A generalized trend of changes that occur when species from large land masses colonize islands. Larger colonizing species often form dwarfed populations and small species generally become larger. On the island, space is more limiting and the number of species present is relatively lower. These very general differences lead to changes in evolutionary pressures on the organism, although individual islands and circumstances may have idiosyncratic differences as well. Plants also have insular trends, however, that may related to similar pressures as those which cause the changes seen in mammals. Plants that are herbaceous annuals on the mainland often become tree-like perennials on islands. Additionally, birds and insects have been known to loose their ability to fly after colonizing islands.
We are reading two papers about the island rule.
Lomolino, M. V.
2005 Body size evolution in insular vertebrates: generality of the island rule. Journal of Biogeography 32:1683 - 1699.
Palkovacs, E. P.
2003 Explaining adaptive shifts in body size on islands: a life history approach. Oikos 103:37 - 44.
The Lomolino paper was optional for week 4 but the Palkovacs paper is required for week 5.
Lomolino's abstract:
"Aim My goals here are to (1) assess the generality of the island rule – the graded
trend from gigantism in small species to dwarfism in larger species – for
mammals and other terrestrial vertebrates on islands and island-like ecosystems;
(2) explore some related patterns of body size variation in insular vertebrates, in
particular variation in body size as a function of island area and isolation; (3)
offer causal explanations for these patterns; and (4) identify promising areas for
future studies on body size evolution in insular vertebrates.
Location Oceanic and near-shore archipelagos, and island-like ecosystems
world-wide.
Methods Body size measurements of insular vertebrates (non-volant mammals,
bats, birds, snakes and turtles) were obtained from the literature, and then
regression analyses were conducted to test whether body size of insular
populations varies as a function of body size of the species on the mainland
(the island rule) and with characteristics of the islands (i.e. island isolation and
area).
Results The island rule appears to be a general phenomenon both with
mammalian orders (and to some degree within families and particular
subfamilies) as well as across the species groups studied, including non-volant
mammals, bats, passerine birds, snakes and turtles. In addition, body size of
numerous species in these classes of vertebrates varies significantly with island
isolation and island area.
Main conclusions The patterns observed here – the island rule and the
tendency for body size among populations of particular species to vary with
characteristics of the islands – are actually distinct and scale-dependent
phenomena. Patterns within archipelagos reflect the influence of island
isolation and area on selective pressures (immigration filters, resource
limitation, and intra- and interspecific interactions) within particular species.
These patterns contribute to variation about the general trend referred to as the
island rule, not the signal for that more general, large-scale pattern. The island
rule itself is an emergent pattern resulting from a combination of selective forces
whose importance and influence on insular populations vary in a predictable
manner along a gradient from relatively small to large species. As a result, body
size of insular species tends to converge on a size that is optimal, or fundamental,
for a particular bau plan and ecological strategy."
Why study the Island Rule in human macroecology?
there are many reasons, but a few include the following:
- Understanding the nature and causes of the Island Rule provides insight into really general features of mammalian evolution. With this comes the potential to generalize the insights to other cases where body size changes. This helps us understand selection in general in addition to life history evolution because of the strong interaction between life history attributes and body size.
- The variables that seem most important for understand the Island Rule include resource availability, predation pressure and/or mortality, population density, and competition. On islands we might be able to isolate, more or less, the sustained effects of changes in these key ecological variables. We want to understand how these variables have affected human evolution via changes in range expansion and body size, among others.
- Something we did not talk about in class is that we also need to understand how humans affect the other species. It is often difficult to link human activities like predation and land disturbance to the disappearance of other species. Yet when humans prey on an animal, they are influencing its mortality rate and when humans disturb habitats they may be altering resource availability via effective territory size. Therefore, there is an important link between two of the important variables in the island rule, resource availability and predation pressure, that might help us understand how humans alter the demographic features of their prey or other species they interact with. If humans hunt animals or take their eggs (and the like) then they are altering their mortality rates. If they disturb habitats they may be limiting resource availability for certain species.
(I had intended to do a lot more with this blog. I hope this helps clarify and keep us all on the same page.)
One limitation to our understanding of body size changes on islands is that it is not usually approached as a life history problem. We'll talk more about that next week as we read the Palkovacs paper.
Have a good weekend
Oskar
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