摘要:

The first part of this paper describes the quantitative characteristics of breeding habitats of sylviid warbler species, chiefly Sylvia and Phylloscopus, in Yorkshire, northern England, and in southern Sweden. The second part of the analysis is made up of the factors that permit or preclude the coexistence of various combinations of warbler species–vegetation structure, food density, and competitive regime. Vegetation structure was measured in 346 warbler territories in the 2 locations, including 151 Phylloscopus territories (P. trochilus, collybita and sibilatrix), 171 Sylvia territories (including S. communis, curruca, atricapilla, borin, nisoria and undata and 16 Hippolais icterina territories. From data on the distribution of the vegetation density over height in territories, discriminant function analysis generates new canonical habitat variables U1and U2that are independent of each other and reflect the correlation structure of the original habitat variables. Now each species' habitat niche can be shown as a family of "confidence ellipses" in the habitat plane of U1and U2, and both species habitat preference and interspecific overlaps easily measured, within and between genera and within and between countries. As a result of this analysis, the following conclusions are drawn. The 3 English canopy—foraging Phylloscopus show some interspecific habitat segregation but more noticeably a great deal of overlap in preferred habitat. This overlap is reflected by the accuracy with which the specific occupant of a habitat patch can be predicted on the basis of the vegetation—structural characteristics of the patch: on average 2/3 of such predictions are verified, 1/3 is wrong. Similarly in Sweden, where 1 Phylloscopus drops out and is replaced by an additional sylviid, Hippolais icterina, predictions on specific habitat occupancy are correct just over 2/3 (72%) of the time. But the positions of habitat niches are rearranged between the 2 countries, because in Sweden the habitats of the absent collybita are taken over partially by sibilatrix but mostly by trochilus, and the newly added icterina occupies habitat that in England would be most typical of trochilus. Despite these differences, canopy warblers in both countries use very similar vegetational criteria in habitat selection, and predictions as to the specific occupant of a habitat based on discriminant function analysis in 1 country but tested by warbler data from the other country are nearly as successful as are within—country tests. Five Sylvia species were studied in each country, with 4 species in common. Interspecific habitat segregation is poorer than in Phylloscopus (54% of habitat occupants correctly classifiable). Four species occupy similar habitats in both countries with but minor habitat shifts, and in addition the Swedish nisoria occupies habitats that are structurally more similar to the English undata than to any other Swedish or English warblers. Again, cross—country predictions are just as successful, or as unsuccessful (@50% correct ), as within—country predictions. The second part of the paper describes and analyses the precise disposition of warbler territories within limited number of study sites in a range of habitat from low scrub to tall woodland. Pilot studies in English habitats show that 3 pairs of species: Phylloscopus trochilus and P. collybita, Sylvia atricapilla and S. borin, and Sylvia communis and S. curruca, species pairs with considerable overlap in preferred habitat type, are strongly interactive and show partial to complete interspecific territoriality. More detailed studies were conducted at 3 Swedish locations. In low woodland/tall scrub at Horby, 4 Sylvia species nested. Sylvia communis is shown to leave some potential territories in the field unoccupied, while S. borin occupies practically all habitat that ranks as highly acceptable. However, just 1 pair of atricapilla is present despite the fact that at least 4 additional atricapilla territories might be accommodated in suitable habitat, and likewise curruca fails to occupy all suitable habitat at the site. The presence of other Sylvia species, in particular the dominant borin, accounts for low densities of curruca and atricapilla, and a combination of statistical analysis of the disposition of territories and direct observation of supplanting and replacement through aggressive behavior confirms that interspecific interactions modify and qualify habitat acceptability. In tall woodland at Hoganas borin and atricapilla interact strongly over habitat, and atricapilla attains high occupancy levels only in habitat that is marginally acceptable to borin. The interaction between them is not affected by the presence or absence of Phylloscopus trochilus. At Bejershamn in low scrubland 5 Sylvia species and 1 each of Hoylloscopus and Hippolais breed. The species display a variety of levels of interspecific interaction, from a direct interspecific territoriality (borin—nisoria, communis—nisoria) to more subtle interactions of partial exclusion (borin—curruca). Here diffuse competition is evident (e.g., icterina is not found with borin and curruca together even though the last 2 occur frequently together and even though Hippolais icterina occurs with either separately). With increasing vegetation height in icterina habitat where more warblers coexist with overlapping territories, from 1— to 2— and 3—species combinations. But only 2/7 one—species combinations, 3/21 two—species combinations and 2/35 3—species combinations are common, stable and compatible. The remainder are precluded by differential habitat selection, by simple competition mitigated by direct behavioral interactions, or by diffuse competition with more subtle avoidance behavior. The role of song convergence in the maintenance of the interspecific interactions and the role of food density and distribution in the formation of compatible species combinations is documented, and further questions about the maintenance and evolution of the system are identified and discussed.

