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Spider Crabitat in the Kelp Forest: a Temporal Comparison of Microhabitat Partitioning of Spider Crabs

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Spider Crabitat in the Kelp Forest: a Temporal Comparison of Microhabitat Partitioning of Spider Crabs SPIDER CRABITAT IN THE KELP FOREST: A TEMPORAL COMPARISON OF MICROHABITAT PARTITIONING OF SPIDER CRABS Zoe Scholz, Adri Sparks, and Henry Vogt Kelp Forest Ecology, UCSC 2015 Abstract One longstanding question in community ecology is how pervasive and persistent habitat partitioning is as a mechanism of species coexistence and the maintenance of biodiversity. We studied habitat associations of five spider crab species of the family Majidae and how these habitat associations differed among species in a kelp forest in central California. To determine if these patterns of partitioning were persistent over time, we compared our results to a previous study of the same system conducted 32 years earlier. Our approach was to compare habitat use of each species with the relative availability of each habitat to quantify habitat associations and to compare those associations (i.e. disproportionate occurrence) among the five species to test for partitioning. Habitats were distinguished by the cover of animal and algal taxa and substratum type and relief. We found that there were strong associations between some species and particular habitats as well as similar species habitat associations between our study and the previously conducted study. Crabs exhibited associations with algal taxa that differed among the four species encountered. Our results indicate that spider crabs in Central California kelp forests partition their benthic habitat and this pattern of habitat partitioning might persist for decades. Our work adds to the many other studies that provide evidence for habitat partitioning as a mechanism of species coexistence. Introduction A fundamental question in ecology is how so many seemingly similar species are able to coexist in a community (Hutchinson 1959). Two hypotheses to explain the maintenance of diversity are niche diversification and resource partitioning (Schoener 1974, Hutchinson 1959), where otherwise similar species are able to divide their habitat and resources to the point that competition with similar species is avoided (MacAurthur 1958). For habitat partitioning, the species should exhibit strong habitat associations, and those associations should differ among the species, such that they avoid competing for the same resources. In addition, for resource partitioning to promote coexistence, those habitat associations need to be persistent through time. However, rarely have studies of resource partitioning been repeated over time to evaluate the persistence of observed patterns of partitioning (Wiens 1977). Kelp forest communities in coastal temperate waters are known for their great species diversity (Carr and Reed 2015). A conspicuous component of these communities is the diverse assemblage of the spider crabs of the family Majidae. Within a kelp bed there can be five or six species co- occurring. We looked at the spider crab family Majidae, a well diversified crab family in the temperate kelp forest. Such species diversity in such a small spatial context could cause significant interspecific overlap in habitat, creating an ideal environment for resource partitioning. We examined the habitat associations between five species of spider crab: Scyra acutifrons, Pugettia richii, Pugettia producta, Pugettia (formerly Mimulus) foliatus, and Loxorhynchus crispatus in the kelp forest and what type of substrate they associate with. By looking not only at where these species associate, but also what the overall substrate composition of the studied kelp forest is, we can get an accurate picture of what type of substrate these species prefer, and if they are disproportionately associating with a particular substrate, showing that they have a defined substrate niche. In addition, we compare the observed patterns of habitat partitioning with those described with a previous study 32 years prior. Our questions therefore are two-fold. First, do the five species of spider crabs exhibit habitat associations and are they distinct among each other. Our second question is if these associations persist over time. Hines (1982) provides data regarding the habitat associations of our target species, however no study has been repeated to determine how consistent his observed patterns are, making our study a useful link in the long term observations on the Majidae species’ habitat associations. We found the comprehensive study of Majidae at Hopkins Marine Station by Hines (1982) a good conceptual framework for our own study, helping guide what kind of questions would be valuable to ask as well as how we might conduct our study. Our findings were compared solely to this study as there was a lack of literature pertaining to spider crab habitat patterns in temperate kelp forests. He examined the differences in microhabitat use, population cycles, carapace size, diet, and predators between the same Majidae species we looked at. Due to restrictions in both time and resources we augmented our study to more accurately fit our question. Replicating Hines (1982), focusing on habitat associations, provides us with valuable insight into whether observed habitat associations of these species have persisted over time. Methods General Approach There are two components to our study: a survey of spider crab species and their associated habitats, and comparing this data to a previous study by Hines, 1982. To accomplish this, we conducted observational field studies at Hopkins Marine Station in Pacific Grove, CA (36"36'N, 12 1°54'W). We conducted swath surveys at intervals throughout the middle of the kelp forest to collect both crab and habitat data. During our first surveys we identified spider crab species and took habitat data surrounding each crab. During our second round of surveys, we collected general habitat data, uncoupled from the crabs, in order to determine available substrate. This allowed us to see if crabs were disproportionately associating with certain habitat types. To compare our study to Hines’ we used similar habitat categories, which included algae, invertebrates, and physical substrate characteristics. System Description Kelp forests are highly productive and species-rich ecosystems. Like other terrestrial and marine systems, it is important to understand what factors allow species to coexist in a given environment. This is especially pertinent to kelp forests because they are home to a staggering diversity of species. One factor that may contribute to the maintenance of diversity is resource partitioning by species habitat associations. The kelp forest at Hopkins Marine Station is a Giant Kelp, Macrocystis pyrifera, forest. The substrate is characterized by high relief granitic reef outcroppings intermixed with beds of sand and shell hash. The inshore side of the reef has areas dominated by leafy red and articulated coralline algae, and slopes up to a zone of Phyllospadix surfgrass in the shallows. In the middle of the reef, Macrocystis, some understory algae, and many benthic invertebrate species cover the rocks, while the sandy areas are home to Diopatra beds and some other invertebrate species. The outer edge of the forest abuts a sand channel at a depth of about 40 ft. The number of habitats formed by the varied topography and different algae and sessile invertebrate species, makes Hopkins an ideal site for studying microhabitat associations of mobile invertebrates. Study Design Our study aims to address the following two hypotheses: 1) Spider crab species at Hopkins Marine Station exhibit habitat associations and these associations differ among species. To determine if the spider crab species included in our study have disproportionate associations with certain microhabitat types, we used swath and UPC surveys. Habitat use was measured by encountering individuals of each species on random transects and recording their surrounding habitat in 0.25m2 quadrats. Habitat availability was measured by recording habitat attributes in randomly distributed 0.25m2 quadrats. The swath survey allowed us to find the crabs and identify the surrounding habitat, while the UPC habitat survey gave us available habitat types and how often they occurred. Assuming there is no habitat association, the crabs should be found on habitats proportionate to what is available. Using PERMANOVA analyses on this data, we were able to see if all the spider crab species, as one group, were found disproportionately on certain habitat types. A P-value of <0.05 rejects our null hypothesis and confirms that there are significant habitat associations. Using Non-metric MDS plots and Pair-Wise analyses, we are able to compare habitat usage between each species, and determine if certain species associate more strongly with certain habitats. Again, P-values of <0.05 mean there is significant evidence to reject the null hypothesis that there are no differences in habitat associations between species. All statistical analyses were conducted with PRIMER-E version 7. 2) These patterns persist over time, in that our findings will be similar to the previous study by Hines, 1982.To compare our data to what Hines presented in 1982, we used the same swath data as the previous hypothesis. Hines’ microhabitat data was presented in bar graphs of how often he found each species in each habitat category. Because we used similar habitat categories, we were able to quantitatively compare our data with his. We were not able to run statistical analyses on how significant our comparisons are because of differences in sampling methods, but we were able to visually compare the data and
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