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Abstract

Invasive species can threaten the natural biodiversity of ecosystems. Biotic resistance gives ecologists insight into what prevents invasive species success. Urban habitats are currently the most common and fastest growing ecosystem on the planet, , so understanding biotic resistance in these areas is of particular importance. Ants are a good study case for urban biotic resistance since they perform important ecosystem functions in both urban and natural habitats. We compared the biotic resistance of urban and natural habitats using data from existing papers and databases of ant distributions in both natural and urban environments. We also examined the effects of latitude and ecosystem type on biotic resistance, and if biotic resistance differs within different urban habitat types. Biotic resistance was found to be higher in natural areas than urban areas, and in a multiple regression, both latitude and ecosystem types predicted 2/3 of the variance in biotic resistance. Urban habitat types were found to differ among four non-mutually exclusive groups, and biotic resistance of these habitats seemed to follow a gradient from least man-made to most man-made. Human-caused change may be an important variable to consider in future studies of biotic resistance with vital implications for urban biodiversity.

Introduction

 Invasive species are a worldwide problem that threaten biological diversity. They can alter critical processes that ecosystems rely on (Buczkowski & Bennett 2008).  Many factors affect how successful these invaders are, but because of the complicated nature of ecosystems there is no accurate way to predict how well invasive species will do in new environments (Kennedy et al. 2002). There has been some question of whether invasive species are the cause of degraded ecosystems or simply “passengers” of non-interactive processes (MacDougall & Turkington 2005). MacDougall & Turkington (2005) conducted experiments on the importance of species competition for invasive success, and found that although competition had some effect, the more common causes of invasion were barriers and environmental stressors that limit native species. A review by Gurevitch & Padilla (2004) found that there is not enough evidence to claim invasive species as the cause of lost biodiversity. However, the relationship between  native species decline and the introduction of invasive species is often simultaneous. The relationship between invasive and native species is important to understand for insight into what makes an ecosystem vulnerable or resistant to invasion.

Some ecosystems are better than others at resisting invasion. When native species reduce the success of invasive species, it is called biotic resistance (Levine et al. 2004). Not all invaders exposed to a new area are successful. Many factors can reduce invasive success, such as predation, competition, and diseases. However, competition was a lower than expected factor in the success of biotic resistance (Levine et al. 2004). Depending on the spatial scale measured, different things account for biotic resistance. When analyzed on a small scale, biodiversity acts as a buffer for invasive species, but on a large-scale there is a positive correlation between invasive species and high biodiversity (Kennedy et al. 2002). Biotic resistance probably does not completely repel invasions, but it can make some difference (Kennedy et al. 2002). Though relationships differ, the link between biodiversity and invasive species is undisputed.

Urban areas are known to have low biodiversity (Shochat et al. 2010). However, they are becoming one of the most common habitat types in the United States and worldwide. For this reason, it is important to understand urban ecology. Future conservation efforts will need to focus on urban systems (Dunn et al. 2006). High biodiversity can act as a buffer to invasives, so non-diverse urban environments could be particularly vulnerable to invasion. One hypothesis postulated for the low biodiversity of urban areas is that there is a high number of invasive species in urban areas that outcompete native species; however, the causal factor is difficult to determine (Shochat et al. 2010). Habitat loss is clearly an important factor, but the competition caused by invasive species may also negatively affect native species. If biotic resistance is studied in urban areas, efforts to conserve biodiversity can be more knowledgeable about one of the most common habitat types. Information on biotic resistance in urban areas could contribute to conservation efforts. Past research concentrates mostly on natural habitats (Shochat et al. 2010). Comparing biotic resistance in urban and natural settings may help us to better understand how invasive species affect both types of ecosystems.

With the increase in urban habitat, species that interact often with humans are the most likely to persist (Menke et al. 2010). Ants specifically take a role in the urban ecosystem that can have some beneficial effects for humans, like removal of food crumbs. Ants are common both in natural and urban areas, and provide ecosystem processes that are central to both types of ecosystems (Menke et al. 2010). Invasive ant species, like Linepethema humile (the Argentine Ant) can negatively affect native ant species and have profound effects on the ecosystems they invade (Buczkowski & Bennett 2008). Additionally, many invasive ant species have significant effects on their ecosystems, and are a target of pest control and conservation efforts (Buczkowski & Bennett 2008). Since ants are present in and central to both urban and natural environments, they are a good study system for comparing biotic resistance between these two ecosystems.

