Species richness

About: Species richness is a research topic. Over the lifetime, 61672 publications have been published within this topic receiving 2183796 citations.

Use of Amazonian Forest Fragments by Understory Insectivorous Birds

Abstract: We sampled understory insectivorous birds in Amazonian forest fragments from before isolation through 9 yr after isolation. We accumulated 3658 mist net captures of 84 insectivorous species in five 1-ha fragments and four 10-ha fragments. Abundance and species richness declined dramatically after isolation, even though fragments were separated from continuous forest by only 70-650 m. Three species of obligate army ant followers disappeared within the first 2 yr after isolation. Mixed-species flocks containing 13 commonly netted species disintegrated within 2-3 yr after isolation, although three species that dropped out of flocks persisted in fragments. Among insectivores not associated with flocks or army ants, only two species of edge specialists were unaffected by frag- mentation. Overall, loss of forest insectivores was not compensated for by an increase in nonforest or previously uncommon species. Secondary vegetation surrounding fragments strongly affected use of fragments after isolation. Fragments surrounded by Vismia, the dominant regrowth where felled forest was burned and temporarily used as cattle pasture, remained depauperate. In contrast, many species returned to fragments by moving through regenerating forest dominated by Cec- ropia, which occurred in areas where the felled forest was not burned. Both 1- and 10-ha fragments surrounded by Cecropia were used by ant followers by 5 yr after isolation. Mixed-species flocks reassembled in 10-ha fragments surrounded by Cecropia by 7-9 yr after isolation, and augmented their group territories by foraging in secondary forest outside fragments. Solitary species were more variable in their responses, although several species returned to 10-ha fragments surrounded by Cecropia. Terrestrial insectivores, such as Scle- rurus leafscrapers and various antbirds, did not return to any fragments, and appear to be the group most vulnerable to fragmentation. Ordination of the insectivore community showed that 1-ha fragments diverged from their pre-isolation communities more than did 10-ha fragments. Communities in 10-ha fragments surrounded by Cecropia were more closely associated with pre-isolation com- munities than those in fragments surrounded by Vismia. Over time, communities in 10-ha fragments surrounded by Cecropia became more like pre-isolation communities, although communities in other fragments generally continued to diverge.

Spatial and Temporal Variation in Tropical Fish Trophic Networks

Abstract: Observed properties of natural food webs have both important theoretical and important management implications Four lowland aquatic food webs were investigated over the course of two years: a large swamp and a small stream in Costa Rica, and a similar swamp and stream in the Venezuelan llanos Each local ecosystem differed from the three others with respect to environmental changes associated with seasonal rainfall Phylogenetic composition and diversity of biotas also varied among systems Volumetric proportional utilization coefficients from fish gut contents were used as estimates of the intensity of predator—prey interactions An annual and two or more seasonal food webs were constructed for each local community Aquatic communities were defined operationally using common fish species as consumers, and using the sink subweb associated with the top predator of each system A computer calculated a variety of food—web statistics and plotted food—web diagrams containing either (a) all observed trophic links (predator—prey interactions), or (b) subsets with weak links eliminated at prescribed thresholds Individual community food webs contained from 58 (stream, Costa Rica) to 104 (swamp, Venezuela) interactive taxonomic units and from 208 to 1243 total trophic links Food—web parameters were very sensitive to changes in level of link threshold Web connectance and related parameters converged near link threshold 004 (utilization coefficients <004 eliminated) in a variety of inter—web comparisons Despite large differences in assemblage composition and attributes of the physical environment, distributions of trophic levels calculated according to a trophic continuum algorithm were very similar among study systems Herbivores, detritivores, and their direct predators formed the largest proportions of fishes in each assemblage, followed by omnivores and secondary carnivores Fishes that fed at more than one trophic interval were extremely common in all food webs Analysis of covariance was used to compare structural features of different webs across a range of link thresholds Extensive among—site variation in food—web parameters was associated with differences in species richness and environmental differences associated with rainfall patterns, physiography, and gross primary production Seasons generally influenced food—web parameters less than did site differences Relative importance of detritus, aquatic primary production, and terrestrial production in aquatic food webs varied seasonally in each system Detritus, derived primarily from aquatic macrophytes, was an important pathway in both tropical swamp ecosystems Aquatic primary productivity comprised the largest fraction of fish diets during the wet season in the Venezuelan swamp, but it formed the major component of fish diets during the dry season at all other sites Based on comparisons using 13 webs, two—thirds of the pairings among six food—web parameters used (number of nodes, compartmentation, connectance, average number of prey per node, average number of predators per node, ratio of consumer nodes to total nodes) were positively intercorrelated Several food—web relationships previously described as constant (eg, connectance x species richness constancy, species scaling law, link—species scaling law) were not confirmed by my data These earlier food—web trends are extremely sensitive to methodological biases, especially decisions regarding the degree of taxonomic lumping of species into trophic units Although food webs have unique emergent properties and spinoff a number of potentially informative macrodescriptors, empirical studies must achieve greater precision and uniformity before analyses can be performed across different systems Several problems and potential resolutions are discussed

