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Article

Comparing Ant Activity and Plants Bearing Extrafloral Nectaries in Rockland Habitats of the Florida Keys with Those of the Everglades and the Bahamas

by
Suzanne Koptur
1,* and
Kathleen H. Keeler
2
1
Department of Biological Sciences, International Center for Tropical Botany, Institute of the Environment, Florida International University, Miami, FL 33199, USA
2
School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588, USA
*
Author to whom correspondence should be addressed.
Diversity 2024, 16(7), 360; https://doi.org/10.3390/d16070360
Submission received: 22 May 2024 / Revised: 20 June 2024 / Accepted: 21 June 2024 / Published: 24 June 2024
(This article belongs to the Special Issue Diversity in 2024)
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Abstract

:
Rocklands are present in South Florida, both on the peninsula and in the Keys. Previous work has shown that extrafloral nectaries on pine rockland plants in the Florida Everglades and surrounding areas play a role in attracting ant protectors. Species with nectaries are more abundant and ant activity greater in more recently burned pine rockland habitats compared with longer undisturbed rockland hammock forest. The floristic composition and prevailing climate of Keys rocklands differs substantially from those on the mainland, so we sought to investigate ant activity and the abundance of plants with extrafloral nectaries in Big Pine Key rockland habitats. Standard baiting methods were used to assess ant abundance and potential predation activity in hardwood hammock, shrubby pine rockland, and open pine rockland sites. Ant activity was the lowest in open pine rockland, greatest in hardwood hammock, and intermediate in shrubby pine rockland, the opposite of what has been observed in previous studies in southern peninsular Florida and on Andros, Bahamas. Sampling vegetation at these same sites revealed a larger proportion of plant species with extrafloral nectaries in the pine rocklands than the hammock, as observed previously in Everglades habitats. Possible reasons for differences in ant activity in Keys rockland habitats are discussed.

1. Introduction

Numerous studies highlight the importance of extrafloral nectaries in ant–plant interactions in many habitats, tropical and temperate (reviewed in [1,2,3,4,5,6,7]), often promoting plant protection [8], though there are exceptions (e.g., [9]), extrafloral nectaries may have other roles. They may attract other predators that can control herbivores [10,11], promote dispersal of seeds [12], serve to distract ants from flowers [13] and/or honeydew-producing herbivores [14], and even encourage ants to nest beneath plants bearing nectaries and so bring nutrients to plants [15,16].
Subtropical south Florida has many species of plants (both native and non-native) bearing extrafloral nectaries, in nature as well as in urban areas [17]. Ants visit nectaries without regard to whether plants are native or not [18], and a large proportion of Florida ants are non-native species (29% [19] p. 11). Though peninsular south Florida has a large proportion of its ant fauna comprised of introduced species (37%), so do the islands of the Florida Keys (38%), both a considerably higher proportion than that of central (20%) and northern Florida (15%) [20].
A larger proportion of introduced species on islands of the Florida Keys than on much of the mainland was attributed to the insular nature of Keys habitats [21]. Wilson noted also that there are many more ants found on the vegetation in the Keys than on the mainland, and the many arboricolous species are of tropical Antillean origin, whereas the terricolous species were of Floridian origin. Wilson suggests that tree-nesting species are more likely to survive inundation, as well as more likely to successfully disperse to the islands, establishing first in coastal mangrove habitats. A thorough inventory of the ants of the Florida Keys is a focal project on Antweb [22]. Ants are a frequently used tool to monitor environmental change and differences [23].
This study is part of an ongoing project to document the distribution of plants with extrafloral nectaries in as many locations as possible. Its objectives were to measure nectar-drinking ant activity and assess the plants of rockland habitats in the Florida Keys to learn which ones have extrafloral nectaries. We measured ant activity in pine rocklands (open and overgrown or shrubby) and hardwood hammock forest and studied the vegetation of each habitat to compare the cover of plants with extrafloral nectaries. We sought to determine how ant activity and plants with extrafloral nectaries vary among these limestone rockland habitats in the Florida Keys, and how they compare with rocklands in the Everglades of peninsular south Florida and similar habitats in the Bahamas.

