摘要:

To determine the effect of lizards on web—spider populations, we conducted an 18—mo field experiment in the Bahamas. Densities of individuals of each common spider species were about three times as high in lizard—removal enclosures as in control enclosures with lizards or in unenclosed plots with lizards; spider densities in control enclosures and unenclosed plots were nearly identical. In the most common spider species, Metepeira datona, lizards reduced juvenile and adult abundance, as well as adult female survivorship and prey consumption. Numbers of spider species were higher where lizards were removed than where they were present; hence, lizards did not promote spider species coexistence in this system. Numbers and biomasses of aerial insects caught in sticky traps were higher in lizard—removal enclosures than in controls; hence, higher spider predation on insects where lizards had been removed did not completely compensate for the lack of lizards. Comparisons between control enclosures and unenclosed plots revealed that the enclosures reduced insect numbers and biomasses. This study and others have demonstrated that lizards play a major role in structuring web—spider communities in the West Indies. Several lines of evidence indicate that predation is the major interaction between lizards and spiders, although some evidence for competition also exists.

ISSN:0012-9615    

DOI:10.2307/1942461

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Energy budgets for the wide—foraging "arthropodivorous" lizard Cnemidophorus tigris were constructed for the reproductive season using doubly labeled water measurements of field metabolic rate (FMR). Rates of body mass change, clutch sizes, and clutch intervals were also investigated. FMRs of both sexes (males, 298 J · g—1 · d—1; females, 247 J · g—1 · d—1) were greater in the reproductive season than during the postreproductive season. This was not due to differences in resting metabolism, but, instead, was due to higher activity costs during the reproductive season. Although males had significantly higher FMR than females, males and females had similar feeding rates (as reflected by water influx rates). The ratio of energy intake to expenditure was higher in females than in males. Females produced eggs but did not grow. Large males did not grow, but small 1st—yr males similar in size to females did grow. Females laid at least two consecutive clutches during a single reproductive season; clutch interval was ≈25 d. Clutch size varied with time of laying (first or second clutch), female body size, and year. We compare the reproductive energetics of C. tigris and other wide—foraging Cnemidophorus with the energetics of ambush iguanids. The wide forager C. tigris and the ambusher Uta stansburiana apparently do not differ in the proportion of the energy budget devoted to reproductive production (reproductive effort, REp). But total reproductive effort, REt, which includes metabolism associated with reproduction, is much lower in Cnemidophorus tigris. Review of the literature indicates Cnemidophorus tend to lay smaller clutches and larger eggs than iguanids, but the number of eggs laid per unit time generally equals that of most iguanids. Cnemidophorus also deposit energy into their eggs at rates almost 60% higher than ambush iguanids. We consider higher rates of production in wide foragers to be permitted by their higher rates of net energy intake while foraging, compared with most ambushers. These higher rates of production in Cnemidophorus apparently allow them either to reach a larger size than iguanids of similar age at first reproduction or to be younger than iguanids of the same size at first reproduction. Life history characters may be influenced by differences in rates of production associated with different food—acquisition modes.

ISSN:0012-9615    

DOI:10.2307/1942462

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

Dense thorn woodlands occupy what are thought to have been grasslands and savannas prior to settlement of the Rio Grande Plains of Texas. However, the tenet that grasslands have been converted to shrublands and woodlands in recent history is controversial and based largely upon conflicting historical accounts. Our objective was to determine how the presumed physiognomic conversion from grassland or savanna to woodlands might have occurred. Some upland landscapes are dominated by closed—canopy woodlands in southern Texas, whereas others have a two—phase pattern of discrete shrub clusters scattered throughout a grassland. More mesic sites are dominated by losed—canopy woodlands. We hypothesized the two—phase landscapes represented an intermediate stage in the conversion of grassland to woodland. As new shrub clusters were initiated and existing clusters expanded and coalesced, a gradual shift from grassland to savanna to woodland would occur. To address this hypothesis, we inventoried herbaceous interspaces for woody colonizers, quantified the composition and distribution of shrub clusters on upland sites, and compared the structure of clusters to that of adjacent, more mesic areas with continuous woody plant cover. To assess the physiognomic stability of the two—phase landscapes, cluster size, density, and cover were quantified for 1941, 1960, and 1983 from aerial photographs. A lone mesquite (Prosopis glandulosa) plant occurred in>80% of the upland clusters, where it was typically the largest individual in terms of basal area, height, and canopy area. The number of woody species per cluster ranged from 1 to 15 and was strongly related to mesquite basal diameter (R2= 0.86). Cluster diversity, evenness, and size were also significantly correlated with mesquite size. The data suggest that mesquite plants invaded grasslands and served as recruitment foci for bird—disseminated seeds of other woody species previously restricted to other habitats. The result was a landscape composed of discrete chronosequences of woody plant assemblages organized about a mesquite nucleus. Within the two—phase portion of the landscape, 50% of the clusters were within 5 m of another and 95% were within 15 m of another. Analysis of the size class distribution of clusters suggested that most had yet to realize their growth potential. Moreover, the herbaceous clearings between clusters contained high densities of woody seedlings, mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha. Coalescence will become increasingly probable if new clusters are initiated and existing clusters expand. This phenomenon appeared to be in progress on one portion of the landscape and had apparently already occurred on others. As clusters developed on the two—phase portion of the landscape, their species composition, dominance, and size class structure became increasingly similar to that of adjacent closed—canopy woodlands on more mesic. Mean cluster size increased from 494 m2in 1941 to 717 m2in 1983. Growth rates of clusters were a function of cluster size and precipitation. During the 1941—1960 period characterized by severe drought, there was a slight decrease in total woody plant cover resulting primarily from the formation of gaps among clusters on the periphery of the site and a 35% decrease in density of clusters<5 m2. These cover losses offset the areal expansion of small (100 m2were an order of magnitude lower than those of clusters5 m2in area are persistent features of the landscape; and (4) the present two—phase pattern is moving toward a monophasic woodland as new clusters are initiated and existing clusters expand and coalesce. As a result, (5) shrub clusters on uplands represent an intermediate stage in the conversion of grassland to woodland, and (6) closed—canopy woodlands on more mesic sites appear to represent portions of the landscape where this has already occurred. Because the conversion of grasslands and savannas to woodlands in the Rio Grande Plains is initiated by mesquite, factors regulating its dispersal, establishment, and role as a facilitator of woody community development are emphasized.

