BACKGROUND
it is generally thought that stable isotope ratios vary across terrestrial landscapes.
the extent of this spatial variation is enigmatic, and not explicitly built into the design of experiments wherein stable isotopes of carbon and nitrogen are employed to infer the trophic biology of consumers  <cit> .
typically, a landscape may be sampled broadly, with samples pooled together based on the assumption that baseline isotopic ratios will not vary appreciably within one kilometer .
nevertheless, to our knowledge there has been no study to test whether baseline stable isotope ratios are consistent across short distances.
moreover, no study has heretofore elucidated the contribution of spatial variation in isotope ratios to the isotopic variation in the tissue of consumers, relative to the effects of biotic factors shaping spatial patterns in isotopic fractionation.
in a tropical rain forest, nearly half of the aboveground primary production falls to the ground as litter, soon to release its carbon and nutrients into the diverse detrital food web  <cit> .
in tropical rain forest leaf litter, a warehouse of biological diversity, little is known about the functional biology of even the most abundant organisms  <cit> .
the processes that govern detrital food web structure, particularly in tropical forests, are poorly known.
caution is required in the use of isotopes to draw inferences about the trophic position of animals across space, as there is great variance introduced by spatial heterogeneity in the baseline isotope ratios and unknown differences in rates of isotopic discrimination  <cit> .
the leaf litter food web of tropical rainforests is replete with ants, the most ecologically dominant guild of predators in this environment.
most ants in the leaf litter are believed to be generalists in diet, consuming prey from a variety of trophic levels  <cit> , and little is known about the environmental factors that may shape diet and isotopic discrimination.
investigations into the variation in the trophic ecology of generalists have focused on effects of heterogeneity in competition  <cit>  as well as variation in the resource base itself  <cit> .
tropical forests demonstrate great heterogeneity in the density of arthropods within leaf litter.
this heterogeneity tracks limiting nutrients, especially phosphorus  <cit> .
because animals are known to shift their diet in response to changes in resource availability, we may expect that spatial variation in the resource base may account for variation in prey selection by ants in leaf litter.
consequently, parameters of the resource base - including phosphorus, the biomass of leaf litter and the rate of its decomposition - may affect isotopic composition of higher-level consumers.
on the other hand, competitor density and diversity are more often considered to influence the trophic position of generalist animals .
animals with broad diets may shift their feeding to underexploited resources when faced with competition  <cit> .
the effect of competitors on diet should covary with density and diversity along environmental gradients  <cit> .
tropical rain forest leaf litter demonstrates great heterogeneity in the density and diversity of ants, which compete with one another for energy and nutrients derived from the resource base, the litter itself  <cit> .
leaf litter ant densities correspond to characteristics of the resource base as well as microclimate and microhabitat  <cit> .
thus we might expect the density or diversity of competing ants to account for any variation in the isotopic composition of a litter-foraging ant species.
stable isotopes are often used to infer the relative trophic position of animals and how trophic position varies across a landscape  <cit> .
to establish relative trophic position of a single species across the landscape of a single forest, stable isotope approaches integrate the trophic position of the entire consumer chain, provided that the baseline isotopic signature is measured for each locality  <cit> .
shifts in the concentration of stable isotope of nitrogen, 15n, have been used to infer that generalist species have variable diets that respond to proximate environmental conditions  <cit> , though it is unclear how environmental conditions altered the isotope ratio of baseline resources.
interspecific studies using 15n have confirmed that species of ants differ in trophic level, with some ant species occupying trophic positions similar to herbivores  <cit> .
tillberg et al.
<cit>  tested for intraspecific δ15n variation in five species of ants.
they found remarkably broad variation within a species, even within a single locality, suggesting that life history and small-scale environmental factors may interact to result in differences in the isotopic composition of ants.
in the present study, we evaluate the carbon and nitrogen isotopic composition of a generalist species, the gyspy ant aphaenogaster araneoides, across the landscape of a single tropical rain forest.
we demonstrate environmental parameters that robustly predict the isotope ratio of a.araneoides, and then account for landscape scale variation to test competing models to account for isotope discrimination factors.
