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Avian persistence in a severely fragmented Afrotropical cloud forest

Toon Spanhove (UGent)
(2012)
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Abstract
Loss and fragmentation of natural habitats rank amongst the biggest threats to global biodiversity. In particular, tropical forests, which are extremely rich in plant and animal species, are under enormous pressure worldwide. In many areas, these forests now remain as small, isolated remnants embedded in a human-dominated landscape that is quite hostile for their original inhabitants. For most forest-dependent species, surviving in these severely fragmented ecosystems is challenging, and few species seem to persist relatively well in these highly altered landscapes. The underlying mechanisms for the endangerment and persistence are highly diverse, being a function of both species-specific and landscape properties. Landscape effects, such as habitat loss, patch isolation, matrix and edge-effects and the configuration of the remaining habitat, as well as patch-scale features, such as habitat quality and size of the remaining patches, may affect demographic and genetic properties of the remaining species, especially when only small population are surviving. Because the high diversity and highly interwoven species interactions in tropical ecosystems, the impact of fragmentation on population persistence are expected to be much more complicated in the tropics compared to temperate zones. Unfortunately, tropical systems remain largely understudied and due to the large differences with temperate regions, mechanisms underlying patterns of species persistence are difficult to extrapolate. This thesis focuses on the persistence of avian populations in the Taita Hills, a severely fragmented montane forest in southeast Kenya that is isolated from similar habitat by at least 50 km of dry bushland. With currently less than 500ha of forest remaining (i.e. less than 2% of the original forest area), the Taita Hills are still considered a site of global conservation importance, harbouring dozens of endemic plant and animal species, including three endemic bird species. While some bird species are critically threatened by the extreme levels of habitat loss and fragmentation, other species seem to cope relatively well with the current levels of habitat modification. We focussed on two critical processes that were expected to strongly affect avian persistence in the Taita Hills forests: nest predation – as a proxy for recruitment – and mobility within and between the remaining forest fragments. In tropical forests, levels of nest predation are typically much higher compared to temperate regions, with mostly between 60% and 90% of the nests being predated. Nest predation is therefore considered a crucial factor in the population dynamics of tropical bird species. Patterns of nest predation were studied for two forest-dependent bird species in the Taita Hills. Based on 179 natural ground nests of the white-starred robin and 228 shrub nests of the Cabanis’s greenbul, we found that on average 70% of the nests were depredated for both species. Predation patterns strongly varied in space and time. While predation rates on white-starred robin nests increased in the course of the breeding season, they decreased for the Cabanis’s greenbul. Also nest concealment and vegetation structure influenced predation rates, although the strength and direction was inconsistent between the two species and between fragments. The most striking and most relevant pattern in the framework of habitat fragmentation was the strong increase in nest predation towards the interior of the forest fragment, opposing the traditional perception of elevated predation rates at the edge of forest fragments. This ‘inverse edge effect’ on nest predation was observed for the two study species, and was further corroborated by an experiment using artificial ground nests and plasticine eggs. Although predation rates were consistently higher in the interior of the forest fragments, the strength of the documented edge effects on nest predation varied between species and between fragments. Besides the variation within single forest fragments, we also documented a strong effect of fragment size on nest predation for the white-starred robin: predation rates were lower – not higher – in the smallest forest fragments. Variation in levels of nest predation significantly affects avian populations inhabiting these forest fragments. For habitat generalists that tolerate breeding in small fragments or at edges, these spatial patterns on nest predation may result in a mitigation – rather than an amplification – of the otherwise negative impact of habitat fragmentation. For habitat specialists – in contrast – the opposite may be true and they may suffer even more from nest predation than when predation rates increase toward forest-matrix ecotones. Such differential impact of nest predators on habitat generalists and specialists may ultimately lead to a shift in the composition of avian meta-communities, further threatening the most vulnerable species. Besides nest predation, mobility of individuals is also expected to play a crucial role for avian persistence in severely fragmented landscapes. When individual (sub)populations become small and isolated, a