In the twentieth century, improvements in public health infrastructure and medicine led to a rapid rise in our population size. The global population is expected to grow by another two billion over the next 60 years. Changes in population size and how densely people are living in their environment are crucial for understanding the ecology and evolution of any species. When people live close together, resources run low, and diseases spread faster, slowing population growth. Studying what causes crowding to impact population growth is important as it helps manage resources and keep community health in check.
A recent study from Deepa Agashe’s lab at NCBS used red flour beetles as a model to uncover how population density shapes reproductive success and survival. The research explored how the number of beetles in an area influences their egg-laying patterns and the survival of their offspring. By examining whether survival or reproduction plays a bigger role in driving changes to population growth under crowded conditions across various environments, the study offers insights into how population density impacts overall growth.
The research team used red flour beetles collected from various parts of India, which had been growing in the lab for over five years. These beetles, notorious pests of stored grains, were primarily raised on wheat, their ancestral habitat. Conducting the study required an extensive experimental setup to manage the beetle populations.“I was working with 60,000 red flour beetle adults. I had an entire incubator full of large boxes of Tribolium (beetle) populations to maintain,” says Shyamsunder Buddh, lead author of the study. Using this setup, the team investigated how these beetles responded to four different types of flour: wheat, corn, finger millet, and sorghum.
Beetles collected from the same site, sharing the same evolutionary history and lineage, were considered a source population. The experiments used their lab-raised descendants. Combining experimental and modelling approaches, the researchers examined how different population densities across different source populations and habitats influenced overall population growth. Shyam placed adults into boxes with different amounts of flour, adjusting the number of beetles per gram of flour to manipulate population density. Once these individuals laid eggs, he let the offspring develop in the same flour for a couple of weeks. The results suggested that population growth was influenced by habitat type, and not by where the beetles were from.
“Reproduction rate and survival are the key ingredients that shape how fast a population grows or shrinks. If you think about it, a population size only changes when there are shifts in births or deaths, right? This way of breaking down population growth is simple yet super powerful—it works in all sorts of situations. Both of these factors can be influenced when the population becomes crowded, as competition for resources like food, space, or other necessities increases.”, Shyam said. The team found that the growth rate of flour beetles was more strongly impacted by how crowding influenced their survival than by changes in their reproduction rate. This pattern was consistent across all five flour habitats.
Shyam developed a population dynamics model for each habitat, using demographic data to create equations that described changes over time. This approach allowed him to link the effects of crowding on survival and reproduction to overall population growth. "Nonlinear dynamical systems often exhibit very complex behaviour. So it's always exciting when a careful experiment uncovers a simple quantitative pattern like the density-dependent behaviour of these beetles. We hope the mathematical models we built based on the data will generate interesting hypotheses about the consequences of this nonlinear density dependence on the population dynamics and evolution, and suggest further experiments to test these hypotheses", said Sandeep Krishna, Professor at NCBS, and a collaborator on this study.
“Ecologists know that many factors are critical for predicting population growth, but clear evidence on which of these are most important is rare. This experiment showed that, despite genetic and trait variation, flour beetle populations respond similarly to increasing density in the same habitat. So, understanding one population can help predict how others will grow, which is reassuring”, says Dr Deepa Agashe, principal investigator of the study.
“We found that while female beetles lay fewer eggs under crowding, egg survival is far more crucial for population growth. The impact of crowding on egg survival varies across habitats, making it key for population growth predictions. We suspect that these results will be similar for other species with similar ecology and life history, such as other pests with very high and fast reproduction rates. Our work suggests that reducing egg survival may be more effective for pest control than altering female fecundity”, she added.
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