The diversity–disturbance relationship: is it generally strong and peaked?

Abstract: The contemporary literature accepts that disturbance strongly influences pat- terns of species diversity, and that the relationship is peaked, with a maximum at inter- mediate levels of disturbance. We tested this hypothesis using a compilation of published species diversity-disturbance relationships that were gleaned from a literature search of papers published from 1985 through 1996 and from references therein. We identified 116 species richness-, 53 diversity-, and 28 evenness-disturbance relationships in the literature, which we grouped according to shape of relationship (nonsignificant, peaked, negative monotonic, positive monotonic, or U-shaped). We tested the relationships between the strength and shapes of these relationships and attributes of the community, disturbance, and sampling and study design. Nonsignificant relationships were the most common, com- prising 35% of richness, 28% of diversity, and 50% of evenness studies. Peaked responses were reported in only 16% of richness, 19% of diversity, and 11% of evenness cases. Explained variation in the three measures of diversity was variable among studies but averaged -50%. It was higher when few samples and few disturbance levels were examined and when organisms within the samples were not exhaustively censused, suggesting that procedural artifact contributes to these relationships. Explained variation was also higher in studies in which disturbance was measured as a gradient of time passed since the last disturbance (mean r2 = 61%), vs. studies of spatial variation in richness (mean r2 = 42%). Peaked richness relationships had the greatest odds of being observed when sampled area and actual evapotranspiration were small, when disturbances were natural rather than an- thropogenic in origin, and when few disturbance levels were examined. Thus, on average, diversity-disturbance relationships do not have consistently high r2 and are not as consis- tently peaked as the contemporary consensus would suggest.

Conservation Conflicts Across Africa

Abstract: There is increasing evidence that areas of outstanding conservation importance may coincide with dense human settlement or impact. We tested the generality of these findings using 1°-resolution data for sub-Saharan Africa. We find that human population density is positively correlated with species richness of birds, mammals, snakes, and amphibians. This association holds for widespread, narrowly endemic, and threatened species and looks set to persist in the face of foreseeable population growth. Our results contradict earlier expectations of low conflict based on the idea that species richness decreases and human impact increases with primary productivity. We find that across Africa, both variables instead exhibit unimodal relationships with productivity. Modifying priority-setting to take account of human density shows that, at this scale, conflicts between conservation and development are not easily avoided, because many densely inhabited grid cells contain species found nowhere else.

An ecosystem engineer, the beaver, increases species richness at the landscape scale

Abstract: Ecosystem engineering - the physical modifi- cation of habitats by organisms - has been proposed as an important mechanism for maintaining high species richness at the landscape scale by increasing habitat heterogeneity. Dams built by beaver (Castor canaden- sis) dramatically alter riparian landscapes throughout much of North America. In the central Adirondacks, New York, USA, ecosystem engineering by beaver leads to the formation of extensive wetland habitat capable of supporting herbaceous plant species not found else- where in the riparian zone. We show that by increasing habitat heterogeneity, beaver increase the number of species of herbaceous plants in the riparian zone by over 33% at a scale that encompasses both beaver-modified patches and patches with no history of beaver occupa- tion. We suggest that ecosystem engineers will increase species richness at the landscape scale whenever there are species present in a landscape that are restricted to engineered habitats during at least some stages of their life cycle.