The flora of the south-western tip of southern Africa, the Cape flora, with some 9000 species in an area of 90000 km2 is much more speciose than can be expected from its area or latitude, and is ...comparable to that expected from the most diverse equatorial areas. The endemism of almost 70%, on the other hand, is comparable to that found on islands. This high endemism is accounted for by the ecological and geographical isolation of the Cape Floristic Region, but explanations for the high species richness are not so easily found. The high species richness is accentuated when its taxonomic distribution is investigated: almost half of the total species richness of the area is accounted for by 33 ‘Cape floral clades’. These are clades which may have initially diversified in the region, and of which at least half the species are still found in the Cape Floristic Region. Such a high contribution by a very small number of clades is typical of island floras, not of mainland floras. The start of the radiation of these clades has been dated by molecular clock techniques to between 18 million years ago (Mya) (Pelargonium) and 8 Mya (Phylica), but only six radiations have been dated to date. The fossil evidence for the dating of the radiation is shown to be largely speculative. The Cenozoic environmental history of southern Africa is reviewed in search of possible triggers for the radiations, climatic changes emerge as the most likely candidate. Due to a very poor fossil record, the climatic history has to be inferred from larger scale patterns, these suggest large-scale fluctuations between summer wet (Palaeocene, Early Miocene) and summer dry climates (Oligocene, Middle Miocene to present). The massive speciation in the Cape flora might be accounted for by the diverse limitations to gene flow (dissected landscapes, pollinator specialisation, long flowering times allowing much phenological specialisation), as well as a richly complex environment providing a diversity of selective forces (geographically variable climate, much altitude variation, different soil types, rocky terrain providing many micro-niches, and regular fires providing both intermediate disturbances, as well as different ways of surviving the fires). However, much of this is based on correlation, and there is a great need for (a) experimental testing of the proposed speciation mechanisms, (b) more molecular clock estimates of the age and pattern of the radiations, and (c) more fossil evidence bearing on the past climates.
AIM: This paper has as its central aim the location of centres of species richness and endemism in the sub‐Saharan African flora. Previous postulation of these centres has been based on an intuitive ...interpretation of distributional data; this paper provides a test of these centres. A second aim is to establish whether the two indices, richness and endemism, locate the same centres. Thirdly the relationship between species richness and endemism, and latitude and rainfall are explored. LOCATION: The study area includes much of sub‐Saharan Africa, but excludes the species‐poor southern margin of the Sahara and the Namib–Kalahari regions. METHODS: Analyses were based on 1818 species, scored on a 2.5 × 2.5 degree grid. Species richness was inferred from a simple grid‐diversity count; endemism was determined by three measures: the number of species restricted to two grids, the sum of the inverse of the ranges of the component species of each grid, and the proportion of the species in each grid that have restricted ranges. RESULTS: The African flora shows a remarkably profound patterning, both in species richness and endemism. The two measures locate largely the same centres, although the rank order among them differs. These centres are: the Cape Floristic Region, East Coast, Congo‐Zambezi watershed, Kivu, Upper and Lower Guinea. Richness is strongly related to maximum rainfall, but there are no obvious correlations between modern climate and endemism. Species richness and endemism north of the equator is much more concentrated into centres than south of the equator. MAIN CONCLUSIONS: There are strongly developed refugia in sub‐Saharan Africa. North of the equator, these refugia are sharply delimited and rather small, separated by large areas of very low endemism. South of the equator endemism tends to be more generally distributed. Variation in species richness in sub‐Saharan Africa can be explained largely by modern rainfall, while endemism may be related to palaeoclimatic fluctuations. Both species richness and endemism show a strong skewing towards the south, indicating that the fluctuations in the Sahara might have influenced the modern distribution of plants in Africa.
