Research Article |
Corresponding author: Paul J. Taillie ( ptaillie@unc.edu ) Academic editor: Sven Bacher
© 2024 Paul J. Taillie, Wesley W. Boone IV, Alexandra L. Wilson-Seelig, Robert McCleery.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Taillie PJ, Boone IV WW, Wilson-Seelig AL, McCleery R (2024) Diet comparison suggests limited competition between invasive black rats (Rattus rattus) and sympatric endangered rodents. NeoBiota 94: 145-158. https://doi.org/10.3897/neobiota.94.121287
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Black rats (Rattus rattus) are one of the most widespread invasive animals and have been implicated in the decline of species representing several wildlife taxa, particularly on islands. However, their impact on more closely related species, i.e. rodents, via competition is less well-understood. Using diet similarity as a metric of competition for food resources, we used stable isotopes to compare diets of two populations of black rats to diets of two endangered populations of rice rats (Oryzomys palustris natator and Oryzomys palustris sanibeli) in southern Florida, USA. Specifically, we analysed hair samples from 32 rice rats and 35 black rats for carbon (δ13C) and nitrogen (δ15N) isotopes. In addition, we analysed samples of 129 potential food items to characterise rodent diets using stable isotope mixing models. Despite considerable overlap in isotope-space, we observed differences between rice rats and black rats in the relative composition of plant and animal foods. Specifically, the diets of both populations of rice rats consisted of mostly animal foods, whereas the diets of black rats consisted mostly of plants. In combination with previous work revealing temporal niche partitioning, our results suggest competition between invasive black rats and endangered native rodents may be limited. As such, expensive and logistically complicated efforts to control black rats may have limited success for conserving endangered rodents.
Florida, island, mangrove, rodent, stable Isotope, Wetland
Invasive species represent a growing threat to global biodiversity (
In addition to direct predation, black rats can affect native fauna through other interspecific interactions, such as competition (
In contrast to the Grinnellian conceptualisation of a species’ niche, where the focus is on the environmental characteristics of a species’ range, the Eltonian conceptualisation of niche focuses on functional traits and interspecific trophic interactions (
Though previously understudied because of lack of data (
In southern Florida, USA, diverse native wildlife communities are being transformed by multiple invasions by introduced species, making this region ideal for examining the role of competition between invasive and native species within taxonomic groups (e.g. Rodentia). Specifically, black rats have been implicated as a threat to multiple native and endangered rodents in southern Florida (
We used stable isotopes to investigate the potential for competition between black rats and native rodents. Specifically, we compared the isotopic niches of invasive black rats and native rice rats (Oryzomys spp.) on two island groups in southern Florida, USA, which we used as a proxy for their Eltonian niches. In addition, we collected potential food items and used stable isotope mixing models to compare the diet composition amongst populations. Due to their more specialised, carnivorous diet (Sharp 1967;
We quantified trophic niches of four rodent populations on two island groups (hereafter: “islands”) in southern Florida, USA. Each of these islands supports a small-ranging subspecies of the marsh rice rat (O. palustris spp.). The Sanibel island rice rat (Oryzomys palustris sanibeli) is endemic to the Sanibel-Captiva barrier island complex on the south-western coast of Florida (Fig.
On both Sanibel and the Lower Keys, we trapped areas known to support the focal subspecies of Oryzomys palustris spp. (i.e. Sanibel Island rice rats and silver rice rats, respectively). All rodent isotope samples were collected between 1 October and 31 December 2021. At a given site, we deployed a grid of 25 Sherman traps for four consecutive nights. Each night, traps were opened within 2 hours of sunset and closed within 3 hours of sunrise the following morning. Upon closing traps, we collected all captured rodents and recorded the species, weight, length, and sex of each. In addition, we used small scissors to collect a ~ 2 mg sample of dorsal guard hairs to be analysed for stable isotopes. Each sample was stored in a sealable plastic bag and was frozen within 12 hours. All trapping and handling methods were approved by the University of Florida Animal Use and Care Committee (#202110390).
At each site, while traps were deployed, we opportunistically collected potential food items, based on previous studies of the diets of Oryzomys palustris spp. (Suppl. material
We first quantified each population’s Eltonian niche by plotting the values of δ13C and δ15N in isotope-space. We made qualitative comparisons amongst the four populations by comparing the relative positions of individuals in isotope-space. In addition, we used an ANOVA to test for statistically significant (p < 0.05) differences of each isotope between rice rats and black rats on each island. We then considered the area of isotope-space occupied by a population as a metric of niche breadth. Specifically, we used the standard ellipse area corrected for small sample sizes (SEAc) to quantify and compare the isotopic niche breadth of rice rats and black rats on each island. To account for variation in sample size amongst populations and to formally account for uncertainty (
To compare the relative position of the ellipses in isotope-space, we used two metrics of ellipse overlap. First, we calculated the proportion of the overlapping area to the total area of the ellipses being compared (hereafter: “total area proportion”). This total area proportion could range from 0 (no overlap) to 1 (total overlap). As black rats are generalists (
To link isotope signatures from rodent tissues to food items and make inferences about rodent diets, we used stable isotope mixing models using the R package simmr (
Our rodent trapping efforts resulted in hair samples from 23 silver rice rats and 18 black rats on the Lower Keys, as well as 9 Sanibel Island rice rats and 17 black rats on Sanibel. Though we trapped 11 sites on Sanibel Island where Sanibel Island rice rats were captured previously, we only detected them at three of these 11 sites. Meanwhile, we collected 78 potential food items, which we supplemented with 51 additional items collected from Sanibel in 2017 (Suppl. material
Broadly, we observed greater values of both δ13C and δ15N in rice rats compared to black rats on their respective islands (Table
The nitrogen and carbon stable isotope values for four populations of rodents on two islands (Lower Keys and Sanibel-Captiva) in southern Florida, USA. The corresponding ellipses represent the posterior distributions of estimated bi-variate ellipses used to compare isotopic overlap. Shown also are the means (black dots) and standard deviations (coloured crosses) of the stable isotope values of the three taxa of potential food items (i.e. sources).
