Search

The seed bank of Neurada procumbens, a prostrate annual common in sandy desert habitats, is vulnerable to surface disturbances. Moreover, its seeds are highly heteromorphic, having both spinose and non-spinose sides, which affects both the vertical distribution of seeds and diaspores, and the precise position in which they lie in the soil. Here we explore the ecological implications of two factors, “safe sites” within habitats and seed orientation or “safe sides”, on seedling establishment and the seed bank. In three natural populations, the vertical distribution of buried, germinable diaspores decreased with soil depth. Most diaspores occurred at the surface (0–1 cm below the surface), while none were observed below 10 cm. Seedling emergence decreased with depth and reached zero in diaspores below 5 cm. Seedling emergence also proved highly sensitive to the orientation of diaspores in soil. Diaspores lying on their smooth side with the spinose upper side directed upwards reached the highest seedling emergence of 61 %. Root length ranged from 10 to 16 cm, and the root to shoot ratio varied between 22.8 and 30.3 cm, depending on diaspore size. Diaspores near the surface may constitute a more transient component of the seed bank, as few of these remained viable for more than a year. Persistence of Neurada seed banks seems to depend not only on vertical movement of diaspores among habitat “safe sites” but also on different seed position alternatives.

Core-satellite theory predicts that, via the “rescue effect”, widespread, abundant species should have reduced risk of local extinctions. We test this hypothesis in southeastern Malagasy littoral forest using data on distribution and abundance of trees and woody understory vegetation in tropical forest fragments along a disturbance gradient. We partition the mortality risk into two kinds of extinction factors, separately operating at demographic (local) and landscape (regional) scales, contrary to core-satellite predictions, for both trees and woody understory vegetation, that the relative number of core (abundant) species declined significantly with increasing disturbance. In the least-degraded forest fragments there was a strong mode of core species, while in the moderately- and severely-degraded fragments the species distributions were essentially log-normal, lacking a substantial core mode. While the rescue effect mitigates one kind of extinction risk, namely local environmental and demographic stochasticity, it may not counterbalance widespread pervasive sources of mortality. The amount of internal forest fragmentation appears to have a much greater effect on species richness and diversity than either fragment size or shape.

Macroecology depends heavily on a comparative methodology in order to identify large-scale patterns and to test alternative hypotheses that might generate such patterns. With the advent and accessibility of large electronic databases of species and their life history and ecological attributes, ecologists have begun seeking generalities, and examining large-scale ecological hypotheses involving core themes of range, abundance and diversity. For example, combinations of ecological, life history and phylogenetic data have been analysed using large species sets to test hypotheses in invasion biology. Analysis of regional species inventories can contribute cogently to our understanding of invasions. Here we examine several ways in which database analysis is effective. We review 19 studies of comparative invasions biology, each using >100 species of plants in their analyses, and show that invader success is linked to seven correlates: short life cycle, abiotic (mostly wind) dispersal, large native range size, non-random taxonomic patterns (emphasizing certain families or orders), presence of clonal organs, occupying disturbed habitats, and earlier time of introduction. These phylogenetically influenced, comparative analyses using regional species inventories are only just beginning and have much potential. © Springer 2006.

Surprisingly few studies have considered the extent to which the nature of the ownership of land is associated with differences in biodiversity. We analysed ownership and other landscape-level effects on rare-species richness for both globally- and regionally-rare biota (including birds, herpetofauna, butterflies, mammals, and plants) in 289 designated natural areas (NAs) in southern Ontario, Canada. Information about each NA −including area, number of plant communities, ownership status and details of species diversity were collected from published sources. Length of perimeter of NA, relative isolation, and an estimate of fragmentation were measured using image analysis and GIS techniques. NAs were in general relatively small, with mean area of 158 ha (median 85 ha, range from 0.9 to 1278 ha) for private NAs; public NAs had mean area of 132 ha (median 16 ha, range from 0.1 to 1481 ha). Mean number of plant communities was 4.6 (median 4, range 1- 13) at private NAs and 3.8 (median 3, range 1-16) at public NAs. Our results show that, of several landscape-level factors, area had the greatest effects on rare-species richness and other biotic indices. Effects of area were followed by effects of plant community diversity, however this was itself significantly affected by area and the extent of perimeter of the NA. Both these factors were followed by effects of ownership of the NA and by effects of isolation of the NA (represented by minimum distance to nearest NA and by number of NAs in 10 km radius). Other landscape- level factors did not appear to have overall significant effects. Variation in area accounted for 0.1% to 29% of variation in number of rare species, with lower values for globally-rare, than for regionally-rare taxa. For all biotic groups, public ownership of NAs was associated with significantly greater rare-species richness compared to private ownership, even after other factors such as area were controlled. For all globally-rare biota except butterflies, area of NA had greater effects on rare-species richness than did ownership. Richness of regionally- rare birds was more affected by plant community diversity than by area of NA. Number of recorded plant communities accounted from 2.1% of variation in number of globally-rare plant species to as high as 31% of variation in regionally-rare butterflies. The diversity of plant communities was itself influenced by total site area (accounting for 45% of variation), extent of elongation of the NA, and both external- and interior- edge perimeters. Public NAs had greatest numbers of rare biota and so should be a significant focus for conservation programs. Smaller, privately-owned patches of natural area dominate (by number and area) in this densely populated region and their significance should not be overlooked.