ABSTRACT
Poaceae (the grasses) is arguably the most successful plant family, in terms of its global occurrence in (almost) all ecosystems with angiosperms, its ecological dominance in many ...ecosystems, and high species richness. We suggest that the success of grasses is best understood in context of their capacity to colonize, persist, and transform environments (the “Viking syndrome”). This results from combining effective long‐distance dispersal, efficacious establishment biology, ecological flexibility, resilience to disturbance and the capacity to modify environments by changing the nature of fire and mammalian herbivory. We identify a diverse set of functional traits linked to dispersal, establishment and competitive abilities. Enhanced long‐distance dispersal is determined by anemochory, epizoochory and endozoochory and is facilitated via the spikelet (and especially the awned lemma) which functions as the dispersal unit. Establishment success could be a consequence of the precocious embryo and large starch reserves, which may underpin the extremely short generation times in grasses. Post‐establishment genetic bottlenecks may be mitigated by wind pollination and the widespread occurrence of polyploidy, in combination with gametic self‐incompatibility. The ecological competitiveness of grasses is corroborated by their dominance across the range of environmental extremes tolerated by angiosperms, facilitated by both C3 and C4 photosynthesis, well‐developed frost tolerance in several clades, and a sympodial growth form that enabled the evolution of both annual and long‐lived life forms. Finally, absence of investment in wood (except in bamboos), and the presence of persistent buds at or below ground level, provides tolerance of repeated defoliation (whether by fire, frost, drought or herbivores). Biotic modification of environments via feedbacks with herbivory or fire reinforce grass dominance leading to open ecosystems. Grasses can be both palatable and productive, fostering high biomass and diversity of mammalian herbivores. Many grasses have a suite of architectural and functional traits that facilitate frequent fire, including a tufted growth form, and tannin‐like substances in leaves which slow decomposition. We mapped these traits over the phylogeny of the Poales, spanning the grasses and their relatives, and demonstrated the accumulation of traits since monocots originated in the mid‐Cretaceous. Although the sympodial growth form is a monocot trait, tillering resulting in the tufted growth form most likely evolved within the grasses. Similarly, although an ovary apparently constructed of a single carpel evolved in the most recent grass ancestor, spikelets and the awned lemma dispersal units evolved within the grasses. Frost tolerance and C4 photosynthesis evolved relatively late (late Palaeogene), and the last significant trait to evolve was probably the production of tannins, associated with pyrophytic savannas. This fits palaeobotanical data, suggesting several phases in the grass success story: from a late Cretaceous origin, to occasional tropical grassland patches in the later Palaeogene, to extensive C3 grassy woodlands in the early–middle Miocene, to the dramatic expansion of the tropical C4 grass savannas and grasslands in the Pliocene, and the C3 steppe grasslands during the Pleistocene glacial periods. Modern grasslands depend heavily on strongly seasonal climates, making them sensitive to climate change.
Endemism in the Australian flora Crisp, M. D.; Laffan, S.; Linder, H. P. ...
Journal of biogeography,
February 2001, Letnik:
28, Številka:
2
Journal Article
Recenzirano
AIM: To detect centres of vascular plant endemism at a continental scale by analysis of specimen‐based distributional data and to relate any pattern to environmental factors and history. LOCATION: ...Australia. METHODS: Presence of 8468 seed plant species‐level taxa throughout continental Australia and Tasmania was mapped on a 1° grid to visualize the pattern of species richness. This sample comprises half the known flora. Three indices of endemism were calculated but we preferred one that is unrelated to species richness, so that these two concepts could be distinguished in practice. Centres of endemism were detected by simple mapping and by spatial autocorrelation analysis (SAC). Linear regression was used to examine the relationship of the patterns of species richness and endemism to latitude, topography and climate. RESULTS: Both species richness and endemism vary greatly across the continent but in most cases the same centres were high in both richness and endemism. Twelve distinct centres were identified. The major centres of both diversity and endemism are south‐west western Australia, the Border Ranges between New South Wales and Queensland, the Wet Tropics near Cairns, Tasmania and the Iron‐McIlwraith Range of eastern Cape York Peninsula. The last centre appears to be more significant than recognized by past authors. Whether this is a true Australian centre of endemism, or is largely an outlier of the flora of Papua New Guinea, is explored. Another centre, in the Adelaide–Kangaroo Island region, has been overlooked altogether by previous authors. Regression analysis did not find a simple climatic explanation of the observed patterns. There was a suggestion that topographic variation within the 1° cells may be positively correlated with endemism, which is consistent with mountainous regions functioning as refugia. One clear result is that all the major centres of endemism are near‐coastal. A likely explanation is that Pleistocene expansions of the central desert have been a powerful limitation on the viability of refugia for narrowly endemic species. All the centres of endemism lie outside the estimated limits of the expanded arid zone at the last glacial maximum (18,000 yr BP). In particular, the ‘Central Australian Mountain Ranges centre of plant diversity and endemism’ of Boden & Given (1995) is detected as a strong centre of species richness, but not at all as a centre of endemism. This is despite good sampling of this region. MAIN CONCLUSIONS: Endemism can be distinguished from species richness by using an appropriate index and mapping of such indices can detect centres of endemism. This study demonstrates the value of specimen based distributional data, such as is held in state herbaria and museums.
