Arthropod Diversity in an Urban Forest from Miami Dade County, Florida, US

Arthropods are excellent candidates for studying how the formation of urban ecosystems impacts the environment for several reasons. Foremost of these is the variety of roles played by arthropods in ecosystem functioning (e.g., food web dynamics, nutrient cycling and redistribution, and pollination) with many of these roles being economically important [e.g. pollination by domestic bees is an $18 billion/year industry in the US alone]. Urban ecosystems have been neglected in ecological research. Arthropods are abundant in urban settings, but little is known about how these animals respond to urbanization. In this project, we present data on species composition and structure of the communities of arthropods from Saint Thomas University Urban Forest, in the city of Miami Gardens, Miami Dade County, State of Florida, USA. In order to conduct our samplings, we set up 16 pitfall traps, as is usually done for invertebrate soil biodiversity. We then sorted and identified collected individuals using a Dissecting Microscope. Data were analyzed using PAST to calculate diversity indices of the sites studied and species organization. We collected a total of 31 species distributed in 21 Families and 13 Orders. Most species were identified up to the species level. Total number of individuals was 2147, representing a high abundance. The most abundant species was Solenopsis molesta, represented by 1313 individuals. Invertebrates are sensitive to climatic and ecological alterations. Their small bodies desiccate easily, a factor which may limit their evolutionary response to accelerated temperature increases. Consequently, the understanding, preservation, and development of urban forests can provide an avenue through which the conservation of local communities is sustained, starting with some of its most numerous and ecologically sensitive species.

Introduction

According to McIntyreet al. (2001), despite being conspicuous and influential features of the biosphere, urban ecosystems have been neglected in ecological research. Arthropods are abundant in urban settings, but little is known about how these animals respond to urbanization. In nature, soil biodiversity has a positive correlation with productivity and sustainability of the system (Hunt & Wall, 2002). The loss in soil biodiversity and simplification of soil community composition led to reduced plant diversity, plant decomposition, nutrient retention and nutrient cycling (Wagget al., 2014).

Arthropods are excellent candidates for studying how the formation of urban ecosystems impacts the environment for several reasons. Foremost of these is the variety of roles played by arthropods in ecosystem functioning (e.g., food web dynamics, nutrient cycling and redistribution, and pollination) with many of these roles being economically important [e.g. pollination by domestic bees is a $18 billion/year industry in the US alone].

Florida is renowned for its expansive natural environments, which contain one of the highest concentrations of endemic flora and fauna in the United States. However, anthropic climate change, pollution, and the introduction of invasive species present threats that are likely to result in severe alterations to Florida biological communities. This is particularly concerning considering the number of endangered species inhabiting Florida’s territories, as well as the state ranking among the worst in the country with respect to preparedness in managing its endangered species (Center for Biological Diversity Action Fund, 2019).

We have not found much information regarding the composition and structure of Arthropod communities in the State of Florida. However, there are two contributions by Cherry (2003), and Cherry and Nuessly (1992) regarding the ant composition of sugarcane plantations in the State, one of them dealing with invasive ants (Cherry & Nuessly, 1992). This seems to be the first attempt to study the soil biodiversity of the in-Campus University Forest.

In this project, we present data on species composition and structure of the communities of arthropods from Saint Thomas University Urban Forest, in the city of Miami Gardens, Miami Dade County, State of Florida, USA.

Materials and Methods

Samplings

In order to conduct samplings, we set up 16 pitfall traps, as is usually done for invertebrate soil biodiversity (Wardet al., 2001) (Fig. 1). We then sorted out and identified individuals collected using a Dissecting Microscope.

Fig. 1. Basic pitfall trap design.

The two traps in each zone were situated approximately 10 feet from each other. For each determined Zone, one trap was equipped with a plant-based bait (banana) and the other a meat-based bait (tuna). The traps were left overnight and inspected the next day for specimen capture. Those with reportable samples were recovered, and their samples were analyzed for taxonomic identification and quantitative representation by species using stereomicroscope imaging.

Data Analysis

Data were analyzed using PAST (Øyvindet al., 2008) to calculate the diversity indices of the sites studied and species organization. Species richness is the simplest diversity index (Ludwig & Reynolds, 1988) and consists of the number of species in a given area. The abundance is given by the number of individuals of a species collected per sampling station. Other ecological indices such as H’ diversity of Shannon and Weaver, diversity of Simpson 1-D, and J [E1, evenness of Pielou], were calculated. Simpson’s index varies from 0 to 1, with being 0 the highest diversity. Pielou Index also ranged from 0 to 1, with the highest values around 1. For more details on the indices, it is recommended the reading of Magurran (1987). All indices calculated are widely used in Community Ecology and are used in other papers by the authors.