RESULTS
mean δ15n of a. araneoides from the seven sample sites within the la selva forest ranged from  <dig> ‰ to  <dig> ‰, and mean δ15n of leaf litter ranged from  <dig> ‰ to  <dig> ‰, and the δ15n discrimination factor  ranged from  <dig> ‰ to  <dig> ‰.
among the soil nutrients evaluated, p accounted for three quarters of the variance of δ15n in a. araneoides , consistent with the strong relationship between soil p and leaf litter δ15n across the landscape .
mean δ13c in ant tissue ranged from - <dig> ‰ to - <dig> ‰, and from - <dig> ‰ to - <dig> ‰ in leaf litter.
no soil nutrient accounted for more than a quarter of the variance in δ13c .
among sampling sites, mean ant δ13c was independent of mean ant δ15n .
soil nutrients were evaluated as stocks in kg/ha to  <dig> cm depth.
δ15n and δ13c values represent mean values of a. araneoides among seven sites.
nearly all of the variance in the δ15n discrimination factor of a. araneoides relative to leaf litter was predicted by local colony growth  and mass of standing leaf litter  .
the most parsimonious model for the c source of a. araneoides indicated that nearly all of the variance was predicted by colony size  and leaf litter decomposition rate  .
the most parsimonious model is indicated by *.
negative relationships with relative trophic position are indicated by italics.
the most parsimonious model is indicated by *.
all significant relationships were positive.
discussion
the variation in δ15n and δ13c in a. araneoides within this 500-ha forest landscape was of equivalent magnitude to that reported from other ant species that were sampled over a much greater geographic extent by tillberg et al  <cit> .
it was surprising to find that soil phosphorus was such a tight predictor of the δ15n of a. araneoides, considering the proximity of sampling sites and multiple unknown trophic links between soil and this common predator.
we found this relationship occurred because the variance in phosphorus throughout the  <dig> ha.
forest was predictive of δ15n of leaf litter, which is the base of the food web for a. araneoides.
across this forest landscape, the standing stocks and rates of decomposition of the leaf litter were principal determinants of the isotopic composition of gypsy ants.
if one assumes that δ15n discrimination indicates relative trophic position and that the trophic shift of 15n is about  <dig>  then gypsy ants range from one to two trophic steps above leaf litter, with a higher trophic position positively associated with the biomass of standing leaf litter.
such assumptions about δ15n discrimination may not withstand continued scrutiny  <cit> .
regardless, the rate of decomposition of leaf litter at each sample site was the environmental parameter that best accounted for the source of c in the diet of the local gypsy ants.
these findings suggest that the basic properties of leaf litter, and the abiotic and biotic factors that govern these properties in tropical rain forests, are essentially regulating the isotopic composition of a generalist predator in the complex environment of the leaf litter food web.
the degree of δ15c discrimination from litter to ants was slight, though the variation that occurred was well accounted by environmental parameters.
we found that ants with greater δ13c discrimination factors came from sites with faster leaf litter decomposition and greater mean colony sizes.
we can infer that the source of c varies across the fertility gradient, though we will have less solid conclusions about the steps in the food web that can account for this variation, as the fractionation of c in litter food webs is open to speculation.
carbon experiences variable rates of fractionation depending on the nature of the trophic step  <cit> .
as the rate of decomposition is expected to be closely tied to rates of microbial respiration, we may expect that the weighted source of c in the litter food web - from air, litter and soil - may vary along the decomposition gradient.
complicating matters further is the possibility that δ13c varies along a vertical gradient, because at the ground level much of the co <dig> assimilated by leaves is of respiratory origin, but in the canopy much of the co <dig> fixed in photosynthesis is of atmospheric origin.
the c in forest-floor leaf litter is from a mélange of leaves from all vertical strata, and we are not able to create a defensible mixing model to evaluate c source.
nevertheless, because δ15n and δ13c tightly covaried among species of collembola along another decomposition gradient  <cit> , we might surmise that the differences in δ13c among sites may be attributed to differences in trophic structure above the level of primary consumers.
this might explain the association with colony size, as nutritional demands of the colony may change with age  <cit> .
the present results reinforce the view that stable isotope ratios for a given species - regardless of its diet or other habits - are apt to vary across space.
we have found that much of this variance may be attributed to ambient variation in the isotopic composition of the base of the food web.
in this circumstance, the discrimination factor of δ15n is governed by p abundance.
the mechanism behind the spatial coupling of p and δ15n remains open to speculation.
two non-competing possibilities include differential rates of n fixation and differential physiological rates of n fractionation under conditions of p limitation  <cit> .
we found that the n stable isotope ratio within gypsy ant tissue varied across the range of  <dig> per mil, a magnitude on par with similar results for other ant species, from collections spanning much larger geographic areas  <cit> .
after accounting for the baseline isotopic composition at each locality within the forest, we found that dietary breadth was not as broad as suggested by the ant-tissue δ15n values alone.
this finding suggests that comparisons of δ15n for sessile organisms  must be constrained to very short distances unless baseline data are collected from each locality.
CONCLUSIONS
phosphorus limitation closely tracked the δ15n of gypsy ants because the δ15n of the base of the food web varied predictably with phosphorus at a fine spatial scale.
the δ15n discrimination factor of gypsy ants was most parsimoniously explained by the mass of leaf litter on the forest floor.
spatial variation in δ13c in ants, relative to the resource base, was almost wholly accounted by colony life history and the rate of leaf litter decomposition.
in terrestrial environments, the failure to incorporate fine-scale information on the isotopic composition of baseline resources may erroneously identify environmental factors predicting trophic position.

 it is generally thought that stable isotope ratios vary across terrestrial landscapes. the extent of this spatial variation is enigmatic, and not explicitly built into the design of experiments wherein stable isotopes of carbon and nitrogen are employed to infer the trophic biology of consumers. typically, a landscape may be sampled broadly, with samples pooled together based on the assumption that baseline isotopic ratios will not vary appreciably within one kilometer. nevertheless, to our knowledge there has been no study to test whether baseline stable isotope ratios are consistent across short distances. moreover, no study has heretofore elucidated the contribution of spatial variation in isotope ratios to the isotopic variation in the tissue of consumers, relative to the effects of biotic factors shaping spatial patterns in isotopic fractionation. in a tropical rain forest, nearly half of the aboveground primary production falls to the ground as litter, soon to release its carbon and nutrients into the diverse detrital food web. in tropical rain forest leaf litter, a warehouse of biological diversity, little is known about the functional biology of even the most abundant organisms. the processes that govern detrital food web structure, particularly in tropical forests, are poorly known. caution is required in the use of isotopes to draw inferences about the trophic position of animals across space, as there is great variance introduced by spatial heterogeneity in the baseline isotope ratios and unknown differences in rates of isotopic discrimination. the leaf litter food web of tropical rainforests is replete with ants, the most ecologically dominant guild of predators in this environment. most ants in the leaf litter are believed to be generalists in diet, consuming prey from a variety of trophic levels, and little is known about the environmental factors that may shape diet and isotopic discrimination. investigations into the variation in the trophic ecology of generalists have focused on effects of heterogeneity in competition as well as variation in the resource base itself. tropical forests demonstrate great heterogeneity in the density of arthropods within leaf litter. this heterogeneity tracks limiting nutrients, especially phosphorus. because animals are known to shift their diet in response to changes in resource availability, we may expect that spatial variation in the resource base may account for variation in prey selection by ants in leaf litter. consequently, parameters of the resource base - including phosphorus, the biomass of leaf litter and the rate of its decomposition - may affect isotopic composition of higher-level consumers. on the other hand, competitor density and diversity are more often considered to influence the trophic position of generalist animals. animals with broad diets may shift their feeding to underexploited resources when faced with competition. the effect of competitors on diet should covary with density and diversity along environmental gradients. tropical rain forest leaf litter demonstrates great heterogeneity in the density and diversity of ants, which compete with one another for energy and nutrients derived from the resource base, the litter itself. leaf litter ant densities correspond to characteristics of the resource base as well as microclimate and microhabitat. thus we might expect the density or diversity of competing ants to account for any variation in the isotopic composition of a litter-foraging ant species. stable isotopes are often used to infer the relative trophic position of animals and how trophic position varies across a landscape. to establish relative trophic position of a single species across the landscape of a single forest, stable isotope approaches integrate the trophic position of the entire consumer chain, provided that the baseline isotopic signature is measured for each locality. shifts in the concentration of stable isotope of nitrogen, 15n, have been used to infer that generalist species have variable diets that respond to proximate environmental conditions, though it is unclear how environmental conditions altered the isotope ratio of baseline resources. interspecific studies using 15n have confirmed that species of ants differ in trophic level, with some ant species occupying trophic positions similar to herbivores. tillberg et al. tested for intraspecific δ15n variation in five species of ants. they found remarkably broad variation within a species, even within a single locality, suggesting that life history and small-scale environmental factors may interact to result in differences in the isotopic composition of ants. in the present study, we evaluate the carbon and nitrogen isotopic composition of a generalist species, the gyspy ant aphaenogaster araneoides, across the landscape of a single tropical rain forest. we demonstrate environmental parameters that robustly predict the isotope ratio of a.araneoides, and then account for landscape scale variation to test competing models to account for isotope discrimination factors.  