The Effect of Shrub Layer Composition on Bird Abundance and Species Richness in Revegetation plantings in Hilltops NSW

The Effect of Shrub Layer Composition on Bird Abundance and Species Richness in Revegetation plantings in Hilltops NSW

Hannah Southwell – Boorowa Central School

Abstract

The intensification of agriculture in the Hilltops region in NSW, has led to the decline in bird populations. Revegetation plantings are one common strategy to deal with this crisis. This study examines how the composition of the shrub layer within revegetation plantings affects species richness and abundance of birds. Vegetation and bird surveys were conducted at 10 revegetation plantings on farms in the Hilltops local government area. The hypothesis that plantings with ahigh shrub density would also have high bird abundance and species richness, was rejected. However, other trends were observed in the data; from these trends, it can be concluded that revegetation plantings in agricultural regions are an indispensable source of habitat for birds and the use of them is highly valuable to conservation efforts. The value of these plantings to birds increases with increasing vegetation density, and more specifically, tree density. It is therefore important to maintain existing areas of established and mature native trees for the benefit of birds.

Literature Review

agricultural landscapes “plantings in general can provide habitat for many species of birds” (Munro et.al., 2011, p. 223). This is further supported by surveys conducted by Greening Australia researchers at a farm in the Hilltops region. Surveys of the native vegetation and birds were conducted several times between 1980 and 2019. They found that as the areas of native vegetation increase due to revegetation, so did the number of birds observed in the surveys. (Hosking and Thackway, 2019). This confirms the importance of revegetation plantings as a strategy for the conservation of bird species in agricultural landscapes. Whilst revegetation plantings are beneficial to birds overall, many factors affect the success of a revegetation planting as bird habitat. Haslem et. al. (2021) found strong evidence that the human management of revegetation plantings strongly influences its suitability as a bird habitat. It is, therefore, crucial that land managers have a well developed understanding of how to best manage factors of plantings that benefit bird species. One such factor is the structure of a revegetation planting, multiple studies (Lindenmayer et al., 2018: Munroet. al., 2011) have found a correlation between the structure of a planting and its suitability for bird habitat. Lindenmayer et al. (2018) found, after conducting bird surveys at 61 sites, that an increase in both the size of a remnant woodland and the amount of understorey within it will positively affect birds. However, they also discovered that increasing the amount of understorey has a negative effect on Noisy Miners (Manorina melanocephala). As Noisy miners are aggressive, their presence often leads to a decrease in other bird species. Munroet. al., 2011 focused on bird species richness and composition in

The Action Plan for Australian Birds 2020 recognises 1 in 6 Australian bird species as nationally threatened in accordance with the IUCN Red List criteria. (Birdlife International,2022). One of the major factors contributing to this is a loss of habitat resulting from the clearing of native vegetation in agricultural areas (Stevens, 2001). Within the Hilltops Local Government area, over 80% of the native vegetation has been cleared for agriculture. (Reid, 2000; NSW National Parks and Wildlife Service, 2002) with the National Parks and Wildlife Service describing the state of the vegetation in the Hilltops region as ‘perilous’. (NSW National Parks and Wildlife Service, 2002, p. v) One of the most common restoration efforts in the Hilltops area is farm revegetation plantings. They can be seen on properties throughout the region and include areas of managed remnant vegetation as well as planted natives. Research has been conducted to determine the suitability of revegetation plantings for bird habitat. A key focus of this research is the comparative value of remnant native vegetation and revegetation plantings to birds. Cunningham et al. found that “remnant native vegetation on farms is critical for many declining bird species” (Cunningham et al.,2008, p. 750) and that whilst farm plantings may offset the loss of remnant vegetation for some species, overall the existing native vegetation is more important for species increase (Cunningham et al., 2008) .However, whilst it is clear that revegetation plantings on farms are not an adequate substitute for remnant vegetation, further research indicates that in heavily modified four different environments: remnant forest, cleared agricultural land, woodlot plantings (containing only native trees) and ecological plantings (containing a wide variety of trees, shrubs and understory plants). The ecological plantings were found to have a larger diversity of bird species than the woodlot plantings (Munro et. al., 2011).

