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Introduction
One interesting observation in landscape ecology is that natural environments occur heterogeneously or exhibit patchiness. This case study examined this hypothesis by evaluating the occurrences of trees and mistletoes in a large remnant area of natural vegetation in Australia.
Mistletoes are generally hemiparasites that feed on trees and shrubs. Therefore, this study provided an opportunity to examine the distribution of species and the ecological relationship between hosts and parasites. In Australian forests and woodlands, Mistletoes and host plants had a fascinating form of relationship in two main ways.
First, Mistletoes are hemiparasites that obtain their nutrients and water from their host plants. Second, a mutualistic relationship exists between the birds and the mistletoes. Specifically, the Mistletoebird is responsible for dispersing seeds of mistletoes while the mistletoe offers its fleshy fruits as food to the bird. In addition to the Mistletoebird, mistletoes also provide food and shelter to other various species of birds in Australian forests and woodlands.
Study Objectives
This study aimed to examine the distribution of the Drooping Mistletoe Amyema pendulum across an urban bushland landscape in relation to variation in vegetation type and the abundance of host tree species. Specifically, the study focused on the following questions:
- Does the density of trees (stems) differ between vegetation types in the study landscape?
- Does the proportion of trees with mistletoe infestation differ between vegetation types?
- What is the spatial pattern of mistletoe parasitism at the level of individual trees?
Study Area
This study was undertaken in the Royal Botanic Gardens, Cranbourne. This is a large area of heathland and heathy woodland at Cranbourne, Australia. In the past, the Royal Botanic Gardens has acted as a study area for many ecological studies, such as a floristic vegetation study, studies of the ecology of Swamp Rats and Southern Brown Bandicoots, resource partitioning by small mammals, the foraging ecology of New Holland Honeyeaters, and studies of frogs.
Study Methodology
There were many types of vegetation identified in the study area. However, this study concentrated on Heathy Woodland (drier vegetation on sandy areas) and Wet Heath (lower lying areas, drainage lines) areas as indicated on the map. There were two major species of trees found in these areas, which included Coast Manna Gum E. pryoriana and Mealy Stringybark E. cephalocarpa.
Stratification of transects
All data were gathered on a single field trip referred to as a snap shot survey. Transects were stratified across the landscape to sample Heathy Woodland and Wet Heath vegetation types. All student groups collected data from all the allocated transects in order to conduct a joint sampling of a number of replicate transects in every vegetation type. All these collected data were later cleaned, classified in classes and analyzed.
Researchers were cautious not to damage the vegetation in the study area.
Data collection
Each group collected data in transect measured at a 100 m by using a tape, which represented 0.4 ha. They collected data from a belt of 20 m wide from all sides of transects. Researchers also methodically gathered data along the transect line. This process involved checking and counting all tree stems, including cases in which a tree had more than one trunk.
Data elements recorded on the data sheet included:
- The total number of tree stems within the transect area
- For each stem that had mistletoes present, record details of
- The number of mistletoe plants present
- Whether the mistletoe plants were alive or dead
- The level of branch on which the mistletoe plant was located (primary, secondary, tertiary)
The Drooping Mistletoe Amyema pendulum was the most common type of mistletoe in the bushland under studies. Researchers observed the foliage of the mistletoe and noted the similarities and the differences between the leaves of the mistletoe and eucalypts. They also checked for any signs of dead trees and possible causes of identified cases and attachment of mistletoes on eucalypt branches.
Another area of interest in this study involved data related to variations across the landscape attributes, such as topography, soils, density of trees, moisture availability and vegetation types.
Discussion
Results from the study confirmed that hypothesis that mistletoe distribution was heterogeneous or depicted patchiness. Thus, the researchers accepted the study hypothesis. The significant difference in the distributions of mistletoes between the dry heath and wet heath showed patchiness in a natural environment.
Past studies have established such patchiness in distributions of mistletoes in Australia (Turner, 1991). Parkes, Newell, and Cheal observed that patches of native vegetation were often heterogeneous and could consist of other several species of plants (Parkes, Newell and Cheal, 2003). In addition, Turner, Clarke, Lewis, and Ostendorf observed variations in vegetation covers and noted that some areas were highly fragmented (Turner, Clarke, Lewis and Ostendorf, 2013).
Several factors could have influenced the distribution of mistletoes at the landscape scale and at the tree level. For example, abiotic factors, such as soils, water, light and biotic factors (dispersal vector) influenced the distribution of the Drooping Mistletoe across the landscape. From the study, several mistletoes were observed in the wet heath (1237) than in the dry heath (807). This indicated that abiotic factors had critical influences on the distribution of mistletoes in natural vegetation.
Some studies also noted that seed dispersal by marsupials altered the spatial distribution patterns of mistletoes (GarcÃa, RodrÃguez-Cabal and Amico, 2009). Thus, biotic factors affected the distribution patterns of mistletoes. Such mammals were responsible for spatial distribution and patchiness in mistletoe offspring at a larger scale (GarcÃa et al, 2009).
Therefore, both abiotic and biotic factors created distribution and dispersal processes that could significantly change the mistletoe spatial distribution in natural vegetation. A mutualistic relationship was observed between mistletoes and other animals. They depended on animals for both pollination and fruit dispersal, and this leads to a broad range of mistletoe-animal interactions (Watson, 2001).
Thus, fruit eating birds, such as Mistletoebirds were responsible for wide dispersion of mistletoe seeds while the preference of birds for infected trees influenced the spread of mistletoes and the spatiotemporal pattern formation of mistletoes (Ward and Paton, 2007). Overall, Watson noted that mistletoes were important resources in forests and woodlands where they were present (Watson, 2001). They offered shelter for both animals and birds in woodlands and forests.
Variations in mistletoe distributions were also observed in the stems of the tree. For instance, primary branches had 10 mistletoes, secondary branches had 135 mistletoes and tertiary branches had 137 mistletoes. Mistletoes survived better in protected areas of the trees such as secondary and tertiary branches. Hence, the use of hosts among mistletoes was also heterogeneously distributed.
In conclusion, this study supported previous observations, which had indicated that vegetations had heterogeneous occurrences in natural environments. The distribution of mistletoes differed significantly in dry and wet heaths and on stems of trees. Variations could have resulted from both abiotic and biotic factors. Overall, mistletoes acted as important resources in their natural habitats.
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