mean δ15n of a. araneoides from the seven sample sites within the la selva forest ranged from ‰ to ‰, and mean δ15n of leaf litter ranged from ‰ to ‰, and the δ15n discrimination factor ranged from ‰ to ‰. among the soil nutrients evaluated, p accounted for three quarters of the variance of δ15n in a. araneoides, consistent with the strong relationship between soil p and leaf litter δ15n across the landscape. mean δ13c in ant tissue ranged from - ‰ to - ‰, and from - ‰ to - ‰ in leaf litter. no soil nutrient accounted for more than a quarter of the variance in δ13c. among sampling sites, mean ant δ13c was independent of mean ant δ15n. soil nutrients were evaluated as stocks in kg/ha to cm depth. δ15n and δ13c values represent mean values of a. araneoides among seven sites. nearly all of the variance in the δ15n discrimination factor of a. araneoides relative to leaf litter was predicted by local colony growth and mass of standing leaf litter. the most parsimonious model for the c source of a. araneoides indicated that nearly all of the variance was predicted by colony size and leaf litter decomposition rate. the most parsimonious model is indicated by *. negative relationships with relative trophic position are indicated by italics. the most parsimonious model is indicated by *. all significant relationships were positive. discussion the variation in δ15n and δ13c in a. araneoides within this 500-ha forest landscape was of equivalent magnitude to that reported from other ant species that were sampled over a much greater geographic extent by tillberg et al. it was surprising to find that soil phosphorus was such a tight predictor of the δ15n of a. araneoides, considering the proximity of sampling sites and multiple unknown trophic links between soil and this common predator. we found this relationship occurred because the variance in phosphorus throughout the ha. forest was predictive of δ15n of leaf litter, which is the base of the food web for a. araneoides. across this forest landscape, the standing stocks and rates of decomposition of the leaf litter were principal determinants of the isotopic composition of gypsy ants. if one assumes that δ15n discrimination indicates relative trophic position and that the trophic shift of 15n is about then gypsy ants range from one to two trophic steps above leaf litter, with a higher trophic position positively associated with the biomass of standing leaf litter. such assumptions about δ15n discrimination may not withstand continued scrutiny. regardless, the rate of decomposition of leaf litter at each sample site was the environmental parameter that best accounted for the source of c in the diet of the local gypsy ants. these findings suggest that the basic properties of leaf litter, and the abiotic and biotic factors that govern these properties in tropical rain forests, are essentially regulating the isotopic composition of a generalist predator in the complex environment of the leaf litter food web. the degree of δ15c discrimination from litter to ants was slight, though the variation that occurred was well accounted by environmental parameters. we found that ants with greater δ13c discrimination factors came from sites with faster leaf litter decomposition and greater mean colony sizes. we can infer that the source of c varies across the fertility gradient, though we will have less solid conclusions about the steps in the food web that can account for this variation, as the fractionation of c in litter food webs is open to speculation. carbon experiences variable rates of fractionation depending on the nature of the trophic step. as the rate of decomposition is expected to be closely tied to rates of microbial respiration, we may expect that the weighted source of c in the litter food web - from air, litter and soil - may vary along the decomposition gradient. complicating matters further is the possibility that δ13c varies along a vertical gradient, because at the ground level much of the co assimilated by leaves is of respiratory origin, but in the canopy much of the co fixed in photosynthesis is of atmospheric origin. the c in forest-floor leaf litter is from a mélange of leaves from all vertical strata, and we are not able to create a defensible mixing model to evaluate c source. nevertheless, because δ15n and δ13c tightly covaried among species of collembola along another decomposition gradient, we might surmise that the differences in δ13c among sites may be attributed to differences in trophic structure above the level of primary consumers. this might explain the association with colony size, as nutritional demands of the colony may change with age. the present results