摘要:

I describe the nonbreeding finch community of several habitats in continental Kenya, East Africa, and compare it with previously described communities of granivorous finches on Galapagos islands. The purpose of the comparison is to explore differences in structure between communities that have evolved on a continent and on an isolated archipelago, and to suggest reasons for the differences. The ultimate goal is to infer the factors important in the evolution of finch communities, of which the majority have a continental origin. Four factors that might be expected to influence finch communities differently in Kenya than Galapagos are: a greater diversity of possible competitors, an abundance of predators, movement between localities, and a greater diversity of resources. Differences predicted on the basis of these four factors are compared with actual differences between Kenya and Galapagos, in order to estimate their relative importance. The communities of Kenya and Galapagos were surprisingly different. Despite the greater age of the Kenya fauna, the diversity of beak sizes was less, the range of seed sizes exploited was less, and the overall utilization of seed resources by finches was incomplete. Species in Kenya were more differentiated by habitat, microhabitat, and seed species, and less differentiated by seed size than finches in Galapagos. Diets were on average more taxonomically specialized, and associations between diet and beak and body dimensions were correspondingly weaker. Kenya finches were sensitive to the proximity of trees and shrubs (cover) while foraging. These results, together with observations from a limited number of other studies, suggest that a dichotomy may exist between the finch communities of continents and isolated archipelagos. However, like Galapagos, local finch density in Kenya was related to food abundance, and diet overlaps between species coexisting in the same locality were low. Comparison of results with predictions from hypotheses based on the four factors suggests that competition among the large diversity of granivorous species in Kenya (i.e., finches and other granivorous birds, rodents, and ants) is a major reason for the differences between the finch communities of Kenya and Galapagos. The narrow range of beak and seed sizes used by finch species in Kenya is consistent with competition from other taxa, and competition between the many finch species constrained to a narrow range of seed sizes may explain the more pronounced habitat, microhabitat, and diet specializations. Thus, while the finch communities of continents and isolated archipelagos may differ in structure, there is an indication that in large part they represent alternative outcomes of the same process. Predation risk is the most likely cause of a preference for feeding near cover in Kenya, and it may explain why certain seed resources are not exploited by finches, such as ground seeds in areas of poor visibility (dense grasses). Most species appeared to be similarly sensitive to risk of predation; for example, finch species showed little partitioning along an axis of distance to cover. Thus, the main effect of predation on community evolution in Kenya may have been to further confine the resource spectrum available to finches, a role similar to that of competition from other taxa. There was no evidence that the effects of competition within localities were swamped by an influx of immigrant individuals and species. However, lesser indirect effects of movement may be present; movement may elevate local species diversity and thereby influence community structure by increasing the diversity of potential competitors and predators. The prediction that a greater diversity of resources in Kenya would lead to a greater diversity of morphological forms was not upheld, suggesting that resource differences are less influential than other factors.

ISSN:0012-9615    

DOI:10.2307/1942538

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Understory fruit and fruit—eating birds were censused monthly for a year in gaps, intact forest, and second—growth sites of a lowland Costa Rican rain forest. Both fruits and birds displayed significant seasonal variation. Peak fruit abundance corresponded with peak fruit—eating bird abundance. Fruits were most abundant in the mid—to—late rainy season (August—January). Crop sizes were larger on second—growth plants than on either gap or intact forest plants. Also, fruit was much more common in second growth than in gaps and more common in gaps than in intact forest. Fruit—eating birds followed the same general patterns of spatial and temporal variation. They were significantly most abundant in second growth, significantly least abundant in intact forest, and most common from October to January. A large increase in the frugivore population in October was due to an influx of temperate and altitudinal migrants. In addition, populations of some resident frugivore species increased concurrently, suggesting altitudinal migration in some of these species as well. The two most common understory frugivores molted during the period of fruit high and bred during the period of fruit low. I suggest that insect abundance may be more important than fruit abundance in determining breeding cycles of these birds. Given the extent of spatial and temporal variation in fruit and frugivore abundance, and the apparent tracking of fruit resources by birds, large areas of diverse habitats are probably necessary for the long—term maintenance of frugivore populations.