Minimum and maximum isotopic values and Standard Ellipse Area with small sample correction for four populations of rodents in southern Florida, USA.
Species | Island | Range δ13C | Range δ15N | Mean δ13C | Mean δ15N | SEAc |
---|---|---|---|---|---|---|
Silver Rice Rat | Keys | -22.8, -18.4 | 4.8, 7.9 | -20.2 | 6.6 | 2.9 |
Black Rat | Keys | -23.5, -20.1 | 1.8, 7.0 | -22.2 | 4.3 | 3.4 |
Sanibel Island Rice Rat | Sanibel | -23.6, -20.1 | 5.5, 8.1 | -22.7 | 6.6 | 2.7 |
Black Rat | Sanibel | -25.2, -21.3 | 1.7, 5.1 | -24.3 | 3.9 | 3.1 |
Despite this overlap, stable isotope mixing models suggested the diets of native rice rats differed from those of black rats on their respective islands. On the Keys, there was a high probability (P > 0.9) that rice rats consumed more animals and fewer plants compared to black rats, which consumed more plants (Fig.
Posterior distributions and boxplots comparing the relative proportion of three food categories (animal, C3 plant and C4 plant) between two genera of rodents (rice rats [Oryzomys] and black rats [Rattus]) in each of two islands in southern Florida, USA (the Lower Keys and Sanibel-Captiva; 2021). Included in each panel is the probability (P) that the food item’s proportional composition of diet is greater for one genus when compared to the other.
Consistent with our expectation, the isotopic niche of black rats was larger than that of rice rats, reflecting a more generalist diet. Both rice rat populations appeared to be more carnivorous than black rats, evidenced by greater δ15N than black rats on their respective islands. However, there was considerable overlap in isotope-space between rice rats and black rats on both islands. Despite this overlap, we observed two important differences in diet. First, rice rats consumed comparable amounts of animal and plant foods, compared to both populations of black rats, which ate mostly plants. On the Keys specifically, rice rats consumed mostly animal foods. These differences in diet suggest competition between native rice rats and exotic black rats may be limited. Second, the diet composition and stable isotope signatures of closely related rodents, as well as the food they consume, differed between the islands we investigated suggesting that the relative impact of black rats on native rodents may vary considerably.
Broadly, we observed that rice rats specialised in animal foods, whereas black rats consumed mostly plants, which is consistent with other studies of rodent diets (
Importantly, we observed some notable differences between the Lower Keys and Sanibel. Though rice rats on both island groups consumed primarily animal foods, plants represented a larger proportion of rice rat diets on Sanibel. Similarly, black rats on Sanibel consumed proportionally more plants compared to black rats on the Lower Keys. These between-island differences could result from differences in marine subsidies that have been shown to be important to mammalian diets (
On both islands, isotope signatures of rice rats and black rats differed more in the values of δ15N than δ13C, suggesting that these differences in diets were driven more by trophic levels than primary producer photosynthetic pathways. The minimal differentiation in δ13C was surprising given that we expected differential consumption of native C4 plants (e.g. grasses), C4 plants in anthropogenic foods (e.g. corn) and marine foods between rodent populations, all of which have been shown previously to influence δ13C (
One important limitation of our study is that we only sampled rodent diets during a single season (late autumn/early winter). Previous studies have shown that stable isotopes can vary seasonally as a function of diet, as well as other factors (
Our results suggest that the degree of competition between black rats and native rodents may vary as a function of environmental factors, such as prey availability and geographical context. Consequently, responses amongst native rodents to invasive black rats and the need for control or eradication efforts may also vary. Therefore, previously documented conservation successes for taxa such as seabirds resulting from invasive rat eradication efforts (
We thank our dedicated field technicians A. Veselka, K. Carey, and A. Merchlinsky who made the project possible. In addition, we thank United States Fish & Wildlife staff (K. Kalasz and Sanda Sneckenberger, among others) for providing assistance with accommodations, access, and logistics. Lastly, we thank the University of Florida Stable Isotope Lab for processing isotope samples.
The authors have declared that no competing interests exist.
No ethical statement was reported.
No funding was reported.
Conceptualization: RM, WWBI, PT. Data curation: WWBI, ALWS, PT. Formal analysis: PT. Funding acquisition: PT, RM. Investigation: ALWS. Methodology: ALWS, RM, WWBI. Project administration: RM, PT. Supervision: WWBI. Writing – original draft: PT. Writing – review and editing: RM.
Paul Taillie https://orcid.org/0000-0001-7172-3589
Wesley W. Boone https://orcid.org/0000-0003-4721-464X
All of the data that support the findings of this study are available in the main text or Supplementary Information.
Data and summary statistics
Data type: docx