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12 - Inverse latitudinal gradients in species diversity
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- By Pavel Kindlmann, Institute of Systems Biology and Ecology, Academy of Sciences of the Czech Republic, University of South Bohemia, Agrocampus Rennes, Iva Schödelbauerová, Institute of Systems Biology and Ecology, Academy of Sciences of the Czech Republic, University of South Bohemia, Anthony F. G. Dixon, University of East Anglia
- Edited by David Storch, Charles University, Prague, Pablo Marquet, Pontificia Universidad Catolica de Chile, James Brown, University of New Mexico
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- Book:
- Scaling Biodiversity
- Published online:
- 05 August 2012
- Print publication:
- 12 July 2007, pp 246-257
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Summary
Introduction
No single pattern of biodiversity has attracted ecologists more than the observed increase in species richness from the poles to the tropics (Pianka, 1966; Rohde, 1992; Rosenzweig & Sandlin, 1997; Gaston & Blackburn, 2000; Willig, Kaufman & Stevens, 2003; Hillebrand, 2004). An obstacle in the search for the primary cause of this latitudinal gradient is the ever-increasing number of hypotheses (Pianka, 1966; Rohde, 1992; Clarke, this volume), their interdependence (Currie, 1991; Gaston & Blackburn, 2000) and lack of rigorous falsification (Currie, Francis & Kerr, 1999; Currie, this volume). However, a general decline in species richness with latitude is commonly observed (Pielou, 1977; Colwell & Hurtt, 1994; Willig & Lyons, 1998; Colwell & Lees, 2000; Zapata, Gaston & Chown, 2003; Colwell, Rahbek & Gotelli, 2004).
Some groups of organisms, however, show an opposite trend: a strong latitudinal decline in species diversity towards the tropics. These trends have been almost neglected in the literature and little is known about their underlying ecological and evolutionary causes. Therefore, the ecological explanations proffered are usually specific to the group in question. Here an account of the most important cases of inverse latitudinal gradients is given. The existing hypotheses explaining this phenomenon are summarized and the evidence that tends to favor one of these is presented.
Mechanisms and evolution of deceptive pollination in orchids
- Jana Jersáková, Steven D. Johnson, Pavel Kindlmann
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- Journal:
- Biological Reviews / Volume 81 / Issue 2 / May 2006
- Published online by Cambridge University Press:
- 28 February 2006, pp. 219-235
- Print publication:
- May 2006
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The orchid family is renowned for its enormous diversity of pollination mechanisms and unusually high occurrence of non-rewarding flowers compared to other plant families. The mechanisms of deception in orchids include generalized food deception, food-deceptive floral mimicry, brood-site imitation, shelter imitation, pseudoantagonism, rendezvous attraction and sexual deception. Generalized food deception is the most common mechanism (reported in 38 genera) followed by sexual deception (18 genera). Floral deception in orchids has been intensively studied since Darwin, but the evolution of non-rewarding flowers still presents a major puzzle for evolutionary biology. The two principal hypotheses as to how deception could increase fitness in plants are (i) reallocation of resources associated with reward production to flowering and seed production, and (ii) higher levels of cross-pollination due to pollinators visiting fewer flowers on non-rewarding plants, resulting in more outcrossed progeny and more efficient pollen export. Biologists have also tried to explain why deception is overrepresented in the orchid family. These explanations include: (i) efficient removal and deposition of pollinaria from orchid flowers in a single pollinator visit, thus obviating the need for rewards to entice multiple visits from pollinators; (ii) efficient transport of orchid pollen, thus requiring less reward-induced pollinator constancy; (iii) low-density populations in many orchids, thus limiting the learning of associations of floral phenotypes and rewards by pollinators; (iv) packaging of pollen in pollinaria with limited carry-over from flower to flower, thus increasing the risks of geitonogamous self-pollination when pollinators visit many flowers on rewarding plants. All of these general and orchid-specific hypotheses are difficult to reconcile with the well-established pattern for rewardlessness to result in low pollinator visitation rates and consequently low levels of fruit production. Arguments that deception evolves because rewards are costly are particularly problematic in that small amounts of nectar are unlikely to have a significant effect on the energy budget of orchids, and because reproduction in orchids is often severely pollen-, rather than resource-limited. Several recent experimental studies have shown that deception promotes cross-pollination, but it remains unknown whether actual outcrossing rates are generally higher in deceptive orchids. Our review of the literature shows that there is currently no evidence that deceptive orchids carry higher levels of genetic load (an indirect measure of outcrossing rate) than their rewarding counterparts. Cross-pollination does, however, result in dramatic increases in seed quality in almost all orchids and has the potential to increase pollen export (by reducing pollen discounting). We suggest that floral deception is particularly beneficial, because of its promotion of outcrossing, when pollinators are abundant, but that when pollinators are consistently rare, selection may favour a nectar reward or a shift to autopollination. Given that nectar-rewardlessness is likely to have been the ancestral condition in orchids and yet is evolutionarily labile, more attention will need to be given to explanations as to why deception constitutes an ‘evolutionarily stable strategy’.