Historical climate change and speciation: neotropical seasonally dry forest plants show patterns of both tertiary and quaternary diversification.
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Historical climate changes have had a major effect on the distribution and evolution of plant species in the neotropics. What is more controversial is whether relatively recent Pleistocene climatic changes have driven speciation, or whether neotropical species diversity is more ancient. This question is addressed using evolutionary rate analysis of sequence data of nuclear ribosomal internal transcribed spacers in diverse taxa occupying neotropical seasonally dry forests, including Ruprechtia (Polygonaceae), robinioid legumes (Fabaceae), Chaetocalyx and Nissolia (Fabaceae), and Loxopterygium (Anacardiaceae). Species diversifications in these taxa occurred both during and before the Pleistocene in Central America, but were primarily pre-Pleistocene in South America. This indicates plausibility both for models that predict tropical species diversity to be recent and that invoke a role for Pleistocene climatic change, and those that consider it ancient and implicate geological factors such as the Andean orogeny and the closure of the Panama Isthmus. Cladistic vicariance analysis was attempted to identify common factors underlying evolution in these groups. In spite of the similar Mid-Miocene to Pliocene ages of the study taxa, and their high degree of endemism in the different fragments of South American dry forests, the analysis yielded equivocal, non-robust patterns of area relationships. (+info)
Tropical forests and global atmospheric change: a synthesis.
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We present a personal perspective on the highlights of the Theme Issue 'Tropical forests and global atmospheric change'. We highlight the key findings on the contemporary rate of climatic change in the tropics, the evidence-gained from field studies-of large-scale and rapid change in the dynamics and biomass of old-growth forests, and evidence of how climate change and fragmentation can interact to increase the vulnerability of plants and animals to fires. A range of opinions exists concerning the possible cause of these observed changes, but examination of the spatial 'fingerprint' of observed change may help to identify the driving mechanism(s). Studies of changes in tropical forest regions since the last glacial maximum show the sensitivity of species composition and ecology to atmospheric changes. Model studies of change in forest vegetation highlight the potential importance of temperature or drought thresholds that could lead to substantial forest decline in the near future. During the coming century, the Earth's remaining tropical forests face the combined pressures of direct human impacts and a climatic and atmospheric situation not experienced for at least 20 million years. Understanding and monitoring of their response to this atmospheric change are essential if we are to maximize their conservation options. (+info)
Species and functional group diversity independently influence biomass accumulation and its response to CO2 and N.
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The characteristics of plant assemblages influence ecosystem processes such as biomass accumulation and modulate terrestrial responses to global change factors such as elevated atmospheric CO(2) and N deposition, but covariation between species richness (S) and functional group richness (F) among assemblages obscures the specific role of each in these ecosystem responses. In a 4-year study of grassland species grown under ambient and elevated CO(2) and N in Minnesota, we experimentally varied plant S and F to assess their independent effects. We show here that at all CO(2) and N levels, biomass increased with S, even with F constant at 1 or 4 groups. Likewise, with S at 4, biomass increased as F varied continuously from 1 to 4. The S and F effects were not dependent upon specific species or functional groups or combinations and resulted from complementarity. Biomass increases in response to CO(2) and N, moreover, varied with time but were generally larger with increasing S (with F constant) and with increasing F (with S constant). These results indicate that S and F independently influence biomass accumulation and its response to elevated CO(2) and N. (+info)
Adaptive radiation from resource competition in digital organisms.
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Species richness often peaks at intermediate productivity and decreases as resources become more or less abundant. The mechanisms that produce this pattern are not completely known, but several previous studies have suggested environmental heterogeneity as a cause. In experiments with evolving digital organisms and populations of fixed size, maximum species richness emerges at intermediate productivity, even in a spatially homogeneous environment, owing to frequency-dependent selection to exploit an influx of mixed resources. A diverse pool of limiting resources is sufficient to cause adaptive radiation, which is manifest by the origin and maintenance of phenotypically and phylogenetically distinct groups of organisms. (+info)
Human population density and extinction risk in the world's carnivores.
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Understanding why some species are at high risk of extinction, while others remain relatively safe, is central to the development of a predictive conservation science. Recent studies have shown that a species' extinction risk may be determined by two types of factors: intrinsic biological traits and exposure to external anthropogenic threats. However, little is known about the relative and interacting effects of intrinsic and external variables on extinction risk. Using phylogenetic comparative methods, we show that extinction risk in the mammal order Carnivora is predicted more strongly by biology than exposure to high-density human populations. However, biology interacts with human population density to determine extinction risk: biological traits explain 80% of variation in risk for carnivore species with high levels of exposure to human populations, compared to 45% for carnivores generally. The results suggest that biology will become a more critical determinant of risk as human populations expand. We demonstrate how a model predicting extinction risk from biology can be combined with projected human population density to identify species likely to move most rapidly towards extinction by the year 2030. African viverrid species are particularly likely to become threatened, even though most are currently considered relatively safe. We suggest that a preemptive approach to species conservation is needed to identify and protect species that may not be threatened at present but may become so in the near future. (+info)
DNA taxonomy of a neglected animal phylum: an unexpected diversity of tardigrades.
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A molecular survey technique was used to investigate the diversity of terrestrial tardigrades from three sites within Scotland. Ribosomal small subunit sequence was used to classify specimens into molecular operational taxonomic units (MOTU). Most MOTU were identified to the generic level using digital voucher photography. Thirty-two MOTU were defined, a surprising abundance given that the documented British fauna is 68 species. Some tardigrade MOTU were shared between the two rural collection sites, but no MOTU were found in both urban and rural sites, which conflicts with models of ubiquity of meiofaunal taxa. The patterns of relatedness of MOTU were particularly intriguing, with some forming clades with low levels of divergence, suggestive of taxon flocks. Some morphological taxa contained well-separated MOTU, perhaps indicating the existence of cryptic taxa. DNA sequence-based MOTU proved to be a revealing method for meiofaunal diversity studies. (+info)
Taxonomic triage and the poverty of phylogeny.
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Revisionary taxonomy is frequently dismissed as merely descriptive, which belies its strong intellectual content and hypothesis-driven nature. Funding for taxonomy is inadequate and largely diverted to studies of phylogeny that neither improve classifications nor nomenclature. Phylogenetic classifications are optimal for storing and predicting information, but phylogeny divorced from taxonomy is ephemeral and erodes the accuracy and information content of the language of biology. Taxonomic revisions and monographs are efficient, high-throughput species hypothesis-testing devices that are ideal for the World Wide Web. Taxonomic knowledge remains essential to credible biological research and is made urgent by the biodiversity crisis. Theoretical and technological advances and threats of mass species extinctions indicate that this is the time for a renaissance in taxonomy. Clarity of vision and courage of purpose are needed from individual taxonomists and natural history museums to bring about this evolution of taxonomy into the information age. (+info)
A taxonomic wish-list for community ecology.
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Community ecology seeks to explain the number and relative abundance of coexisting species. Four research frontiers in community ecology are closely tied to research in systematics and taxonomy: the statistics of species richness estimators, global patterns of biodiversity, the influence of global climate change on community structure, and phylogenetic influences on community structure. The most pressing needs for taxonomic information in community ecology research are usable taxonomic keys, current nomenclature, species occurrence records and resolved phylogenies. These products can best be obtained from Internet-based phylogenetic and taxonomic resources, but the lack of trained professional systematists and taxonomists threatens this effort. Community ecologists will benefit most directly from research in systematics and taxonomy by making better use of resources in museums and herbaria, and by actively seeking training, information and collaborations with taxonomic specialists. (+info)