Additionally, birds found in the ecological plantings were “shrub-associated birds” (Munro et. al., 2011, p. 223) compared to the “generalist bird species” (Munro et. al., 2011, p. 223) found in the woodlot plantings.

In relation to the specific types of birds benefitted by revegetation plantings; Lindenmayer et. al. (2018) examined the changes to bird species in old-growth woodland, regrowth woodland and restoration plantings over 13 years. They observed an overall increase in smallbodied species of birds particularly in restoration plantings (Lindenmayer et. al., 2018). A substantial amount of research has demonstrated the extensive value of revegetation plantings for bird abundance and diversity. The characteristics of plantings that benefit birds are still being investigated. However, the general theme of the literature suggests that having more ‘shrub’ species in the understorey of a revegetation planting may lead to increased bird diversity. Although further research is clearly required to confirm these findings.

Scientific Research Question

How does the composition of the shrub layer in revegetation plantings affect the abundance and species richness of birds in Hilltops NSW?

Scientific Hypothesis

Within revegetation plantings, the density of the shrub layer will have a strong positive correlation with bird abundance and species richness. The literature suggests that a greater percentage of the population will be small-bodied birds because these species rely on native plants for food and shelter from predators

Methodology

Plantings

10 revegetation plantings were located within the Hilltops local government area. To ensure validity, a revegetation planting was defined as; an area of less than 2 hectares consisting primarily of native vegetation, that includes some tree species and has been managed (planted, fenced off or protected in some way) by the land owner or by an organisation on the land owner’s behalf. The area may include existing woodland remnants but is isolated from large areas of scrub. All sites were a minimum distance of 100m from all other sites to ensure independent and valid results.

Table 1- The revegetation plantings used in this experiment.

Vegetation Data

For each revegetation planting two researchers collaborated to collect the vegetation data. The length and width of the planting was measured using a trundle wheel and Google maps. This was practical and accurate to the nearest metre. Then the number of alive and dead shrubs, trees and thickets inside the planting were counted. A shrub was defined as a plant that is between 1 and 3 m tall and at least 50cm wide (with fairly dense foliage). Any plant taller than a shrub was recorded as a tree. A thicket was defined as an area of shrubs that are growing so close together that their individual stems cannot be counted and their leaves form a single canopy. A horticulturalist was consulted to create a list of the most common native plants found in each planting and any introduced plant species.

Bird Survey

At each site a researcher used the Birdata app to record a survey using the 20 minute search method (sensu Loyn, 1986). This method was chosen because of its validity to the objectives of the study. Any bird species that were seen or heard in the 20 minute period were recorded directly into Birdata while in the field. Birds that flew over the area were only counted if they were obviously using the area (eg. Birds of prey, hunting in the planting). Bird’s that could not be identified were listed as unknown species so as not to affect the accuracy. All surveys were conducted during the day and the permission of the landowner was obtained before entering the property. Over the next 2months, 4 surveys were recorded at each site. Surveys were not conducted in high wind, rain or fog as these conditions can affect results (Field et. al, 2002). A safety induction was completed at each property to ensure the safety of the researchers.

Analysis

Following the collection of vegetation data, shrubs per hectare, trees per hectare and plants per hectare. The results of the experiment were analysed by creating scatter plot graphs and calculating the Pearson’s Correlation coefficient. This allowed the correlations between variables to be clearly seen. The Birdata (Birdlife, 2022) website was used to create a polygon of the Hilltops area. The website contained all of the data from bird observations submitted to Birdlife Australia, which is a highly reliable source. All species were then classified into the following categories: water birds, ground birds, birds of prey, small-bodied birds and large-bodied birds. This was compared to the data from the experiment using a Chi-squared test.

Results

Figure 1- Graph representing the correlation between the average number of individual birds observed in each planting (abundance) and the number of shrubs per hectare. There is a very weak linear correlation between the two variables.

Figure 2- Graph representing the correlation between the average number of bird species observed in each planting (diversity) and the number of shrubs per hectare. There is a very weak linear correlation between the two variables.