ISSN:0012-9615    

DOI:10.2307/1942539

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

ISSN:0012-9615    

DOI:10.2307/1942540

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

In cloud forest at Monteverde, Costa Rica, I investigated the reproductive consequences of avian seed dispersal for three species of gap—dependent plants: Phytolacca rivinoides (Phytolaccaceae), Witheringia solanacea, and W. coccoloboides (Solanaceae). Of six bird species that consumed fruits of these plants, only three (Myadestes melanops [Muscicapidae], Phainoptila melanoxantha [Ptilogonatidae], and Semnornis frantzii [Capitonidae]), dispersed seeds in viable condition. Other species discarded most seeds before swallowing fruit pulp, or destroyed seeds in the gut. I estimated the quality of dispersal service provided by the "legitimate" disperser species by comparing the seed shadows they produced with the spatial and temporal distributions of establishment sites for the plants. I estimated seed shadows from data on gut passage rates of seeds and on movement patterns of radio—tracked birds. Seed shadows produced by all three effective dispersers were extensive, with few seeds deposited near the parent plant, and some seeds moved>500 m. Seeds of the species examined establish in forest gaps formed by treefalls or landslides. Establishment success varies with gap size and age, but the relationship is different for each species; both Witheringia species establish well in gaps as small as 15 m2or as old as 6 mo, whereas P. rivinoides establishes well only in gaps>70 m2or<4 mo old. Consequently, establishment sites for all three plants are both rare and ephemeral, but to differing degrees. Seeds that are not dispersed to suitable habitat patches can remain dormant in the soil until a gap is formed overhead; seed dormancy experiments showed no significant decrease in viability of seeds buried for up to 27 mo. To determine consequences of dispersal and dormancy for plant reproductive success, I developed a simulation model that uses data on seed shadows, germination requirements, seed dormancy, and forest dynamic processes to estimate reproductive output (total offspring produced during an individual plant's lifetime) and relative "fitness" (an estimator that discounts the contribution of offspring produced after a long period of dormancy). Results show that (1) dispersal by any of the three legitimate dispersers increases reproductive output 16—36 times, even without seed dormancy. (2) Dormancy capabilities up to 2 yr enhance both reproductive output and "fitness," but greater capabilities increase only reproductive output. (3) Without dispersal, dormancy has little effect on either reproductive output or fitness. Thus, both dispersal and dormancy ("dispersal" in time) are essential to these gap—dependent plants, but long—term seed dormancy may be relatively unimportant.

ISSN:0012-9615    

DOI:10.2307/1942541

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

According to the habitat—matrix model, arboreal microhabitat specialists are adapted behaviorally and morphologically for locomotion in different subsets of the vegetation, each characterized by its three—dimensional structure, and these adaptations help explain morphological patterns among coexisting species. I tested this model's predictions for Anolis lizards near Monteverde, Costa Rica. Anolis humilis, A. tropidolepis, and A. woodi were active at different heights in the shaded forest understory, and A. insignis inhabited the canopy. Gap specialists, A. altae and A. intermedius, resembled one another in microhabitat use but were largely separated by elevation. Adult males of forest—understory and gap species were active higher above ground than adult females, which averaged higher than juveniles. In their different structural environments, species, sexes, and age classes differed in proportional use of locomotor modes: running, jumping, and crawling. During field observations of forest—understory and gap species, frequency of crawling was highest for anoles that used slender, widely spaced supports, though much variation in crawling frequency was unexplained. Frequency of jumping increased as mean support size and average distance between supports decreased; the latter variable, expressed relative to body length, accounted for most of the variation in jump frequency. With enclosure experiments, I assessed proximate effects of microhabitat features on locomotor behavior and removed these effects to test for interspecific differences in intrinsic locomotor tendencies. Jump frequency of A. altae consistently increased with decreasing distance between supports, whereas the effects of support diameter were more complex, and varied with spacing of supports. Support diameter exerted both a surface—area effect (a lower tendency to jump from larger supports compared with smaller ones) and, where supports were widely spaced, a target—size effect (a higher tendency to jump to larger supports compared with smaller ones). Compared in the same array of supports, A. tropidolepis, A. altae, and A. intermedius, although similar in body size, differed in frequency of jumping. Thus, differences in locomotor behavior among species reflected not only proximate influences of vegetation structure but also intrinsic tendencies. Morphological traits were strongly associated with locomotor behavior and microhabitat specialty. Differences in limb proportions of primarily running anoles (A. altae and A. intermedius), crawling anoles (A. insignis), and anoles that jumped frequently (A. humilis, A. tropidolepis, and A. woodi) accorded with predictions from biomechanics. Body size was also functionally related to locomotion and was correlated with microhabitat structure. Thus, ability to exploit various structural environments may depend not only on body shape but also size. Nonrandom patterns of interspecific differences in morphology suggested limiting similarity without revealing the underlying ecological process(es). The mechanistic basis for these patterns,however, appears to lie, at least in part, in the functional relationships between morphology and habitat structure.

