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Vasco Lepori
ETH Zurich
Global Change Biology
What determines the limits of a species range?
Date: April 24th 2017
Supervisor: Harald Bugmann
Introduction
Understanding the factors and processes that define the range (distribution) of a species is one of the oldest
questions in ecology, and one that is yet to be definitively answered. Multiple factors have been identified
as significant, but their relative strength, spatial variation and interplay are not yet fully understood (Gaston
2009). Nonetheless, this question deserves keeping being investigated, as it is of importance not only for
advancing scientific understanding, but also for ecosystem services and thus for society at large. Being able
to understand how range limits function and predict how they behave will be crucial to face current and
future challenges like biological invasions, climate change and spread of diseases (Gaston 2009).
Question
What are the causes limiting the range of species at the global scale?
What are the genetic and ecological processes acting at range limits?
Results
Geographic ranges can be understood as the spatial reflection of a species’ niche (Hargreaves et al. 2014).
But rarely do species occupy the full geographic extent of their niche, because factors such as dispersal and
history influence their current distribution. This condition, called range disequilibrium, can be easily observed
by looking at the success of invasive species in non-native environments, which confirms that they were
dispersal-limited on a global scale. Furthermore, their climatic niche tends to remain similar in the new range
(Petitpierre 2012), though niche shifts do happen sometimes (through ecological or evolutionary change).
Causes of range limits
Physical barriers: At the global scale, most species are dispersal-limited (Gaston 2003). Barriers include
mountain ranges, oceans, ocean flows, rivers, or large lakes.
Abiotic conditions: Climate may be the most important factor in regulating species distributions. Several
aspects of climate can be limiting, such as mean temperatures, precipitation, extreme values,
monthly distribution, growing season length, etc. Other abiotic factors, some related to climate, can
determine range limits: light, geology, nutrient availability, soil conditions (e.g., pH), fire and other
disturbances.
Biotic conditions: Interspecific interactions are also important, possibly more so in warmer and in more
species-rich environments (Hargreaves et al. 2014). Parasites and predators for example, especially
specific ones, are limited by the distribution of their hosts and prey, respectively. Many range limits
are also created by competition resulting in the exclusion of the weakest competitor. It was thought
that predators and parasites would not drive their prey/hosts to local extinction since they would go
extinct themselves or would need to switch prey/host (Gaston 2003). However, this view has recently
been challenged, and prey ranges may also be restricted by top-down effects of predation and
parasitism when predators are generalists and/or have sufficiently large dispersal (Gaston 2009).
Other interactions, such as pollination, may also influence ranges.
Vasco Lepori
ETH Zurich
Global Change Biology
Multiple factors are likely to interact in defining a range limit, which makes identifying causes and predicting
ranges a complex task. Recently developed Species Distribution Models (SDM) try to answer this question by
becoming increasingly elaborate and considering at the same time multiple processes and factors (such as
species interactions or habitat availability), while classical SDMs only used data on current distribution and
environmental parameters to predict distribution.
Processes acting at range limits
From an ecological standpoint, species ranges can be interpreted as a question of population dynamics at
the range edges. Ranges coincide with the habitats where a species can maintain a non-negative growth rate
to sustain a persisting population, and range limits are the edges where this condition can no longer be
fulfilled. Thus, any limiting factor(s) must act on the species ecological parameters:
Nt+1 = Nt + b - d + i – e
For example, climate seems to restrict ranges by acting on reproduction (births) more frequently than by
increasing mortality, although there are examples of high mortality induced by extreme weather events
(Gaston 2003). Range equilibrium, and the vital rate determining the range limit, can be tested through
beyond-range transplant experiments. Hargreaves et al. (2014) found that in 75% of the cases, performance
decreased beyond the range. However, only 46% of the times did the range limit coincide with the niche
limit, which suggests that disequilibrium due to other factors at play (dispersal), is frequent. The downside
of these experiments is that they may only consider limited time-scales (often one to a few years), thus might
be missing important and rare events affecting survival beyond ranges (e.g. climatic extremes, fire).
Even a negative growth rate can create a window of opportunity for adaptive evolution (Sexton et al. 2009).
In fact, the study of range limits can also be approached from an evolutionary biology / genetic viewpoint:
what prevents a species from adapting to conditions beyond its range, and evolving traits that would allow
for range expansion? Possible explanations comprise: low genetic variation in small, marginal populations at
range limits, genetic swamping by maladaptive gene flow from central populations, low trait heritability,
physiological limit, and trade-offs between stress-resistant genotype and generally favourable genotype. But
evidence leaning towards any of these over the others is still scarce (Gaston 2003).
Climate change and range limits
Species ranges are already shifting under climate change as they track their climatic niche by moving
polewards and upwards. Chen et al. (2012) found that species are moving at an average pace of 17 latitudinal
km per decade, and 11 meters per decade upwards. Observed range shifts match the predicted ones (based
on climatic parameters change) for latitudinal movements, but lag behind in altitudinal shifts. This suggests
that dispersal can not keep up with the speed of climate change, or that other non-climatic factors prevent
range shifts (such as habitat availability, biotic factors or physical barriers). Biotic interactions will likely
influence (impede or accelerate) range shifts under climate change (HilleRisLambers et al. 2013). Much faster
range expansion happens during biological invasions mediated by human vectors.
Conclusion
Despite having been the object of investigation by experts from different sub-disciplines of biology for
decades, species range limits are still to be definitively explained. Ranges are determined by dispersal,
population history, and several sets of biotic and abiotic factors that act on population dynamics. On top of
this, genetic processes between and within populations determine whether adaptation at range bounds is
possible. This complexity of factors at play explains why the topic is still relatively poorly understood
nowadays. Biological invasions and climate change-induced range shifts will offer great natural experiments
to advance our understanding, as well as bringing important challenges.
Vasco Lepori
ETH Zurich
Global Change Biology
References
Chen, I.-C., 2012. Rapid Range Shifts of Species Associated with High levels of Climate Warming. Science,
1024(2011), pp.17–20.
Gaston, K.J., 2009. Geographic range limits: achieving synthesis. Proceedings of the Royal Society B,
276(1661), pp.1395–1406.
Gaston, K.J., 2009. Geographic range limits of species. Proceedings. Biological sciences / The Royal Society,
276(1661), pp.1391–3. Available at:
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2677225&tool=pmcentrez&rendertype=
abstract.
Gaston, K.J., 2003. The stucture and dynamics of geographic ranges, Oxford University Press.
Hargreaves, A.L., Samis, K.E. & Eckert, C.G., 2014. Are species’ range limits simply niche limits writ large? A
review of transplant experiments beyond the range. The American naturalist, 183(2), pp.157–73.
Available at:
http://www.journals.uchicago.edu/doi/10.1086/674525%5Cnhttp://www.ncbi.nlm.nih.gov/pubmed/
24464192.
Hillerislambers, J. et al., 2013. How will biotic interactions influence climate change-induced range shifts?
Annals of the New York Academy of Sciences, 1297, pp.112–125.
Petitpierre, B., 2012. Climatic Niche shifts are rare among terrestrial plant invaders. Science, 335(March),
pp.1344–1348.
Sexton, J.P. et al., 2009. Evolution and ecology of species range limits. Annual Review of Ecology &
Systematics, 40, pp.415–436.