We chose to use ants to examine the factors that determine the success of invasions. We are looking at both biotic resistance as a biotic factor, and latitude as a correlate for abiotic factors. Biotic resistance is often studied in natural systems, but less commonly in urban systems. For example, different natural ecosystems have differing levels of biotic resistance that are documented, but urban habitat types have not been extensively studied (Shochat et al. 2010). We want to determine if there is a significant difference between the biotic resistance of these two types of ecosystems, as well as habitat types within urban systems. We also chose to examine latitude with climate change in mind. Changing climates will affect the abiotic factors, like temperature by latitude, which can affect species assemblages and vulnerability to invasion (Chapin III et al. 2000).  One example of this climate change effect is the Argentine Ant, which is limited in its northern range by soil temperatures. Should the temperatures increase on a latitudinal gradient, this harmful species may move further north (Brightwell et al. 2010).

Our first hypothesis will be to determine if there is a difference between the biotic resistance in urban and natural areas. Our second hypothesis will be to see if biotic resistance varies by latitude. Our third hypothesis will be to determine if there is a difference in biotic resistance between different types of habitats within urban ecosystems. To quantify biotic resistance, we use ratios of the number of invasive species to the number of total species in an area. By looking at both biotic resistance and how it interacts with latitude, we hope to learn more about the factors that promote invasion.

Methods

Data Collection

As a class, we requested data from 13 articles on urban ant distributions. The data we requested was presence/ absence matrices for every species of ant recorded and GPS coordinates of all sites where ant data were collected. Not all papers kept track of the specific data. Many of the papers did not have GPS coordinates for sites, and so presence/absence matrices were calculated by city. Our study required both city wide data, and within city data for analyses. We looked at the combined data collected by the class to choose what was appropriate for analysis. We eliminated data that did not have GPS coordinates, as well as data from papers that only collected samples in more natural areas, like forests. This left us with data on 10 cities from eight different papers (Appendix A). We submitted the latitude values of each city to our professor, Dr. Sean Menke, and he provided us with presence/ absence data on ants in natural habitats for similar latitudes. This left us with 10 urban data sites, and 10 natural data sites.

For the data on different urban habitat types, we used only data from the paper on Raleigh, NC (Menke, et al. 2010). We chose to use only this data because the habitat types were the best recorded, and it had the most data points. We had no method to compare what one paper labeled a park to what another paper labeled a park, so we kept our data to one paper for consistency.

Data Analysis

For all cities, the number of invasive species was counted. Invasive species were determined using data for North America (Menke et al. 2010; Invasive Species). We then created a ratio of the number of invasive species in an area to the total number of species in an area. Since we planned on using parametric tests, we transformed the data using the equation: ArcSine(Square root(#invasive species/ # total species)), to create the variable which we labeled “transformed ratio.”

To test our first hypothesis, we ran an independent samples t-test on the mean transformed ratios comparing urban and natural data. To test our second hypothesis, we ran a linear regression of latitude on transformed ratios. We also conducted a multiple regression model of latitude and ecosystem on transformed ratios. To test our third hypothesis, we ran a one-way ANOVA test comparing the transformed ratios of different habitat types. We ran a post-hoc Tukey HSD test after finding the ANOVA significant.

Results

Hypothesis 1. Urban ecosystems averaged significantly more invasive species per total species than natural ecosystems (0.45 ± 0.21SD, 0.07 ± 0.11SD respectively) (t18 = 5.18, p < 0.0005) ( Fig. 1).

Hypothesis 2. We ran a regression of latitude on transformed ratios, which was non-significant, R2=.003, F1,87-0.261, p=.611. We ran a multiple regression of latitude and habitat type on the transformed ratios, which was found to be significant, R2=0.673, F2,17=17.474, p<.0005. In this model, ecosystem type was a significant factor (t= -5.545, p<0.0005) and latitude was marginally significant (t=-1.964, p=0.066) (Fig. 2).