Results

Species Composition

We collected a total of 31 species distributed in 21 Families and 13 Orders (Fig. 2). Most species were identified up to the species level. Further details on distribution and other relevant information about the species roster are listed in Table I. Species collected comprised two Phyla, four Classes, and 12 Orders.

Fig. 2. Solenopsis molesta, represented by 1,313 individuals.

Table I. Composition of Higher Categories

Category	No. of species
Class insecta
Order blattodea	5
Order diptera	5
Order coleoptera	8
Order hemiptera	1
Order hymenoptera	4
Order orthoptera	1
Order parasitiformes	1
Order phasmatodea	1
Class arachnida
Order araneae	1
Class diplopoda
Order spirobolida	2
Class malacostraca
Order isopoda	2
Total	31

A major breakdown into distribution categories is presented in Table II. There we can see that the forest harbors 13 endemic species, and three local ones, although the number of invasive is large as well, with five invasive species present.

Table II. Distribution Categories

Category	No. of species
Cosmopolitan	3
Local	3
Endemic	13
Invasive	5
Unknown	7
Total	31

The total number of individuals was 2147, representing a high abundance (Table III). The most abundant species was the thief ant Solenopsis molesta, represented by 1313 individuals. Out of the total we found five species that turned out to be invasive. IUCN (2017) stated that invasive species are recognized as a major threat to biological diversity and ecosystem services. The threat of invasive species is likely to increase with growing trade and travel so more action is needed. However, the capacity of many countries to manage and improve border control and quarantine invasive species is not yet sufficient to achieve the Aichi Biodiversity Target 9 –‘By 2020, invasive alien species and pathways are identified and prioritized, priority species are controlled or eradicated, and measures are in place to manage pathways to prevent their introduction and establishment.

Table III. Species Abundances and No. of Habitats

Species	Abundance	No. of habitats
Aedes sp.	1	1
Aethina tumida	5	3
Anadenobolus monilicornus	7	5
Attagenus unicolor	3	1
Anisomorpha buprestoides	1	1
Blatta orientalis	2	1
Calosoma sayi	4	1
Camponotus floridanus	35	8
Diptera sp.	7	1
Dorymyrmex insanus	114	8
Drosophila melanogaster	296	8
Eurycotis floridana	5	2
Isopoda sp.	18	2
Isoptera sp.	1	1
Leiodes sp.	149	7
Muscoidea sp.	2	1
Narceus americanus	1	1
Oncopeltus sp.	18	1
Oniscoidea	12	1
Parasitiformes sp.	2	2
Pasimachus strenuus	34	5
Pasimachus sublaevis	1	1
Peltotrupes profundus	4	4
Periplaneta fuliginosa	4	3
Phyllophaga sp.	3	1
Pycnoscelus surinamensis	26	2
Salticidae sp.	2	1
Romalea guttata	1	1
Sarcophaga sp.	11	3
Solenopsis molesta	1313	8
Tetramorium bicarinatum	65	5
Total general	2147	90

Species collected were found in eight major habitats. The habitat with the highest number of individuals was the Orchid Shelter, with 719 individuals, followed by the Casuarina-Pinus transitional Forest, with 520 individuals.

Community Structure

Our results on Community Structure are included in Table IV. Shannon Index was slightly low probably due to the uneven distribution of abundances among all species collected. Probably for the same reason Simpson Index had a medium to low value of 0.6 (Between 0, highest, and 1, lowest). Pielou’s Equitability is from Medium to low (J = 0.44), which is due to the asymmetry among the abundance distribution per species.

Table IV. Ecological Indicators of Community Structure

Indexes	Values
Taxa S	31
Individuals	2147
Dominance D	0.4006
Shannon H	1.523
Simpson 1-D	0.5994
Evenness e^H/S	0.1433
Equitability J	0.4393

In the context of environmental pressures developing, conservation efforts must explore every possible investigative angle to devise methods to secure and heal communities affected by pollution and deforestation. One ecological factor of interest is that of the urban forest. These forests, fractured and contained within the broader stretches of urban development, can present opportunities of interest to both conservation and local community projects. Monitoring the health and diversity of invertebrate taxa is fundamental to understanding and maintaining forest communities, as these animals play a crucial role in fundamental ecological relations, be they as part of the foundation of food chains, leaf litter detritivores, or pollinators, among other roles. However, multiple factors can impact the viability of invertebrate populations.

