectar‐sucking adults), parasitic Hymenoptera (mainly endoparasitoids, especially of other insects) and aphids (sap‐sucking plant parasites) – how the terms ‘generalist’, usually equated with levels of phagy (oligo‐ and polyphagy), and ‘specialist’ (monophagy), still in widespread parlance, have often been misrepresented. Probably, the reality of generalism, be that herbivorous, predatory and parasitic, can only be demonstrated by detailed empirical field observations plus the use of high‐resolution molecular (DNA) markers, including sequencing, and thereby determining whether the organism in question is really a homogeneous species population over a wide geographical range, or rather comprises a series of morphologically similar/identical cryptic, host‐adapted specialist populations. In the case of insects, the largest group of terrestrial animals on the planet, even if it can be shown that certain species are indeed polyphagous and feed on a variety of plant hosts (herbivores) or prey species (predators and parasitoids), nevertheless, the range of these food items may be highly selective and restricted, depending on morphological–genetical (biochemical/chemical)–behavioural constraints. In the end, while some animals appear to be generalist, this situation may well be illusory. Our present recognition of the term is at best inappropriate, and at worse, inaccurate, as we demonstrate in the examples given, mostly insects. In the meantime, we suggest that the terms used should be re‐defined as four broad classes of specialism–generalism, although the apparent ‘generalism’ is itself conditional on proof following further empirical analyses.     

Foraging of rocky habitat cichlid fishes in Lake Malawi: coexistence through niche partitioning?

The haplochromine cichlid fish communities of the rocky habitats of Lake Malawi are highly diverse; however, many species live side by side with apparently very similar resource requirements. There is a long-standing debate concerning whether these species partition their resources on a finer scale than has been previously reported or if species that are truly ecologically indistinguishable can coexist. A field study of food resource use was conducted to determine whether coexisting species segregate their diet and foraging sites. Significant differences between species were found, yet considerable inter-specific resource use overlap was commonplace. The data indicate that these cichlid species coexist both with and without niche differentiation. We propose that alternatives to niche differentiation should be considered to explain how many species coexist in Lake Malawi cichlid communities. Received: 5 October 1998 / Accepted: 30 June 1999     

Resource partitioning between sympatric wild and domestic herbivores in the Tarangire region of Tanzania

The effect of the introduction of an exotic species (cattle) into a native African herbivore assemblage was investigated by studying resource partitioning between zebu cattle, wildebeest and zebra. Resource partitioning was investigated by analysing grass sward characteristics (such as sward height and percentage nitrogen in leaves) of feeding sites selected by the different herbivore species. Linear discriminant analysis was used to determine whether a distinction could be made between feeding sites selected by the different animal species or whether the animal species showed overlap in resource use by selecting similar feeding sites. Wildebeest and zebra did not show overlap in resource use except in the wet season when resources were ample. Cattle showed overlap in resource use with zebra in the early wet season and with wildebeest in the early dry season, seasons when food limitation is likely. In the wet season, cattle showed overlap in resource use with both zebra and wildebeest. Implications of these results for competitive relationships between livestock and wildlife are discussed. We suggest that interpretation of overlap in resource use may be different for an assemblage of long-term coexisting native species as compared to an assemblage of native and exotic species. Among native herbivores, overlap in resource use is not expected based on evolutionary segregation. In a native assemblage to which an exotic species has been introduced, however, overlap in resource use can occur under food-limited conditions and consequently implies competition. Received: 1 September 1998 / Accepted: 20 April 1999     

Europe‐wide biogeographical patterns in the diet of an ecologically and epidemiologically important mesopredator,the red fox Vulpes vulpes: a quantitative review

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Spatial heterogeneity and scale‐dependent habitat selection for two sympatric raptors in mixed‐grass prairie

Sympatric predators are predicted to partition resources, especially under conditions of food limitation. Spatial heterogeneity that influences prey availability might play an important role in the scales at which potential competitors select habitat. We assessed potential mechanisms for coexistence by examining the role of heterogeneity in resource partitioning between sympatric raptors overwintering in the southern Great Plains. We conducted surveys for wintering Red‐tailed hawk (Buteo jamaicensis) and Northern Harrier (Circus cyanea) at two state wildlife management areas in Oklahoma, USA. We used information from repeated distance sampling to project use locations in a GIS. We applied resource selection functions to model habitat selection at three scales and analyzed for niche partitioning using the outlying mean index. Habitat selection of the two predators was mediated by spatial heterogeneity. The two predators demonstrated significant fine‐scale discrimination in habitat selection in homogeneous landscapes, but were more sympatric in heterogeneous landscapes. Red‐tailed hawk used a variety of cover types in heterogeneous landscapes but specialized on riparian forest in homogeneous landscapes. Northern Harrier specialized on upland grasslands in homogeneous landscapes but selected more cover types in heterogeneous landscapes. Our study supports the growing body of evidence that landscapes can affect animal behaviors. In the system we studied, larger patches of primary land cover types were associated with greater allopatry in habitat selection between two potentially competing predators. Heterogeneity within the scale of raptor home ranges was associated with greater sympatry in use and less specialization in land cover types selected.     

围封对温带半干旱典型草原群落种间关联的影响

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浙江丽水虎纹蛙形态特征的两性异形和食性

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