Spatial dynamics of anthropogenically altered dispersal patterns : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Albany, New Zealand
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Date
2017
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Massey University
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Abstract
The thesis put forward in this study is that anthropogenic disruption of natural
dispersal is of central importance for the conservation of biodiversity. The main rationale
being that dispersal, as a fundamental life history trait, is directly linked to, and affected by
three of the main threats to biodiversity: Invasive species, habitat fragmentation and
climate change.
In terms of biological invasions, the introduction of exotic taxa into a foreign habitat
is performed by people, and the spread of these species within the invasion range is often a
result of human aided dispersal. Surprisingly though, not much emphasis is put on
anthropogenic dispersal when modelling biological invasions.
Habitat fragmentation is the direct consequence of habitat destruction. In a
fragmented landscape, remnant natural patches host populations with varying degrees of
inter-connectivity. Dispersal through degraded habitat is the key to maintaining viable
meta-populations in a fragmented landscape. To facilitate dispersal in highly fragmented
landscapes such as cities, a clear knowledge of the roles played by different land cover
features (e.g. density of trees or traffic) is the starting point to make informed urban
planning decisions.
The main issue with climate change is not that climate conditions are disappearing,
but that they are moving. As a consequence of this, organisms also need to move along
with the climatic conditions they are adapted to. This process, named range shift, is being
observed worldwide on several taxa. Researchers, however, put very little attention to the
problems arising when “climate migrants” encounter dispersal barriers (e.g.
anthropogenic land cover) on their path. The main body of work presented here consists in both the practical analysis of
specific systems and the development of conceptual and methodological frameworks to
address general issues. Among the former, I used population and landscape genetics
approaches to analyse the case of Copper skinks (Oligosoma aeneum) in Auckland (New
Zealand) as a recent and intensive habitat fragmentation scenario, with the goal of
detecting the land cover types that allow the highest dispersal in urban settings. I also
analysed the case of the Australian Litoria frogs (L. aurea, L. raniformis and L. ewingii) in New
Zealand with the intent of quantifying the anthropogenic dispersal component of their
invasion. As per the latter, I developed a novel method for estimating the anthropogenic
component of biological invasions, infer their natural dispersal parameters, and forecast
future developments of biological invasions. I also developed the novel concept of C-trap,
whereby the shape and spatial orientation of the interface between natural and
anthropogenic land cover types may originate traps for climate migrants, and used it to
determine where, on a global scale, their high densities can further threaten endangered,
endemic animal species.
My results identified an early stage fragmentation scenario for urban Copper skinks in
Auckland. Relatively high population genetic structure formed as a result of segregation
by a motorway. However, the roles of other land cover types in controlling population
connectivity could not be determined. Of all Litoria frog populations recorded in New
Zealand, about 30% were found to be of anthropogenic origin. Even though the borders
of the distribution range occupied so far are unlikely to expand, the density of populations
within the range is expected to rise with new, suitable patches being found. Tests on
virtual species with the novel methodology for the modelling of biological invasions
showed good performance (accuracy in estimating natural dispersal kernel and
anthropogenic contribution), also in the presence of challenging limitations in the input dataset. Finally, the global scale application of the C-traps concept to endangered,
terrestrial species highlighted the high potential for conservation issues among climate
migrants in Eastern Europe and Southern Asia.
Although it is challenging to quantify the consequences for biodiversity conservation
of anthropogenic alterations to natural dispersal, results from the applied studies confirm
the initial thesis on their critical role as biodiversity threats; while the novel concepts and
methodologies provide a valuable tool for better incorporating these aspects in ecological
studies and conservation management.
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Keywords
Animals, Dispersal, Spatial ecology, Mathematical models, Landscape ecology, New Zealand, Fragmented landscapes, Environmental aspectsd, Oligosoma, Litoria, Dispersal, R (Computer program language)