Figure 3- Graph showing the correlation between the percentage of observations that were small-bodied bird species and the number of shrubs per hectare. There is a very weak linear correlation between the two variables.

Table 2-The statistical results of the experiment, all of the results are very low, indicating that there is only a very weak correlation between variable

Discussion

This experiment examined the effect of shrub density on bird abundance and species richness in revegetation plantings in the agricultural region of Hilltops, NSW.

The revegetation plantings (appendix 1) chosen represented a wide variety of different planting structures, although some were similar. Planting 6 had the highest total number of plants and also had a comparatively high number of dead trees and shrubs. Only a few of the plantings contained thickets, however those that did tended to have a high number of them. These included plantings3, 6, 7 and 10. Planting 4 consisted of eucalypt trees with only 5 shrubs counted. The most common species across all plantings were eucalyptus and acacia species. Callistemons, grevilleas and hakeas were also counted in smaller numbers. Plantings located closer together did not necessarily contain similar vegetation, demonstrating the effect of human management.

The highest number of birds was observed in planting 7 whilst planting 2 displayed a very low abundance of birds (appendix 2). A very similar number of birds was found in plantings1 and 8. No water birds were seen during the surveys and only 2 ground birds were observed. 3 birds of prey were seen and all other birds were in the small-bodied bird or large-bodied bird categories. The highest number of species observed in one planting over the 4 surveys was 18 in planting 3 (appendix 2). Plantings 2 and 7 both had the lowest species richness of 7 species. Planting 8 had the highest percentage of small-bodied bird species and also the lowest percentage of largebodied bird species.

When bird abundance and species richness were graphed against shrubs per hectare, the correlation was positive but extremely low (Figures 2 and 3). Interestingly the correlation coefficient was the same for both graphs. This may indicate a relationship between bird abundance and species richness. The correlation coefficient for the percentage of small birds and the abundance and species richness of small birds was higher, indicating as lightly stronger correlation, although still not significant. Accordingly, the null hypothesis must be accepted and the alternate hypothesis rejected. It is therefore concluded that there is no statistically significant difference between the density of shrubs in a revegetation planting and the diversity and abundance of birds observed there. This contradicts the common findings in the literature.

Limitations

The most significant source of error in this investigation was in the definition and classification of both plants and birds. For this experiment, shrubs were defined as ‘a plant that is between 1 and 3 m tall and at least 50cm wide (with fairly dense foliage)”. Any plant taller than a shrub was recorded as a tree. The purpose of classifying the vegetation in this way was to isolate the different structures within revegetation plantings in order to relate patterns in bird observations to one specific type of plant. However, identifying a plant as either a shrub or a tree was still dependent on the discretion of the observer. This may have impacted the accuracy of this investigation. Furthermore, ‘trees’ encompassed both mature eucalypts and younger trees which have very different ecological functions. In future experiments, different methods of quantifying vegetation density should be explored. For example a habitat complexity score (Tay,2019).

Birds were also classified for this investigation. This was done based roughly on their size, taxonomic group and ecological niche. However this investigation would be more valid if the birds had been grouped using a more systematic approach. For example, if all birds with a wingspan less than 20cm were listed as small-bodied birds. Finally, this investigation would be more reliable if more bird surveys were conducted, although this would be less practical and achievable.

Other Findings

Whilst no correlation was observed between shrub cover and bird abundance and species richness, there were other patterns in the data collected. As can be seen in Figure 5, there was a high correlation between the number of trees per hectare and the abundance of smallbodied birds and an even higher correlation between the number of plants per hectare (including all plants counted) and small-bodied birds. This correlation suggests that tree density and total vegetation is more important for bird species than shrub density.

Figure 5- Graph showing a high positive correlation between abundance of smallbodied birds and trees per hectare. This relationship has an R2 value of 0.758.

Figure 6-graph showing a very high positive correlation between abundance of small bodied birds and plants per hectare. This relationship has an R2 value of 0.771.