Changing agricultural practices have dramatically altered the arable flora of Europe since the Second World War. We conducted a meta‐analysis of the available literature to assess the dynamics of ...species richness and species traits in the arable flora across Europe during this time period. We found a total of 32 publications, yielding 53 data sets with an average number of 252 studied plots per data set. Average species number per plot of arable plants across all data sets declined by about 20%. However, twelve data sets showed an increase in average species number per plot, including all studies starting after 1980. Plant species preferring nutrient‐rich sites, neophytes and monocotyledons generally increased since 1980, while characteristic or threatened species of arable weed communities further declined. This temporal development of the European arable flora suggests that the changes happening in agricultural management since the 1980s, such as organic farming and reduced pesticide input, may have helped slow the decline of the arable flora in terms of species number, but not in terms of characteristic or threatened arable weeds. Hence, more specific measures are necessary to stop decline of the latter, making sure that these measures are advantageous for rare and characteristic arable species, but not for harmful weeds.
Areas of endemism are central to cladistic biogeography. The concept has been much debated in the past, and from this has emerged the generally accepted definition as an area to which at least two ...species are endemic. Protocols for locating areas of endemism have been neglected, and to date no attempt has been made to develop optimality criteria against which to evaluate competing hypotheses of areas of endemism. Here various protocols for finding areas of endemism are evaluated—protocols based on both phenetic and parsimony analyses, on both unweighted data and data weighted by various criteria. The optimality criteria used to compare the performance of the methods include the number of species included in the areas of endemism, the number of areas delimited, and the degree of distributional congruency of the species restricted to each area of endemism. These methods are applied to the African Restionaceae in the Cape Floristic Region. Parsimony methods using weighted data are shown to perform best on the combination of all three optimality criteria. By varying the weighting parameters, the size of the areas of endemism can be varied. This provides a very useful tool for locating areas of endemism that satisfy prespecified scale criteria.
EVOLUTIONARY HISTORY OF POALES Linder, H.P; Rudall, P.J
Annual review of ecology, evolution, and systematics,
01/2005, Letnik:
36, Številka:
1
Journal Article
Recenzirano
The predominantly wind-pollinated order Poales includes about one third of all monocot (Angiosperm) species, with c. 20,000 species dominating modern savanna and steppe vegetation. Recent ...improvements in understanding relationships within the order allow phylogenetic optimizations of habitat preferences and adaptive character states, enabling exploration of the factors that have influenced evolution in this successful order. Poales probably originated in the late Cretaceous in wet nutrient-poor sunny habitats. By the Paleogene the lineage had diversified into swamps, the forest understory, epiphytic habitats, and nutrient-poor heathlands. The Neogene saw major diversifications of the grasses and possibly the sedges into fire-adapted vegetation in seasonal climates and low atmospheric $CO_{2}$. Diversification into these habitats was facilitated by morphological features such as the sympodial habit and physiological factors that allowed frequent evolution of $CO_{2}-concentrating$ mechanisms.
Plant species richness and endemism on oceanic islands is dependent on island age, size, elevation and distance to nearest source of migrants. Mainland ‘island’ systems, such as the cool-adapted ...tropical-alpine flora of the mountains in Africa, are less well studied. Here we analyse the tropical Afroalpine flora, found on highly isolated mountains straddling the equator. Using the β-sim index in a hierarchical clustering approach combined with ordination methods and mantel tests, we locate four geographical groups: West Africa, northern East Africa, western East Africa and eastern East Africa. We show that these groupings are better explained as the consequence of geographical isolation rather than environmental filtering. We further show that the species richness of the tropical Afroalpine mountain regions—similar to oceanic islands—can be explained by a model including age, area size, elevation and isolation. Levels of endemism are best explained by species richness in combination with area and isolation. Overall we develop a comprehensive model of the species richness, endemism and composition of the tropical Afroalpine flora.
