Mechanisms: Structure and Function of Individuals (Week 9)
Brown, J.H. 1995. Macroecology. University of Chicago Press, Chicago.
Chapter 7 discusses mechanisms for the natural macroecological patterns.
Raia, P., Meiri, S. 2006. The Island Rule in Large Mammals: Paleontology Meets Ecology. Evolution. 60:1731-1742
The Island Rule in Large Mammals: Paleontology Meets Ecology
The authors finished by suggesting that in order to predict the direction and magnitude of the evolution of size there must be an understanding of internal ecological characteristics of island populations.
Chapter 7 discusses mechanisms for the natural macroecological patterns.
- Body size results in dietary and habitat specialization because of the effect it has the habitat and nutrients that are available to species.
- Allometric influences result in lower rates of nutrient consumption in smaller individuals; however, the intake per unit body mass and the quality of the nutrient intake is greater than the larger relatives.
- Allometry has four macroecology consequences:
- Smaller size should correlate with diet specificity, a result from consuming a limited range of food.
- Small species adaptation is for types of habitat rather than food type.
- Species in the modal range maintain densities that are comparable or higher than larger relatives.
- Space requirements for modal sized species occur alongside high population densities.
- As the size of the body diminishes, organisms must obtain high-quality nourishment
- Hypothesis: species can only do this to some crucial size
- Predictions:
- below the critical body size, territory size is inversely proportional to body size
- the minimum size of the geographic range should decrease as body size falls below the mode.
- Optimal Size Model:
- large herbivores could use resources that are not used by acquiring smaller size and a more discriminatory diet.
- Smaller species have a reduced time to first reproduction and an increase in litter size.
- Constraint of Flight
- aerodynamics severely limit the shape of organisms as well as the body mass
- Four predictions
Raia, P., Meiri, S. 2006. The Island Rule in Large Mammals: Paleontology Meets Ecology. Evolution. 60:1731-1742
The Island Rule in Large Mammals: Paleontology Meets Ecology
The authors in this paper cite different hypothesis to explain the Island rule, a tendency of large species on islands to experience dwarfism on islands, and offer a new explanation that includes both ecological interactions and resources as factors. Their explanation consist of four hypotheses: 1. biological interaction effect hypotheses, 2. Overdispersion hypothesis, 3. carnivore resource-competition hypothesis, and 4. sexual size dimorphism hypothesis. To test these hypotheses they used the data in current literature and compared island fauna to mainland fauna in the Mediterranean area.
The results stated in the paper supported two of the hypotheses, the biological interaction effect and overdispersion hypotheses. However, the data on carnivore size refuted their hypothesis that competition, predation, and diet plays an important part in the patters for such organisms. The lack of sexual dimorphism in size when compared to mainland relatives also refuted hypothesis number four.The authors finished by suggesting that in order to predict the direction and magnitude of the evolution of size there must be an understanding of internal ecological characteristics of island populations.
One thing the paper showed is that dietary differences influence body size change of mammals. I wonder if there has ever been a case where a species switches its diet completely (i.e., go from herbivore to carnivore) in order to fit into a body size range.
ReplyDeleteAlso, the paper only looked at mammals. I would expect there to be a degree of competition of small herbivore with birds that end up on islands. I wonder how looking at interactions with birds would have changed/strengthened their results?
Also, Brown suggests that there is an optimal size for mammals: around 100 g. Is there a size a species wouldn't want to be because it'd make individuals more obvious to predators? For large herbivores, what came first: the big size to escape from predators, or food requiring a larger gut?
One thing refreshing about both the paper and the chapter is how much biotic factors are considered instead of just abiotic factors!
In response to Meghan, I known anecdotally that there does appear to be a bimodal distribution of "prey" size, either too big to take down easily, or small enough to avoid detection. I believe Felisa has said the middle ground of body size would be like painting a big target on the animal, it's like saying "come eat me".
ReplyDeleteWhich is a stronger pressure on an herbivore? To avoid predation, or to have a large gut for digestion? My gut instinct is that predation would serve as a strong proximal cause for getting bigger, and having a larger gut comes along with that larger size.
What kind of competitive pressures do very large herbivores place on small herbivores, even in non-island situations? Do they impart some kind of selective pressure?
Meghan, your first question makes me immediately think of early carnivorous ancestors to artiodactlys.
ReplyDeleteMel: I thought about the same thing. With predators, there's always a trade off between hunting the prey and the energy derived from the prey. So if a prey item were to undergo dwarfism on an island with a predator present, is there a certain size at which it becomes beneficial to be smaller? Kind of like with mimicry; there's a cost valley between the benefits of being cryptic and looking like you're poisonous. Is there a similar valley between being big enough to avoid predation and being small enough for predators to just not be bothered? If there is, how does this effect predator size? Do the predators get smaller to capitalize on the food source, or remain large, consume more prey and eventually change food sources when the prey become too small to be worth catching?
ReplyDeleteAs far as carnivores changing into herbivores, my first thought was to bats and primates. Early bats were most likely insectivorous, and later evolved into frugivory. Similar story with primates, although I'm not so clear on their evolution.
More applicably, all of the first true dinosaurs were carnivorous, many of which evolved into herbivory.
I wonder why they chose these methods of analysis to study this question. It seems like they are putting a lot of layers of categories between the actual data and the analysis. It seems like there must be a better way to study this question. This gets to an overarching challenge in macroecological research - since it is a fairly new field, there is a dearth of good analysis methods in macroecology.
ReplyDeleteI think it would add to this study to include both small mammals and birds. It seems like, for a place that has been studied as long as the Mediterranean has, there should be a decent fossil record for small mammals.
The reading this week really interests me!! I love learning more about island biogeography and whats even cooler is paleo-island biogeography!! Couple thoughts on the paper: if carnivores can subdue prey 5 x's their size then shouldn't we see more extreme dwarfism in carnivores even though they rely on resource abundance? Which leads me to believe I am not fully comprehending resource abundance to it's fullest? I was somewhat skeptical towards the methods just because (an I know the reasons behind this) I find it necessary to include small mammals and island area when looking at dwarfism in herbivores and carnivores. The results were a bit unclear to me as well; do ungulates only dwarf if no competition or predators present? Or are there other factors I didn't get?
ReplyDeleteOnto the chapter: Makes sense that smaller animals are short-lived, reproduce early, and have larger clutch sizes, but how do we explain the existence of the Broad-tailed hummingbird and other insectivorous bats? What is a best-fit power function exactly? I have plenty of other thoughts to discuss in class. See you all then!!