Overlap in resource use, and interspecific competition

Microhabitat use by bisexual and unisexual fishes (Poeciliopsis: Poeciliidae) in an artificial stream

Two all-female species of viviparous fish depend on males of bisexual species for sperm in order to reproduce. The unisexual P. monacha-latidens mates with males of P. latidens (the sexual host) while the unisexual P. monacha-lucida mates with males of P. lucida (its sexual host). Ecological divergence of each unisexual from its sexual host is promoted by any competitive interactions while ecological convergence of the unisexual toward its sexual host is promoted by the unisexual's sperm requirements. This study measured the habitat use in an artificial stream of five Poeciliopsis species (P. monacha-latidens, P. monacha-lucida, P. latidens, P. lucida and P. monacha) when alone and in several combinations. The habitat use of both unisexuals was examined for evidence of 1) competition with their sexual hosts, 2) use of monacha-like behavior, and 3) competition with each other. The two unisexuals interacted differently with their sexual hosts. P. monachalatidens competed with P. latidens. These two species differed more in their habitat use when they were sympatric than when they were allopatric. They frequented different habitats when they were sympatric, P. latidens preferring fast-water areas and P. monacha-latidens preferring slowwater areas. The distribution of P. monacha-latidens was more similar to P. monacha when the unisexual was with its sexual host than when it was alone, indicating that this unisexual may use monacha-like behavior to escape from competition with its sexual host. In contrast, P. monachalucida did not compete with P. lucida. These two species were more similar in their habitat use when they were sympatric than when they were allopatric. The habitat use of P. monacha-lucida when alone and when with P. lucida showed no consistent pattern when compared with the habitat use of P. monacha. Apparently, P. monacha-lucida did not use monacha-like behavior when with its sexual host. The two unisexuals did not compete. Thus, forces favoring ecological divergence are more important for P. monachalatidens and forces favoring ecological convergence are more important for P. monacha-lucida.

Feeding patterns in grasshoppers (Orthoptera: Acrididae): Factors influencing diet specialization

Diets of grasshopper species from two arid grassland communities in Trans-Pecos, Texas, were determined by gut analysis. Species-specific food plant choice and niche breadths are presented for each of these species. As a group, grasshoppers range from monophagous to polyphagous feeders although most species fall in the oligophagous to polyphagous group. Phylogenetic constraints are evident such that gomphocerinae are primarily grass feeders while melanoplinae feed predominantly on forbs; the oedipodinae show less clearcut tendencies.Feeding patterns are remarkably constant from site to site and overall, community niche breadth distributions between sites do not differ greatly. Individual species tend to eat the same plant species at various sites and maintain similar niche breadths. Species with relatively specialized diets tend to feed on predictable plant species such as grasses and long-lived perrenial forbs.Grasshopper feeding patterns present some problems to the current theory of herbivore diet specialization since forb feeding melanoplines tend to be polyphagous (contrary to predictions). Life history patterns unrelated to tracking host plants may explain some aspects of diet breadth since diet selectivities are presumably adjusted according to the probability of finding suitable food plants.

Niche relations among dung-inhabiting beetles

The whole dung-inhabiting (adult) beetle community living in southern Finland was studied with reference to the seasonal (6 summer months), macrohabitat (open field, half-open pine forest and closed spruce forest) and successional (30 days) gradients. The material comprised 50 coprophagous and 129 carnivorous species, represented by 26,650 and 35,850 individuals, respectively. The most important characteristics of each species are given in an Appendix.In the coprophages the species-abundance relations fitted the lognormal distribution well, but in the carnivores the distribution was strikingly less even. A great number of other differences (see below) apparent between the two trophic groups were explained as the result of a difference in the degree of specialization, apart from the difference in their trophic position: coprophages are microhabitat (dung) and food specialists, while carnivores are microhabitat generalists (and therefore their "critical" niche space was not completely covered in this study).In the coprophages, two important species guilds were observed: the first occurred at the very geginning of the succession (mean position 2.5 days), and consisted mainly of true dung specialists; the second guild contained species with wider utilization along the successional dimension (mean position 6.5 days), and species of more generalist nature (saprophages). The utilization of carnivores along the successional gradient was more continuous, probably because of their more diversified food resources. The total niche width among the coprophages increased from early successional species to species occurring mainly later on, but was more constant in carnivores. The environmental dimensions proved to be totally independent of each other among the carnivores, while the coprophages showed a clear supplementary relationship along them. Among the carnivores, a spherical niche shape dominated, but among the coprophages elongated and disk-liked shapes were also well represented, especially in specialist species. In both groups, a significant positive correlation existed between niche width and dominance, but not between niche width and abundance. It is suggested that the patchy distribution and transient occurrence of suitable microhabitats contributes to the coexistence of many ecologically similar species.