In this experiment, for a plant to be defined as a ‘tree’ it had to be taller than 3m and therefore well established. The correlation between increased trees per hectare and increased bird observations suggests there may be a connection between the age of a planting and bird diversity and abundance. This could confirm the findings of Cunningham et. al.(2008), by supporting the idea that remnant vegetation is more valuable as bird habitat than revegetation plantings.

Further conclusions can be drawn using the Hilltops polygon obtained from Birdlife Australia data. On average, 38.4% of individual birds observed in the Hilltops area were small-bodied birds compared to between 52-97% small-bodied birds observed in this experiment. Additionally, 28.4% of species in the Hilltops polygon were small-bodied birds compared to between 40-85% in the experiment. Chi squared tests were performed to compare the ratio of small-bodied birds to largebodied birds in the Hilltops polygon and the data from this experiment to determine if this difference is statistically significant (appendix 3). This was done for each planting and for both abundance and species richness data. Every planting showed a p value of significantly < 0.05 for both tests with the exception of the abundance test for planting 5 which had a p value of 0.13. This means the ratio of small to medium birds and bird species is significantly different in agricultural revegetation plantings compared to the average ratio found in the Hilltops region. This suggests revegetation plantings are hugely beneficial to small-bodied bird species.

The most common species observed throughout the surveys was the Superb Fairy-wren (Malurus cyaneus) with 258 counted (appendix 4). They made up 32.09% of birds sighted in the surveys compared to 3.4% in the Hilltops region. This indicates that revegetation plantings are valuable habitat for the Superb Fairywren (Malurus cyaneus). The second most common species was the Yellowrumped Thornbill (Acanthiza chrysorrhoa) followed by the Crested Pigeon (Ocyphaps lophotes)and then the Yellow Thornbill (Acanthiza nana). The high number of crested pigeon sightings is suspected to be due to the repeated sighting of a large flock in the same planting. The presence of 3 small-bodied bird species in the 4 most common species observed further affirms the benefits of revegetation plantings for small birds. Further research could involve developing a better understanding of the value of revegetation planting to these species. Most other species were present in insignificant ratios compared to the polygon.

Conclusion

The results of this experiment refute the hypothesis that revegetation plantings with a higher density of shrubs also support a greater bird abundance and species richness. The correlation between shrub density and both bird abundance and species richness was positive but very weak (R2= 0.003 for both). The correlation between the percentage of birds that were smallbodied and the shrub density was slightly higher (R2= 0.197) but still insignificant. There were limitations to the accuracy of this study including the methods of classifying both birds and vegetation. In future research, alternate methods of measuring vegetation type should be employed. Whilst the hypothesis was rejected, other trends in the results suggest that the vegetation within a revegetation planting is important for bird abundance and species richness. A strong correlation was found between bird abundance and tree density and an even stronger correlation was present between bird abundance and plant density. From this it can be concluded that an increase in overall vegetation density does lead to an increase in bird abundance and species richness. Comparing the results of this study to the data from Birdata allows further conclusions to be drawn. As shown by Chi squared tests, there is a statistically significant difference between abundance and species richness in revegetation plantings and the average abundance and species richness in the Hilltops region. Therefore, revegetation plantings are important for birds, particularly small-bodied species such as the Superb Fairy-wren (Malurus cyaneus). Further research is required to establish the reason that plantings are so beneficial to small-bodied birds. Further study in this area of research could include focusing on the plant species in revegetation plantings and the genera that compose plantings that are particularly suitable for bird habitat. Similar studies to this should also be conducted in regions that specialise in different sectors of the agricultural industry such as broad-acre cropping.

In conclusion, within the agriculturally intense region of Hilltops, NSW, revegetation plantings provide valuable habitat for birds. Whilst they cannot replace native vegetation cleared for agriculture, their implementation on farms can increase bird abundance and species richness. No link has been established between shrub density and bird abundance and species richness, although there is evidence to suggest that higher vegetation density supports greater bird abundance.

Acknowledgements

I would like to thank Eleanor Lang from the Australian National University for her advice regarding this study and for providing feedback on it. I would also like to thank Jayden Gunn from Birdlife Australia for his suggestions and assistance with the preliminary surveys. Also thanks to Dr Stuart Browne and the CSIRO Agricultural Research Station Boorowa, for allowing me to use their tree plantings. Finally, I wish to thank Craig Southwell for assisting in completing the vegetation surveys and providing knowledge of plant species.

