Lambir lizard and insect abundance project

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Nocturnal Lizard and Insect Abundance in a Lowland Mixed Dipterocarp Forest

Chris Anderson and Rachel Hawkins

Contents

Abstract

Lizards are both a charismatic and crucial part of a tropical ecosystem, serving as predators and prey in their habitats. They are ubiquitous in lowland dipterocarp forests and feed upon many small insects and arthropods. Their distribution within the larger scale of the forest may be influenced by the availability of prey items. In this study, we examine whether there is a correlation between areas in which lizards have been observed and the amount of insect abundance, diversity, and frequencies of favored prey in a lowland mixed dipterocarp forest (Lambir Hills National Park, Sarawak, Malaysia). Our results show that although there may be a weak correlation between insect abundance and lizard absence, there does appear to be a relationship between site similarity and overall insect composition. We suggest that insect composition could be a topic for further study and that it may play more of a role in lizard distribution than was initially suspected.

Introduction

Figure 1: Gonocephalus grandis, territorial lizard

A healthy ecosystem has wide-ranging interactions between organisms, and the predator-prey dynamic is among them, often shaping where organisms dwell and An analysis of these dynamics call tell us about how animals distribute themselves within their macrohabitats, an area of ongoing study which is a long-standing ecological conundrum. In our study, we focus on lizards and their interactions with their potential food sources as a factor that may influence their distribution.

Lizards and geckos (which hereafter are referred to as “lizards”) are an important part of the rainforest ecosystem, playing a role as both predators and prey to many other animals. Their habitats are wide-ranging, varying from human dwellings to primary forest. They feed primarily upon small insects and arachnids, such as ants, grasshoppers, and spiders, and occasionally feed upon other geckos. It has been observed in the field by the authors that some lizards appear to be territorial, often remaining on the same tree or branch multiple nights in a row (see Figure 1).

Question

  • Is there a correlation between areas in which lizards can be found nocturnally and the amount of insect abundance, insect diversity, ant frequency, orthopteran frequency, or spider frequency found in that area?

Methods

Field methods

We went out into the field on a main trail leading to the Latak Waterfall from 2300 to 2400 on two nights (8/6/2010 and 10/6/2010) on the main trail and look for lizards. We marked 5 sites where lizards were observed and attempted to photograph the animals.

In addition to the lizard-observed plots, we also randomly chose an equal number of sites in which no lizards were observed and performed the same vigorous process and collected the insects. We used a random method to determine these sites. Random numbers were generated to determine the number of steps taken into the forest to the site from the beginning of the trail, and whether the number was even or odd determined the left or right side of the trail. The 2m by 2m area centered directly in front of the walker off the trail was the area for sampling. All random sites were sufficiently similar to the sites where lizards were found (brush, small saplings, and trees present) and were not within 5 meters of another site.

We returned to the sites of the nocturnally observed lizards and the random sites at night (2300 to 0000) to test for insects. We swept each site with a small butterfly net in an area of 2m x 2m for 16 vigorous sweeps through the brush and small saplings, a modification of the method used by Sodhi (2002). We swept from 1m high to the ground level through the vegetation. We repeated the process twice at each site (once per night) and collected the insects from each site. We visited the sites in the same order each night to ensure that each was visited at approximately the same time.

We froze the samples from each site for at least 3h in order to immobilize the insects. We hand-sorted the samples and identified the arthropods to order, recording the numbers of each.

Analysis

Table 1: Table of Findings

For our analysis, we used the statistical analysis platform R to examine our data (see Table 1). We used a poisson-family general linear model (glm) to see if the presence or absence of a lizard was correlated with 5 sets of data: gross insect abundance, order diversity, ant frequency, orthopteran frequency, and spider frequency.

We also created a dendrogram within R using the bray method for calculating similarity to visually analyze the insect composition similarity of each site (see Figure 2).
Figure 2: Figure showing similarity between sample sites

Results

In examining gross abundance, we found a p-value of 0.0613. This result, while not strictly significant, might suggest a correlation between lizard distribution and gross insect abundance. Specifically, it could suggest that in areas where lizards are absent, there is higher insect abundance (see Figure 3).
Figure 3: Figure showing correlation between insect abundance and lizard presence

In examining insect order diversity, we found a p-value of 0.857. This result is not significant, suggesting no correlation between lizard distribution and insect diversity. The insect diversity seems to be homogenous throughout the sites we sampled. We did not reject the null hypothesis (no correlation between lizard distribution and insect diversity).

In examining ant frequency, we found a p-value of 0.6. This result is not significant, suggesting no correlation between lizard distribution and ant frequency. Ant frequencies seem to be homogenous throughout our sampled sites. We did not reject the null hypothesis (no correlation between lizard distribution and ant frequency).

In examining orthopteran frequencies, we found that our data set was not robust enough to support a valid glm test. We decided not to consider the orthopteran frequencies as part of our analysis.

In examining spider frequencies, we found a p-value of 0.8186. This result is not significant, suggesting no correlation between lizard distribution and spider frequencies. Spider frequencies seem to be homogenous throughout our sampled sites. We did not reject the null hypothesis (no correlation between lizard distribution and spider frequencies).

Our dendrogram reveals that sites with lizards were generally more similar to other lizard sites with respect to insect composition. The same holds true for sites without lizards (see Figure 2).

Discussion

While our overall results were not highly significant, this can partly be attributed to the low number of sample sites which we were able to consider. Considering more sites will potentially yield stronger results based on more lizard habitats. The limited time which we had to conduct the experiment also gave our data a weakness. More data over a longer period of time, through different environmental seasons (dry and rainy) and biological seasons (mating vs. non-mating) will perhaps lend more robust data results. Different types of sampling (Burlese funnels, tree-bark fogging, and/or pitfall traps) might also yield a more comprehensive ensemble of insects. Another factor which may have skewed our data was a heavy downpour right before sampling on Jun 9 2010, which may have suppressed the true number of insects per site.

The p-value of the correlation between lizard absence and gross number of insects is especially interesting, given its closeness (0.0613) to a significant value. While it does not fall under the standard 0.05 value, it does suggest that there could be a relationship between these two factors. This result opens up this question to more comprehensive, thorough investigation which may indeed yield a significant result. Our low number of sample sites was able to yield this low p-value; more sites (and thus a higher power of analysis) would clear up this ambiguity. A relationship between lizard absence and gross number of insects can be explained by the fact that lizards feed upon small insects and subsequently keep their populations in check in areas where they are present. This is a topic for further study.

The dendrogram is perhaps the strongest result from this study, suggesting that while there does not appear to be an overwhelming correlation between insect abundance or diversity and lizard presence, there does appear to be a similarity of insect composition between similar sites. Perhaps in examining the types of habitats which lizards prefer, insect composition, rather than abundance or diversity, should be a topic of further investigation.

References

  • Das, Indraneil. A Photographic Guide to Snakes and Other Reptiles of Borneo. New Holland: London, 2006.
  • Sodhi, N S. (2002). The effects of food-supply on Southeast Asian forest birds. Ornithological Science 1: 89:93.

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