Harvesting of plant biomass, particularly woody debris, is noted to have a significant effect on local invertebrate populations as many species rely on hardwood residues for consumptive, protective, and reproductive uses (Grodskyet al., 2018). Invertebrates are sensitive to climatic and ecological alterations. Their small bodies desiccate easily (Andrewartha & Birch, 1954), a factor which may limit their evolutionary response to accelerated temperature increases. Indeed, studies in Drosophila indicate limits in evolutionary response to dehydration and heat resistance (Arthuret al., 2008). Many species’ population distributions are also influenced by the distributions of their host plants (Hengeveld, 1990), a factor which may present another challenge due to studies indicating a gradual northward shift by some hardwood species in response to global warming. Consequently, the understanding, preservation, and development of urban forests can provide an avenue through which the conservation of local communities is sustained, starting with some of its most numerous and ecologically sensitive species.

Discussion

Some arthropod taxa prove to be hardy generalists which can adapt themselves to a variety of urban environments, including residential, industrial, agricultural, and desert remnant environments, but many others form community structures specific to particular forms of urban land (McIntyreet al., 2001). Some studies on specific taxonomic groups have found negative correlations between species diversity and increasing urbanization (Hansenet al., 2005, Sattleret al., 2010), although other studies have indicated that urban settings themselves may not be obstacles for arthropod populations and that the stressors produced by the process of urban development may instead be at least partly to blame for temporal changes in biodiversity patterns. Still, other data indicates that urbanization produces either a positive influence on local arthropod diversity or no discernible influence at all (McIntyre, 2000; McKinney, 2008). The matter is complicated further by the development of sub habitats within these already fragmented urbanized ecosystems, some of which can be surprisingly effective in supporting local arthropod communities. For instance, neighborhood lawns can be maintained in such a way as to allow them to function as viable microhabitats which have been shown to enhance local arthropod species richness. Additionally, the maintenance of vacant lots as accessible habitats for local wildlife as well as their conversion into agroecosystems can provide their own distinctive benefits, including enhancing contributions to biological pest control by species that benefit under these modified environments.

These varying observations indicate contrasting interrelations between local species and the different forms of urbanization each taxa faces. This habitat structure specificity is crucial in the context of different urban ecosystems, as altered arthropod communities will yield distinct effects on local environments, from soil quality and nutrient cycles to pollinator services and food web systems. Urban forests can thus function as reservoirs of beneficial arthropod biodiversity within urban settings and disturbed environments which may otherwise prove difficult for the subsistence of some species. For example, in comparisons between urban forests, vacant lots, and lots converted to urban agroecosystems, ant species richness was found to be greatest in forest settings despite the accessibility that both undisturbed and ecologically modified lots can offer (Unoet al., 2010). Additionally, the stable communities that form within urban forests can result in lower pest densities compared to city landscapes (Longet al., 2019), and urban forests in Dera Ghazi Khan were found to harbor significant abundances of beneficial arthropod species, including native predatory insects and various detritivores (Mohsinet al., 2022). Consequently, across urban settings around the world, it is apparent that urban forests provide unique and irreplaceable environments for many arthropod species, even in comparison to urban settings reconverted into ecologically beneficial models.

Conclusion

A total of 31 arthropod taxa were identified across 16 sample sites distributed throughout the St. Thomas University urban forest. Among the 31 taxa, 16 were comprised of confirmed native populations while 5 were invasive species. Additionally, three taxa consisted of species with widespread global distributions. Seven further taxa were confirmed to represent species distinct from all other groups, but could not be categorized at sufficient taxonomic specificity to determine their native or exotic status. Asymmetric abundances across species groups were observed and this was reflected in our diversity indices; higher representations in localized ant species were a primary factor behind this, although the disturbed nature of the forest (adjacent to a construction site) may be a contributing factor as well. Additionally, subhabitats with transitional environmental features and partial shading from sunlight (an orchid shelter and a Casuarina-Pinus transitional forest) yielded the highest number of collected individual specimens. Many species were reported from a small proportion of sampled habitats, indicating how even relatively small urban forests can contain environments conducive to various ecological niches.