Hypothesis 3. We ran a one-way ANOVA to see if invasive species/ total species ratio varied by urban habitat type (Fig. 3). There were large differences in the ratios of different habitat types (Table 1, Fig. 4). The ANOVA was significant and post-hoc Tukey HSD tests were  run ( F6,89=7.70, p<.0005) (Table 2). Post-hoc Tukey HSD tests showed four non-mutually exclusive groups. Group 1 included forest, greenway, and park. Group 2 included greenway, park, and residential. Group 3 included park, residential, business, and agriculture. Group 4 included  residential, business, agriculture, and industrial (Table 3, Fig. 5).

Discussion

We found a significant difference between the means of biotic resistance in urban and natural areas. Additionally, we discovered that latitude alone did not predict biotic resistance, but that latitude and ecosystem together can predict biotic resistance of an area. We also found some significant differences between the mean biotic resistances of different urban habitat types. All of these findings support our three hypotheses.

The findings on our first hypothesis are not surprising. The natural areas had higher biodiversity in general than the urban areas. Since our data were site level collected in a series of small studies, it makes sense that this biodiversity served as a buffer of sorts to the success of invasive species. These results, however, could have been an artifact of the data collection methods. Since urban habitats are studied less often than natural habitats, the data may have simply been less thorough, resulting in lower diversity. However, despite sampling techniques, the number of invasive species in urban environments was much higher than that in natural environments.

Our regression model gave some interesting results. Latitude alone was not a significant predictor of biotic resistance. This could have been due to low power, since we only had 10 sites for each type of ecosystem, resulting in a total of 20 samples. However, we decided to run a second multiple regression to include both ecosystem and latitude. The regression model was highly significant and a good predictor of transformed ratios, accounting for 2/3 of the variation in biotic resistance among ants. The significance of this model revealed why the first model may have been different. Though latitude seemed to have an effect, the overall starting point for biotic resistance ratios in urban areas was higher (Fig. 2). This detail makes sense in light of the results of our first hypothesis. With increasing latitudes, biotic resistance seemed to be strengthened. The slopes of the lines did not appear to differ significantly. This information gives us some insight to a possible latitudinal gradient in biotic resistance, in which latitudes higher north have higher biotic resistance, shown by lower invasive species: total species ratios. This gradient is interesting due to the known diversity gradient in latitude, in which latitudes farther north seem to have less overall diversity.

Our third hypothesis had some very illuminating results. The data were limited in that it was collected in only one city, so we must be cautious when drawing conclusions. However, the biotic resistance in the urban area almost seems to follow a gradient of more urban-like to more natural-like environments. The order of biotic resistance from lowest resistance to highest resistance is: industrial, agricultural, business, residential, park, greenway, forest. When one sees the category agricultural field, it does not seem to fit in as an “urban-like” environment. However, when one takes into account the level of man-made change in each ecosystem, agriculture ranks high, close to industrial. This result shows that not only does biotic resistance differ between urban and natural areas, but also within different urban habitat types. Additionally, the level of man-made change in an area may possibly have an effect on the success of invasive species there. The Tukey HSD test showed that not every environment differed significantly from the others, however, the significant differences also showed the urban-natural gradient (Table 2). For example, industrial habitats differed significantly from park, greenway, and forest habitats, they three areas with the highest biotic resistance.

The interesting result that our data yields is that man-made change in an ecosystem could be a factor in determining the success of invasive species. Examining this factor could be a productive research direction for the field. While the findings of this study are interesting, it is important to note methodological limitations. We did not collect the data ourselves, so discrepancies in data collection methods between studies may have affected the data. Ideally, all areas would have been equally sampled using the same methods for sampling ants. We also were only able to employ correlational analyses, so we cannot attribute cause-effect relationships to these variables. Additionally, we did not include all known data on urban ant distributions because of missing information from some of the studies, like GPS coordinates. With proper resources, large-scale data sampling of urban and natural environments on similar latitudes in the future would be a good way to reanalyze our hypotheses. Even better would be an experimental manipulation of the level of man-made change or “urbanness” of an area to see if a causal relationship is involved. Examining this variable may lead to further knowledge on how invasive species establish success in an ecosystem. The more we can learn about invasive species, the more knowledgeable conservation efforts targeting these species will be.

