Effects of Food Availability on Population Density; a Long-term Study of Wild Great Reed Warblers (Acrocephalus arundinaceus)

Abstract

Food availability is known to influence animal distribution and abundance, playing a crucial role in population dynamics. However, this correlation remains somewhat unclear due to interactions with several other factors such as climate, predation pressure, competition, or even human influence. Therefore, understanding the mechanisms underlying these processes is crucial for ecologists to effectively conserve wildlife populations and predict potential environmental effects on biodiversity.

This study examines the impact of food availability on the density of breeding Great Reed Warblers across two spatial scales in the natural reserve of Kvismaren. Key findings reveal a significant correlation and parallel fluctuations between annual emerging insect biomass and the number of breeding bird Great Reed Warblers at the landscape scale. This suggests that the availability of highly energetic prey could potentially limit bird populations. However, territory choice is a complex process, and the effect of this correlation was not as pronounced at the local scale. This indicates that other factors may also influence the birds’ habitat selection behaviour.

In terms of climate, temperature does not appear to significantly affect the number of invertebrates in the lakes over the years. However, Kvismaren is characterized by human-induced changes and disturbances, which might influence variations in insect abundance and wetland quality, thereby indirectly affecting bird populations.

The results of this study, together with existing literature, suggest a strong relation between insect biomass and bird numbers. Although food availability interacts with other factors in shaping populations, it still has a significant impact on the overall suitability of a wetland, provides insights into its quality and influences bird population sizes.

Popular Abstract

Insect availability affect the abundance of marshland songbirds

In a world where resources are limited, birds must adapt and manage to survive according to the characteristics of the habitat they live in. The Great Reed Warblers (GRW), inhabitants of the marshland take constitute the Kvismaren nature reserve, are no exception. These birds need to consider several factors when selecting their breeding sites, and they are expected to chooe the area that maximises theirproduction and quality of offspring. However, understanding the mechanisms affecting selection processes is not simple due to a complex interplay of many different factors that can affect bird behaviour, like weather, predation pressure, food abundance and human activities (e.g. use of pesticides, habitat destruction).

Food availability is well known to be an important factor, vital for population growth, that can directly affect the distribution and survival of individuals among populations of birds. GRW rely heavily on the biomass of large water-living insects that live in the reed beds they breed in, which serves as their primary food source. Consequently, birds would be expected to choose high-quality areas (e.g. habitats with more food availability) to gain more benefits. Therefore, when the insect populations decline, bird numbers should decline as well, and therefore (strong) positive correlations between predators and prey can be predicted.

To understand if this is the case for the Great Reed Warblers, to assess the impact of food availability on bird populations, and to gain insights into habitat selection processes, we conducted a study where we examined the relationship between the abundance of insects and the number of breeding GRW over a period spanning 24 years, this allowed me to evaluate how food availability may limit the abundance of this bird population and if there also can be be other factors that indirectly limit bird and insect populations.

I found a significant positive correlation between the number of breeding GRW and the abundance of emerging insects in the whole wetland area of Kvismaren. More birds decided to breed in the wetland in years with higher food availability. In addition, my analyses suggested that the GRW appeared to take previous experience into account when choosing whether to breed or not in the wetland, thus not only relying on knowledge of the food abundance the current year. This suggests that tradition and experience play an important role in decisions of where to breed in GRW, and such a behaviour would increase the likelihood of returning and breeding again in the same area if the previous year was rich in terms of food availability.

I also tried to investigate the positive relationship between insect and bird abundance at a smaller spatial scale, focusing on the different small lakes (marshes) that form the wetland of Kvismaren. Within the lakes over the study period, no strong correlations were found, suggesting that food availability might limit the birds but only at the larger, whole wetland, scale. Birds can seek food outside their territories, and move between lakes, but within the wetland. Therefore, the choice of territory may be influenced by factors other than insect abundance that may be more important at the time when they select the specific breeding site within the wetland.

Furthermore, it is known that some insects are sensitive to pollution and quality changes in their habitat. Anoxic events or increases in the nitrogen content of a wetland can pose stress on them. Given their crucial role in wetlands and birds' diet, these stressors can have detrimental effects on their abundance, which can then indirectly affect bird populations. Kvismaren has always been associated with agricultural practices, and over the years, it has experienced the intensification of agriculture. Land conversion, overuse of pesticides, and the use or diversion of water have a significant impact on the health of a wetland and its inhabitants which have led to losses in Kvismaren’s biodiversity.

In this context, it is interesting that my results showed that dragonflies and damselflies were the insects that had the largest effect on the number of breeding birds. Dragonflies and damselflies are known to be reliable bioindicators of an area’s quality. Their presence and abundance can provide valuable insights into the health of the lake and the quality of its water. This allows us to gain insights into the quality of Kvismaren and suggests that years with superior water quality may have attracted more birds to breed. Such information should be crucial for wetland ecologists and managers, because it provides valuable guidance for the implementation of effective conservation strategies.

Ultimately, the study reveals that bird populations, such as the Great Reed Warblers in the Kvismaren nature reserve, are influenced by a multitude of factors. These include the quality of their habitat, the impact of human activities or even their own past experiences. While food availability, especially insect biomass, plays a crucial role in the birds’ breeding decisions and success, it is just one component in a complex ecological equation. The dynamics of bird populations over time reflect the interaction of these various factors, emphasizing the complex balance of wildlife and the flexibility of species in response to different environmental changes. I also showed that territory selection behaviour is scale-dependent, and that we can find differences depending on the perspective we choose.

My research not only sheds light on the life of the Great Reed Warblers but also underscores the importance of maintaining biodiversity and ecological balance in our natural reserves.

Master’s Degree Project in Conservation Biology, 45 credits 2024
Department of Biology, Lund University

Advisors: Maja Tarka, Dennis Hasselquist, Javier Pineda Pampliega
Department of Biology, Lund University

• author: Jiménez Ruiz, Marina
• supervisor: Maja Tarka; Dennis Hasselquist; Javier Pineda Pampliega
• organization: Degree Projects in Biology
• course: BION02 20231
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9149294
• date added to LUP: 2024-04-02 14:12:06
• date last changed: 2024-04-02 14:12:06