negative spiral of demographic and genetic may bring them to the brink of extinction. Movements between forest fragments may counteract these threats, either by numerical increasing the remaining populations or due to exchange of genes between small populations. We studied the mobility of the twelve most common forest understory species based on the analysis of a long-term capture-recapture ringing programme, complemented with expert judgement of movements into the non-forest matrix. Species that seem to cope well with current level of fragmentation either combined high levels of inter-fragment dispersal with a high frequency of movements in the non-forest matrix, or at least showed sufficient high levels of one of these mobility characteristics. Both mechanisms were expected to increase the functional population size, either by linking subpopulations into one metapopulation in the case of high interfragment dispersal levels, or by increasing the resources that can be utilised –thus increasing the potential size of the (sub)population itself – in case of high levels of movements into the non-forest matrix. Species that combined low levels of inter-fragment dispersal with high levels of habitat specialisation, i.e. rarely venturing in the non-forest matrix, are the most vulnerable to extreme levels of habitat fragmentation. If the endangered bird populations of the Taita Hills want to be preserved for future generations, a conservation programme is urgently needed. Several actions can help to safeguard the imperilled populations. For example, the conversion of exotic plantations around indigenous forest remnants may drastically increase the amount of habitat available for the remaining bird populations. Besides, the transformation of as little as five hectares agricultural land into indigenous forest habitat may even create a new, large (but elongated) forest fragment that connects six existing forest remnants with a continuous corridor. These straightforward and feasable conservation action may mean the different between successful conservation or losing critical species – forever.
Keywords
forest, bird, habitat fragmentation, nest predation, dispersal, movement, Kenya, Taita Hills

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Citation

Please use this url to cite or link to this publication:

Chicago
Spanhove, Toon. 2012. “Avian Persistence in a Severely Fragmented Afrotropical Cloud Forest”. Ghent, Belgium: Ghent University. Faculty of Sciences.
APA
Spanhove, T. (2012). Avian persistence in a severely fragmented Afrotropical cloud forest. Ghent University. Faculty of Sciences, Ghent, Belgium.
Vancouver
1.
Spanhove T. Avian persistence in a severely fragmented Afrotropical cloud forest. [Ghent, Belgium]: Ghent University. Faculty of Sciences; 2012.
MLA
Spanhove, Toon. “Avian Persistence in a Severely Fragmented Afrotropical Cloud Forest.” 2012 : n. pag. Print.
@phdthesis{3106350,
  abstract     = {Loss and fragmentation of natural habitats rank amongst the biggest threats to global biodiversity. In particular, tropical forests, which are extremely rich in plant and animal species, are under enormous pressure worldwide. In many areas, these forests now remain as small, isolated remnants embedded in a human-dominated landscape that is quite hostile for their original inhabitants. For most forest-dependent species, surviving in these severely fragmented ecosystems is challenging, and few species seem to persist relatively well in these highly altered landscapes. The underlying mechanisms for the endangerment and persistence are highly diverse, being a function of both species-specific and landscape properties. Landscape effects, such as habitat loss, patch isolation, matrix and edge-effects and the configuration of the remaining habitat, as well as patch-scale features, such as habitat quality and size of the remaining patches, may affect demographic and genetic properties of the remaining species, especially when only small population are surviving. Because the high diversity and highly interwoven species interactions in tropical ecosystems, the impact of fragmentation on population persistence are expected to be much more complicated in the tropics compared to temperate zones. Unfortunately, tropical systems remain largely understudied and due to the large differences with temperate regions, mechanisms underlying patterns of species persistence are difficult to extrapolate.
This thesis focuses on the persistence of avian populations in the Taita Hills, a severely fragmented montane forest in southeast Kenya that is isolated from similar habitat by at least 50 km of dry bushland. With currently less than 500ha of forest remaining (i.e. less than 2\% of the original forest area), the Taita Hills are still considered a site of global conservation importance, harbouring dozens of endemic plant and animal species, including three endemic bird species. While some bird species are critically threatened by the extreme levels of habitat loss and fragmentation, other species seem to cope relatively well with the current levels of habitat modification. We focussed on two critical processes that were expected to strongly affect avian persistence in the Taita Hills forests: nest predation -- as a proxy for recruitment -- and mobility within and between the remaining forest fragments.
