4. Discussion

We found that Pisonia had the highest N and Tabebuia had lowest P. Also, Ficus had higher arthropod abundance, while Pisonia and Ficus had higher arthropod richness than the other plant species. Species composition of detritivore arthropods was different among plant species, and three clusters were formed: arthropod species composition under Ficus, Pisonia, and Tabebuia, species composition under Coccoloba, and species composition under Conocarpus. We also found that morphotypes that grouped Ficus, Pisonia, and Tabebuia were located toward the high N side of the vector, while those unique to Coccoloba and Conocarpus were at the low N side of the vector. These data suggest that physicochemical foliar traits of plants directly influence litter arthropods on the lower trophic levels of the decomposer food web.

#### 4.1. Nutrients

When compared to species growing in other dry forests, the five tree species in this study are within the range for N and for P at the lower end [32] corroborating the data of Lugo and Murphy [17]. We found that green leaf nutrients varied among species. In Guánica, for a mature stand and pooled leaves from a sample, Lugo and Murphy [17] reported 16.4 mg/g N and 0.64 mg/g P. For N, our pooled average, 16.9 (2.7) mg/g, was similar to Lugo and Murphy, while our average for P was higher, 0.92 (0.2) mg/g, than in Lugo and Murphy. At the species level, N was higher in Pisonia and lower in Conocarpus, while the other three species were similar to the reported value. We found P to be similarly higher in all species when compared to Tabebuia. In addition, Lugo and Murphy reported that N:P ratio (on a dry weight basis) was 25, while in this study, we found the pooled average of N:P to be 20.4 suggesting that the plants near the coastal cliff grow with similar soil P limitation than plants uphill. For Pisonia, Medina and Cuevas [18] report nutrient concentration values that are similar to those found in this study, 18.9 mg/g N and 0.95 mg/g P. For Tabebuia growing in the Luquillo Experimental Forest (wet forest), Sánchez et al. [33] reported N 12–16 mg/g and P 0.8–1.3 mg/ g. The similarity of N and P concentration in Tabebuia between two contrasting sites, such as dry and wet forests, shows the plasticity of the species to adapt to different climatic regimes. The P limitation in Guánica (dry forest) is due to the high P fixing capacity of the substrate, while the P limitation in Luquillo (wet forest) is due to highly weathered soils with low P availability due to iron (Fe) fixation.

#### 4.2. Arthropods

Total arthropod abundance was similar among plant species, but four arthropod orders were more abundant under specific plant species. Milcu et al. [34] found that decomposer species performed better under some plant species than under others because of resource quality and because of the presence of other decomposer species. These data suggest that the higher abundance of these four orders might be related to interactions with other soil fauna species and to resource quality. We also found that richness and identity of arthropods were different among plant species (38 morphotypes common to all plant species, 17 unique to Coccoloba, 20 to Conocarpus, 39 common to Ficus and Pisonia, and 33 to Ficus and Tabebuia) [28]. These data suggest that plant species identity differently influence the number and identity of arthropod species associated to the decomposing organic matter produced by each plant. De Deyn et al. [35] found that the identity of the plant species (i.e., resource quality) was the most important factor for soil nematode diversity; these findings support the idea that, similarly to nematodes, arthropod diversity is influenced by the plant species identity.

These data suggest that arthropods that depend directly on resource quality, and thus have a tight relationship with the resource, were significantly affected by the identity of plant species. In addition, it also suggests that arthropods in higher trophic levels, such as predators, are more generalist; that plant species identity effect does not cascade up; and that the exposed rocky terrain that separates the individual trees does not constitute a barrier for them to move among tree species.

#### 4.3. Idiosyncratic effects

graph (near the intercept of X and Y) such as G-002, G-003, G-004, G-007, and G-274, while those that occurred only in high N species (e.g., Ficus and Pisonia) are located in the upper half of the graph such as G-109, G-102, G-118, G-301, and G-119. Those detritivores that occurred

We found that Pisonia had the highest N and Tabebuia had lowest P. Also, Ficus had higher arthropod abundance, while Pisonia and Ficus had higher arthropod richness than the other plant species. Species composition of detritivore arthropods was different among plant species, and three clusters were formed: arthropod species composition under Ficus, Pisonia, and Tabebuia, species composition under Coccoloba, and species composition under Conocarpus. We also found that morphotypes that grouped Ficus, Pisonia, and Tabebuia were located toward the high N side of the vector, while those unique to Coccoloba and Conocarpus were at the low N side of the vector. These data suggest that physicochemical foliar traits of plants directly