We investigated effects of landscape-level factors on measures of biodiversity using published descriptions for 98 significant natural areas along the Niagara Escarpment. This is a 725 km, largely forested, Paleozoic limestone escarpment that, excepting the Great Lakes, is the most prominent topographical feature of southern Ontario, Canada. Results show highly significant differences in mean site size and extent of forest interior among natural areas of different ownership classes, with larger and more forested sites being under mixed (private + public) ownership, but no significant difference between sites of public and private ownership. Analysis of covariance demonstrated that after controlling for differences in landscape-level factors (total size of natural area, extent of forest interior, extent of landform heterogeneity and geographic location), most measures of biotic diversity (including the number of vegetation community types, provincially rare vascular plants, and regionally and locally rare breeding birds) differed significantly among sites of private, public and mixed ownership. In general, values at public and mixed ownership sites were greatest, with significantly lower biodiversity values at privately-owned sites. Furthermore it would seem not to be a product of public bodies having historically purchased the largest sites or most-forested sites, since there is no significant difference between the mean size of publically-owned and privately-owned sites. Results of stepwise multiple regression confirm the well known relation between size of a natural area and variation in both total, and rare species diversity. Since public sites have generally more species than private sites, they are essential elements of any conservation network.

In the coastal littoral forest of extreme southeastern Madagascar, we studied tree diameter at breast height (DBH) $ 10 cm in 20, 50 3 50 m plots in each of four forest fragments, and understory woody vegetation (DBH , 10 cm, $1 m tall) in 60, 10 3 10 m plots in three of the fragments. One forest fragment was located in the highly degraded Lokaro region, and three in the nearby Sainte-Luce forest. A total of 3476 trees, representing 169 species in 55 families, were recorded in the 50 3 50 m plots, and 10 282 understory stems, representing 195 species in 54 families, were found in the 10 3 10 m plots. For each tree, DBH was recorded. Mean tree diameter and patterns of tree size class distribution did not differ among the four forest fragments. However, the fragments differed significantly in both tree and understory stem densities, species richness and diversity values, and family richness values, with the Lokaro fragment having the lowest values for all measures. Furthermore, floristic patterns, family importance values, and community similarity measures revealed that the species composition at the Lokaro fragment was very different from the Sainte-Luce fragments. Anthropogenic disturbance appears most pronounced in the isolated Lokaro forest, where biotic resources are limited to this single fragment.

Until fairly recently, regional-scale ecological and evolutionarypatterns have tended to be ignored as conservation efforts have been concernedwith species and their habitats. Here we compare frequencies in the Egyptianflora of particular rank sizes (order, family and genus) with patterns ofspecies abundance (classified as very rare, rare, common, or very common) and anarray of life-history attributes. The angiosperm flora of Egypt is representedby 2446 taxa (2088 species), including taxa in 10 subclasses, 51 orders, 120families, and 742 genera. A high degree of monotypism was observed: four ordersare monotypic (each existing as single species), and have very rare overallabundances; 30 families are monotypic (17 of which are very rare or rare); and 354genera are monotypic (over 70% of which are very rare or rare). Fourteenfamilies (in particular the Resedaceae and Zygophyllaceae) have at leastone-fifth of their global species represented in the Egyptian flora. Introducedspecies in general, and tree, aquatic herb and liana life forms all are especially well represented among monotypic genera. Native taxa are highlyrepresented among rare and very rare abundance classes, while introduced taxadid not differ significantly in their abundance patterns, compared to overallflora values. Few large genera (>20 spp.) occur in the flora, with mostspecies concentrated in genera containing 8–19 species per genus.Similarly, few families were highly speciose. Annual and herbaceous species weresignificantly over-represented, mainly among large, speciose genera andfamilies. However, perennials, trees, shrubs, aquatic herbs, lianas and parasiticspecies were found mainly in families and genera having very few taxa.Life-history attributes may have important implications to speciation rates.Taxonomically based results, involving abundances and life-history attributes,are discussed in the context of biodiversity and conservation.

Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility, or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy (males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant sex morphs. Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability. Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or nonexistent—the reproductive mode recognized as dioecy. Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates, sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode of pollination (insector wind-pollinated) and other ecological factors.