by Abelardo Correa, Evelyn Erickson, Amy O’Donnell, Steven Zbaida
 

INTRODUCTION:

The relationship between monocultures and herbivory is important for many reasons. With the advent of agriculture, people have been cultivating crops and transforming habitats to support a growing population.  Traditionally, dense populations of the same species, have been vulnerable to plagues of insects and disease.  In spite of this monocultures are found in nature.  Presumably, these situations are successful because both plants and predators have evolved together, arriving at this high-density situation.

In disturbed areas, such as the cuffs of roads and edges of settlements, early successional species allocate more energy to reproduction then to defense.  Subsequently, these species receive heavy pressure from a large host of general feeders.  Regardless, such species may be very successful, swelling in number and creating large stands.

This study focuses on the relationship between density and the level of herbivory in Florecilla (Baltimora recta), a successional plant found along the roadsides in Palo Verde.  The hypothesis is that the level of predation would increase with plant density.  Additionally,  the study examines biodiversity in relation to density and herbivory.
 
 

METHODOLOGY:

Three levels of density were identified:  low( 1 – 5 plants), medium (10 – 25 plants), and high ( 50 or more plants). Three study plots from each density area were marked.  On day one, twenty leaves from each of the 9 sites were randomly collected.

  An overhead projector was used in order to determine if  the various sizes of leaves were proportional. Three different size leaves were chosen. The area of each leaf was compared to the area of a rectangle circumscribing it.  The leaves and rectangles were traced on graph paper and individual boxes were counted in order to find the average area occupied by a leaf.  The average area occupied by a leaf was calculated (Z-computed as 0.6).  The following formula was used In order to calculate  the area of each of  the leaves collected in the study: Length x Width x Z. Two people estimated the level of leaf damage and an average was recorded.
 
The afternoon of day one, six of the nine sites were swept 10 times with an insect net.  Insects were soaked in gasoline and brought back to the research station.  The following afternoon all remaining sites were swept at the same time of day as the previous collections. Insects were classified into two categories:  herbivores and predators/pollinators.  All samples were alpha indexed and tallied.  A distinction was made between biodiversity and abundance.
 
 

RESULTS:

Graph one (Percent Predation) shows a significant difference in leaf damage between the low and high-density plots.  Though, no significant difference was found between the medium and high plots.  Biodiversity remained constant between low, medium, and high-density plots.  However, insect abundance was approximately the same between low and medium density areas.  Abundance in the high-density plot was nearly twice the level found in low and medium plots (see graph two).
 
 

DISCUSSION:

There is an apparent discrepancy between the level of damage detected in the area of lower density and the abundance of species at the site.  However, this may be easily explained by the fact that leaf collection in our project represents an integrated index of damage over a long period of time, whereas the insects collected represents a snap-shot in time.  The type and number of insects present at the site may vary during different times of day and during different seasons. It is possible that the insects collected were not the ones responsible for the damage observed.  An effort was made in order to find areas that met our definition of high, medium, and low density.  However, at times, areas of medium damage were hard to define.  They were usually part of a continuous area of high density that was naturally broken into smaller segments.  Therefore, pseudo-replication may have effected the computed results.  Despite the fact that an effort was made to sweep an area that contained only the Florecilla, it was impossible to find areas in which it was completely isolated.  Therefore, a portion of the insect specimens may have inadvertently been gathered from neighboring vegetation.

Our results indicate that there is a significant difference between the level of damage in high and low-density areas.  Contrary to our hypothesis, we find a possible indirect relationship between the density and the level of damage.  Further studies are necessary in order to establish this relationship.  Here are some possible explanations for this phenomenon:

1. As insects fly they may stand a higher chance of landing on individual sites of low density, due to the increased surface area, and thus, cause a higher level of damage.

2. Insects move to areas of higher density once they consume a significant amount of vegetation in an area that has low density.

The results of the study indicate that 34 different species were collected.  This study acknowledges that a higher level of biodiversity may be present that went undetected because of instrumentation or technique.  The consistent level of biodiversity found might be predicted since all plots were located in the same general area.  Despite the fact that this plant grows in a disturbed area it provides resources for a wide variety of species.
 
 

ACKNOWLEDGEMENTS:

 Jim Cole and Ted Stiles for all their help, support, and expertise.