The Greater Cape Floristic Region J. Born; Linder, H. P.; P. Desmet
Journal of biogeography,
01/2007, Letnik:
34, Številka:
1
Journal Article
Recenzirano
Aim The Cape Floristic Region (CFR) (Cape Floristic Kingdom) is currently narrowly delimited to include only the relatively mesic Cape fold mountains and adjacent intermontane valleys and coastal ...plains. We evaluate the floristic support for expanding the delimitation to include the whole winter-rainfall area (arid and mesic climates) into a Greater CFR. Location Southern Africa, particularly the south-western tip. Methods The initial divisive hierarchical classification analysis TWINSPAN used the presence/absence of vascular plant genera to obtain major floristic groupings in southern Africa. For the more detailed analyses, we scored the flora as present/ absent within a set of centres, among which the floristic relationships were investigated (agglomerative methods, UPGMA and minimum spanning trees). These analyses were conducted with species, genera and families separately. The centres were grouped into five regions. The species richness and endemism was calculated for the centres, regions and combination of regions. The dominant floristic components of each region were sought by calculating the percentage contribution of each family to the flora. Results The divisive method showed that the winter-rainfall areas are floristically distinct from the rest of southern Africa. The species- and generic-level analyses revealed five regions: CFR, Karoo Region, Hantam-Tanqua-Roggeveld Region, Namaqualand Region and Namib-Desert Region. The CFR has the highest endemism and richness. However, the combination of the CFR, the Hantam-Tanqua-Roggeveld Region and the Namaqualand Region results in a higher total endemism. Combined, these three regions almost match the region delimited by the TWINSPAN analysis, and together constitute the Greater CFR. Main conclusions The CFR constitutes a valid floristic region. This is evident from the endemism and the distinctive composition of the flora. However, the total endemism is higher for the whole winter-rainfall area, and this supports the recognition of the larger unit. If floristic regions are to be delimited only on endemism, then the Greater CFR is to be preferred. If floristic regions are delimited on the composition of their floras at family level, then the support for such a grouping is weaker.
Aim The flora characteristic of the Cape Floristic Region (CFR) is dominated by a relatively small number of clades that have been proposed as 'Cape clades'. These clades have variously been ...suggested to have African or Austral affinities. Here we evaluate the support for these conflicting hypotheses. In addition, we test the hypothesis that these clades share a common time of differentiation from their geographical neighbours. Location The Cape Floristic Region, South Africa Methods We use both published and unpublished phylogenetic information to investigate the geographical sister areas of the Cape clades as well as the timing and the direction of biogeographical disjunctions. Results Almost half of the Cape clades for which unambiguous sister areas could be established show a trans-Indian Ocean disjunction. The earliest trans-Indian Ocean disjunction dates from 80 Ma. Other disjunctions date from various times in the Cenozoic, and we suggest that the process of recruiting lineages into the Cape flora might be ongoing. Relatively few Cape clades show a sister relationship with South America and tropical Africa, despite their relative geographical proximity. Numerous Cape clades contain species also found on tropical African mountains; in all cases tested, these species are shown to be embedded within the Cape clades. While many Cape clades show a relationship with the Eurasian temperate flora, this is complicated by their presence in tropical Africa. The single case study addressing this to date suggests that the Cape clade is nested within a European grade. Main conclusions Although many Cape clades show Austral rather than African relationships, there are numerous other patterns suggestive of a cosmopolitan flora. This spatial variation is echoed in the temporal data, from which, although there is wide variance around the dates of disjunctions, it is clear the Cape flora has been assembled over a long time period. There is no simple hypothesis that can account for the geographical sources of the currently distinctive Cape flora. The phylogenetic positions of Afromontane members of Cape clades suggest a history of dispersal from the CFR, rather than the reverse.
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