ISSN:0012-9615    

DOI:10.2307/2937239

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

Breeding bird communities in spruce—fir forests of the southern Appalachian mountains are considerably less diverse than those of similar northern forests. Censuses also show more widespread coexistence of congeneric and confamilial species in the north. Long—distance migrants dominate the northern forests while resident species dominate in the south. In a 4—yr study of these bird communities, I used observations of foraging strategies and resource partitioning to test hypotheses that might explain this probematic latitudinal gradient in species diversity. I conducted quantitative studies of foraging behavior of arboreal insectivorous birds at sites with very similar plant communities in the Great Smoky Mountains of North Carolina and in northwestern Maine. Six species that breed in both north and south had consistently more generalized foraging in the south, as measured by spatial distribution of foraging, variety of techniques used, directionality in probing and flights, and patchiness of habitat use. These north—south differences in foraging behavior result in part from greater behavioral diversity of each individual. Five exclusively northern species showed greater foraging generalization at the northern site than their sympatric congeners that breed in the south as well. The 11 species in the north occupied a smaller total foraging niche volume than the 6 species in the south. Each species overlapped more broadly with others in the north, and maximum overlap was greater. The southern avifauna is a nearly totally included subset of the northern. Northern diversity is produced by the addition of very similar (congeneric), widely overlapping generalists. This "stacking" of generalists differs considerably from tropical patterns of community diversification. Ecological, biogeographic, and evolutionary hypotheses explaining tropical diversity and island diversity are inadequate to account for the reversal of this latitudinal gradient and the species—packing characteristics of northern diversification. Populations in the depauperate south apparently released from competition with exclusively northern congeners did not show stronger generalization than populations of species that are the sole representatives of their families north and south, contrary to expectation. This is better explained by a common response, predicted by foraging optimization theory, to lower food levels in the south than by ecological release. Seasonal compression of productivity in northern spruce—fir forests, suggested by geophysical and climatological data, produces stronger pulses of food production than in the south, as shown by arthropod sampling. Compressed productivity and winter limitation of both northern residents and migrants probably produce higher food levels relative to consumer biomass in the north. Seasonal diversity is probably enhanced in northern forests by strong, predictable resource oscillations that are difficult for consumers to track Cyclic relaxation of limits to ecological similarity through seasonally abundant resources creates a higher diversity peak than under weaker seasonality. In the more seasonally constant south, residents are probably better able to track resources, and dominance by these groups is enhanced. Predictable seasonal peaks of production have probably promoted the evolution of opportunistic migrants and reduced their competition with winter—limited residents. I discuss this hypothesis in light of a theory proposing that variable conditions may often lead to greater diversity than more stable conditions. This study and others indicate the importance of seasonal species and temporal complexity in natural communities.

ISSN:0012-9615    

DOI:10.2307/2937240

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY

摘要:

We examined the pattern of development of assemblages of coral reef fishes on artificial reefs at One Tree Reef, Great Barrier Reef. Two sets of 8 reefs were built. To investigate the effects of habitat structure on species diversity, each set consisted of replicate pairs of 4 different reef types: plain (no holes), small—holed, medium—holed, and large—holed reefs. To examine seasonal differences in colonization, 1 set was established in summer (October 1971) and the other set in winter (July 1972). Visual censuses of the reefs' fish assemblages were made at monthly intervals over 32 mo (summer set) and 23 mo (winter set). We recorded a total of 105 resident species from the artificial reefs. Although less diverse, the fish assemblages differed little in terms of species composition from those of small natural patch reefs. Artificial reefs showed a high between—reef variability in species composition that was largely unrelated to habitat structure. Competitive interactions between species appeared unimportant in explaining the distribution of species between reefs. Likewise, possible positive associations between species were indicated for only a few species. Almost all settlement of the artificial reefs was by juvenile fishes. Recruitment was markedly seasonal and occurred mainly over summer (September—May). Juvenile recruitment patterns showed little year—to—year constancy. Most species tested showed nonrandom, clumped patterns of settlement on replicate reefs. More than half of species tested showed no significant differences on numbers of individuals which settled on different reefs, but the remainder showed significantly greater settlement on some reefs, suggesting habitat selection. For a few species, there was also greater recruitment to reefs already occupied by conspecifics, suggesting possible social facilitation of settlement. For most species, there were no significant interspecific interactions between adults and newly settling juveniles. Between—census variability in species composition was high and species turnover ranged from °17% of species/mo in winter to °39% in summer. Maximum times of persistence for most species were<12 mo, and for many individuals survivorship was of the order of only a few months. Much of the high turnover appears due to losses through predation. There was no evidence that a persistent species equilibrium was attained during colonization. Because of predation and the seasonally varying, uncertain nature of recruitment, we see the development of any long—term species equilibrium as unlikely. We suggest that nonequilibrium conditions are a characteristic of coral reef fish communities and that because of these conditions, high within—habitat diversities are maintained.

ISSN:0012-9615    

DOI:10.2307/2937241

出版商:Ecological Society of America    

年代:1978

数据来源: WILEY