When compared to species growing in other dry forests, the five tree species in this study are within the range for N and for P at the lower end [32] corroborating the data of Lugo and Murphy [17]. We found that green leaf nutrients varied among species. In Guánica, for a mature stand and pooled leaves from a sample, Lugo and Murphy [17] reported 16.4 mg/g N and 0.64 mg/g P. For N, our pooled average, 16.9 (2.7) mg/g, was similar to Lugo and Murphy, while our average for P was higher, 0.92 (0.2) mg/g, than in Lugo and Murphy. At the species level, N was higher in Pisonia and lower in Conocarpus, while the other three species were similar to the reported value. We found P to be similarly higher in all species when compared to Tabebuia. In addition, Lugo and Murphy reported that N:P ratio (on a dry weight basis) was 25, while in this study, we found the pooled average of N:P to be 20.4 suggesting that the plants near the coastal cliff grow with similar soil P limitation than plants uphill. For Pisonia, Medina and Cuevas [18] report nutrient concentration values that are similar to those found in this study, 18.9 mg/g N and 0.95 mg/g P. For Tabebuia growing in the Luquillo Experimental Forest (wet forest), Sánchez et al. [33] reported N 12–16 mg/g and P 0.8–1.3 mg/ g. The similarity of N and P concentration in Tabebuia between two contrasting sites, such as dry and wet forests, shows the plasticity of the species to adapt to different climatic regimes. The P limitation in Guánica (dry forest) is due to the high P fixing capacity of the substrate, while the P limitation in Luquillo (wet forest) is due to highly weathered soils with low P

Total arthropod abundance was similar among plant species, but four arthropod orders were more abundant under specific plant species. Milcu et al. [34] found that decomposer species

influence litter arthropods on the lower trophic levels of the decomposer food web.

only under low P species (e.g., Tabebuia) include G-234, G-236, and G-272.

4. Discussion

12 Tropical Forests - New Edition

4.1. Nutrients

availability due to iron (Fe) fixation.

4.2. Arthropods

Aboveground plant species composition was the best predictor of arthropod assemblages [36], and arthropod species with specific requirements were associated to specific habitats [37]. Similarly, one can expect belowground arthropod assemblages to be best predicted by plant species and litter arthropod species to have specific nutrimental requirements. In our study, unique arthropod species in Ficus and Pisonia were located toward the high N vector, while unique arthropod species in Coccoloba and Conocarpus toward the low N vector. These data suggest that unique arthropod species respond to high nutritional content in high-quality plant species, while unique arthropod species respond to low nutritional content in lowquality plant species.

Litter decomposes faster in areas dominated by the plant species that produced it, the home-field advantage effect [38]. Home-field advantage has been related to the specialization of biota on litter produced by their plant through specialized enzymes, feeding on specialized fungi or animals using litter fragments in survival activities [39], and is also most pronounced in lowquality litter [40]. In decomposer food webs, lower trophic levels influence plant productivity more than higher trophic levels, and given that there is high redundancy within trophic groups, plant productivity is independent of what species are present as long as all of the trophic groups are present [41]. In addition, identity of plants affected the response of arthropods. For example, collembolans were positively affected by grasses and negatively by legumes, while earthworms were positively affected by legumes, suggesting that arthropod response varies depending on the group and nutrients [34]. Our data can be thus interpreted as arthropod species composition of lower trophic groups responds to variations in plant species characteristics, and the response depends upon the nutritional characteristics of the plant, in this case high or low N, which are correlated with the nutritional characteristics of the detritus the plant produces [4].

## 5. Conclusions

We expected that arthropod abundance, richness, and species as well as trophic composition would be differentially affected by the identity of the plant species. We found that the abundance of four arthropod orders was affected; also, total arthropod richness and species composition varied significantly specifically due to the response that detritivores had to physicochemical foliar traits (the only trophic group that differed among plant species). The CCA indicated that detritivore response is linked to aboveground nutritional content of plants. Wardle [3] suggests that the decomposing fauna is tightly associated to the detritus produced by plant species so that this association maximizes the decomposition and nutrient cycling. Therefore, differences in quality among plant species potentially influence litter-feeding arthropods. On the other hand, St. John et al. [42] found that mite assemblages were not affected by the identity of the grass species that mites inhabited neither in abundance, richness, or the composition. Our data support Wardle's ideas [3]. When pooled together our data suggest that litter arthropods in the lower trophic levels, such as detritivores (e.g., Acari, Psocoptera, and Diplopoda), perform better under specific plant species (therefore supporting Milcu et al.'s [34] findings) possibly because they are tied to resource quality (therefore supporting Wardle's ideas).

