Spatial distribution of rare species in lotic habitats

Abstract

Abstract. Species rarity is a common phenomenon in the biological world. Although rare species have always interested biologists, the meaning of ‘rare’ has not always been clear with the definition of rarity often arbitrary. In the current study, we investigate rarity in stream ecosystems using black flies (Diptera: Simuliidae). We defined rare species a priori as those species found ≤ 10% of stream sites examined (n = 111 streams for ‘summer collections’; n = 88 collection for ‘spring’ collections). Hence, we are exploring only one axis of rarity, restricted range. We first consider the distribution of each rare species separately to determine if the mean (euclidian) distance among streams (habitats) for each rare species differs from a random model. We next took a collective approach by pooling all rare species to determine the influence of stream conditions, niche breadth, and distance among habitats on rarity. Even within this biologically uniform group of flies, dispersal, range limits, and stream conditions all might play a role in rarity, and the importance of each of these factors appear to vary among species. Rather than looking for broad causes of rarity, future studies might be more fruitful if they looked at species‐specific causes.