2. Materials and Methods

2.1. Study Sites

We sampled ants and plants in the National Key Deer Refuge on Big Pine Key during the early dry season in January 2008. This protected natural area has many private landholdings within its boundaries, and a large amount of urban–wildland interface. We chose two pine rockland sites, each with a characteristic understory type. One was open, with only few shrubs and a diverse array of herbaceous plants (N 24°71.297′, W 081°38.769′); the other was shrubby, with many large shrubs and fewer herbs due to the shrub cover (N 24°42.311′, W 081°23.266′). In addition, we studied a hammock site (N 24°42.338′, W 081°23.160′), long without fire, with a diverse overstory of hardwoods with some shrubs and few herbs. We will refer to these as open pineland, shrubby pineland, and hammock sites.

2.2. Ant Activity

Ant activity has often been measured using baiting to attract foraging ants so they may be counted and collected [24]. The speed of discovery of baits is an indicator of ant activity. We placed ten honey baits and ten potted meat baits in two linear transects at each site. A small dab of these foods was dotted on a white paper card (2 cm × 2 cm) on the ground at intervals of approximately 1.5 m along the transect. Honey baits measured the activity of ants that drink nectar, as with other research (e.g., [25,26,27]). Potted meat baits were used to attract other predatory ants.
We checked the baits every five minutes for one hour and recorded ants and other arthropods at the bait at that moment. We recorded ant numbers (more than ten individuals of one species were recorded as “many”), describing the different types, at each interval for each bait. Once we saw an ant on a bait, we said it was “discovered”; if we observed more than five ants of the same type, we said it was evidence of “recruitment”. Representatives of each ant (presumed) taxon were sampled at the end of each baiting period. We tallied the average time-to-discovery for each type of bait, the proportion of each type of bait discovered, and the proportion of baits with recruitment, for each habitat.
We also made observations of ants on plants as we conducted our study, recording species observed and collecting individuals for vouchers. Ant species were determined with the help of Dr. Jaeson Clayborn (FIU and Miami-Dade College) and confirmed by Dr. Mark Deyrup (Archbold Biological Station). Vouchers are deposited in the Florida State Collection of Arthropods.

2.3. Plants

In each habitat we used linear transects to measure plant species’ richness. Two 25 m transects were employed in each habitat, and plants intercepted by the transect were recorded for each odd-numbered meter. We detected extrafloral nectaries by examining the plants carefully all over, using hand lenses and dissecting microscopes as well as our botanical knowledge of taxa to guide our inspections. We may have missed some nectaries if the plants were not in the appropriate stage of development, or if glands were not discernable, especially pore-like ones that were not actively secreting nectar. All plant species were determined using vouchered collections deposited at Fairchild Tropical Botanic Garden; these can be viewed at http://www.virtualherbarium.org/ (accessed on 20 June 2024).

2.4. Statistical Comparisons

We compared time to discovery of baits among sites (habitats) using univariate analysis of variance (ANOVA), as observations of each bait were independent of the other baits, and times to discovery were normally distributed. We therefore had two degrees of freedom (3 habitat types minus one). Significant differences among habitats were detected using post-hoc Dunnett C and Tukey HSD tests (IBM SPSS Statistics, version 23).
We compared the number of baits discovered in each transect/habitat with the Pearson chi-square test df = 2 (3 habitats). Recruitment data (that had many zero values) were arcsine transformed prior to analysis.
We combined the two plant transects for each habitat and compared the proportion of individuals with extrafloral nectaries, and the proportion of species with nectaries. The data presented for individual counts therefore represent two transects per habitat.

3. Results

3.1. Ants Encountered

We found eight species (in seven genera) of ants visiting baits and extrafloral nectaries in rockland habitats on Big Pine Key (Table 1). These ant species all occur also in southern peninsular Florida [28,29], and most are considered native to Florida or the Keys.
  • Camponotus floridanus (Buckley)—Florida carpenter ant—a native occurring in a wide variety of natural habitats [19] as well as disturbed and human habitats. It is a generalist predator and feeds also on nectar and honeydew, guarding their resources, plant [30] and animal [31]. They nest subterraneanly or in rotten wood on the ground.
  • Camponotus planatus Roger—compact bicolored carpenter ant—regarded as a recent Florida exotic spread by humans from the New World tropics [19]. It visits nectar resources and can protect Costus woodsonii from predispersal seed predators in Panama [32], and flowers and fruits of Myrmecophila (Schomburgkia) tibicinus in Mexico [33]. It nests primarily in dead wood, hollow twigs, and grass stems [29].
  • Forelius pruinosus (Roger)—frosty asbestos ant—this native is most often found in open, natural habitats. Foragers can dominate nectar sources and baits, using their chemical repellents to exclude other ants, also on dead insects [19]. Their small nests are usually built in soil under rocks or other items in direct contact with the ground.
  • Odontomachus ruginodis M.R. Smith—rough petiole snapping ant of the New World tropics—may be native to the Florida Keys but is suspected to be exotic in Florida due to its preference for disturbed areas where it nests in the ground [19].
  • Paratrechina longicornis (Fabricius)—the longhorn crazy ant is an exotic that occurs throughout the tropics and is considered a “tramp species” [34,35] It is often found in disturbed habitats (a pest around buildings) but also on beaches. These ants nest in rotten wood (fallen tree branches, logs, and stumps), and soil under objects, and in piles of litter and trash.
  • Pheidole floridana Emery—Florida big-headed ant—considered native. Most often found in open areas, sometimes nesting at base of pine trees. Known to scavenge dead arthropods and are drawn to both sweet and meats [19]. They are ground nesters, especially in sandy soil.
  • Pseudomyrmex elongatus (Mayr)—the mangrove slender twig ant—occurs throughout south Florida but is not known from Cuba or the Bahamas and is considered native in Florida. However, Deyrup [19] concludes that this species might be a long-established exotic in Florida, or that it came via natural dispersal around the Gulf of Mexico during a warmer climate period (though there are no other tropical Florida species that are not found in the Caribbean). They nest in dead branches and twigs and other cavities in plant material.
  • Solenopsis geminata (Fabricius)—big-headed native fire ant—considered both native and introduced in Florida [19]. This soil-nesting species is often displaced by the red imported fire ant (Solenopsis invicta).

3.2. Discovery of and Recruitment to Baits by Ants

Honey baits in hardwood hammocks were discovered faster than those in either shrubby pineland or open pineland (Figure 1); shrubby pineland honey baits were discovered significantly more rapidly than those in open pineland. Meat baits showed the same trend, but the only significant difference was between the quickly discovered baits in the hammock vs. those in the open pineland; the shrubby pineland was not significantly different from either of the other habitats. Only in hammocks was every bait discovered within the hour baiting period; most baits in pineland sites were also discovered within the hour.
In tropical lowland sites [26], it is not unusual for all baits to be discovered within the hour, but this study site is in the subtropics. Average discovery time of less than fifteen minutes for hardwood hammock baits was notably rapid in this subtropical site. Substantially fewer baits were discovered in shrubby and open pinelands (Table 2). We measured recruitment because more ants may provide more protection to plants with extrafloral nectaries. In the hardwood hammock transects, all the meat baits and 80% of the honey baits experienced recruitment (Table 2). Shrubby pineland baits showed intermediate levels of recruitment, and less recruitment was observed in open pineland transects. One interesting non-ant visitor was observed on both kinds of baits, an ant-mimicking broad-headed bug Hyalymenus longispinus (Hemiptera: Alydidae), whose nymphs look like workers of the Camponotus spp. [36]. These were not included in the ant counts for discovery and recruitment, for consistency.

3.3. Plants with Extrafloral Nectaries

Of the 49 taxa of plants recorded in transect samples in all habitats (open pineland, shrubby pineland, and hardwood hammock), nine have extrafloral nectaries (18% of all species; Table 3). Open pineland had the largest total number of plant species in our transects (31), more than did shrubby pineland (28) or hardwood hammock (21), but fewer of these taxa bear extrafloral nectaries (5 species, or 16% for open pineland, vs. 7 species, or 25% for shrubby pineland, and only three for hardwood hammock, 14%; Table 3). As for the number of individuals of all species encountered in our samples, this indicates more realistically the presence of plants (i.e., cover of plants) bearing extrafloral nectaries. Open pinelands, with 13% of individuals having nectaries, displayed a smaller proportion of individuals with nectaries versus the 26% seen in shrubby pinelands; hardwood hammocks also have roughly 13% of individuals with nectaries, equivalent to the open pinelands in our transects (Table 3).
Seven families of plants (all angiosperms) in our sampling bear extrafloral nectaries (Table 4). Six of these species also occur in Everglades pine rocklands, but two are endemic to the pine rocklands of the lower Florida Keys (Chamaecrista lineata var. keyensis and Pithecellobium keyense).

4. Discussion

Florida is the “ant-iest” of the eastern states in the US, and ants are species-rich in the state due to the warm and humid climate and proximity to the Caribbean [19]. Hammocks occur on slightly higher ground, made higher in part by accumulated leaf litter and other organic materials. Fire and fire history are the chief determinants of the vegetation on the rocklands of the lower Keys [37]. In addition to fire, hurricanes may periodically knock down trees and precipitate tidal inundation, making a changing mosaic of disturbance history. It is likely that disturbance history structures the ant community.
Ant activity was greater than previously observed in forested rockland (hardwood hammocks) in our sites on Big Pine Key, and lower than expected in open pine rocklands, following our earlier studies in the Everglades and the Bahamas. This was especially surprising as the lowest percentage of plant species with extrafloral nectaries occurred in the hardwood hammocks, though the coverage of plants with extrafloral nectaries was equivalent to that of open pine rocklands.
The largest percentage of individual plants with extrafloral nectaries found in shrubby pinelands of the Keys is likely due to the ecotonal nature of pine rocklands that have long remained unburned. Without frequent fires, woody plants grow up and eventually shade out the understory, transforming the land into hardwood hammock as pine trees are outcompeted. It is not surprising that there are more species with extrafloral nectaries as these shrubby pinelands have both herbaceous and woody components, and this leads to greater coverage of plants with nectaries in these habitats, as larger plants with nectaries are encountered in vegetation transects.
Previous studies have found that nectaries on some species present in these Keys habitats gain protection from ants against plants from herbivores [38] and increase fruit set [39]. Several congeners of previously studied plants (Acacia, Chamaecrista, Passiflora, Pithecellobium, and Senna) also occur in Keys rockland habitats and we predict that extrafloral nectaries on those plants may promote plant protection as has been shown in their relatives [40,41,42,43]. Ant protection has been shown to be more effective when plants with extrafloral nectaries grow in sunnier more open habitats [44,45], places where herbivore pressure from insects is also greater.
Most of the ants we encountered with our baiting are ground nesters. Perhaps if we had placed baits on plants as well as on the ground, sampling ants on vegetation, we may have obtained better representation of arboricolous species. Wilson [21] reasoned that tree-nesting species would be more common on these islands subject to periodic saltwater flooding. A thorough assessment of the ant fauna is best done with multiple sampling methods [46].
As development has proceeded in the Keys, there have been concomitant effects on the ant fauna. A survey of ants throughout the Florida Keys [47] found that sites with more native ant species also had more non-native ant species and that non-native ant species’ richness was increased with the amount of development. In particular, the red imported fire ant was more abundant near roads and greater development [48]. Ninety-four ant species were documented in the Keys in 2014 [49]; there may be more today. This may eventually decrease native ant biodiversity as invasive ants often outcompete natives at nectar and other food resources [50].
It is not surprising that our results differ from those obtained by similar methods in pine rocklands of the Everglades [27] (Table 3). The lower Florida Keys experience much lower rainfall than the Everglades and the rock substrate differs in its more pavement-like character, where peninsular rocklands are much more pitted with solution holes. Fire frequency is likely to be much lower, as lightning is less frequent [51]. Comparisons with the Bahamas pine rocklands show the effects of frequent fires on Andros Island (Bahamas) [52] contrasting with much less frequent fire on Big Pine Key; the more recent the fire in the Bahamas, the greater the ant activity (Table 3).
The objectives of this study were met and, while some species of plants and ants are shared, the proportions of plant taxa with extrafloral nectaries are lower in the Keys than in either the Everglades or the Bahamas. The cover of these species in the Keys, however, is comparable to these other areas, and ants readily visit baits in all habitats. The difference appears to be that, in the Keys, ant activity is greater in shadier areas than in open areas, the opposite pattern of what has been observed in studies in the Everglades and the Bahamas. Pine rocklands in the Lower Keys are changing due to rising seas [53] and impacts of hurricanes [54], with pine trees fewer and farther apart. The intense sun exposure due to the lower density of trees in pine rocklands of the Florida Keys [55] may be tempered by more shrubby vegetation, providing more optimal conditions for foraging by ants in those habitats intermediate between earlier successional pine rocklands and late successional hardwood hammocks.

