Question

1) Why is incorporating dispersal into range models important, and what are some of the approaches used to do this?

2) Why might migration rates at the end of the last ice age be different from today?

Answer 1

Species distribution models (SDMs) are one of the most important GIScience research areas in biogeography and are the primary means by which the potential effects of climate change on species’ distributions and ranges are investigated. Dispersal is an important ecological process for species responding to changing climates, however, SDMs and their subsequent spatial products rarely reflect accessibility to any future suitable environment. Dispersal-related movement can be confounded by factors that vary across landscapes and climates, as well as within and among species, and it has therefore remained difficult to parametrise in SDMs. Here we compared 20 models that have previously been used (or have the potential to be used) to represent dispersal processes in SDM to predict future range shifts in response to climate change. We assessed the different dispersal models in terms of their accuracy at predicting future distributions, as well as the uncertainty associated with their predictions. Atlas data for 50 bird species from 1988 to 1991 in Great Britain were treated as base distributions (t₁), with the species–environment relationships extrapolated (using three commonly used statistical methods) to 2008–2011 (t₂). Dispersal (in the form of the 20 different models) was simulated from the base distribution (t₁) to 2008–2011 (t₂). The results were then combined and used to identify locations that were both abiotically suitable (obtained from the statistical methods) and accessible (obtained from the dispersal models). The accuracy of these coupled projections was assessed with the 2008–2011 atlas data (the observed t₂ distribution). There was substantial variation in the accuracy of the different dispersal models, and in general, the more restrictive dispersal models (e.g. fixed rate dispersal) resulted in lower accuracy for the metrics which reward correct prediction of presences. Ensemble models of the dispersal methods (generated by combining multiple projection outcomes) were created for each species, and a new Ensemble Agreement Index (EAI), which ranges from 0 (no agreement among models) to 1 (full agreement among models) was developed to quantify uncertainty among the projections. EAI values ranged from 0.634 (some areas of disagreement and therefore medium uncertainty among dispersal models) to 0.999 (large areas of agreement and low uncertainty among dispersal models). The results of this research highlight the importance of incorporating dispersal and also illustrate that the method with which dispersal is simulated greatly impacts the projected future distribution. This has important implications for studies aimed at predicting the effects of changing environmental conditions on species’ distributions.

1. As biodiversity hotspots are often characterized by high human population densities, implementation of conservation management practices that focus only on the protection and enlargement of pristine habitats is potentially unrealistic. An alternative approach to curb species extinction risk involves improving connectivity among existing habitat patches. However, evaluation of spatially explicit management strategies is challenging, as predictive models must account for the process of dispersal, which is difficult in terms of both empirical data collection and modelling.
2. Here, we use a novel, individual‐based modelling platform that couples demographic and mechanistic dispersal models to evaluate the effectiveness of realistic management scenarios tailored to conserve forest birds in a highly fragmented biodiversity hotspot. Scenario performance is evaluated based on the spatial population dynamics of a well‐studied forest bird species.
3. The largest population increase was predicted to occur under scenarios increasing habitat area. However, the effectiveness was sensitive to spatial planning. Compared to adding one large patch to the habitat network, adding several small patches yielded mixed benefits: although overall population sizes increased, specific newly created patches acted as dispersal sinks, which compromised population persistence in some existing patches. Increasing matrix connectivity by the creation of stepping stones is likely to result in enhanced dispersal success and occupancy of smaller patches.
4. Synthesis and applications. We show that the effectiveness of spatial management is strongly driven by patterns of individual dispersal across landscapes. For species conservation planning, we advocate the use of models that incorporate adequate realism in demography and, particularly, in dispersal behaviours.
5. Movement behaviour has become increasingly important in dispersal ecology and dispersal is central to the development of spatially explicit population ecology. The ways in which the elements have been brought together are reviewed with particular emphasis on dispersal distance distributions and the value of mechanistic models.
6. 2

There is a continuous range of movement behaviours and in some species, dispersal is a clearly delineated event but not in others. The biological complexities restrict conclusions to high‐level generalizations but there may be principles that are common to dispersal and other movements.

7. 3

Random walk and diffusion models when appropriately elaborated can provide an understanding of dispersal distance relationships on spatial and temporal scales relevant to dispersal. Leptokurtosis in the relationships may be the result of a combination of factors including population heterogeneity, correlation, landscape features, time integration and density dependence. The inclusion in diffusion models of individual variation appears to be a useful elaboration. The limitations of the negative exponential and other phenomenological models are discussed.

8. 4

The dynamics of metapopulation models are sensitive to what appears to be small differences in the assumptions about dispersal. In order to represent dispersal realistically in population models, it is suggested that phenomenological models should be replaced by those based on movement behaviour incorporating individual variation.

The conclusions are presented as a set of candidate principles for evaluation. The main features of the principles are that uncorrelated or correlated random walk, not linear movement, is expected where the directions of habitat patches are unpredictable and more complex behaviour when organisms have the ability to orientate or navigate. Individuals within populations vary in their movement behaviour and dispersal; part of this variation is a product of random elements in movement behaviour and some of it is heritable. Local and metapopulation dynamics are influenced by population heterogeneity in dispersal characteristics and heritable changes in dispersal propensity occur on time‐scales short enough to impact population dynamics.

Answer 2-  

What would happen if there was an ice age today?

We may have delayed the onset of the next ice age for now, but if another one came it would have pretty big consequences for human civilisation.

Besides the fact it would be an awful lot colder, huge regions where hundreds of millions of people live would become completely uninhabitable. They'd be covered in thick ice sheets and subject to an inhospitable climate.

"Assuming it was similar to the last one, then north America would be covered in ice, the whole of northern Europe, the whole of northern Asia would be covered in ice," Dr Phipps said.

There would be a lot less agricultural land available, so it would be very difficult to support the human population, Dr Phipps warned.

And the physical shape of the continents would look completely different across the whole planet.

A huge drop in sea level of up to 120 metres would close down marine channels - the Mediterranean Sea, Torres Strait, Bass Strait and Bering Strait - and create new areas of land that could be used for habitation or agriculture.

Ocean ports would no longer be on the ocean, and anyone wanting water views would need to relocate large distances.

Ice ages have had an absolutely enormous impact on human evolution.

During the last ice age, which ran from about 110,000 years ago to 10,000 years ago, the lower sea levels allowed humans to move out across the entire world.

"There was no Bering Straits, so north America and Asia were joined and that's actually how humans first roamed into the Americas, they just walked over the land bridge," Dr Phipps said.

While there was still some water between Asia and Australia it took just a few short canoe trips to bring the first humans to Australasia.

"They would have come over towards New Guinea. There was no Torres Strait so humans could have just walked from New Guinea to the Australian mainland. And there was no Bass Strait so humans could have walked from the Australian mainland over to Tasmania," he said.

The whole dispersal of humans around the world during the last 100,000 years was made entirely possible by the fact we were in an ice age at the time.