Class Order Morpho. Coccoloba Conocarpus Ficus Pisonia Tabebuia Arachnida Acari G-004 470 (462) 320 (385) 340 (284) 490 (431) 1330 (3231) Arachnida Acari G-002 460 (723) 80 (92) 340 (299) 210 (328) 1200 (1624) Arachnida Acari G-105 200 (249) 90 (120) 340 (259) 280 (220) 710 (1186) Arachnida Acari G-271 150 (440) 120 (114) 310 (493) 40 (126) 210 (321) Arachnida Acari G-274 50 (108) 10 (32) 250 (756) 10 (32) 30 (48) Arachnida Acari G-188 40 (84) 40 (84) 90 (160) 130 (279) 180 (193) Arachnida Acari G-147 30 (67) 60 (70) 380 (278) 270 (374) 160 (143) Arachnida Acari G-087 20 (42) 90 (185) 30 (67) 130 (189) 20 (42) Arachnida Acari G-244 20 (42) 150 (372) 210 (354) 320 (452) 140 (158) Malacostraca Isopoda G-010 10 (32) 290 (882) 30 (48) 170 (177) 20 (63) Arachnida Acari G-120 10 (32) 30 (67) 90 (145) 110 (185) 150 (409) Arachnida Acari G-231 10 (32) 50 (108) 60 (84) 100 (94) 60 (84) Arachnida Acari G-262 140 (443) 30 (95) 10 (32) — — 110 (348) Arachnida Acari G-226 20 (42) 110 (99) 60 (170) — — 130 (211) Arachnida Acari G-187 10 (32) 50 (127) 20 (42) — — 60 (84) Malacostraca Isopoda G-184 20 (42) 10 (32) — — 30 (67) — — Arachnida Acari G-288 20 (63) 20 (63) — — —— 10 (32) Arachnida Acari G-279 10 (32) — — 20 (42) 20 (63) 30 (67) Arachnida Acari G-280 20 (63) — — 30 (95) 10 (32) 40 (126) Arachnida Acari G-202 10 (32) — — 10 (32) 60 (107) 10 (32) Arachnida Acari G-185 20 (63) — — 10 (32) —— — — Hexapoda Blattodea G-020 10 (32) — — 10 (32) —— — — Arachnida Acari G-269 10 (32) — — — — —— 40 (97) Hexapoda Psocoptera G-211 10 (32) — — — — —— — — Arachnida Acari G-277 10 (32) — — — — —— — — Arachnida Acari G-287 10 (32) — — — — —— — — Hexapoda Psocoptera G-029 — — 10 (32) 50 (127) 70 (134) 40 (84) Malacostraca Isopoda G-039 — — 10 (32) 50 (158) 90 (166) — — Arachnida Acari G-100 — — 10 (32) — — 10 (32) 30 (48) Arachnida Acari G-083 — — 20 (63) — — —— 30 (95) Arachnida Acari G-150 — — 20 (63) — — —— 10 (32) Hexapoda Psocoptera G-289 — — 10 (32) — — —— — — Hexapoda Psocoptera G-102 —— —— 90 (129) 60 (84) 10 (32) Hexapoda Psocoptera G-118 —— —— 130 (287) 10 (32) — — Malacostraca Isopoda G-085 —— —— 40 (97) — — 20 (63) Hexapoda Blattodea G-119 —— —— 10 (32) —— — —

Physicochemical Foliar Traits Predict Assemblages of Litter/Humus Detritivore Arthropods

http://dx.doi.org/10.5772/intechopen.75076

15

## Acknowledgements

Funds for this research came from the CREST-Center for Applied Tropical Ecology and Conservation of the University of Puerto Rico at Rio Piedras Campus, grant NSF-HRD-0206200 through a fellowship to MFBA. Also, funds and logistic support came from the CREST-Center for Applied Tropical Ecology and Conservation of the University of Puerto Rico at Rio Piedras Campus and from the USDA Forest Service-International Institute of Tropical Forestry.
