Wageningen University
March 2002
Email: r.etienne@biol.rug.nl
Striking the metapopulation balance;
Mathematical Models & Methods Meet Metapopulation Management
There are two buzz words in nature management: fragmentation and connectivity. Not only (rail) roads, but also agricultural, residential and industrial areas fragment previously connected (or even continuous) habitat. Common sense tells us that the answer to habitat fragmentation is defragmentation and hence much effort is put into building corridors, of which fauna crossings are just one example. Corridors are conduits connecting two pieces of habitat through an environment of hostile non-habitat. Needless to say, there are good reasons for building corridors. Yet, there are some valid arguments against connecting everything. The risk of spreading of infectious diseases through these corridors is one of the most prominent arguments. The spread of the effects of (natural) catastrophes such as fire is another. But even when dismissing such negative effects of connectivity, there may be other mitigating measures which are much more efficient (and less expensive) than building corridors. The question whether this is the case and how alternatives should be compared stimulated the work for this thesis.
Metapopulation theory seems the appropriate tool to answer these questions. A metapopulations is a collection of (local) populations, each of which runs a considerable risk of extinction, but can also be recolonized by dispersers from other populations. Through a balance of local extinctions and recolonization, the metapopulations can persist for a very long time although the local populations cannot. Using a stochastic patch occupancy metapopulations model, I was able to formulate the following rules of thumb: to optimize metapopulation extinction time, decreasing the risk of local extinction is preferable over increasing colonization probability and this should generally be done in the least extinction-prone patches; if changing local extinction risk is impossible, then increasing the colonization probability between the two least extinction-prone patches is most preferable. When extinction and colonization are related to patch size and interpatch distance by mechanistic submodels of the corresponding processes, the last two of these rules transform into: the preferred strategies to optimize the metapopulation extinction time and the basic reproduction number are, firstly, increasing the size of the largest patch (which is least extinction-prone) and, secondly, decreasing the effective interpatch distance between the two largest patches. These rules are less strongly supported than the rules in terms of extinction and colonization probabilities. And if absolute (instead of relative) increases in patch size are considered, the smallest patch should be increased. The reason for this is that in the mechanistic submodel for local extinction a large patch requires a large increase in size to substantially alter its local extinction probability. Since it is not a priori clear whether increases in patch sizes must be compared on an absolute or a relative basis, final conclusions cannot be drawn. We still need a socio-politico-economic study taking into account e.g. the costs of habitat creation in relation to the size of the patch to which habitat is added. We also require additional work on the important biological question how ecoducts and the like change the effective interpatch distance; this is usually merely hidden in the parameters. Although the answers are not final, at least more light has been shed on the range of possible final conclusions and, more importantly, the conditions under which they are valid.
Two chapters of my thesis deal with the questions above. I also looked into the problem whether it is better to have a few large or many small habitat patches. Furthermore, I studied several extensions of the simplest metapopulation model, the Levins model, and the influence of timelags on metapopulation extinction time. The last part of the thesis deals with applications of stochastic patch occupancy models to real metapopulation. One is concerned with predicting the impact of reinstating an old railway track on two amphibian species, the other introduces a Bayesian approach to parameter estimation which is applied to a tree frog metapopulation.
Parts of the thesis were co-authored by Hans Heesterbeek (University of Utrecht, The Netherlands), Kees Nagelkerke (University of Amsterdam), Lia Hemerik and Marjolein Lof (Wageningingen University), Claire Vos (Alterra, Wageningen, The Netherlands), Michiel Jansen and Cajo ter Braak (Biometris, Wageningen, The Netherlands).
My present employer is the Laborary for Plant Ecology, University of Groningen, The Netherlands, but I am stationed at Tropical Nature Conservation and Vertebrate Ecology, Wageningen University and Research Centre, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands. Email: Rampal.Etienne@wur.nl. Phone: +31 317 484672. Fax: +31 317 484845.
PhD-Thesis Index
NVTB - Home Page