reinforce the view that stable isotope ratios for a given species - regardless of its diet or other habits - are apt to vary across space. we have found that much of this variance may be attributed to ambient variation in the isotopic composition of the base of the food web. in this circumstance, the discrimination factor of δ15n is governed by p abundance. the mechanism behind the spatial coupling of p and δ15n remains open to speculation. two non-competing possibilities include differential rates of n fixation and differential physiological rates of n fractionation under conditions of p limitation. we found that the n stable isotope ratio within gypsy ant tissue varied across the range of per mil, a magnitude on par with similar results for other ant species, from collections spanning much larger geographic areas. after accounting for the baseline isotopic composition at each locality within the forest, we found that dietary breadth was not as broad as suggested by the ant-tissue δ15n values alone. this finding suggests that comparisons of δ15n for sessile organisms must be constrained to very short distances unless baseline data are collected from each locality.  phosphorus limitation closely tracked the δ15n of gypsy ants because the δ15n of the base of the food web varied predictably with phosphorus at a fine spatial scale. the δ15n discrimination factor of gypsy ants was most parsimoniously explained by the mass of leaf litter on the forest floor. spatial variation in δ13c in ants, relative to the resource base, was almost wholly accounted by colony life history and the rate of leaf litter decomposition. in terrestrial environments, the failure to incorporate fine-scale information on the isotopic composition of baseline resources may erroneously identify environmental factors predicting trophic position. 
after taking into account landscape-scale variation in baseline resources, the most parsimonious model indicated that colony growth and leaf litter biomass accounted for nearly all of the variance in the δ15n discrimination factor, whereas the δ13c discrimination factor was most parsimoniously associated with colony size and the rate of leaf litter decomposition. the δ15n discrimination factor of a higher order consumer in this food web was structured by bottom-up influences - the quantity and decomposition rate of leaf litter. we found that 75% of the variance in the fraction of 15n in the tissue of a. araneoides was accounted by one environmental parameter, the concentration of soil phosphorus. stable isotope studies on the trophic biology of consumers may benefit from explicit spatial design to account for edaphic properties that alter the baseline at fine spatial grains. we test competing models to account for relative influence of resources and competitors to the carbon and nitrogen isotopic enrichment of gypsy ants, taking into account site-specific differences in baseline isotope ratios. the isotopic composition of generalist consumers may be expected to vary in space as a consequence of spatial heterogeneity in isotope ratios, the abundance of resources, and competition. landscape, soil phosphorus accounted for spatial variation in baseline nitrogen isotope ratios. we aim to account for the spatial variation in the carbon and nitrogen isotopic composition of a generalized predatory species across a ha. there was no indication that competitor density or diversity accounted for spatial differences in the isotopic composition of gypsy ants.
 the isotopic composition of generalist consumers may be expected to vary in space as a consequence of spatial heterogeneity in isotope ratios, the abundance of resources, and competition. we aim to account for the spatial variation in the carbon and nitrogen isotopic composition of a generalized predatory species across a ha. tropical rain forest landscape. we test competing models to account for relative influence of resources and competitors to the carbon and nitrogen isotopic enrichment of gypsy ants, taking into account site-specific differences in baseline isotope ratios.  we found that 75% of the variance in the fraction of 15n in the tissue of a. araneoides was accounted by one environmental parameter, the concentration of soil phosphorus. after taking into account landscape-scale variation in baseline resources, the most parsimonious model indicated that colony growth and leaf litter biomass accounted for nearly all of the variance in the δ15n discrimination factor, whereas the δ13c discrimination factor was most parsimoniously associated with colony size and the rate of leaf litter decomposition. there was no indication that competitor density or diversity accounted for spatial differences in the isotopic composition of gypsy ants.  across a ha. landscape, soil phosphorus accounted for spatial variation in baseline nitrogen isotope ratios. the δ15n discrimination factor of a higher order consumer in this food web was structured by bottom-up influences - the quantity and decomposition rate of leaf litter. stable isotope studies on the trophic biology of consumers may benefit from explicit spatial design to account for edaphic properties that alter the baseline at fine spatial grains. 