Lambir water strider project

From BiodivBorneo09

Intraspecific density dependent variation in water strider territory

Jessica Lavash, Mindy Tuan, and Alexandra Torres

Abstract

This observational study measured the relationship between population density and individual territory size within one species of water strider. Data was collected in situ by measuring individual body size, mapping territory, estimating density, and analyzing the data with a statistical correlation test. There was a positive correlation between body size and territory in dense populations. Reduced territory size due to density was not observed, but with improved sampling methods and further study this relationship may become apparent.

(See summary of argument)

Introduction

Individuals within a population will have varying body sizes, and body size influences territory size. Larger individuals may be more dominant or more physically able to patrol larger spaces. An individual’s territory size may also be influenced by intraspecific competition within a population. If space is a limiting resource, individuals may compete with conspecifics for territory space. In a less densely populated habitat, space is not as limited and therefore individual territory sizes may be larger. Thus, determining if population density restricts territory size, estimated by a positive correlation between body size and territory, is necessary in understanding dominance behavior and social structure within the species.

Question

Does density of water strider populations restrict the territory size of an individual?

Methods

Field Methods

• Data was collected during three days.
• Along the same stream, five sites hosting our selected species were chosen to eliminate pseudoreplication. Over the course of data collection, the stream changed width due to rains: one site had a stream width of 1.5 meters, two with a width of 4 meters, and two with a width of approximately 6 meters. This change in environmental conditions meant that certain aspects of strider populations were different each day. Population density was observed to have an inverse relationship with stream width, so stream width was used as a measure of strider density.
• Two observers mapped the movement of each strider on graph paper independently for a timed interval of 1 minute. This was done to collect a mean estimate of the territory of each target strider as well as control for individual bias.
• Observers also noted whether an intraspecific interaction occurred, and counted the number of other conspecifics within a 50cm radius around the targeted individual to measure territory boundaries and to obtain an additional measure of population density.
• After the area was calculated, the targeted water strider was caught with a small fish net and measured from the eyes lengthwise to the end of the abdomen.
• Striders were immediately released back into its habitat after measurements were recorded. This was done to reduce impact on the density and social behavior of other striders within the plot.
• Total sample size collected in the study was 45 water striders.

Analysis

• Our statistical hypothesis is that an increase in water strider population density causes individual territory size to decrease. Our null hypothesis is that density is not significantly correlated to individual territory size.
• Analysis of data was done using the statistical software “R” to test our hypothesis with Pearson’s product-moment correlation test. Boxplots and general linear models were also constructed to examine significance.

Results

Our research illustrates the following findings:

1. At high population densities, there is a significant correlation between individual body size and territory size (Figure 1). At medium and lower population densities, this correlation was not observed (Table 1).
2. A significant reduction in territory size was not observed when an individual interacted with a conspecific.
3. A significant reduction in territory size was not observed regardless of how many conspecifics were within a 50cm radius of its territory.

Discussion

Contrary to our original hypothesis, no significant change in territory size was observed in relation to population density. This could be due to error in sampling: it was easy to map the area when a strider was actively patrolling its territory, but if a strider was foraging or resting, its territory might not have been accurately mapped. Further studies would require different methods of territory mapping and could be improved upon with manipulation studies to get a better delineation of territorial borders and a better understanding of behavioral changes of other striders in response to caught-and-released striders.

However, we were able to observe a significant relationship between territory size and body size when the river was low. After a strong rainstorm passed, the river level changed dramatically and increased the habitat size from a width of 1.5 meters to a width of more than 6 meters. Consequently, the surface area on the river increased and presumably the population of striders remained the same. This means that population density in a given area would be reduced as they would be more spread out. The following day, there was slight rain, and the river width was 4 meters. When the river level was medium, the positive relationship was not observed, and when the river level was at the highest level, the correlation was even less significant. This might be explained by the strider behavior that we observed each day. It appeared that in denser populations, striders were actively patrolling and defending their territorial boundaries, while in less dense populations, striders were not observed to be patrolling and defending as actively, instead spending their time foraging and resting for majority of the time that we were there. This could explain why we didn’t observe a correlation between body size and territory size in less dense populations.

Further studies could include determining what factors influence territory choice, examining interspecific interactions and influences, as well as determining differences in behavior between water striders of the same species in different size classes. Video techniques could also be used to strengthen the precision of territory area measurements.

References

• Hill, D. S. & Abang, F. (2005), The Insects of Borneo (Including South East & East Asia), Universiti Malaysia Sarawak, Kota Samarahan, Sarawak, Malaysia.

p=0.736805, r=-0.06288756