Population Response to Habitat Fragmentation in a Stream-Dwelling Brook Trout Population

Abstract

Fragmentation can strongly influence population persistence and expression of life-history strategies in spatially-structured populations. In this study, we directly estimated size-specific dispersal, growth, and survival of stream-dwelling brook trout in a stream network with connected and naturally-isolated tributaries. We used multiple-generation, individual-based data to develop and parameterize a size-class and location-based population projection model, allowing us to test effects of fragmentation on population dynamics at local (i.e., subpopulation) and system-wide (i.e., metapopulation) scales, and to identify demographic rates which influence the persistence of isolated and fragmented populations. In the naturally-isolated tributary, persistence was associated with higher early juvenile survival (∼45% greater), shorter generation time (one-half) and strong selection against large body size compared to the open system, resulting in a stage-distribution skewed towards younger, smaller fish. Simulating barriers to upstream migration into two currently-connected tributary populations caused rapid (2–6 generations) local extinction. These local extinctions in turn increased the likelihood of system-wide extinction, as tributaries could no longer function as population sources. Extinction could be prevented in the open system if sufficient immigrants from downstream areas were available, but the influx of individuals necessary to counteract fragmentation effects was high (7–46% of the total population annually). In the absence of sufficient immigration, a demographic change (higher early survival characteristic of the isolated tributary) was also sufficient to rescue the population from fragmentation, suggesting that the observed differences in size distributions between the naturally-isolated and open system may reflect an evolutionary response to isolation. Combined with strong genetic divergence between the isolated tributary and open system, these results suggest that local adaptation can ‘rescue’ isolated populations, particularly in one-dimensional stream networks where both natural and anthropogenically-mediated isolation is common. However, whether rescue will occur before extinction depends critically on the race between adaptation and reduced survival in response to fragmentation.

Contribution of conservation genetics in assessing neotropical freshwater fish biodiversity

“…Populations structured by habitat fragmentation due to barriers such as roads, dams, waterfalls and climate (drought and rain) have their abundance, dispersal and population size reduced, thereby increasing the risk of extinction (Gross et al, 2004;Letcher et al, 2007). This fragmentation can lead to the total or partial isolation of a population, conditioning the response of the individuals.…”

Monitoring, Restoration, and Source Water Protection: Canadian Community-Based Environmental Organizations’ Efforts towards Improving Aquatic Ecosystem Health

“…Excessive sedimentation poses a major threat to aquatic ecosystems [82][83][84] and "is one of the primary causes of stream corridor degradation" [46] (p. 442). When fragmentation restricts movement within aquatic ecosystems preventing species from accessing critical habitat, it has harmful effects on biodiversity that can lead to population decline or even extinction [85][86][87][88][89]. The removal of riparian zones leaves streams more susceptible to terrestrial influences, and can negatively impact in stream habitat and water quality [47,52,53,57].…”

Potential impacts of climate change on brook trout (Salvelinus fontinalis) populations in streams draining the Laurel Hill in Pennsylvania

“…The resulting fragmentation would confine remaining populations to small isolated patches vulnerable to extirpation due to loss of avenues of connectivity for potential colonizers (Fleebe et al 2006;Letcher et al 2007). The strong correlation between air/stream temperatures in the cool-water receiving streams (Figure 8) indicates potentially narrowing windows for movement of brook trout and other cold-water species through their respective tributary networks, creating the potential for genetic isolation.…”