ISSN:0012-9615    

DOI:10.2307/1942463

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY

摘要:

There are two alternate community states in the rocky subtidal of the Atlantic coast of Nova Scotia, an unproductive sea urchin/coralline alga community, and highly productive kelp beds dominated by Laminaria longicruris. Disease—induced mortality of the sea urchins (Strongylocentrotus droebachiensis) triggered a switch from the first state to the second and provided a unique opportunity to study (1) the ability of L. longicruris to recover its former dominant status, and (2) its stability when competing with other seaweeds and when perturbed by storms and grazers other than urchins. Rates of recolonization of L. longicruris depended on the proximity of a refugial source of spores. When reproductive plants were nearby, a closed canopy developed within 18 mo of urchin mortality. When a reproductive population was several kilometres away, there was sparse recolonization for 3 yr, then a massive recruitment occurred with closure of the canopy in the 4th yr. Laminaria is clearly the competitive dominant in the seaweed community. Manipulative experiments showed that the kelp limits the abundance of several understory species, but there was no evidence that the abundant annual seaweeds limited kelp recruitment. When sea urchins were rare, the density and growth rates of Laminaria were influenced mostly by intraspecific competition. When the canopy of adult plants was removed there was a dramatic increase in kelp recruitment, but the recruits that grew in dense patches in the clearings were significantly smaller than those of a similar age that grew more sparsely beneath the canopy. Once the kelp recovered from destructive grazing and formed a mature forest, it was able to maintain its dominance, even in habitats subject to severe nutrient stress for 8 mo of the year. For most of the year mortality and erosion of laminae outweighed the effects of recruitment and growth, and the canopy declined, especially during winter when storms were frequent. Erosion was exacerbated by grazing of the gastropod Lacuna vincta. However, in late winter and early spring, recruitment and rapid growth restored the canopy. When severe storm damage was simulated by completely removing Laminaria in patches, the kelp rapidly recolonized and soon outgrew other seaweeds. Unlike the competitive dominants in kelp bed systems in the northeast Pacific, L. longicruris in Nova Scotia manifests multiple patterns of adaptation that enable it to dominate early and late stages of succession in a range of habitats of different levels of nutrient stress and of disturbance from storms and grazers. The principal threat to the stability of the kelp beds is destructive grazing by sea urchins. We suggest that the considerable differences between the dynamics of kelp beds in Nova Scotia and those of the northeast Pacific, and the high degree of stability of L. longicruris stands in Nova Scotia, is attributable to the low diversity of kelps and therefore low levels of competition in Nova Scotia, and to the multiple adaptations of L. longicruris that enable it to tolerate several stresses and disturbances. We argue that the dynamics of community organization, and therefore the stability properties of this system are determined primarily by biological interactions and not by physical variables. This differs from the kelp communities in the northeast Pacific, in which both biological and physical factors influence dynamics significantly at a primary level. We offer a qualitative model of the dynamics of community structure in Nova Scotia that may be viewed as a set of deterministic "subroutines," in which each subroutine describes the outcome of a particular biological interaction. The subroutine(s) that predominate at one point in time and space are probably determined mostly by physical hydrographic variables that have a large stochastic component.

ISSN:0012-9615    

DOI:10.2307/1942464

出版商:Ecological Society of America    

年代:1988

数据来源: WILEY