Browsing by Author "Midgley, J J"
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- ItemOpen AccessClassification and dynamics of the forest vegetation of Hluhluwe Nature Reserve, Kwa Zulu-Natal(1996) West, Adam; Midgley, J JThe forests of Hluhluwe Nature Reserve, Kwa Zulu-Natal, were examined to determine composition, grain, classification, dynamics and conservation value. Field sampling took the form of recording DBH for all species in twenty-one 0.04 ha circular plots, walking 5-metre wide transects in mature and marginal forest to record size-classes and analysis of twenty gaps. Data were analysed by the multivariate programs, TWINSPAN and CANOCO. The composition of the canopy and subcanopy is distinct. Four canopy and two subcanopy communities can be detected. Community succession in the mature forest takes the form of almost random replacement of canopy trees through two subcanopy communities. Size-class distributions indicate a prevalence of shade-intolerant species. This forest is deemed to be coarse-grained and can be classified as coastal scarp forest. Comparisons with other forests shows Hluhluwe to have slightly above average species richness. A removal of anthropogenic disturbance by the creation of the reserve may have resulted in present forest canopy structure. Management for conservation of species diversity should encourage disturbance. There is potential for harvesting of certain shadetolerant species by local communities. This forest is a pristine fragment of a rapidly diminishing vegetation type and efforts should be made to conserve its entire expanse, within Hluhluwe Nature Reserve, in order to prevent species loss.
- ItemRestrictedExperimental evidence for bird pollination and corolla damage by ants in the short-tubed flowers of Erica halicacaba (Ericaceae)(Elsevier, 2012) Turner, R C; Midgley, J J; Barnard, P; Simmons, R E; Johnson, S DUnrelated plants pollinated by similar animals tend to show convergent evolution of floral traits. Floral syndromes have been used successfully to develop hypotheses about pollination systems but can be misleading when plants have unusual floral morphology or mechanisms of pollen transfer. A case in point is Erica halicacaba, a local endemic shrub on the Cape Peninsula of South Africa. Its short-tubed greenish-yellowish flowers with a narrow aperture have been considered to be insect pollinated. However, field observations, selective exclusion experiments and analysis of pollen loads all indicate that its primary pollinator is the Orange-breasted Sunbird (Anthobaphes violacea). These sunbirds were common visitors and netted individuals carried large numbers of E. halicacaba pollen tetrads on their culmens, rather than on their head plumage as is typical for Cape Erica species with longer tubular flowers. Plants of E. halicacaba from which vertebrates were excluded had a lower incidence of anther tripping and set significantly fewer seeds than those exposed to both birds and insect visitors. Nectar in E. halicacaba is present in small concentrated amounts and, typical of sunbird-pollinated plants, is dominated by sucrose. Flowers often had holes in their corollas and we identified ants, specifically Myrmicaria nigra, as the likely agents. This was supported by reduced rates of corolla damage on branches from which crawling insects such as ants were experimentally excluded. These findings indicate that considerable caution should be applied when attempting to predict pollinators from floral syndromes, and highlight the role that crawling insects can play in damaging flowers.
- ItemRestrictedHow do fires kill plants? The hydraulic death hypothesis and Cape Proteaceae "fire-resisters"(Elsevier, 2011) Midgley, J J; Kruger, L M; Skelton, RThe actual mechanism which causes plant death after having been burned has been poorly studied. One possibility is that fire causes direct, or indirect, fatal damage to plant hydraulic systems. If true, this suggests that burned plants ultimately die of drought. This hypothesis was tested on the post-fire response of a "fire-resister" species of the Cape Proteaceae, as well as by analysing its morphology. Fire-resisters are plants which are incapable of resprouting, but nevertheless survive some fires. Mortality of the studied fire-resister appears to be compatible with a hydraulic death hypothesis because i) most post-fire mortality occurred within days, ii) it occurred from the base-upwards and iii) correlated negatively with stem diameter rather than plant height. Higher levels of survival of the fire-resister is probably due to absolutely thicker bark than co-occurring re-seeder species of the same age. Since this bark has not evolved to protect buds, it has probably evolved to protect stem hydraulic systems.
- ItemOpen AccessLife-history evolution as an explanation for the absence of the tree life-form in Cape fynbos(2008) Midgley, J J; Rebelo, A GInvasive exotic trees are able to grow tall (more than 15 m) in the fynbos environment of the southwestern Cape of South Africa, whereas the tallest fynbos plants, generally from the Proteaceae, have much shorter maximum stature (less than 5 m). Analysis of the Protea Atlas Database for the Cape shows a positive correlation between age at maturity and maximum plant size. Monographs of the Proteaceae indicate a shorter period to maturity for reseeders than resprouters. We propose an evolutionary hypothesis for the absence of trees in the fynbos; predictable, frequent fires have favoured early maturity and this has discriminated against any strategies with delays to maturity (such as experienced by resprouters and trees).
- ItemOpen AccessNo adverse signs of the effect of environmental change on tree biomass in the Knysna forest during the 1990s(2006) Midgley, J J; Seydack, AWe analysed the growth of all trees greater than 10 cm diameter at breast height in 108 plots of 0.04 ha each in an unharvested nature reserve in the Knysna forest for the period 1991-2001. Gross growth rates (in-growth plus growth of survivors) of trees were extremely slow (<1%) and thus the forest is extremely sensitive to changes in mortality of large trees. Over the 10-year study period, total basal area and biomass increased by 2%, in part because overall stem density increased by 1.2%. Rainfall in the 1990s was slightly below the longterm mean for the study area. At this stage, therefore, the Knysna forest biomass is not showing a negative response to environmental change.
- ItemRestrictedResponse to Matthews et al. (2001) vegetation of the Tembe Elephant Park, Maputaland, South Africa(2003) Kirkwood, D; Midgley, J JThe contribution of Matthews et al. (2001) to our knowledge of the vegetation of Tembe Elephant Park (TEP) is both valuable and timely. Tembe is at the epicentre of the Maputaland Centre of Endemism (Van Wyk 1994), which, together with the Pondoland Centre, is likely to be recognized in the near future as a global biodiversity hotspot by Conservation International (S Frazee pers. comm.). Currently there are only 25 of these areas of exceptional diversity and endemism which are also substantially transformed by humans (Myers 1990).
- ItemRestrictedVariation in seed set amongst populations of a redent pollinated geophyte, Colchicum coloratum(2009) Kleizen, C; Midgley, J J; Johnson, S. D.Agricultural activities around the rural village of Nieuwoudtville in the Succulent Karoo region of South Africa has lead to the confinement of many plant and animal species to fragmented patches of relatively untransformed habitat. The geophyte Colchicum coloratum subsp. coloratum (Colchicaceae) was studied in five patches of variable size in and around Nieuwoudtville. This species is dependent on rodent visitation for seed production. The influence of variation in population size and plant size on seed set was investigated, as well as whether there is pollen limitation in this species. A pollen-supplementation experiment indicates that there is pollen limitation in C. coloratum, and that much of the natural seed set could be the result of pollinator-mediated selfing. The five populations appeared to have different rodent abundances, however, neither population size nor the abundance of rodents in the area have an effect on seed set. This suggests that the mutualism between C. colchicum and its rodent pollinators is robust, and that habitat fragmentation in Nieuwoudtville has not yet affected the seed production of this geophyte.
- ItemOpen AccessWere Malagasy Uncarina fruits dispersed by the extinct elephant bird?(2009) Midgley, J J; Illing, NWe hypothesise that the spiny fruits of the endemic Madagascar genus Uncarina (Pedaliaceae) are trample burrs that evolved to be dispersed on the feet of the extinct elephant bird (Aepyornis). Our evidence is : i) the morphology of the fruit with its large grapple hooks is more likely to attach to a foot than to adhere to fur and ii) the presentation of mature fruits on the ground rather than in the canopy. These differences to adhesive burrs make lemurs unlikely dispersers. We argue, given the absence of other large terrestrial mammals in Madagascar, that the most likely dispersers of Uncarina fruits were the extinct large birds. If correct, our hypothesis has implications for conservation of Uncarina, the biogeography of the elephant birds and dispersal biology. For example, we predict that the demography of Uncarina will be skewed towards adult plants, and that the dispersal mutualism could possibly be rescued by domestic animals.
- ItemOpen AccessWhy do elephants damage savanna trees?(2005) Midgley, J J; Balfour, D; Kerley, G IElephants damage and kill trees. THIS behaviour often appears to be excessive because their immediate, subsequent consumption of edible parts of these trees is often negligible. Some trees later resprout after snapping and toppling by elephants and thus produce forage at the animals’ preferred feeding-height. We argue that this and other ‘farming’ hypotheses are group-selectionist and are thus not evolutionarily stable strategies. We suggest that excessive damage to trees is more likely to be due to social or sexual factors. More behavioural analyses and experiments are needed to understand this phenomenon and its implications for conservation