Reference List

Birdlife International. (2022).The Action Plan forAustralian Birds 2020 reveals that one in sixare nationally threatened. BirdLife Data Zone. Retrieved13 July 2022, fromhttp://datazone.birdlife.org/sowb/case study/the-action-plan-for-australian-birds2020-reveals-that-one-in-six-arenationally-threatened.

Cunningham, R., Lindenmayer, D., Crane, M., Michael, D., Macgregor, C., Montague-drake, R.,& Fischer, J. (2008). The Combined Effects of Remnant Vegetation and Tree Planting on Farmland Birds. Conservation Biology,22(3), 742-752.doi: 10.1111/j.1523-1739.2008.00924.x

Field, S., Tyre, A., & Possingham, H. (2002). Estimating bird species richness: How should repeat surveys be organized in time?. Austral Ecology,27(6), 624-629. doi:10.1046/j.1442-9993.2002.01223.x

Google Maps (https://www.google.com/maps/@- 34.571022,148.9207696,11z) was used for identifying and measuring plantings.

Haslem, A., Clarke, R., Holland, G., Radford, J., Stewart, A., & Bennett, A. (2021). Local management or wider context: What determines the value of farm revegetation plantings forbirds?.Journal Of Applied Ecology,58(11), 25522565.https://doi.org/10.1111/13652664.13988

Hosking, G., & Thackway, R. (2019). Birds, Biodiversity and Agricultural Land. Soils For Life. Retrieved 13 July 2022, fromhttps://soilsforlife.org.au/therelationship-of-habitat-and-biodiversityon-agricultural-land/.

Lindenmayer, D., Blanchard, W., Crane, M., Michael, D., & Florance, D. (2018). Size or quality.What matters in vegetation restoration for bird biodiversity in endangered temperate woodlands?.Austral Ecology,43(7), 798806. https://doi.org/10.1111/aec.12622

Lindenmayer, D., Lane, P., Westgate, M., Scheele, B., Foster, C., & Sato, C. et al. (2018). Tests of predictions associated with temporal changes in Australian bird populations. Biological Conservation,222, 212-221. https://doi.org/10.1016/j.biocon.2018.04.007

Loyn, R. (1986). The 20 Minute Search –A Simple Method For Counting Forest Birds.Corella,10(2), 58-60. Retrieved fromhttps://absa.asn.au/corella_documen ts/the-20-minute-search-a-simplemethod-for-counting-forest-birds/

Munro, N., Fischer, J., Barrett, G., Wood, J., Leavesley, A., & Lindenmayer, D. (2011). Bird's Response to Revegetation of Different Structure and Floristics-Are “Restoration Plantings” Restoring Bird Communities?. Restoration Ecology,19(201), 223235.https://doi.org/10.1111/j.1526100x.2010.0070.x

NSW National Parks and Wildlife Service. (2002).The Native Vegetation of Boorowa Shire(p.2). Hurstville, NSW.https://www.environment.nsw.gov.a u/resources/nature/sbsnssscopeboorowa. pdf

Reid, J. (2000). Threatened and Declining Birds in The New South Wales SheepWheat Belt (p.1). Canberra: CSIRO.https://www.environment.nsw.gov .au/resources/nature/reportBirdsWheatSh eepBeltComplete.pdf.

Stevens, W. (2001). Declining Biodiversity and Unsustainable Agricultural Production-Common Cause, Common Solution?. Parliamentary Library. https://www.aph.gov.au/About_Parliamen t/Parliamentary_Departments/Parliament ary_Library/pubs/rp/rp0102/02RP02.

Tay, Y. S. (2019). Bird species richness and abundance: The effects of structural attributes, habitat complexity and tree diameter. ANU Undergraduate Research Journal,9, 123–137. Retrieved from https://studentjournals.anu.edu.au/index.p hp/aurj/article/view/1

Appendices

Appendix 1

Appendix 2

Appendix 3

Appendix 4