References

 Brightwell, R.J., P.E. Labadie,  & J. Silverman. 2010. Northward expansion of the invasive Linepithema humile (Hymenoptera: formicidae) in the eastern United States is constrained by winter soil temperatures. Environmental Entemology 39: 1659-1665

Buczkowski, G. & G.W. Bennett. 2008. Detrimental effects of highly efficient interference competition: Invasive Argentine Ants outcompete native ants at toxic baits. Environmental Entemology 37: 741-747

Chapin III, F.S., E.S. Zavaleta , V.T. Eviner, R.L. Naylor, P.M. Vitousek, H.L. Reynolds, D.U. Hooper, S. Lavorel, O.E. Sala, S.E. Hobbie, M.C. Mack, & S. Diaz. 2000. Nature 405: 234-242

Dunn, R.R., M.C. Gavin, M.C. Sanchez, J.N. Solomon. 2006. The pigeon paradox: Dependence of global conservation on urban nature. Conservation Biology 20: 1814-1816

Gurevitch, J. & D.K. Padilla. 2004. Are invasive species a major cause of extinctions? Trends in Ecology and Evolution 19: 470-474

Invasive species: Information, images, videos, distribution maps. 2011. Center for Invasive Species and Ecosystem Health http://www.invasive.org/species/insects.cfm

Kennedy, T.A., Naeem, S., Howe, K.M., Knops, J.M.H., Tilman, D. & Reich, P. 2002. Nature 417: 636-638

Levine, J.M., P.B. Adler & S,G. Yelenik. 2004. A meta-analysis of biotic resistance to exotic plant invasions. Ecology Letters 7: 975-989

MacDougall, A.S, & R, Turkington. 2005. Are invasive species the drivers or passengers of change in degraded ecosystems? Ecology 86: 42-55

Menke, S.B., W. Booth, R.R. Dunn, C. Schal, E.L. Vargo, & J. Silverman. 2010. Is it easy to be urban? Convergent success in urban habitats among lineages of a widespread native ant. PLoS ONE 5: 1-8

Menke, S.B., B. Guenard, J.O. Sexton, M.D. Weiser, R.R. Dunn, & J. Silverman. 2010. Urban areas may serve as habitat and corridors for dry-adapted, heat tolerant species; an example from ants. Urban Ecosystems 14:135-163.

Shochat, E., S.B. Lerman, A.M. Anderies, P.S. Warren, S.H. Faeth, & C.H. Nilon. 2010. Invasion, competition, and biodiversity loss in urban ecosystems. BioScience 60: 199-208

Appendix A

Clarke, K.M., B.L. Fisher, & G. LeBuhn. 2008. The influence of urban park characteristics on ant (Hymenoptera, Formicidae) communities. Urban Ecosystems 11:317–334.

Field, H.C., W.E. Evans Sr., R. Hartley, L.D. Hansen, & J.H. Klotz. 2007. A Survey of Structural Ant Pests in the Southwestern U.S.A. (Hymenoptera: Formicidae). Sociobiology 29:1-14.

Forys, E.A & C.R. Allen. 2005. The Impacts of Sprawl on Biodiversity: the Ant Fauna of the Lower Florida Keys. Ecology 79:2041-2056

Menke, S.B., B. Guenard, J.O. Sexton, M.D. Weiser, R.R. Dunn, & J. Silverman. 2010. Urban areas may serve as habitat and corridors for dry-adapted, heat tolerant species; an example from ants. Urban Ecosystems 14:135-163.

Pecaravic, M., J. Danoff-Burg, & R.R. Dunn. 2010. Biodiversity on Broadway - Enigmatic Diversity of the Societies of Ants (Formicidae) on the Streets of New York City. PLoS ONE 5: 1-8

Sanford, M.P., P.N. Manley, & D.D. Murphy. 2008. Effects of Urban Development on Ant Communities: Implications for Ecosystem Services and Management. Conservation Biology 23:131-141

Suarez, A.V., D.T. Bolger, & T.J. Case. 1998. Effects of Fragmentation and Invasion on Native Ant Communities in Coastal Southern California. Ecology 79:2041-2056

Uno, S., J. Cotton, & S.M. Philpott. 2010. Diversity, abundance, and species composition of ants in urban green spaces. Urban Ecosystems 13:425–441