In tropical forests, levels of nest predation are typically much higher compared to temperate regions, with mostly between 60\% and 90\% of the nests being predated. Nest predation is therefore considered a crucial factor in the population dynamics of tropical bird species. Patterns of nest predation were studied for two forest-dependent bird species in the Taita Hills. Based on 179 natural ground nests of the white-starred robin and 228 shrub nests of the Cabanis{\textquoteright}s greenbul, we found that on average 70\% of the nests were depredated for both species. Predation patterns strongly varied in space and time. While predation rates on white-starred robin nests increased in the course of the breeding season, they decreased for the Cabanis{\textquoteright}s greenbul. Also nest concealment and vegetation structure influenced predation rates, although the strength and direction was inconsistent between the two species and between fragments. The most striking and most relevant pattern in the framework of habitat fragmentation was the strong increase in nest predation towards the interior of the forest fragment, opposing the traditional perception of elevated predation rates at the edge of forest fragments. This {\textquoteleft}inverse edge effect{\textquoteright} on nest predation was observed for the two study species, and was further corroborated by an experiment using artificial ground nests and plasticine eggs. Although predation rates were consistently higher in the interior of the forest fragments, the strength of the documented edge effects on nest predation varied between species and between fragments. Besides the variation within single forest fragments, we also documented a strong effect of fragment size on nest predation for the white-starred robin: predation rates were lower -- not higher -- in the smallest forest fragments. Variation in levels of nest predation significantly affects avian populations inhabiting these forest fragments. For habitat generalists that tolerate breeding in small fragments or at edges, these spatial patterns on nest predation may result in a mitigation -- rather than an amplification -- of the otherwise negative impact of habitat fragmentation. For habitat specialists -- in contrast -- the opposite may be true and they may suffer even more from nest predation than when predation rates increase toward forest-matrix ecotones. Such differential impact of nest predators on habitat generalists and specialists may ultimately lead to a shift in the composition of avian meta-communities, further threatening the most vulnerable species.
Besides nest predation, mobility of individuals is also expected to play a crucial role for avian persistence in severely fragmented landscapes. When individual (sub)populations become small and isolated, a negative spiral of demographic and genetic may bring them to the brink of extinction. Movements between forest fragments may counteract these threats, either by numerical increasing the remaining populations or due to exchange of genes between small populations. We studied the mobility of the twelve most common forest understory species based on the analysis of a long-term capture-recapture ringing programme, complemented with expert judgement of movements into the non-forest matrix. Species that seem to cope well with current level of fragmentation either combined high levels of inter-fragment dispersal with a high frequency of movements in the non-forest matrix, or at least showed sufficient high levels of one of these mobility characteristics. Both mechanisms were expected to increase the functional population size, either by linking subpopulations into one metapopulation in the case of high interfragment dispersal levels, or by increasing the resources that can be utilised --thus increasing the potential size of the (sub)population itself -- in case of high levels of movements into the non-forest matrix. Species that combined low levels of inter-fragment dispersal with high levels of habitat specialisation, i.e. rarely venturing in the non-forest matrix, are the most vulnerable to extreme levels of habitat fragmentation.
If the endangered bird populations of the Taita Hills want to be preserved for future generations, a conservation programme is urgently needed. Several actions can help to safeguard the imperilled populations. For example, the conversion of exotic plantations around indigenous forest remnants may drastically increase the amount of habitat available for the remaining bird populations. Besides, the transformation of as little as five hectares agricultural land into indigenous forest habitat may even create a new, large (but elongated) forest fragment that connects six existing forest remnants with a continuous corridor. These straightforward and feasable conservation action may mean the different between successful conservation or losing critical species -- forever.},
  author       = {Spanhove, Toon},
  isbn         = {9789461971012},
  language     = {eng},
  pages        = {131},
  publisher    = {Ghent University. Faculty of Sciences},
  school       = {Ghent University},
  title        = {Avian persistence in a severely fragmented Afrotropical cloud forest},
  year         = {2012},
}