ISSN:0012-9615    

DOI:10.2307/1942542

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Spatial and temporal variation in dispersal and recruitment of the kelps Macrocystis pyrifera and Pterygophora californica and of filamentous brown algae (FBA) consisting primarily of Ectocarpus siliculosus were examined off southern California, USA. These three taxa of sublittoral brown algae show enormous differences in adult morphology and demography, but release propagules near the bottom that are similar in size, shape, and motility. Their weekly variation in recruitment was measured on replicate frosted glass slides placed at different distances from stands of fertile adults at three sites. For all taxa, significant temporal variation was observed in recruitment at the zoospore source as well as at distances out to 4000 m from the source. For the kelps, this variation was due to variable rates of both zoospore settlement and early postsettlement mortality. In general, recruitment density of both kelps rapidly declined with distance from the adult stand; significantly lower recruitment was observed at as little as 3 m away. In contrast, dispersal of FBA was much greater; no significant decrease in recruitment density was observed out to 500 m. The differences in dispersal between these two algal groups may result from behavioral differences of their zoospores. Unlike those of kelps, zoospores of FBA are positively phototatic. This enables FBA zoospores to remain in the water column longer, thus increasing the distances over which they disperse. The predominant pattern of limited dispersal in the two kelps, however, was not constant over time. Dispersal was greatly expanded during episodic periods of high recruitment that coincided with winter storms. Turbulent water flower associated with these storms may resuspend kelp zoospores normally occurring near the bottom and allow them to be transported greater distances by prevailing currents. Although species characteristics of the adult stand appeared to influence the distribution of recruits, the effect disappeared within 10 m of the stand. At a short—range study site, decreases in spatial variation in recruitment density between the zoospore source and 3 and 10 m away indicated that the distribution of zoospores became more uniform with distance. Likewise at a long—range site, recruitment out to 4000 m appeared to occur uniformly and coincided with recruitment at the zoospore source and at all intermediate distances. This uniformity and coincidence of recruitment indicated that dispersal over longer distances was probably via individual zoospores rather than via clumps of drifting plants or reproductive fragments as suggested in previous studies. Although episodic, such dispersal is probably essential in promoting the persistence of these plants in highly disturbed and unpredictable environments.

ISSN:0012-9615    

DOI:10.2307/1942543

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

The dynamics of zooplankton herbivores in Lake Valencia, Venezuela, were studied over a 5—yr interval. Lake Valencia is a large, warm—monomictic, eutrophic lake. Copepods were the dominant group; rotifers were more abundant than the cladocerans, which were not present in all years. Variation between years in herbivore biomass bore no statistical relationship to patterns in algal biomass. The average annual dry mass production of the herbivores was high (7.0 mg · L—1 · d—1), but less than expected given the high primary productivity of the lake. The annual ratios of production to biomass for individual species were well below the maxima expected for steady growth, and thus imply growth suppression. Feeding capacity of Chaoborus, the principal primary carnivore, exceeded herbivore production for 66% of the sampling dates. Annual overturn broke the coupling between predator and prey by causing extensive mortality in both herbivores and carnivores; herbivores recovered from this suppression more rapidly than carnivores. Herbivores followed three basic strategies for coexistence with their predators in Lake Valencia: (1) two Brachionus species were able to match predation losses by reproductive output; (2) the copepods, which experienced intense predation pressure as adults, were able to sustain the high losses because the adult stage was subsidized by recruitment from earlier, less vulnerable developmental stages; (3) Keratella and the cladocerans were opportunistic in that they became abundant only in response to a decline in the predator population. The opportunistic species were most abundant during the mixing season, which was the period of minimum abundance for Chaoborus. Predation strongly influenced the herbivore populations, but population data indicate that inadequate food resources also played a role. Growth suppression was the result of qualitative rather than quantitative inadequacy in the algal food base. The intensity of control by the two factors was strongly affected by abiotic factors associated with mixing events. Partial mixing provided brief respite from growth suppression for certain species (typically rotifers), but did not alter predation pressure. Annual overturn resulted in improvement of food quality, but because mortality of herbivores coincided with relaxation of predation, the herbivores were delayed in exploiting qualitative improvements in the food resources. Periods of simultaneous release from growth suppression and predation were infrequent and brief. Herbivores were thus controlled simultaneously by predation and quality of food resource.

ISSN:0012-9615    

DOI:10.2307/1942544

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY