Erin S. Lindquist

Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition

Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100xa0m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabsxa0m−2) than in the inland zone (0.76±0.78 crabsxa0m−2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.

Land crabs as key drivers in tropical coastal forest recruitment

Plant populations are regulated by a diverse assortment of abiotic and biotic factors that influence seed dispersal and viability, and seedling establishment and growth at the microsite. Rarely does one animal guild exert as significant an influence on different plant assemblages as land crabs. We review three tropical coastal ecosystems–mangroves, island maritime forests, and mainland coastal terrestrial forests–where land crabs directly influence forest composition by limiting tree establishment and recruitment. Land crabs differentially prey on seeds, propagules and seedlings along nutrient, chemical and physical environmental gradients. In all of these ecosystems, but especially mangroves, abiotic gradients are well studied, strong and influence plant species distributions. However, we suggest that crab predation has primacy over many of these environmental factors by acting as the first limiting factor of tropical tree recruitment to drive the potential structural and compositional organisation of coastal forests. We show that the influence of crabs varies relative to tidal gradient, shoreline distance, canopy position, time, season, tree species and fruiting periodicity. Crabs also facilitate forest growth and development through such activities as excavation of burrows, creation of soil mounds, aeration of soils, removal of leaf litter into burrows and creation of carbon‐rich soil microhabitats. For all three systems, land crabs influence the distribution, density and size‐class structure of tree populations. Indeed, crabs are among the major drivers of tree recruitment in tropical coastal forest ecosystems, and their conservation should be included in management plans of these forests.

Effects of Power-line Maintenance on Forest Structure in a Fragmented Urban Forest, Raleigh, NC

Abstract n With the increase in urban development, forest fragments are becoming more prevalent. In urban areas, there is a tendency to hide power-lines within or on the edges of these fragmented forests; however, it is unknown how the maintenance of vegetation under and along power-lines impacts the forest composition and structure of an adjacent fragmented, urban forest. An urban, fragmented maple-oak-hickory forest is located on the Meredith College campus, Raleigh, NC. A 1-ha plot with a hundred 10- × 10-m subplots was established in 2007 to initiate a long-term project supporting undergraduate research. An adjacent meadow is cut and maintained regularly up to the forest and plot edge for power-line clearance and access. We identified, tagged, and measured all of the trees with a diameter at breast height (DBH) ≥ 5 cm in this permanent plot, and compared the tree species richness (S), Shannon-Weiner diversity index (H), Sorensons similarity index (Ss ), DBH, stem density, and basal area along the 100-m gradient from the forest edge. We also used a non-metric multidimensional scaling (NMS) analysis to describe how species composition changed along the gradient. Our findings showed that S, H, and Ss did not change along the 100-m gradient. The NMS confirmed that species composition was not different in the edge subplots (0–10 m from edge) compared to all other subplots and therefore was not impacted by continual, local disturbance along forest edges. However, we found that forest structure changed along the gradient with the exception of mean DBH; stem density and total basal area varied along the 100-m gradient. There was greater stem density along the edge of the forest (0–5 m and 10–20 m from edge) compared to the other interior subplots. Some of the interior subplots (10–20 m and 60–70 m from the edge) had a higher total basal area than the remaining plots. As expected, we also found that there was a negative linear relationship between DBH and stem density for all subplots. Our results confirm trends found in previous studies that community structure parameters (stand density and basal area) differ between forest edges and their respective forest interiors, but did not agree with previous research, which found species composition to be affected by edges. We believe the regular pruning of the forest edge adjacent to the power-lines explains our observed differences in forest structure, but tree species richness, diversity, similarity, and composition may be determined by the disturbance of larger-scale ecological processes. Our results show how power-line placement within a fragmented urban forest can affect the structure of the adjacent forest, and we recommend that the ecological effects of power-line corridors should be further investigated and incorporated into the larger body of literature on forest fragmentation.

Utilization of Woody Debris by Peromyscus leucopus in a Fragmented Urban Forest

Abstract n Small nocturnal mammals, such as Peromyscus leucopus (White-footed Mouse), tend to avoid open spaces due to the threat of predation. Previous studies have shown that Peromyscus and other small-mammal species are captured at higher frequencies at mature forest sites with higher densities of woody debris. Reported trapping frequencies along forest edges relative to continuous forest have varied in previous literature, possibly due to regional differences in forest composition and Peromyscus distribution. We hypothesized that mice in an urban forest setting would be captured at a higher frequency in trapping sites with higher volumes of woody debris and capture rates would be lower at the edge than the interior. We trapped mice in 100 to 121 Sherman live traps in a permanent 1-ha plot in an urban, fragmented forest on the Meredith College campus in Raleigh, NC over a two-year period. We also measured volume of woody debris at each trapping site in one (2007) of the two years. Between the two years (2007 and 2008), trapping rates of P. leucopus were lower in 2008 than in 2007, but we estimated a higher population size in 2008 than in 2007. We found no correlation between volume of woody debris and number of P. leucopus captured in 2007 and that capture rates did not vary with distance from the forest edge in both years. Our results support previous findings that P. leucopus are nonspecific users of microhabitat, but are contrary to other research that found a positive correlation between amount of woody debris and abundance of Peromyscus.

Effects of urbanization on the population structure of freshwater turtles across the United States: Urbanization Effects on Turtles

Landscape-scale alterations that accompany urbanization may negatively affect the population structure of wildlife species such as freshwater turtles. Changes to nesting sites and higher mortality rates due to vehicular collisions and increased predator populations may particularly affect immature turtles and mature female turtles. We hypothesized that the proportions of adult female and immature turtles in a population will negatively correlate with landscape urbanization. As a collaborative effort of the Ecological Research as Education Network (EREN), we sampled freshwater turtle populations in 11 states across the central and eastern United States. Contrary to expectations, we found a significant positive relationship between proportions of mature female painted turtles (Chrysemys picta) and urbanization. We did not detect a relationship between urbanization and proportions of immature turtles. Urbanization may alter the thermal environment of nesting sites such that more females are produced as urbanization increases. Our approach of creating a collaborative network of scientists and students at undergraduate institutions proved valuable in terms of testing our hypothesis over a large spatial scale while also allowing students to gain hands-on experience in conservation science.