Do predators eat each other? Do they only eat herbivores? These questions have been in my mind for a while. Whenever we hear or read the word “lions” we immediately connect it to savagery toward people or fierceness and sometimes to Penn State which is, of course, because of Penn State Logo. However, writing about Penn State’s connection with Lions is not what made me write this post. What brought lions, predators in general, to my attention is that we often hear stories or watch videos that are only about predators attacking or predating on humans or herbivores. So I asked myself if predators do really eat other predators and if yes, why is it not common and what are the reasons we do not hear about cases where predators eat each other, and finally whether there are any correlations or causation relationships. So I decided to write about this topic and do some research on the internet.
The null hypothesis being tested: Predators do not predate on other predators.
The Alternative hypothesis being tested: Predators do predate on other predators.
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According to a post written by Darren Naish, cases where predators predate upon other have been noticed by biologists and ecologists many times. This phenomenon was called intraguild predation by Polis et al (1989). An observational study on intraguild predation was conducted by Palomares & Caro (1999). They recorded 27 cases that were reported as intraguild predation cases and studied them. They noticed that the most common case among the other cases was the one in which adults predated on babies of the same species. However, that the study did not include an analysis of why predators would do that nor did it mention any information about the animals themselves, the atmosphere (i.e. if they are in a typical condition), or whether or notthey are starving at that time and had no other option but to predate on the younger one, as it known and natural for a starving animal to eat another animal from the same species in order to survive. In addition, I think that the number of cases recorded was very small. Furthermore, because the study did not say a lot about the what was measured, I cannot say if it suffers from the Texas Shooter Problem, however, the study got published, therefore the results do not suffer from the File Drawer Problem. In my opinion, I think that the findings of the study did not help me make a decision on whether or not I should reject the null hypothesis. So I decided to do more research about this matter.
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I found another article in an academic website called Wiley Online Library written by FABRIZIO SERGIO and FERNANDO HIRALDO, two scientists who decided to do a meta- analysis study on previous studies that focused on intraguild predation. The data they found was “for 39 experimental studies on 63 populations belonging to 11 killer species and 15 victim species.” After analyzing these studies, they found out that the results were almost consistent with the results of the study done by Palomares & Caro even.
The study results suggested that intraguild predation is very common in their natural habitants whenever external factors that might make predators predate on other predators are present, such as predation pressure which includes the habitant size in the experiment, the risk of predating the prey, and the number of predators and non-predators’ species in the experiment. Individuals of the prey species responded to predation pressure through direct spatial avoidance, risk-sensitive habitat. The study also suggests that there is a correlation between the number of killer (predators) and victim (herbivores) species in the provided space or habitant in an experiment. They noticed that the higher the number of herbivories in the experiment, the lower the number of intraguild predation cases recorded and vice versa.
While the study results can be due to chance, I think that the study was well- conducted and that it is highly unlikely that these study results are false negative, in other words, were due to chance or due to another confounding variable such as an odd biological variable in the animals being tested in the study that made them eat each other and that predators do not in fact predate on other predators, as the study is a controlled trial experiment, which is, as we learned in class, more reliable than an observational study. Also, not only did the researchers study use a big number of animals in the experiments, but they also done the experiments in different locations, which, as Andrew mentioned in class can eliminate any third variables that could appear in small or observational studies. And of course the study does not suffer the File Drawer Problem because it got published nor the Texas Shooter Problem, because the study is experimental and not observational.
I also think that the study should have focused on only one species so that they can more accurate and precise results, as it is known that an animal behavior significantly differs from one to another. In addition, studying one species can help them examine the effects that could results from the intraguild predation phenomenon among animals in that species. Other than this change in the study, I think that everything else was well-done.
Finally, the meta-analysis study results are consistent with the alternative hypothesis and show that predators do hunt each other; however, this phenomenon often happens because of predation pressure such as lack of food or the population size in their habitat and seldom happens in identical conditions, where the number of herbivories is significantly higher than the number of predators in a certain habitant. Furthermore, many well conducted meta-analysis studies were done by researchers that also showed consistency with the alternative hypothesis such as in an article named as the biological control: theory and practice, which supported the hypothesis and even provided evidence of why it happens and how it is actually a good thing!!
SERGIO, F. and HIRALDO, F. (2008), Intraguild predation in raptor assemblages: a review. Ibis, 150: 132–145. doi:10.1111/j.1474-919X.2008.00786.x
Lion vs Cheetahs
Lion vs Lion
biological control: theory and practice
Kindlmann, P. & Houdková, K. Popul Ecol (2006) 48: 317. doi:10.1007/s10144-006-0006-4