Author Contributions

Conceptualization, S.K. and K.H.K.; methodology, S.K. and K.H.K.; formal analysis, S.K.; investigation, S.K. and K.H.K.; resources, S.K. and K.H.K.; data curation, S.K.; writing—original draft preparation, S.K.; writing—review and editing, K.H.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Upon publication, the data will be freely available in the FIU Research Data Portal at https://doi.org/10.34703/gzx1-9v95/JXVPLR accessed on 20 June 2024.

Acknowledgments

We greatly appreciate the taxonomic expertise of Jaeson Clayborn and Mark Deyrup, who aided insect identification. We thank the staff of the NKDR for access to sites, logistical support, and friendly enthusiasm during the research. This work was conducted under permit # FF04RFKD-2015-002. This is contribution # 1730 to the Institute of the Environment at FIU.

Conflicts of Interest

The authors declare no conflicts of interest.

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Diversity 16 00360 g001
Figure 1. Mean time to discovery (in minutes ± SE) of (A) honey baits and (B) meat baits in Big Pine Key rockland habitats. Lower ant activity is indicated by greater time to discovery. Univariate ANOVA for honey baits: F (2,1) = 20.15, p < 0.0001. For meat baits: F (2,1) = 8.06, p = 0.001. Significantly different times to discovery are represented by different lower-case letters with post-hoc test (p < 0.05); bars with the same letters do not differ significantly.
Figure 1. Mean time to discovery (in minutes ± SE) of (A) honey baits and (B) meat baits in Big Pine Key rockland habitats. Lower ant activity is indicated by greater time to discovery. Univariate ANOVA for honey baits: F (2,1) = 20.15, p < 0.0001. For meat baits: F (2,1) = 8.06, p = 0.001. Significantly different times to discovery are represented by different lower-case letters with post-hoc test (p < 0.05); bars with the same letters do not differ significantly.
Diversity 16 00360 g001
Table 1. Big Pine Key ants and locations encountered May–June 2008. Baits of honey and tuna were employed to sample ant occurrence and abundance (see methods). * = exotic species.
Table 1. Big Pine Key ants and locations encountered May–June 2008. Baits of honey and tuna were employed to sample ant occurrence and abundance (see methods). * = exotic species.
SpeciesOpen PinelandShrubby PinelandHardwood HammockHoney BaitsMeat Baits
Camponotus floridanus XX
Camponotus planatus * XXXX
Forelius pruinosusX X
Odontomachus ruginodisX X
Paratrechina longicornis *XXXX
Pheidole floridanaX XX
Pseudomyrmex elongatus X
Solenopsis geminata X X
Hyalymenus longispinus
(ant-mimicking bug)		 X XX
Table 2. Ant visits to honey and meat baits on Big Pine Key. Numbers in the table are the percentage of twenty baits monitored for one hour experiencing discovery and recruitment.
Table 2. Ant visits to honey and meat baits on Big Pine Key. Numbers in the table are the percentage of twenty baits monitored for one hour experiencing discovery and recruitment.
Open PinelandShrubby PinelandHardwood Hammock
HoneyMeatHoneyMeatHoneyMeat
Discovery
(% baits discovered)		45%65%75%80%100%100%
Recruitment
(% baits with recruitment)		15%40%50%75%100%80%
Table 3. Plant taxa and individuals with extrafloral nectaries in three habitats on Big Pine Key, Florida, and comparative data from previous studies. Data from all transects were combined for each habitat. # = number of; % = proportion of.
Table 3. Plant taxa and individuals with extrafloral nectaries in three habitats on Big Pine Key, Florida, and comparative data from previous studies. Data from all transects were combined for each habitat. # = number of; % = proportion of.
Habitat# Species Encountered in Sampling% Species with Extrafloral Nectaries# Individuals Encountered in Sampling% Individuals with Extrafloral Nectaries
Open pineland315%13113%
Shrubby pineland287%15326%
Hardwood hammock213%7613%
Below: comparative numbers from previous studies with similar habitats
Everglades (Koptur 1992)
Pineland3327%35334%
Hardwood hammock2322%19723%
Andros, Bahamas (Koptur, Williams, and Olive 2002)
Pineyard5225%25618%
Coppice3931%17720%
Table 4. Plants with extrafloral nectaries in sampling transects on Big Pine Key.
Table 4. Plants with extrafloral nectaries in sampling transects on Big Pine Key.
FAMILY
Latin Name, Authority		Common NameNectary Position
COMBRETACEAE
Conocarpus erectus L.buttonbushpetiolar
EUPHORBIACEAE
Croton linearis Jacq.grannybush, pineland crotonpaired on leaf bases, abaxial surface
FABACEAE
Chamaecrista lineata (Sw.) Greene var. keyensis (Pennell) H.S.Irwin and BarnebyBig Pine partridge pea, Key cassia, narrowpod sensitive peaon rachis
Pithecellobium keyense Britton ex Britton and RoseFlorida Keys black beadon rachis
Senna mexicana (Jacq.) H.S.Irwin and Barneby var. chapmanii (Isely) H.S.Irwin and BarnebyBahama senna, Chapman’s wild sensitive planton rachis between lowest pair of leaflets
MYRSINACEAE
Myrsine cubana A.DC.myrsine, colicwoodbase of leaf, abaxial surface
PASSIFLORACEAE
Passiflora suberosa L.corky-stem passionflower, juniper-berrypetiolar
POACEAE
Eragrostis elliottii S. Watsonlovegrassin inflorescence
RUBIACEAE
Morinda royoc L.wild mulberry, rhubarb, mouse’s pineapplepostfloral—on ovary after corolla falls off
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MDPI and ACS Style

Koptur, S.; Keeler, K.H. Comparing Ant Activity and Plants Bearing Extrafloral Nectaries in Rockland Habitats of the Florida Keys with Those of the Everglades and the Bahamas. Diversity 2024, 16, 360. https://doi.org/10.3390/d16070360

AMA Style

Koptur S, Keeler KH. Comparing Ant Activity and Plants Bearing Extrafloral Nectaries in Rockland Habitats of the Florida Keys with Those of the Everglades and the Bahamas. Diversity. 2024; 16(7):360. https://doi.org/10.3390/d16070360

Chicago/Turabian Style

Koptur, Suzanne, and Kathleen H. Keeler. 2024. "Comparing Ant Activity and Plants Bearing Extrafloral Nectaries in Rockland Habitats of the Florida Keys with Those of the Everglades and the Bahamas" Diversity 16, no. 7: 360. https://doi.org/10.3390/d16070360

APA Style

Koptur, S., & Keeler, K. H. (2024). Comparing Ant Activity and Plants Bearing Extrafloral Nectaries in Rockland Habitats of the Florida Keys with Those of the Everglades and the Bahamas. Diversity, 16(7), 360. https://doi.org/10.3390/d16070360

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